Chapter 1 Introduction to Digital Radiography and PACS

Chapter 1 Introduction to Digital Radiography and PACS
Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Objectives Define the term digital imaging.
Explain latent image formation for conventional radiography. Describe the latent image formation process for computed radiography. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Objectives Compare and contrast the latent image formation process for indirect capture digital radiography and direct capture digital radiography. Explain what a PACS (picture archiving and communication system) is and how it is used. Define digital imaging and communications in medicine. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Computed radiography
DICOM (digital imaging and communications in medicine) Digital imaging Digital radiography Direct capture DR Indirect capture DR PACS Teleradiology Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Conventional Radiography
Method is film-based. Method uses intensifying screens. Film is placed between two screens. Screens emit light when x-rays strike them. Film is processed chemically. Processed film is viewed on lightbox. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Imaging Digital imaging is a broad term.
Term was first used medically in 1970s in computed tomography (CT). Digital imaging is defined as any image acquisition process that produces an electronic image that can be viewed and manipulated on a computer. In radiology, images can be sent via computer networks to a variety of locations. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Historical Development of Digital Imaging
CT coupled imaging devices and the computer. Early CT scanners required hours to produce a single slice. Reconstruction images took several days to produce. First CT scanners imaged the head only. First scanner was developed by Siemens. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Magnetic resonance imaging (MRI) became available in the early 1980s. Lauterbur paper in 1973 sparked companies to research MRI. Many scientists and researchers were involved. Advancements in fluoroscopy occurred in the 1970s as well. Analog-to-digital converters allowed real-time images to be viewed on TV monitors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Fluoroscopic images could also be stored on a computer. Ultrasound and nuclear medicine used screen capture to grab the image and convert it digitally. Eventually, mammography converted to digital format. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography Development
Concept began with Albert Jutras in Canada in the 1950s. Early PACS systems were developed by the military to send images between Veterans Administration hospitals in the 1980s. Development was encouraged and supported by the U.S. government. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography Development
Early process involved scanning radiographs into the computer and sending them from computer to computer. Images were then stored in PACS. Computed and digital radiography followed. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Computed Radiography Uses storage phosphor plates
Uses existing equipment Requires special cassettes Requires a special cassette reader Uses a computer workstation and viewing station and a printer Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Computed Radiography Storage phosphor plates are similar to intensifying screens. Imaging plate stores x-ray energy for an extended time. Process was first introduced in the United States by Fuji Medical Systems of Japan in 1983. First system used a phosphor storage plate, a reader, and a laser printer. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Computed Radiography Method was slow to be accepted by radiologists.
Installation increased in the early 1990s. More and more hospitals are replacing film/screen technology with digital systems. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography Cassetteless system
Uses a flat panel detector or charge-coupled device (CCD) hard-wired to computer Requires new installation of room or retrofit Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography Two types of digital radiography
Indirect capture DR Machine absorbs x-rays and converts them to light. CCD or thin-film transistor (TFT) converts light to electric signals. Computer processes electric signals. Images are viewed on computer monitor. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography Direct capture DR Photoconductor absorbs x-rays.
TFT collects signal. Electrical signal is sent to computer for processing. Image is viewed on computer screen. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography First clinical application was in 1970s in digital subtraction. University of Arizona scientists applied the technique. Several companies began developing large field detectors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography DR used CCD technology developed by the military and then used TFT arrays shortly after. CCD and TFT technology developed and continues to develop in parallel. No one technology has proved to be better than the other. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Comparison of Film to CR and DR
For conventional x-ray film and computed radiography (CR), a traditional x-ray room with a table and wall Bucky is required. For DR, a detector replaces the Bucky apparatus in the table and wall stand. Conventional and CR efficiency ratings are about the same. DR is much more efficient, and image is available immediately. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Latent image formation is different in CR and DR. Conventional film/screen Film is placed inside of a cassette that contains an intensifying screen. X-rays strike the intensifying screen, and light is produced. The light and x-ray photons interact with the silver halide grains in the film emulsion. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

An electron is ejected from the halide. Ejected electron is attracted to the sensitivity speck. Speck now has a negative charge, and silver ions will be attracted to equal out the charge. Process happens many times within the emulsion to form the latent image. After chemical processing, the sensitivity specks will be processed into black metallic silver and the manifest image is formed. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

A storage phosphor plate is placed inside of CR cassette. Most storage phosphor plates are made of a barium fluorohalide. When x-rays strike the photosensitive phosphor, some light is given off. Some of the photon energy is deposited within the phosphor particles to create the latent image. The phosphor plate is then fed through the CR reader. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR, continued Focused laser light is scanned over the plate, causing the electrons to return to their original state, emitting light in the process. This light is picked up by a photomultiplier tube and converted into an electrical signal. The electrical signal is then sent through an analog-to-digital converter to produce a digital image that can then be sent to the technologist review station. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

No cassettes are required. The image acquisition device is built into the table and/or wall stand or is enclosed in a portable device. Two distinct image acquisition methods are indirect capture and direct capture. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

DR, continued Indirect capture is similar to CR in that the x-ray energy stimulates a scintillator, which gives off light that is detected and turned into an electrical signal. With direct capture, the x-ray energy is detected by a photoconductor that converts it directly to a digital electrical signal. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Processing Conventional radiography CR and DR
Image is determined by the film itself and the chemicals. CR and DR Image processing takes place in a computer. For CR, the computer is located near the readers. For DR, the computer is located next to x-ray console, or it may be integrated within the console, and the image is processed before moving on to the next exposure. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Latitude or Dynamic Range
Conventional radiography Based on the characteristic response of the film, which is nonlinear. Radiographic contrast is primarily controlled by kilovoltage peak. Optical density on film is primarily controlled by milliampere-second setting. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Latitude or Dynamic Range
CR and DR Contain a detector that can respond in a linear manner. Exposure latitude is wide, allowing the single detector to be sensitive to a wide range of exposures. Kilovoltage peak still influences subject contrast, but radiographic contrast is primarily controlled by an image processing look-up table. Milliampere-second setting has more control over image noise, whereas density is controlled by image-processing algorithms. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Scatter Sensitivity It is important to minimize scattered radiation with all three acquisition systems. CR and DR can be more sensitive to scatter than screen/film. Materials used in the many CR and DR image acquisition devices are more sensitive to low-energy photons. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Picture Archival and Communication Systems
Networked group of computers, servers, and archives to store digital images Can accept any image that is in DICOM format Serves as the file room, reading room, duplicator, and courier Provides image access to multiple users at the same time, on-demand images, electronic annotations of images, and specialty image processing Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Custom designed for each facility Components/features can vary based on the following: Volume of patients Number of interpretation areas Viewing locations Funding Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Early systems did not have standardized image formats. Matching up systems was difficult. Vendors kept systems proprietary and did not share information. DICOM standards helped change this by allowing communication between vendors’ products. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

First full-scale PACS Veterans Administration Medical Center in Baltimore used PACS in 1993. PACS covered all modalities except mammography. Shortly after, PACS was interfaced with radiology information systems, hospital information systems, and electronic medical records. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

PACS Uses Made up of different components Reading stations
Physician review stations Web access Technologist quality control stations Administrative stations Archive systems Multiple interfaces to other hospital and radiology systems Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

PACS Uses Early PACS seen only in radiology and some cardiology departments. PACS now can be used in multiple departments. Archive space can be shared among departments. PACS reading stations may also have image processing capabilities. PACS allows radiologists to reconstruct and stitch images in their offices. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

PACS Uses Orthopedic workstations are available for the following:
Surgeons can plan joint replacement surgery. Specialized software allows matching of best replacement for patient with patient anatomy. System saves time and provides better fit. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 4 Cassette-Based Equipment: The Computed Radiography Cassette, Imaging Plate, and Reader

