1. Fundamentals of Digital Radiology
George David
Medical College of Georgia

2. Filmless Department What we mean by Digital Digital Radiographs PACS
Picture Archival & Communication Systems
Reading from Monitors

3. What is a digital image?
2D array of #’s representing some image attribute such as
optical density
x-ray attenuation
Radiography
Fluoroscopy
CTDI
echo intensity
Magnetization T1 T2
Proton Density
125 25
311
111
182
222
176
199
192 85 69
133
149
112 77
103
118
139
154
120
145
301
256
223
287
225
178
322
325
299
353
333
300

4. Number Array Forms Digital Image
194 73 22

5. Digital Image Formation
The finer the mesh, the better the digital rendering

6. What is this?
12 X 9 Matrix

7. Same object, smaller squares
24 X 18 Matrix

8. Same object, smaller squares
48 X 36 Matrix

9. Same object, smaller squares
96 X 72 Matrix

10. Same object, smaller squares
192 X 144 Matrix

11. Display of Digital Image
Each number of a digital image (pixel value) assigned a gray shade
Assignments can be changed
Window/level
Pixel values cannot

12. Computer Storage Computer image file is array of numbers
Same as any computer file
125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300
125 25
311
111
182
222
176
199
192 85 69
133
149
112 77
103
118
139
154
120
145
301
256
223
287
225
178
322
325
299
353
333
300

13. Digital Copies
125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300
125, 25, 311, 111, 182, 222, 176, 199, 192, 85, 69, 133, 149, 112, 77, 103, 118, 139, 154, 125, 120, 145, 301, 256, 223, 287, 256, 225, 178, 322, 325, 299, 353, 333, 300 =
If you’ve got the same numbers you have an IDENTICAL copy
Analog copies are never identical

14. All Digital Copies are Originals=

15. Image Matrix & Image Size
Doubling the matrix dimension quadruples the # pixels
111 87
118
155
125 25
311
111
199
192 85 69 77
103
118
139
145
301
256
223
4 X 4 Matrix
16 pixels
2 X 2 Matrix
4 pixels

16. Doubling the matrix dimension quadruples # pixels
Image Matrix
Doubling the matrix dimension quadruples # pixels
Matrix # Pixels
512 X => ,144
1024 X1024 => 1,048,576
2048 X2048 => 4,194,304
A matrix compared to a 5122 matrix quadruples
disk storage requirements
image transmission time
digital image manipulation

17. Matrix Size & Resolution
More pixels = better spatial resolution

18. Pixel Values & The Bit Bit=>Fundamental unit of computer storage
Only 2 allowable values 1
Computers do all operations with 0’s & 1’s BUT Computers group bits together

19. Abbreviations Review Bit (binary digit) Byte Kilobyte Megabyte
Smallest binary unit; has value 0 or 1 only
Byte
8 bits
Kilobyte
210 or 1024 bytes
sometimes rounded to 1000 bytes
Megabyte
213 or 1,048,576 bytes or 1024 kilobytes
sometimes rounded to 1,000,000 bytes or 1,000 kilobytes

20. # of unique values which can be represented by 1 bit2

21. # of unique values which can be represented by 2 bits1 2
4 unique combinations / values 3 4

22. # of unique values which can be represented by 3 bits5 1 6 2 7 3 8 4
8 unique combinations / values

23. Digital Image Bit Depth
the number of computer bits (1’s or 0’s) available to store each pixel value
Values
Bits
# Values 1 2 3 . 8
0, 1
00, 01, 10, 11
000, 001, 010, 011, 100, 101, 110, 111 .
, ,
2 1 = 2
2 2 = 4
2 3 = 8 .
2 8 = 256

24. Bit Depth and Pixel Presentation on Image
Indicates # of possible brightness levels for a pixel
presentation of brightness levels
pixel values assigned brightness levels
brightness levels can be manipulated without affecting image data
window
level

25. Bit Depth & Contrast Resolution
The more bits per pixel the more possible gray shades and the better contrast resolution.
2 bit; 4 grade shades
8 bits; 256 grade shades

26. Computer Storage / Image Size
Storage = # Pixels X # Bytes/Pixel
Example: 512 X 512 pixels; 1 Byte / Pixel
512 X 512 pixel array
# pixels = 512 X 512 = 262,144 pixels
262,144 pixels X 1 byte / pixel =
262,144 bytes
256 Kbytes
0.25 MBytes

27. Image Size Depends on both matrix size & bit depth
Larger (finer) matrix requires more storage
doubling matrix size quadruples image size
Larger bit depth requires more storage
doubling bit depth (theoretically) doubles image size

