Comparison of Film v. Digital Image Display

Process of data capture All image recording systems rely on differential absorption within the patient to produce a radiographic image Each system analog (film/screen) versus digital (CR and DDR) records photon intensity and displays the data. Differences in photon density become radiographic contrast

Radiographic Contrast Radiographic contrast is dependent on subject contrast and technical factors Subject contrast is basically fixed for each patient. Within certain body/organ systems we can alter subject contrast by introducing contrast media. Some technical factors include; kVp Grid ratio Screen speed Processor temperature Altering technical factors can improve or degrade radiographic contrast.

Film Films produce an analog image that allows edges to gradually blend into one another. This is how we see images normally. Conversely, digital images are broken up into pixels (tiny squares). Because the image consists of pixels there is an inherent loss of spatial resolution when compared to analog film. 2.5 – 5 lp/mm versus 10 lp/mm

Computed Radiography Photostimulable plate (PSP) Barium-fluorohalide doped with europium When the PSP is struck by a photon electrons are released in the crystal lattice and stored in F centers within the lattice In the CR reader, a helium-neon laser (red light) stimulates the crystals of the PSP. Once stimulated the crystals emit blue-violet light.

The amount of light emitted is proportional to the amount of radiation absorbed by the PSP. The emitted light is read by a photomultiplier tube within the CR reader producing an electronic signal. The electronic signal is then fed through an analog-to-digital converter (ADC usually 12 bit) producing a digital signal. Essentially, the PSP is a OSL radiation monitor

ADC While converting the analog signal to a digital the ADC also breaks the pieces or squares that represent pixels in the final image.

Digital Radiography (DR) Direct DR X-ray photons are absorbed by the DR plate. The active ingredient is usually amorphous selenium. The a-Se is ionized by the photons and the released electrons are stored in the TFT (thin film transistor) array. The TFT consists of multitude of individual elements that represent the pixels of the final image

Direct TFT

Indirect DR Similar to direct DR but x-ray photons are converted to light photons (cesium iodide) and then pass through a photo-multiplier tube coupled with amorphous silicon. Cesium iodide crystals resemble needles in appearance resulting in very little light spread and high spatial resolution.

Indirect DR w/CCD

Indirect DR w/CMOS

Indirect DR Deterioration of image quality due to light diffusion

Device Active Ingredient Grayscale/ dynamic range Processor Film/ Screen Phosphorescent screens and silver halide film emulsion 1000 Wet or dry chemical action CR Barium fluorobromide doped with europium 14 bit 16000 Helium-neon red laser stimulates the PSP to emit light Direct DR Amorphous selenium 14 to 16 bit 16000 - 65000 shades of gray Indirect DR Cesium iodide scintillator coupled with amorphous silicon

Film/screen Optical density ranges from 0 to 3 This represents a range of 3 orders of magnitude or 1000. View boxes can only display 30 shades of gray Film/screen grayscale is 1000

CR Four orders of magnitude are possible CT and MR CR and DR 10000 shades of gray CT and MR 12 bit or 4096 CR and DR 14 bit or 16384 Digital Mammography 16 bit or 65536 shades of gray

Contrast Resolution Principal descriptor Grayscale Dynamic range

Window level Window and leveling of the digital image allows us to see the entire grayscale of the digital image. Even though the eye is limited to 30 shades of gray by manipulating the window width and level we can determine where the 30 shades occur. We determine the center point of the 30 shades, level, and the width is how many grays will be displayed.

Window Width and Level

Processing the Image Image pre-processing Contrast enhancement Find the pertinent image (histogram) Scale data to appropriate range Contrast enhancement Anatomy specific gray scale manipulation Spatial frequency enhancement

Raw Image Inherent subject contrast displayed Contrast inverted PSL signal amplitude log amplified

Preprocessed raw image Scaled and inverted: Unprocessed image

Contrast Enhancement Optimize image contrast via non-linear transformation curves Unprocessed images have linear ‘subject contrast’ Gradiation processing – Fuji Tone scaling – Kodak MUSICA - Agfa

Contrast Enhancement

Scaled and inverted: Unprocessed image Contrast enhanced

CsI crystals

Spatial Resolution and Monitor Performance In the analog environment, spatial resolution is a affected by SID, OID, RSV, FFS, and type of film to name a few. In the digital domain, the viewing monitor also affects spatial resolution as well as contrast resolution or dynamic range.

Monitor Factors Affecting Image Quality Monitor brightness The brighter the monitor the better the image quality Color v/ B & W For CR and DR color monitors do not offer enough dynamic range Matrix size Most PC monitors are 1024 x 1280 or 1200 x 1600. These will allow you to view CT and MR at resolution.

Diagnostic Monitors Matrix size Black and white not color Brightness 2048 x 2560 at minimum Black and white not color Brightness 600 cd/m2 v 300 cd/m2

What are radiation protection and safety issues? Unique characteristics of screen/film imaging systems “self limitation” of patient exposure concept of "speed" defined and understood New considerations for digital radiography no “self limitation” as in screen/film systems no consensus on “speed” "inefficient" systems possible? TERPSSC 2001 Robert M. Gagne

Film/Screen “Self Limitation” Imaging task with large dynamic range Be careful not to under or over expose film “Self limitation” of patient exposure TERPSSC 2001 Robert M. Gagne

Film/Screen “Speed” Film/screen “speed” Difference in speed of about 2 Film/screen “speed” speed = 100/E where E is exposure in mR to produce an optical density of 1.0 position on exposure axis dependent on “speed” higher “speed” number translates to lower patient exposure TERPSSC 2001 Robert M. Gagne

DR “Speed” DR operating point equivalence to film/screen “speed” set at installation? no “self limitation” except at extreme ends of the gray-scale transfer curve patient exposure increase / decrease / equivalence compared to film/screen? TERPSSC 2001 Robert M. Gagne

Elements of DR Imaging Systems Capture element The element/material used to capture the x-ray photon Coupling element Transfers the x-ray generated signal to the collection element Fiber optics, lens, contact layer, or a-Se Collection element Photodiode, CCD, TFT, CMOS Photodiodes, CCDs, and CMOS are light sensitive devices that collect light photons TFT collect electrons

Digital Radiography Type CR Indirect DR Direct DR Capture Element Barium fluorohalide CsI CsI – cesium iodide/GdOD gadolinium oxysulfide CsI – cesium iodide a-Se amorphous selenium Coupling element Lens or fiber optics a-Si Amorphous silicon Fiber optics or lens a-Se Collection element CMOS TFT CCD

Indirect w/ CCD and fiber optics Multiple detectors Single detector CCD Fiber optic coupling CsI capture element

Indirect w/CMOS and lens

Indirect TFT a-Si Coupling element

Direct TFT

Review of Indirect DR Basically there are three types of indirect systems CCD A multitude of small CCD (charged coupled devices) receive light from the cesium iodide scintillator. TFT The light from scintillator is recorded by a TFT (thin film transistor) that is also called a flat panel detector. CMOS (complementary metal oxide semiconductor) A lens focuses light on the CMOS chip

Flat panel technology (TFT) indirect and direct DR applications