Objectives Describe the basic construction of a computed radiography cassette. Describe the construction of a computed radiography imaging plate. Identify the various layers of the imaging plate. Describe the purpose of each layer of the imaging plate. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Objectives Explain the process of photostimulation in the imaging plate. Describe the process of laser beam formation. Explain the process of reading the imaging plate. Compare conventional radiographic screen and film speed to computed radiography systems. Discuss how an image is erased from the imaging plate. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Analog-to-digital conversion Backing layer Barcode
Barium fluorohalide Cassette Color layer Conductive layer Electron-to-light conversion Imaging plate Laser Phosphor center Phosphor layer Photomultiplier Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Photostimulable luminescence Photostimulable phosphor
Protective layer Reflective layer Speed Support layer Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Introduction Cassette-based or computed radiography (CR) systems differ from conventional radiography in that the cassette is simply a lightproof container to protect an imaging plate from light and handling. The imaging plate takes place of radiographic film and is capable of storing an image formed by incident x-ray photon excitation of phosphors. The reader releases the stored light and converts it into an electrical signal, which is then digitized. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Computed Radiography Equipment
Cassette Imaging plate Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Cassette Looks like a film/screen cassette
Durable, lightweight plastic Backed by aluminum No intensifying screens Antistatic material Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging Plate Construction
Image recorded on a thin sheet of plastic known as the imaging plate Consists of several layers: Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging Plate Protective layer—a very thin, tough, clear plastic for protection of the phosphor layer Phosphor, or active, layer A layer of photostimulable phosphor that “traps” electrons during exposure Usually made of phosphors from the barium fluorohalide May also contain a dye that differentially absorbs the stimulating light to prevent as much spread as possible Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging Plate Reflective layer—a layer that sends light in a forward direction when released in the cassette reader May be black to reduce the spread of stimulating light and the escape of emitted light Some detail lost in this process Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging Plate Conductive layer—a layer of material that will absorb and reduce static electricity Color layer—Newer plates may contain a color layer, located between the active layer and the support that absorbs the stimulating light but reflects emitted light. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging Plate Support layer—a semirigid material that gives the imaging sheet some strength Backing layer—a soft polymer that protects the back of the cassette Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Cassette and Imaging Plate
Cassette contains a window with a barcode label or barcode sticker on the cassette. Label enables technologist to match the image information with the patient-identifying barcode on the exam request. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Cassette and Imaging Plate
For each new exam, check the following: Patient identifying barcode and the barcode label on the cassette must be scanned and connected to the patient position or exam menu. Cassette also is labeled: With stickers indicating the top and left side of the cassette, or With a label on the back of the cassette indicating the top and right sides of the patient Stickers serve to orient the cassette to the top of the patient and the patient’s right side so that the image orientation is in line with the computer algorithm. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Acquiring and Forming the Image
Patient is radiographed exactly the same way as in conventional radiography. Patient is positioned using appropriate positioning techniques. Cassette is placed on the tabletop or within the table Bucky. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Patient is exposed using the proper combination of kilovoltage peak, milliampere-seconds, and distance. Difference lies in how the exposure is recorded. In CR, remnant beam interacts with electrons in the barium fluorohalide crystals contained within the imaging plate. Interaction stimulates, or gives energy to, electrons in the crystals. Crystals enter into the conductive layer, where they are trapped in an area of the crystal known as the color or phosphor center. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

This trapped signal remains for hours, even days. Deterioration begins almost immediately. Trapped signal is never completely lost. A certain amount of an exposure remains trapped so that the imaging plate can never be completely erased. Residual trapped electrons are so few in number that they do not interfere with subsequent exposures. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Reader With CR systems, no chemical processor or darkroom is necessary. Cassette is fed into a reader: Removes the imaging plate Scans it with a laser, releasing the stored electrons Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Laser Laser stands for light amplification by stimulated emission of radiation. Laser creates and amplifies a narrow, intense beam of coherent light. Atoms or molecules of a crystal such as a gas, liquid, or substances such as ruby or garnet are excited so that more of them are at high energy levels rather than low energy levels. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Laser Surfaces at both ends of the laser container
Reflect energy back and forth as atoms bombard each other Stimulate the lower-energy atoms to emit secondary photons in the same frequency as the bombarding atoms When the energy builds sufficiently, the atoms discharge simultaneously as a burst of coherent light Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Laser Laser requires a constant power source to prevent output fluctuations. Laser beam passes through beam-shaping optics to an optical mirror, which directs the laser beam to the surface of the imaging plate. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Using the Laser to Read the Imaging Plate
Cassette is put into the reader. Imaging plate is extracted. Plate is scanned with a helium laser beam or solid-state laser diodes. Beam is about 100 µm wide with a wavelength of 633 nm, or 670 to 690 nm for solid-state. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Reader scans the plate with red light in a zigzag, or raster pattern. Laser gives energy to the trapped electrons. Red laser light is emitted at approximately 2 eV, which is necessary to energize the trapped electrons. Extra energy allows the trapped electrons to escape the active layer, where they emit visible blue light at an energy of 3 eV as they relax into lower energy levels. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging plate moves through the reader. Laser scans across the imaging plate multiple times. Process is known as translation. Scanning produces lines of light intensity information detected by a photomultiplier. Photomultiplier amplifies the light and sends it to a digitizer. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Translation speed of the plate must be coordinated with the scan direction of the laser, or the spacing of the scan lines will be affected. The action of moving the laser beam across the imaging plate is much like holding a flashlight at the same height and moving it back and forth across a wall. The more angled the beam is, the more elliptical the shape of the beam. The same thing happens with the reader laser beam as it scans. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

