Digital Radiographic Imaging 101

Terms Digital radiography (DR), image receptors, film digitizer, noise, signal to noise ratio, area beam, xenon gas detectors, scintillation detectors, fan beams, pre and post patient collimation, slit radiography, translation, interrogation time, attenuation profile, extinction time, Computed radiography (CR), photostimulable phosphor image plate, barium fluorohalide, europium, IP reader, photomultiplier (PM) tube, photocathode, photoemission, dynode, CR workstation, linear energy response, duel image format, charged couple devices (CCD), amorphous silicon & selenium image receptors, thin film transistor (TFT), active matrix array (AMA), direct radiography.

Digital Acquisition Methods 1. Digitize radiographs with a film digitizer Similar to a paper scanner, only for film Film is read by a laser and the image file is sent to a designated secondary device

* Converts films to digital files * Teleradiology * Teaching files and Uses * Converts films to digital files * Teleradiology * Teaching files and presentations Advantages * Inexpensive * Easy to use * Small Disadvantages * Conversion of analog to digital adds a step that degrades image quality * Impractical for converting large archives

1. Digitize radiographs with a film digitizer Digital Acquisition Methods 1. Digitize radiographs with a film digitizer 2. Digitize the video signal with an ADC Advantages: Inexpensive Easy to install Disadvantages: Noisy cameras Poor signal-to-noise ratio (SNR) Area beam (Scatter) Small matrix size (525)

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) * Greatly reduces the area of the beam, and scatter * Replaces the camera with detectors Fan shaped beam Depth of beam may be a cm, or smaller

Xenon Gas Detector From a CT Scanner Xenon gas chamber

Ring of Xenon Detectors in a CT Scanner

SPR used for CT Scout Films Stationary X-ray tube Detectors

CT Scout Views Acquired by SPR, to produce a digital radiograph Scatter radiation is greatly reduced

Detectors 1. Xenon Gas 2. Scintillation

Detectors 1. Xenon Gas 2. Scintillation Photomultiplier (PM tube) Crystal

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) 4. Computed Radiography (CR) Photostimulable image plate (IP) technology Barium Fluorohalide doped with Europium

CR Advantages Uses existing radiographic hardware So is relatively inexpensive to purchase Reduced number of repeats Increased latitude Filmless capture The CR IP looks like a conventional intensifying screen, and is housed in a conventional looking cassette.

CR Facts 300 RSV Only one speed (no detail or high speed) Standard film sizes Laser Film – wet or dry processing

Step 1. Make the exposure like any other radiographic Computed Radiography Step 1. Make the exposure like any other radiographic exposure, only use an IP instead of film. *Remnant photons strike plate *Photoelectric interaction causes barium fluorohalide to fluoresce as electron is ejected. *Electrons (that are of no more use in film radiography) are trapped in the energy traps created by the europium IP

Reading the IP Converting the stored energy to an electric current, point by point.

CR Workstations

Posterior bases obscured Problems Inherent to Conventional Chest Radiography Under- exposed Retrocardiac clear- space overexposed Posterior bases obscured by diaphragm on PA

CR Film

Latitude Logarithmic response of film Linear response of CR Maxed out Yet to respond Linear response of CR

Analog is continuous. Image is fixed in film Digital is discrete Analog is continuous. Image is fixed in film Digital is discrete. Image may be manipulated 1 15 15 1

CR Postprocessing & Characteristic Curves

Histogram

Processing Algorithms Poorly exposed image plates may be corrected by software to some extent

Patient Dose Calculated and displayed Total energy absorbed by IP Fuji S number (200 ave) Low number = high exposure Kodak (1800-2200) Low number = low exposure

Cassetteless Readers Chest units. In table

Laser, Dry Image Hardcopy Devices

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) 4. Computed Radiography (CR) 5. Charge Coupled Devices (CCD)

Charge-Coupled Device (CCD) CCD’s have replaced the Vidicon tube in camcorders

Charge-Coupled Device (CCD) Read-out row Photoelectric detectors embedded in layers of silicon Each pixel is 6 to 25 microns in size, and can store 10,000 to 50,000 electrons.

Charge-Coupled Device (CCD) .. . … .. .. . . . ……. . . . . . . .. .. . . … …. .. .. .. .. .. . . . . Incident light creates a charge in the pixels

Charge-Coupled Device (CCD) .. . … .. .. . . . ……. . . . . . . .. .. . . … …. .. .. .. .. .. . . . . A shutter closes to stop further interaction of light on the pixels The frame is ready for reading

Charge-Coupled Device (CCD) Charges shift from one pixel to another as they move to the readout row

Charge-Coupled Device (CCD) Charges shift from one pixel to another as they move to the readout row

Charge-Coupled Device (CCD) Charges move along the readout row, and exit the chip.

Charge-Coupled Device (CCD) Charges move along the readout row, and exit the chip.

Charge-Coupled Device (CCD) Question: When the charges leave the CCD chip, where do they go? Charges move along the readout row, and exit the chip. Answer: If the CCD was functioning as a camera, they could be sent directly to an analog monitor as the video signal, or...

Charge-Coupled Device (CCD) ALU CU Primary Memory Secondary (RAM) ADC DAC DAC ADC or...they could be sent to an ADC, on to RAM, displayed on a digital monitor, and stored in a secondary memory device

Digital Acquisition Methods 1. Scan radiographic films 2. Digitize the video signal 3. Scan projection radiography (SPR) 4. Computed Radiography (CR) 5. Charged Couple Devices 6. Flat Panels Amorphous Silicon & Amorphous Selenium

Thin film transistors (TFT) in an Active Matrix Array (AMA), are incorporated in a “flat panel” detector that is used in place of a film cassette.

Thin Film Transistors (TFT) 139 microns (half a hair) Diodes connected to rows Current flows out columns

Amorphous Silicon Cesium iodide (CsI) scintillator converts X-rays to light Light is converted to a charge by a photodiode at a TFT junction.

Amorphous Selenium Photon in Interaction creates electron-hole pairs (called Direct Radiography) Positive charge + + + + + + + + Electrode with a bias voltage Photoconductor material Photon in - - - - - - - - - - - Negative charge TFT Interaction creates electron-hole pairs Signal out

* Method by which the stored, latent electronic image is discharged Digital Radiography (DR) Receptor Reader* Energy Comment Transformations Video Target of camera Electron gun x-ray to light to Use limited by charged globules noise of camera to video signal SPR Xenon Interrogations of x-ray to ionized Dedicated cxr successive detectors electrons and CT scouts Scintillation Interrogations of x-rays to light successive detectors to current CR Photostimulable Helium-neon x-ray to light to Only portable phosphorIP laser to trapped electrons receptor to light to current CCD IC Point by point discharge x-ray to light to Potential next of photoelectric trapped electrons generation of IIs detectors (pixels) to current Amorphous TFT AMA point by point discharge x-ray to light Called direct silicon Flat panel of TFTs to current radiography Amorphous TFT AMA point by point discharge x-ray to current Called direct selenium Flat panel of TFTs radiography * Method by which the stored, latent electronic image is discharged