1. Comparison of Film v. Digital Image Display

2. 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

3. 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.

4. 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

5. 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.

7. 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

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

9. 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

10. Direct TFT

11. 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.

12. Indirect DR w/CCD

13. Indirect DR w/CMOS

14. Indirect DR
Deterioration of image quality due to light diffusion

16. 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 shades of gray
Indirect DR
Cesium iodide scintillator coupled with amorphous silicon

17. 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

18. 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 shades of gray

19. Contrast Resolution
Principal descriptor
Grayscale
Dynamic range

20. 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.

21. Window Width and Level

22. 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

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

26. Preprocessed raw image
Scaled and inverted:
Unprocessed image

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

28. Contrast Enhancement

29. Scaled and inverted:
Unprocessed image
Contrast enhanced

32. CsI crystals

33. 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.

34. 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 These will allow you to view CT and MR at resolution.

35. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

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

48. Indirect w/CMOS and lens

49. Indirect TFT
a-Si
Coupling
element

50. Direct TFT

51. 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

52. Flat panel technology (TFT) indirect and direct DR applications