If this change in the beam shape were ignored, output of the screen would differ from the middle to the edges. Result would be differing spatial resolution and inconsistent output signals depending on the position and angle of the laser beam. To correct, beam is “shaped” by special optics that keep the beam size, shape, and speed largely independent of the beam position. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Beam deflector moves the laser beam rapidly back and forth across the imaging plate to stimulate the phosphors. Mirrors are used to ensure that the beam is positioned consistently. Because the type of phosphor material in the imaging plate has an effect on the amount of energy required, the laser and the imaging plate should be designed to work together. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The light collection optics direct the released phosphor energy to an optical filter and then to the photodetector. Although there will be variances between manufacturers, the typical throughput is 50 cassettes per hour. Some manufacturers claim up to 150 cassettes per hour, but based on average hospital department workflow, 50 cassettes per hour is much more realistic. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digitizing the Signal When we talk about digitizing a signal, such as the light signal from the photomultiplier, we are talking about assigning a numeric value to each light photon. As human beings, we experience the world analogically. We see the world as infinitely smooth gradients of shape and colors. Analog refers to a device or system that represents changing values as continuously variable physical quantities. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digitizing the Signal A typical analog device is a watch in which the hands move continuously around the face and is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a finite number of times. In the process of digitizing the light signal, each phosphor storage center is scanned, and the released electrons enter a digitizer that divides the analog image into squares (matrix) and assigns each square in the matrix a number based on the brightness of the square. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digitizing the Signal Each square is called a pixel or picture element. The typical number of pixels in a matrix range from about 512 × 512 to 1024 × 1024 and can be as large as 2500 × 2500. The more pixels there are, the greater the image resolution. The image is digitized by position (spatial location) and by intensity (gray level). Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digitizing the Signal Each pixel contains bits of information, and the number of bits per pixel that define the shade of each pixel is known as bit depth. If a pixel has a bit depth of 8, then the number of gray tones that pixel can produce is 2 to the power of the bit depth, or 28 or 256 shades of gray. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digitizing the Signal So, how bright a pixel is determines where it will be located in the matrix in conjunction with the amount of gray level or bit depth. Some CR systems have bit depths of 10 or 12, resulting in more shades of gray. Each pixel can have a gray level between 0 (20) and 4096 (212). The gray level will be a factor in determining the quality of the image. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Resolution Spatial resolution refers to the amount of detail present in any image. Phosphor layer thickness and pixel size determines resolution in CR. The thinner the phosphor layer is, the higher resolution. Film/screen radiography resolution at its best is limited to 10 line pairs per millimeter (lp/mm). CR resolution is 2.55 lp/mm to 5 lp/mm, resulting in less detail. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Resolution CR dynamic range, or the number of recorded densities, is much higher, and lack of detail is difficult to discern. More tissue densities on the digital radiograph are seen, giving the appearance of more detail. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Resolution For example, an anteroposterior knee radiograph typically does not show soft tissue structures on the lateral aspects of the distal femur or proximal tibia or fibula. An anteroposterior knee digital image shows not only the soft tissue but also the edge of the skin. This is due to the wider dynamic recording range and does not mean that there is additional detail. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Speed In conventional radiography, speed is determined by the size and layers of crystals in the film and screen. In CR, speed is not exactly the same because there is no intensifying screen or film. The phosphors emit light according to the width and intensity of the laser beam as it scans the plate, resulting in a relative speed that is roughly equivalent to a 200-speed film/screen system. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Speed CR system speeds are a reflection of the amount of photostimulable luminescence given off by the imaging plate while being scanned by the laser. For example, Fuji Medical Systems reports that a 1-mR exposure at 80 kVp and a source-to-image distance of 72 inches will result in a luminescence value of 200, hence the speed number. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Speed In CR, most cassettes have the same speed; however, there are special extremity or chest cassettes that produce greater resolution. These are typically 100 relative speed. Great care must be taken when converting to a CR system from a film/screen system to adjust technical factors to reflect the new speed. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Speed For example, if the technique for a knee was 20 mA-s at 70 kVp in the Bucky with a 400 screen speed system, then the new CR technique would be 10 mA-s at 70 kVp if the grid ratios are equal. If not, then a grid conversion factor would be used. More detail about exposure settings for CR systems is discussed in Chapter 5. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Erasing the Image The process of reading the image returns most of electrons to a lower energy state. Reading effectively removes the image from the plate. Imaging plates are extremely sensitive to scatter radiation. Plates should be erased to prevent a buildup of background signal. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Erasing the Image Plates should be run at least once a week under an erase cycle to remove background radiation and scatter. Erasure mode allows the surface of imaging plate to be scanned without recoding generated signal. Systems automatically erase the plate by flooding it with light to remove any electrons still trapped after the initial plate reading. Cassettes should be erased before use if the last time of erasure is unknown. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Preprocessing, Processing, and Forwarding the Image
Once the imaging plate has been read, the signal is sent to the computer. Image is preprocessed. A monitor displays the image so that the technologist can do the following: Review the image Manipulate it if necessary (postprocessing) Send it to the quality control station and ultimately to the picture archiving and communications system (PACS) Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary The cassette-based imaging system consists of a specially designed cassette made of durable, lightweight plastic. The imaging plate is multilayered with protective, phosphor, reflective, conductive, color, support, and backing layers. Barcodes are used to identify the cassette or imaging plate and exam request to link the imaging plate with the patient exam. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Barium fluorohalide crystals in the imaging plate release light energy that is stored in the conductive layer. The imaging plate reader uses a laser to scan the imaging plate, releasing the energy stored in the conductive layer as blue light. A photomultiplier amplifies the light and sends it to a signal digitizer. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary The digitizer assigns a numeric value to each pixel in a matrix according to the brightness of the light and its position. Spatial resolution of the digital image is determined by the thickness of the phosphor layer and the number of pixels, which also effects resolution. Cassette-based spatial resolution is approximately 2.55 lp/mm to 5 lp/mm (lower than 10 lp/mm of conventional radiography). Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Because so many more densities are recorded in CR (wide dynamic range), images appear more detailed. Because energy stored in the imaging plate is lost over time, imaging plates should be read as quickly as possible to avoid image information loss. Imaging plates are erased by exposing them to bright light such as fluorescent light. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary The image is sent to the quality control station where it is analyzed and sent to PACS for long-term storage. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 5 Cassette-Based Image Acquisition

Objectives Discuss the importance of matching the body part being examined to the exam menu. Discuss the selection of technical factors for density, contrast, and penetration. Relate imaging plate size selection to radiographic exams. Describe the grid selection process. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Objectives Relate the importance of preprocessing collimation.
Discuss the importance of patient side markers. Compare exposure indicators for the major computed radiography (CR) manufacturers and vendors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Artifacts Automatic data recognition Collimation EI
Exposure index Exposure indicators Fixed mode Grid focus Grid frequency Grid ratio Histogram kVp Logarithm of the median mAs Matrix Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Multiple manual selection mode Operator errors
Quantum mottle Quantum noise Reader errors S Semiautomatic mode Sensitivity Shuttering Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Computed Radiography Image Acquisition
Part selection Technical factors Equipment selection Collimation Side/position markers Exposure indicators Image data recognition and preprocessing Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Part Selection Once the patient has been positioned and the plate has been exposed, you must select the exam or body part from the menu choices on your workstation. If you are performing a skull exam, you then select “skull” from the workstation menu. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Part Selection Selecting the proper body part and position is important for the proper conversion to take place. Image recognition is accomplished through complex mathematical computer algorithms, and if the improper part and/or position is selected, the computer will misinterpret the image. For example, if a knee exam is to be performed and the exam selected is for skull, the computer will interpret the exposure for the skull, resulting in improper density and contrast and inconsistent image graininess. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Part Selection It is not acceptable to select a body part or position different from that actually being performed simply because it looks better. If the proper exam/part selection results in a suboptimal image, then service personnel should be notified of the problem and the problem should be corrected as soon as possible. Improper menu selections may lead to overexposure of the patient and to repeats. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Technical Factors Kilovoltage peak selection
Milliampere-second (mAs) selection Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Kilovoltage Peak Selection
Kilovoltage peak, milliampere-second, and distance are chosen in exactly the same manner as for conventional film/screen radiography. Kilovoltage peak must be chosen for penetration and the type and amount of contrast desired. In the early days of CR, kilovoltage peak minimum values were set at about 70. This is no longer true or necessary. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Kilovoltage peak values now range from around 45 to 120. It is not recommended that kilovoltage peak values less than 45 or greater than 120 be used because those values may be inconsistent and may produce too little or too much excitation of the phosphors. The k-edge of phosphor imaging plates ranges from 30 to 50 keV so that exposure ranges of 60 to 110 kVp are optimum. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

However, exposures outside that range are widely used and depend on the quality desired. Remember, the process of attenuation of the x-ray beam is exactly the same as in conventional film/screen radiography. It takes the same kilovoltage peak to penetrate the abdomen with CR systems as it did with a film/screen system. It is vital that the proper balance between patient dose and image contrast be achieved. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Milliampere-Second Selection
Milliampere-second is selected according to the number of electrons needed for a particular part. Too few electrons and no matter what level of kilovoltage peak is chosen, the result will be a lack of sufficient phosphor stimulation. When insufficient light is produced, the image will be grainy, a condition known as quantum mottle or quantum noise. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Milliampere-Second Selection
CR systems typically use automatic exposure controls just as many film/screen systems do. Backscatter from the cassette/detector influences the number of milliampere-seconds necessary to create the image. When converting from film/screen systems to a CR system, it is critical that the automatic exposure control be recalibrated. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Equipment Selection Imaging plate selection Grid selection

Imaging Plate Selection
Two important factors should be considered when selecting the CR imaging cassette: type and size. Most manufacturers produce two types of imaging plates: standard and high resolution. Cassettes should be marked on the outside to indicate high-resolution imaging plates. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Typically, high-resolution imaging plates are limited to size range and are most often used for extremities, mammography, and other exams requiring increased detail. In conventional film/screen radiography, one is taught to select a cassette appropriate to the size of the body part being imaged. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR cassette selection is the same, but even more critical. CR digital images are displayed in a matrix of pixels, and the pixel size is an important factor in determining the resolution of the displayed image. The CR reader scans the imaging plate at a relatively constant frequency, about 2000 × 2000 pixels. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Use of the smallest imaging plate possible for each exam results in the highest sampling rate. When the smallest possible imaging plate is selected, a corresponding matrix is used by the computer algorithm to process the image. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

A 2000 × 2000 matrix on an 8 × 10-inch cassette results in much smaller pixel size, thereby increasing resolution. If, for example, a hand were imaged on a 14 × 17-inch cassette, the entire cassette would be read according to a 14 × 17-inch matrix size with much larger pixels, and the resultant image would be very large. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Postexposure manipulation of the image to a smaller size reduces the resolution. Appropriate image plate selection for the exam also eliminates scatter outside the initial collimation and increases image resolution. In addition, the image size on hard copy and soft copy is affected by cassette selection. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Some units use newer CR imaging plate technology but are cassetteless. These units are typically used for chest imaging, and the imaging plate is enclosed within the unit. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The storage phosphors have a needle-like structure that allows light to be guided with little light spread. Combined with line-scan readouts and charge-coupled device detectors, these units have a complex reader within the fixed system. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Grid Selection Digital images are displayed in tiny rows of picture elements or pixels. Grid lines that are projected on the imaging plate when using a stationary grid can interfere with the image, resulting in a wavy artifact known as a moiré pattern. This pattern occurs because the grid lines and the scanning laser are not parallel. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Grid Selection The oscillating motion of a moving grid or Bucky blurs the grid lines and eliminates the interference. Because of the ability of CR imaging plates to record a very high number of x-ray photons, the use of a grid is much more critical than in film/screen radiography. Appropriate selection of stationary grids reduces this interference as well. Grid selection factors are frequency, ratio, focus, and size. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Grid Frequency Grid frequency refers to the number of grid lines per centimeter or lines per inch. The higher the frequency or the more lines per inch, the finer the grid lines in the image and the less they interfere with the image. Typical grid frequency is between 80 and 152 lines per inch. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Grid Frequency Some manufacturers recommend no less than 103 lines per inch and strongly suggest grid frequencies greater than 150 lines per inch. The higher the frequency, the less positioning latitude is available, increasing the risk for grid cutoff errors, especially in mobile radiography. The closer the grid frequency is to the laser scanning frequency, the greater likelihood of frequency harmonics or matching and the more likely the risk of moiré effects. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Ratio The relationship between the height of the lead strips and the space between the lead strips is known as grid ratio. The higher the ratio, the more scatter radiation is absorbed. The higher the ratio, the more critical the positioning is, such that high grid ratio is not the appropriate choice for mobile radiography. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Ratio A grid ratio of 6:1 would be proper for mobile radiography, whereas a 12:1 grid ratio would be appropriate for departmental grids that are more stable and less likely to be mispositioned, causing grid cutoff errors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Grid Focus Most grids chosen by radiography departments are parallel and focused. Parallel grids are less critical to beam centering but should not be used at distances less than 48 inches. Focused grids consist of lead strips angled to coincide with the diversion of the x-ray beam and must be used within specific distances using a precisely centered beam. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Size The physical size of the grid matters in CR exams.
The smaller the cassette being used, the higher the sampling rate will be. When using cassettes that are 10 × 12 inches or smaller, it is important to select a high-frequency grid to eliminate scatter that will interfere with quality image interpretation by the computer algorithm. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Collimation When an exposure is made and radiation enters the patient, the larger the volume of tissue being irradiated and the greater the kilovoltage peak used, the more likely the production of Compton interactions or scatter. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Collimation Although the use of a grid decreases the amount of scatter that exits the patient from affecting latent image formation, properly used collimation reduces the area of irradiation and reduces the volume of tissue in which scatter can be created. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Collimation This results in increased contrast because of the reduction of scatter as fog and reduces the amount of grid cleanup necessary for increased resolution. Through postexposure image manipulation known as shuttering, a black background can be added around the original collimation edges, virtually eliminating the distracting white or clear areas. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Collimation However, this technique is not a replacement for proper preexposure collimation. Shuttering is an image aesthetic only and does not change the amount or angles of scatter created. There is no substitute for appropriate collimation, for collimation reduces patient dose. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Side/Position Markers
One can easily mark images with left and right side markers or other position or text markers after the exposure has been made, at the time of processing. Conventional lead markers should be used the same way they were used in film/screen systems. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Side/Position Markers
Marking the patient exam at the time of exposure not only identifies the patient’s side, but also identifies the technologist performing the exam. This is also an issue of legality. If the exam is used in a court case, the marker with the technologist’s markers allows the possibility of the technologist’s testimony and lends credibility to his or her expertise. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators The amount of light given off by the imaging plate is a result of the radiation exposure that the plate has received. The light is converted into a signal that is used to calculate the exposure indicator number, which is a different number from one vendor to another. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators The base exposure indicator number for all systems designates the middle of the detector operating range. For Fuji, Phillips, and Konica systems, the exposure indicator is known as the S, or sensitivity, number. The S number is the amount of luminescence emitted at 1 mR at 80 kVp, and it has a value of 200. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators The higher the S number with these systems, the lower the exposure. For example, an S number of 400 is half the exposure of an S number of 200, and an S number of 100 is twice the exposure of an S number of 200. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators The numbers have an inverse relationship to the amount of exposure so that each change of 200 results in a change in exposure by a factor of 2. Kodak uses exposure index, or EI, as the exposure indicator. A 1 mR exposure at 80 kVp combined with aluminum/copper filtration yields an EI number of 2000. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators An EI number plus 300 (EI + 300) is equal to a doubling of exposure, and an EI number of minus 300 (EI − 300) is equal to a halving of exposure. The numbers for the Kodak system have a direct relationship to the amount of exposure so that each change of 300 results in change in exposure by a factor of 2. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators This is based on logarithms, only instead of using 0.3 (as is used in conventional radiographic characteristic curves) as a change by a factor of 2, the larger number 300 is used. This is also a direct relationship; the higher the EI, the higher the exposure. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators The term for exposure indicator in an Agfa system is the lgM, or logarithm of the median exposure. An exposure of 20 µGy at 75 kVp with copper filtration yields an lgM number of 2.6. Each step of 0.3 above or below 2.6 equals an exposure factor of 2. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators An lgM of 2.9 equals twice the exposure of 2.6 lgM, and an lgM of 2.3 equals an exposure half that of 2.6. The relationship between exposure and lgM is direct. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Exposure Indicators These ranges depend on proper calibration of equipment and represent the minimum and maximum exposure numbers that correspond with radiation exposure within the diagnostic range. Exposure numbers outside the range indicate overexposure and underexposure. Pediatric exam ranges vary, as do specific body part indices according to manufacturer. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Data Recognition and Preprocessing
The image recognition phase is important in establishing the parameters that determine collimation borders and edges and histogram formation. All CR systems have this phase, and each has a specific name for this process. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Agfa uses the term collimation, Kodak uses the term segmentation, and Fuji uses the phrase “exposure data recognition.” All systems use a region of interest to define the area where the part to be examined is recognized and the exposure outside the region of interest is subtracted. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Each vendor has a specific tool for different situations—such as neck, breasts, and hips, or pediatrics—in which the anatomy requires some special recognition. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Automatic Data Recognition
In automatic data recognition, the image recording range is automatically determined. When the automatic mode is selected, the radiographer must also select whether the field is divided for multiple exposures and in what pattern the exposure will be made. This mode automatically adjusts reading latitude (L) and S number. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Collimation is automatically recognized, and a complete histogram analysis occurs. Good collimation practices are critical because overcollimation or undercollimation leads to data recognition errors that affect the histogram. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Lead markers must be in the exposure area, and overlapping of exposures must be avoided because overlaps are interpreted as areas of increased exposure and negatively affect the histogram. Each of the exposure regions is processed to identify the shape of the field and the approximate center. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Data recognition then occurs from the center out diagonally, and when the value of the pixels exceeds a preset threshold, those points are interpreted as collimation. Exposure data outside the collimation points is subtracted in the histogram analysis. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Semiautomatic Mode In the semiautomatic mode, the L value of the histogram is fixed, and only a small reading area is used. There is no collimation detection, and the proper kilovoltage must be used to maintain subject contrast because the L value does not change. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Semiautomatic Mode Semiautomatic mode is especially useful for exams of the odontoid, L5/S1 spot film, sinuses, and any other tightly collimated exams. Precautions must be taken when using this mode to carefully center the part to be examined, and the mode is not recommended for high-absorption objects such as prostheses. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Semiautomatic Mode Selection of several different semiautomatic modes may be available where the size of the region of interest is different (5 × 5 cm, 7 × 7 cm, 10 × 10 cm) or with multiple areas of interest where the values of the areas are determined and the resultant maximum value is used. This could be the “semi” mode selected for the chest. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Multiple Manual Selection Mode
Fuji Systems calls this mode the Semi-X mode in which the user selects from nine areas of the imaging plate. The technologist selects the area of interest, and the image is derived from the selected areas imaged in semiautomatic mode. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Multiple Manual Selection Mode
The same precautions apply as in semiautomatic mode. The cassette orientation label must be noted with relation to the area of interest. This mode is helpful in cross-table exams in which the body part may not align with automatically selected imaging plate regions. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Fixed EI and L value are fixed in that the user selects the EI and S value, and the L value is set by the menu selection. There is no histogram analysis and no recognition of imaging plate division; using fixed mode is like using film screen; the density of the image directly reflects the technique that is used. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Fixed This mode is useful when imaging cross-table hips, C7-T1 lateral view of the cervical spine, any body part with a lot of metal, and parts that cannot be centered. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Common CR Image Acquisition Errors
As with film screen, artifacts can detract and degrade images. Imaging plate artifacts Plate reader artifacts Image processing artifacts Printer artifacts Operator errors Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Imaging Plate Artifacts
As the imaging plate ages, it becomes prone to cracks from the action of removing and replacing the imaging plate within the reader. Cracks in the imaging plate appear as areas of lucency on the image. The imaging plate must be replaced when cracks occur in clinically useful areas. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Adhesive tape used to secure lead markers to the cassette can leave residue on the imaging plate. If static exists because of low humidity, hair can cling to the imaging plate. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Backscatter created by x-ray photons transmitted through the back of the cassette can cause dark line artifacts. Areas of the lead coating of the cassette that are worn or cracked allow scatter to image these weak areas. Proper collimation and regular cassette inspection helps to eliminate this problem. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Plate Reader Artifacts
The intermittent appearance of extraneous line patterns can be caused by problems in the electronics of the plate reader. Reader electronics may have to be replaced to remedy this problem. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Horizontal white lines may be caused by dirt on the light guide in the plate reader. Service personnel need to clean the light guide. If the plate reader loads multiple imaging plates in a single cassette, only one of the plates will usually be extracted, leaving the other to be exposed multiple times. The result is similar to a conventional film/screen double-exposed cassette. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Incorrect erasure settings result in a residual image left in the imaging plate before the next exposure. Results vary depending on how much residual image is left and where it is located. Orientation of a grid so that the grid lines are parallel to the laser scan lines of the plate reader results in the moiré pattern error. Grids should be high frequency, and the grid lines should run perpendicular to the laser scan lines of the plate reader. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Printer Artifacts Fine white lines may appear on the image because of debris on the mirror in the laser printer. Service personnel need to clean the printer. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Operator Errors Insufficient collimation results in unattenuated radiation striking the imaging plate. The resulting histogram is changed so that it is outside the normal exposure indicator range for the body part selected. Using the smallest imaging plate possible and proper collimation, especially on small or thin patients, eliminates this error. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Operator Errors If the cassette is exposed with the back of a cassette toward the source, the result is an image with a white grid-type pattern and white areas that correspond to the hinges. Care should be taken to expose only the tube side of the cassette. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Operator Errors Underexposure produces quantum mottle, and overexposure affects contrast. The proper selection of technical factors is critical to patient dose and image quality and to ensure the appropriate production of light from the imaging plate. The Figure on the left shows an underexposed image caused by insufficient milliampere-seconds, and The Figure on the right shows an overexposed image caused by excessive kilovoltage peak, resulting in quantum mottle and decreased contrast, respectively. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Menu choices are critical to proper image acquisition. The menu choice must match the part being examined. Kilovoltage peak should be selected for the type and amount of contrast desired. Beam attenuation is the same in digital radiography as it is in film/screen radiography. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Sufficient photons are necessary to form any x-ray image. Insufficient photons result in quantum noise or mottle. Care must be taken not to overuse milliampere-seconds to avoid quantum mottle. Imaging plate selection is important to ensure the proper matrix and resolution. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Because of the wide range of densities produced, grid use is critical. Specific guidelines should be followed as to the frequency, ratio, focus, and size. Care should be taken to avoid the moiré grid error. Collimation not only reduces the area irradiated, thereby reducing scatter production, but also reduces the grid cleanup necessary. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary A black background can be added postexposure, eliminating distractive light-transmitting borders. Side or position markers should always be used regardless of the postexposure ability to add them. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary S, EI, and lgM are terms used by manufacturers to indicate the amount of exposure. The exposure range numbers represent the maximum to minimum diagnostic exposures. The middle value in that range represents the S, EI, or lgM number. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Image recognition takes place through computer algorithms that determine collimation borders and edges and histogram formation. Typical recognition programs are automatic, semiautomatic, multiple manual selection, and fixed modes for Fuji systems. Common CR acquisition errors include imaging plate, plate reader, image processing, and printer artifacts, as well as operator errors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 6 Cassetteless Equipment and Image Acquisition

Objectives Describe the construction of direct and indirect cassetteless systems. Differentiate between direct and indirect image capture. List the steps for x-ray-to-digital conversion with anamorphous silicon detectors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Objectives Discuss the function of a charged-coupled device.
Compare detector detective quantum efficiency to cassette-based systems. Explain the importance of detector size and orientation. Discuss factors that affect spatial resolution in cassetteless systems. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Cesium iodide scintillator Charge-coupled device
Complementary metal oxide silicon Detective quantum efficiency Detector size Direct conversion Field-effect transistor Flat-panel detector Indirect conversion Rare-earth scintillator Thin-film transistor Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography Digital radiography (DR) is another way to record x-ray exposure after it has passed through the patient. DR includes computed radiography and direct or indirect methods of digital image capture The term digital radiography is used to describe images recorded on an electronically readable device. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiography DR is hard-wired. DR is cassetteless.
Detectors are permanently enclosed inside a rigid protective housing. Thin-film transistor (TFT) detector arrays may be used in direct- and indirect-conversion detectors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Flat-Panel Detectors Consist of a photoconductor Amorphous selenium
Holds a charge on its surface that can then be read out by a TFT Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Direct Conversion X-ray photons are absorbed by the coating material.
Photons are immediately converted into an electrical signal. The DR plate has a radiation-conversion material or scintillator. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Direct Conversion DR Scintillator
Typically made of amorphous selenium Absorbs x-rays and converts them to visible photons Converts photons to electrical charges Charges stored in the TFT detectors Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Direct Conversion DR TFT
The TFT is a photosensitive array. The TFT uses small (about 100 to 200 µm) pixels. TFT converts light into electrical charges. Each pixel contains a photodiode. Generates electrical charges Absorbs light from scintillator Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

A field-effect transistor (FET) or silicon TFT Isolates each pixel element Reacts like a switch to send the electrical charges to the image processor Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

A million-plus pixels can be read and converted to a composite digital image in under a second. A line of TFT switches, each associated with a photodiode, allows electrical charge information to discharge when switches are closed. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The information is discharged onto the data columns and is read out with dedicated electronics. Specialized silicon integrated circuits are connected along the edges of the detector matrix. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

On one side, integrated circuits control the line scanning sequence, and on the other side, low-noise, high-sensitivity amplifiers perform the readout, amplification, and analog-to-digital conversion. High-speed digital electronics are then used to achieve fast image acquisition and processing. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Indirect Conversion Similar to direct detectors in that the TFT technology is also used Two-step process: X-ray photons are converted to light. Light photons are converted to an electrical signal. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Indirect Conversion A scintillator converts x-rays into visible light.
Light is then converted into an electrical charge by photodetectors such as amorphous silicon photodiode arrays or charge-coupled devices, or CCDs. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Indirect Conversion X-ray photons striking the dielectric receptor are absorbed by a scintillation layer in the imaging plate that converts the incident x-ray photon energy to light. A photosensitive array, made up of small (about 100 to 200 µm) pixels, converts the light into electrical charges. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Indirect Conversion Each pixel contains a photodiode that absorbs the light from the scintillator and generates electrical charges. An FET or silicon TFT isolates each pixel element and reacts like a switch to send the electrical charges to the image processor. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Indirect Conversion More than a million pixels can be read and converted to a composite digital image in under a second. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Amorphous Silicon Detector
This type of flat-panel sensor uses thin films of silicon integrated with arrays of photodiodes. These photodiodes are coated with a crystalline cesium iodide scintillator or a rare-earth scintillator (terbium-doped gadolinium dioxide sulfide). When these scintillators are struck by x-rays, visible light is emitted proportional to the incident x-ray energy. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The light photons are then converted into an electric charge by the photodiode arrays. Unlike the selenium-based system used for direct conversion, this type of indirect-conversion detector technology requires a two-step process for x-ray detection. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The scintillator converts the x-ray beams into visible light, and light is then converted into an electrical charge by photodetectors, such as amorphous silicon photodiodes. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Cesium Iodide Detectors
A newer type of amorphous silicon detector uses a cesium iodide scintillator. The scintillator is made by growing very thin crystalline needles (5 µm wide) that work as light-directing tubes, much like fiber optics. This allows greater detection of x-rays, and because there is almost no light spread, there is much greater resolution. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

These needles absorb the x-ray photons and convert their energy into light, channeling it to the amorphous silicon photodiode array. As the light hits the array, the charge on each of the photodiodes decreases in proportion to the light received. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Each photodiode represents a pixel, and the amount of charge required to recharge each photodiode is read electronically and converted to digital data. This process is very low noise and very fast (approximately 30 million pixels per second). Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Charge-Coupled Devices
The oldest indirect-conversion DR system is based on CCDs. X-ray photons interact with a scintillation material, such as photostimulable phosphors, and this signal is coupled or linked by lenses or fiber optics, which act like cameras. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

These cameras reduce the size of the projected visible light image and transfer the image to one or more small (2 to 4 cm2) CCDs, which convert the light into an electrical charge. This charge is stored in a sequential pattern and released line by line and sent to an analog-to-digital converter. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Even though CCD-based detectors require optical coupling and image size reduction, they are widely available and relatively low in cost. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Complementary Metal Oxide Silicon
Developed by NASA, complementary metal oxide silicon (CMOS) systems use specialized pixel sensors that, when struck with x-ray photons, convert the x-rays into light photons and store them in capacitors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Complementary Metal Oxide Silicon
Each pixel has its own amplifier, which is switched on and off by circuitry within the pixel, converting the light photons into electrical charges. Voltage from the amplifier is converted by an analog-to-digital converter also located within the pixel. This system is highly efficient and takes up less fill space than CCDs. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Detective Quantum Efficiency
How efficiently a system converts the x-ray input signal into a useful output image is known as detective quantum efficiency, or DQE. DQE is a measurement of the percentage of x-rays that are absorbed when they hit the detector. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The linear, wide-latitude input/output characteristic of computed radiography (CR) systems relative to screen/film systems leads to a wider DQE latitude for CR, which implies that CR has the ability to convert incoming x-rays into “useful” output over a much wider range of exposure than can be accommodated with screen/film systems. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

In other words, CR records all of the phosphor output. Systems with higher quantum efficiency can produce higher-quality images at a lower dose. Indirect and direct DR capture technology has increased DQE over CR. However, DR direct capture technology, because it does not have the light conversion step and consequently no light spread, increases DQE the most. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

There is no light to blur the recorded signal output; a lower dose is required than for CR, and higher-quality images are produced. Newer CMOS indirect capture systems may be equal to DR image acquisition because of the crystal light tubes, which also prevent light spread. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The DQE of detectors changes with kilovoltage peak, but generally the DQE of selenium- and phosphor-based systems is higher than for CR, CCD, and CMOS systems. CCD in particular has problems with low-light capture. The area of a TFT array is limited due to the structure of the matrix. This also affects the size and number of pixels available. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Known as the fill factor, the larger the area of the TFT photodiodes, the more radiation can be detected and the greater amount of signal generated. Consequently, the greater the area of the TFT array, the higher the DQE. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Detector Size The actual physical size—length and width—of the x-ray detector is critical. It must be large enough to cover the entire area to be imaged and small enough to be practical. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Detector Size For chest radiographs, the detector needs to be at least 17 × 17 inches so that lengthwise and crosswise exams are possible. Special exams such as leg length and scoliosis series may require dedicated detectors. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Resolution Depending on the physical characteristics of the detector, spatial resolution can vary a great deal. Spatial resolution of amorphous selenium for direct detectors and cesium iodide for indirect detectors is higher than CR detectors but lower than film/screen radiography. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Resolution Excessive image processing, in an effort to alter image sharpness, can lead to excessive noise. Digital images can be processed to alter apparent image sharpness; however, excessive processing can lead to an increase in perceived noise. The best resolution is achieved by using the appropriate technical factors and materials. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Pixel Size and Matrix Size
Matrix size is determined by the size of the pixels and the spacing between them, or pixel pitch. More pixels do not always mean better resolution because of the amount of x-ray scatter, light scatter, or both within the receptor. Larger matrices combined with small pixel size increase resolution; however, it may not be practical to use large matrices. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Pixel Size and Matrix Size
The larger the matrix, the large the size of the image, the greater the space needed for network transmission and picture archival and communication system (PACS) storage. Typically, 2000 pixels per row are adequate for most diagnostic exams. Smaller pixel sizes may be necessary for mammographic exams. Pixel size in TFT displays is related to the design of the capacitance elements and also to the fill factor of these devices. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Technical Factor and Equipment Selection
Kilovoltage peak, milliampere-second, distance, collimation, and anatomic markers are the same for cassetteless systems as they are for cassette-based systems. Typically only one exposure is made at a time on the image receptor, but that does not mean that collimation is unnecessary. Collimation may be more critical because the cassetteless systems are more sensitive to scatter radiation. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Kilovoltage Peak, Milliampere-Second, Distance, Collimation, and Anatomic Markers
When grids are used in any digital imaging system, there is always the possibility that the grid lines will interfere with the pixel rows, resulting in the moiré- pattern error. Grid interaction artifacts are not always easy to identify and can decrease image quality, so caution and proper selection of the grid is advised. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Potential Cassetteless Image Acquisition Errors
Although the conversion of x-rays to digital signal occurs quickly, each step of the conversion has the potential of signal loss. The major cause of noise in this system is electronic noise and is the main factor limiting quality. The more time allowed for signal conversion, the more precise the pixel values. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Incomplete charge transfer causes inaccuracies in pixel values in subsequent exposures, reducing image quality. Additionally, if exposures are taken in too-rapid sequences, there may not be enough time for each previous exposure to transfer the entire signal, resulting in what is known as electronic memory artifact. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The detector readout may have built-in safeguards against this, but it would be wise to know whether these protective measures are in place. Not all cassetteless systems are appropriate for high- speed, rapid-succession imaging such as fluoroscopy. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary There are two types of cassetteless digital imaging systems: direct and indirect. Direct sensors are TFT arrays of amorphous silicon coated with amorphous selenium. Direct sensors absorb x-ray photons and immediately convert them to an electrical signal. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Indirect-conversion detectors use a scintillator that converts x-rays into visible light, which is then converted into an electrical charge. CCDs act as miniature cameras that convert light produced by x-ray interaction with photostimulable phosphors into an electrical charge. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Pixel and matrix size are important in determining the amount of resolution and the size of the image to be stored in the PACS system. In TFT technology, pixel and matrix size are determined by the amount of area available to “fill” with photons. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Technical and equipment factors in cassetteless systems are equivalent to cassette-based systems but may be more critical in terms of grid use and collimation. Incomplete transfer of the signal generated in the cassetteless receptor or the amount of signal retained by the receptor can cause artifacts, especially with short acquisition or rapid-succession acquisitions. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 7 Digital Radiographic Image Processing and Manipulation

Objectives Describe formation of an image histogram.
Discuss automatic rescaling. Compare image latitude in digital imaging with film/screen radiography. List the functions of contrast enhancement parameters. State the Nyquist theorem. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Objectives Describe the effects of improper algorithm application.
Explain modulation transfer function. Discuss the purpose and function of image manipulation factors. Describe the major factors in image management. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Archive query Automatic rescaling Contrast manipulation
Edge enhancement High-pass filtering Histogram Image annotation Image orientation Image sampling Image stitching Latitude Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Key Terms Look-up table Low-pass filtering Magnification Manual send
Modulation transfer function Nyquist theorem Patient demographics Shutter Smoothing Spatial frequency resolution Window and level Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital Radiographic Image Processing and Manipulation
In cassette-based and cassetteless systems, once the x-ray photons have been converted into electrical signals, these signals are available for processing and manipulation. The reader is used only for cassette-based systems, but the processing parameters and image manipulation controls are similar for both systems. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Preprocessing Preprocessing takes place in the computer where the algorithms determine the image histogram. Postprocessing is done by the technologist through various user functions. Digital preprocessing methods are vendor-specific. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Reader Functions The computed radiography (CR) imaging plate records a wide range of x-ray exposures. If the entire range of exposure were digitized, values at extremely high and low ends of range would also be digitized. This would result in low-density resolution. To avoid this, exposure data recognition processes only the optimal density exposure range. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Reader Functions Data recognition program searches for anatomy recorded on the imaging plate as follows: Finding collimation edges Eliminating scatter outside the collimation Failure of the system to find the collimation edges can result in incorrect data collection. Images may be too bright or too dark. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Reader Functions Data within collimation result in generation of a graphic representation called a histogram. Because information within the collimated area is signal used for image data, the information is the source for a vendor-specific exposure data indicator. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Image Sampling Plate is scanned.
Image location and orientation is determined. Size of the signal is determined. Value is placed on each pixel. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Image Sampling Histogram is generated that allows system to find useful signal by locating the minimum (S1) and maximum (S2) signal within the anatomic regions of interest in the image. Histogram identifies all densities on the imaging plate in the form of a graph: X-axis is related to amount of exposure. Y-axis displays the number of pixels for each exposure. Graphic representation appears as a series of peaks and valleys and has a pattern that varies for each body part. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Image Sampling Low energy (kilovoltage peak) gives a wider histogram. High energy (kilovoltage peak) gives a narrow histogram. Histogram shows the distribution of pixel values for any given exposure. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

CR Image Sampling For example:
Pixels have a value of 1, 2, 3, and 4 for a specific exposure. Histogram shows the frequency of each of those values and actual number of values. Histogram sets the minimum (S1) and maximum (S2) “useful” pixel values. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Histogram Analysis Analysis is complex.
Shape of the histogram stays fairly constant for each part exposed (anatomy specific). For example: Shape of histogram for a chest radiograph on a large adult patient looks different from a knee histogram generated from a pediatric knee exam. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Histogram Analysis It is important to choose the correct anatomic region on the menu before exposing the patient. Raw data used to form the histogram are compared with a “normal” histogram of the same body part by the computer. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Nyquist Theorem Theorem states that when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal so that the reconstruction of the original image will be nearly perfect. At least twice the number of pixels needed to form the image must be sampled. If too few pixels are sampled, the result is a lack of resolution. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Nyquist Theorem The number of conversions in CR—electron to light, light to digital information, digital to analog signal—results in loss of detail. Some light is lost during the light-to-digital conversion because of the spreading out of light photons. Because there is a small distance between the phosphor plate surface and the photosensitive diode of the photomultiplier, some light spreads out there as well, resulting in loss of information. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Nyquist Theorem The longer the electrons are stored, the more energy they lose. When laser stimulates electrons, some lower-energy electrons escape the active layer. If enough energy was lost, some lower-energy electrons are not stimulated enough to escape and information is lost. All manufacturers suggest that imaging plates be read as soon as possible to avoid this loss. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

The Nyquist Theorem Indirect and direct radiography lose less signal to light spread than conventional radiography. The Nyquist theorem is still applied to ensure that sufficient signal is sampled. Because sample is preprocessed by the computer immediately, signal loss is minimized but still occurs. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Aliasing Spatial frequency is greater than the Nyquist frequency.
Sampling occurs less than twice per cycle. Information is lost. Fluctuating signal is produced. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Aliasing Wraparound image is produced.
Image appears as two superimposed images slightly out of alignment. Aliasing results in a moiré effect. Aliasing can be problematic because of the same effect occurring with grid errors. It is important that the technologist remembers to look at both. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Automatic Rescaling Exposure is greater than or less than what is needed to produce an image. Automatic rescaling occurs to display the pixels for the area of interest. Images are produced that have uniform density and contrast regardless of the amount of exposure. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Automatic Rescaling Problems occur with rescaling:
When too little exposure is used, resulting in quantum mottle When too much exposure is used, resulting in loss of contrast and loss of distinct edges because of increased scatter production Rescaling is no substitute for appropriate technical factors. Danger exists of using higher than necessary milliampere-second values to avoid quantum mottle. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Look-Up Table The look-up table (LUT) is a reference histogram.
LUT is used as a cross-reference to transform the raw information. LUT is used to correct values. LUT has a mapping function: All pixels are changed to a new gray value. Image will have appropriate appearance in brightness and contrast. LUT is provided for every anatomic part. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Look-Up Table LUT can be graphed as follows:
Plotting the original values ranging from 0 to 255 on the horizontal axis Plotting new values, also ranging from 0 to 255 on the vertical axis Contrast can be increased or decreased by changing the slope of this graph. Brightness (density) can be increased or decreased by moving the line up or down the y-axis. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Latitude Latitude is the amount of error that still results in a quality image. Histograms show a wide range of exposure because of automatic rescaling of the pixels. Actual exposure latitude is slightly greater than that of screen/film exposures. In CR, if exposure is more than 50% below ideal exposure, quantum mottle results. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Latitude If exposure is more than 200% above ideal exposure, contrast loss results. Biggest difference between digital and film/screen radiography lies in the ability to manipulate the digitized pixel values, which results in what seems like greater exposure latitude. Proper kilovolt and milliampere-second values prevent mottle and contrast loss. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Enhanced Visualization Image Processing
Kodak Takes image diagnostic quality to a new level Increases latitude while preserving contrast Process decreases windowing and leveling Virtually eliminates detail loss in dense tissues Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Modulation Transfer Function
Modulation transfer function (MTF) is the ability of a system to record available spatial frequencies. Sum of the components in a recording system cannot be greater than the system as a whole. When the function of any component is compromised because of interference, the overall quality of the system is affected. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

MTF is a way to quantify the contribution of each system component to the overall efficiency of the entire system—e.g., ratio of the image to the object. A perfect system would have an MTF of 1 or 100%. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Digital detectors X-ray photon energy excites a phosphor. Phosphor produces light. Spreading out of the light will always occur. Light spread reduces system efficiency. The more light spread, the lower the MTF. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Quality Control Workstation Functions
Image processing parameters Contrast manipulation Spatial frequency resolution Spatial frequency filtering Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Processing Parameters
Digital systems have greater dynamic range than film/screen imaging. Initial digital image appears linear when graphed because all shades of gray are visible. Digitalization gives the image a wide latitude. If all shades were left in the image, contrast would be too low. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Processing Parameters
To avoid this, digital systems make use of various contrast-enhancement parameters. Names differ by vendor; Agfa uses MUSICA, Fuji uses Gradation, and Kodak uses Tonescaling. Purpose and effects are basically the same. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Contrast Manipulation
Contrast-enhancement parameters convert the digital input data to an image with appropriate density and contrast. Image contrast is controlled by using a parameter that changes the steepness of the exposure gradient. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Contrast Manipulation
Density can be varied at the toe and shoulder of the curve, removing the extremely low and extremely high density values using a different parameter. Another parameter allows density to remain unchanged while contrast is varied. These parameters should be used to enhance the image only; no amount of adjustment takes the place of proper technical factor selection. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Workstation Screen Showing Contrast Manipulation Choices

Spatial Frequency Resolution
Sharpness control is referred to as spatial frequency processing. Sharpness is controlled in film/screen by various factors such as focal spot size, screen and/or film speed, and object image distance. Digitized images can be further controlled for sharpness. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Frequency Resolution
Controls are available for the following: Structure to be enhanced Degree of enhancement for each density to reduce image graininess How much edge enhancement is applied If improper algorithms are applied, image formation is affected. It is possible to degrade image information if algorithms are improperly applied. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Spatial Frequency Filtering
Edge enhancement Smoothing Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Edge Enhancement When the signal is obtained, averaging of the signal occurs to shorten processing time and storage. The more pixels involved in the averaging, the smoother the image appears. Signal strength of one pixel is averaged with the strength of adjacent pixels or neighborhood pixels. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Edge Enhancement Edge enhancement occurs when fewer pixels in the neighborhood are included in the signal average. The smaller the neighborhood, the greater the enhancement. When frequencies of areas of interest are known, they can be amplified and other frequencies can be suppressed. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Edge Enhancement Amplification, also known as high-pass filtering, results in an increase of contrast and edge enhancement. Suppression of frequencies, also known as masking, can result in loss of small details. This technique is useful for enhancing large structures such as organs and soft tissues but can be noisy. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Smoothing Smoothing is another type of spatial frequency filtering.
Smoothing is also known as low-pass filtering. Smoothing results from averaging of the frequency of each pixel with surrounding pixel values to remove high-frequency noise. Result is a reduction of noise and contrast. Low-pass filtering is useful for viewing small structures such as fine bone tissues. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Basic Functions of the Processing System
Image manipulation Image management Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Manipulation Window and level Background removal or shutter
Image orientation Image stitching Image annotation Magnification Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Window and Level Window and level are the most common controls for brightness and contrast. Window controls how light or dark the image is. Level controls the ratio of black to white, or contrast. User is able to manipulate quickly through use of the mouse. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Window and Level One direction, vertical or horizontal, controls brightness, and the other direction, contrast. To control density and contrast further, contrast enhancement parameters are used. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Background Removal or Shutter
Unexposed borders around the collimation edges allow excess light to enter the eye. Effect is known as veil glare. Glare causes oversensitization of a chemical within the eye called rhodopsin. This results in temporary white light blindness. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Eye recovers quickly enough so that viewer recognizes only that the light is very bright. Glare is a great distraction that interferes with image reception by the eye. In film/screen radiography, black cardboard glare masks or special automatic collimation view boxes were used to lessen the effects of veil glare, but no techniques were entirely successful or convenient. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

In CR, automatic shuttering is used to blacken out the white collimation borders. This eliminates veil glare. Shuttering is a viewing technique only. Shuttering should never be used to mask poor collimation practices. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Removal of the white unexposed borders results in an overall smaller number of pixels. This reduces the amount of information to be stored. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Orientation Image reader scans and reads the image from the leading edge of the imaging plate to the opposite end. Image is displayed exactly as it was read. Different vendors mark the cassettes in different ways. Cassette must be oriented so that the image is processed to display as expected. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Orientation Fuji uses a tape-type orientation marker.
Kodak uses a sticker. Some exams require unusual orientation of the cassette. Reader must be informed of the orientation of the anatomy with respect to the reader. In digital radiography, the position of the part should correspond with the marked top and sides of the imaging plate. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Stitching Stitching is used for anatomy or areas of interest too large to fit on one cassette. Multiple images can be “stitched” together. Sometimes, special cassette holders are used and positioned vertically, corresponding to foot to hip or entire spine radiography. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Stitching Images are processed in computer programs that nearly seamlessly join the anatomy. Computer displays one single image. Process eliminates the need for large (36-inch) cassettes previously used in film/screen radiography. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Annotation Information other than standard identification must be added to the image. In screen/film radiography, additional information is marked by the following: Time and date stickers Grease pencils Permanent markers Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Annotation Annotation function allows selection of preset terms and/or manual text input. Annotation can be useful when such additional information is necessary. Annotations overlay the image as bitmap images. Annotations may not transfer to picture archival and communication system (PACS). Input of annotation for identification of the patient’s left or right side should never be used as a substitute for technologist’s anatomy markers. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Magnification Two basic types of magnification techniques are standard with digital systems: One type functions as a magnifying glass: A box is placed over a small segment of anatomy on the main image. Box shows a magnified version of the underlying anatomy. The size of the magnified area and the amount of magnification can be made larger or smaller. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Magnification Other technique is “zoom.”
Zoom allows magnification of the entire image. Image can be enlarged enough that only parts of it are visible on the screen. Those parts can be seen through mouse navigation. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Image Management Patient demographics input Manual send Archive query

Patient Demographics Input
Proper identification of the patient is even more critical. Retrieval can be nearly impossible if image is not properly and accurately identified. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Demographic information about the patient includes the following: Name Health care facility Patient identification number Date of birth Exam date Other pertinent information Input or linked via barcode label scans, before the start of the exam and before the processing phase Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Occasionally, errors are made and demographic information must be altered. If technologist performing the exam is absolutely positive that image is of the correct patient, then demographic information can be altered at the processing stage. This function should be tracked and changes should be linked to the technologist altering the information to ensure accuracy and accountability. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Problems occur if the patient name is entered differently from visit to visit or exam to exam. For example: Patient’s name is Jane A. Doe and is entered that way. Name must be entered that way for every other exam. If name is entered as Jane Doe, then system will save it as a different patient. Merging of files can be difficult. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Problems: Several versions of the name are given. Suppose the patient gives a middle name on one visit but has multiple exams under his or her first name. Retrieval of previous files will be difficult. The right images must be placed in the correct data files. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Manual Send Because the quality control workstation is networked to the PACS, it also has the capability to send images to local network workstations. The manual send function allows the quality control technologist to select one or more local computers to receive images. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Archive Query PACS archive can be queried for historical images.
Function allows retrieval of images from the PAC system based on the following: Date of exam Patient name or number Exam number Pathologic condition Anatomic area Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Archive Query Example:
Technologist could query PACS to retrieve all chest radiographs for a particular date or range of dates. Technologist could query retrieval of all of a patient’s images. Multiple combinations of query fields are possible: Can generate general retrieval Specific recovery of images Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Recognition of exposure data involves processing only the optimal density exposure range and generates a graphic representation or histogram of the optimal densities. The plate is scanned, and the image location and orientation are determined. A value is place on each pixel, and the histogram is generated displaying the minimum and maximum diagnostic signal. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary The histogram is anatomic region specific and remains fairly constant from patient to patient. Automatic rescaling allows pixel display for the area of interest, regardless of the amount of exposure unless the exposure is too low or too high. In those cases, quantum mottle or contrast loss occurs. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary There is no substitute for proper kilovoltage peak and milliampere-second settings. Images cannot be created from nothing; that is, insufficient photons, insufficient penetration, or overpenetration will result in loss of diagnostic information that cannot be manufactured by manipulation of the image parameters. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Exposure latitude is slightly greater with digital imaging than that of film/screen imaging because of the wide range of exposures recorded with digital systems. Contrast-enhancement parameters allow enhancement of the image by controlling the steepness of the exposure gradient, density variance, and contrast amount. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Spatial frequency resolution is controlled by focal spot, object image distance, and computer algorithms. The Nyquist theorem is applied to digital images to ensure that sufficient signal sampling occurs so that maximum resolution is achieved. MTF refers to the contribution of all system components to total resolution. The closer the MTF value is to 1, the better the resolution. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Edge enhancement is accomplished by limiting the number of pixels in a neighborhood of the matrix. Known area of interest frequencies can be amplified or high-pass filtered to increase contrast and edge enhancement. Suppression of frequencies of lesser importance, known as masking, can cause small detail loss. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Low-pass filtering or smoothing is the result of pixel averaging to remove high-frequency noise. Contrast and noise are decreased, allowing small structures to be seen. Window and level parameters control pixel brightness and contrast. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Shuttering is a process that removes or replaces the background in order to block distracting light surrounding a digital image. This does not take the place of proper collimation and can be removed to show proper collimation. Digital imaging cassettes are marked for orientation to the top and right sides. This ensures that images are displayed correctly. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Image stitching is a computer program process that allows multiple images to be joined when the anatomy is too large for one exposure. The result is a nearly seamless, single image. Magnification techniques are available with digital systems that allow small area enlargement or whole image enlargement. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Proper patient demographic input is the responsibility of the technologist performing the exam. Any alterations of patient demographics should be avoided unless absolute identification is possible. The manual send function allows images to be sent to one or more networked computers. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Summary Historical study of patient exams can be accomplished through the archive query function. Retrieval of radiographic studies can be specific as to patient name, date, and exam or broad such as date ranges and combinations of anatomic areas. Elsevier items and derived items © 2008 by Mosby, Inc., an affiliate of Elsevier Inc.

Chapter 1 Introduction to Digital Radiography and PACS

Similar presentations

Presentation on theme: "Chapter 1 Introduction to Digital Radiography and PACS"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Chapter 1 Introduction to Digital Radiography and PACS

Similar presentations

Presentation on theme: "Chapter 1 Introduction to Digital Radiography and PACS"— Presentation transcript:

Similar presentations

About project

Feedback