28. Image Compression jpg gif (20) 37’s
reduction of digital image storage size by application of algorithm
for example, repetitive data could be represented by data value and # repetitions rather than by repeating value
jpg
37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37
gif
(20) 37’s

29. Image Compression Image Decompression Compression Ratio
calculating original digital image from previously compressed data
Compression Ratio
original image size compressed image size
ratio depends upon
data to be compressed
algorithm

30. Compression Types Reversible Compression Non-reversable Compression
Image decompresses to original pixel values
Low compression ratios only
Non-reversable Compression
Decompressed image’s pixel values not necessarily identical to original
much higher compression ratios possible
variation from original image may or may not be visible or clinically significant

31. Non-Reversable Compression
variation from original image generally increases with increasing compression ratio
but a higher compression ratio means less storage requirements
variation less noticeable for dynamic (moving) images than for still images such as radiographs

32. Computed Radiography (CR)
Re-usable metal imaging plates replace film & cassette
Uses conventional bucky & x-ray equipment
                                                                 

33. CR Exposure & Readout

34. CR Readout

35. Another View: CR Operation

36. Computer Radiography (CR)
photostimulable phosphor plate
radiation causes electrons to move to higher energy states
Excitation
Plate’s structure traps electrons in higher energy states
Form latent image H i g h e r E n e r g y - E l e c t r o n S t a t e P h o t o n p u m p s e l e c t r o n t o X - R a y h i g h e r e n e r g y s t a t e P h o t o n - - - L o w e r E n e r g y - - - - - - E l e c t r o n - - - - - - - - - S t a t e - - - - - - - - -

37. Reading Imaging Plate laser scans plate with
laser releases electrons trapped in high energy states
electrons fall to low energy states giving up energy as visible light
light intensity is measure of incident radiation
Lower Energy Electron State

38. Reading Imaging Plate Reader scans plate with laser
Beam moved using rotating mirror
Plate pulled through scanner by rollers
Light emitted by plate measured by PM tube & recorded by computer

39. Laser & Emitted Light are Different Colors
Phosphor stimulated by laser light
Intensity of emitted light indicates amount of radiation incident on phosphor at each location
Only light emitted by phosphor measured by PMT
Filters remove laser light

40. CR Erasure after read-out, plate erased using a bright light
plate can be erased and re-used
Erasure re-use cycle can be repeated without limit
Plate life defined not by erasure cycles but by physical wear

41. CR Resolution
Some vendor CR spatial resolution depends upon plate size.
Smaller pixels
More pixels / mm

42. CR Throughput Generally slower than film processing
CR reader must finish reading one plate before starting the next
Film processors can run films back to back

43. CR Latitude Much greater latitude than screen/film
Plate responds to many decades of input exposure
under / overexposures unlikely
Computer scale inputs exposure to viewable densities
Unlike film, receptor separate from viewer

44. Film Screen vs. CR Latitude
CR Latitude: .01 – 100 mR
Unlike film, small changes in incident radiation result in CR signal
100

45. Digital Radiography (DR)
Digital electronic bucky

46. DR Formats Electronic bucky incorporated into x-ray equipment
Electronic wireless cassettes

47. Digital Radiography (DR)
Receptor provides direct digital output
No processor / reader required
Images available virtually immediately
Far fewer steps for radiographer

48. TFT = THIN-FILM TRANSISTOR ARRAY
Types of DR Receptors
TFT = THIN-FILM TRANSISTOR ARRAY

49. Digital Radiography (DR)
High latitude as for CR
DR portables now in available
Radiographer immediately sees image

50. Digital Raw Image Unprocessed image as read from receptorCR
Intensity data from PMT’s as a result of scanning plate with laser DR
Raw Data read directly from TFT array
Not a readable diagnostic image
Requires computer post-processing
Specific software algorithms must be applied to image prior to presenting it as finished radiograph

51. Enhancing Raw Image (Image Segmentation)*
Identify collimated image border
Separate raw radiation from anatomy
Apply appropriate tone-scale to image
Done with look-up table (LUT)
This process is specific to a particular body part and projection

52. Image Segmentation Computer then defines anatomic region
Computer establishes location of collimated border of image based on exam type and view provided by operator
Computer then defines anatomic region
Finished image produced by tone scaling
Requires histogram analysis of anatomic region

53. Histogram Graph showing how much of image is exposed at various levels
# pixels

54. Tone Scaling Post-Processing
Body part & projection-specific algorithms determine average exposure
Must correctly identify anatomical region
Look Up Table (LUT) computed to display image with proper
Density
Contrast

55. Film/Screen Limited Latitude
Film dictates proper radiation exposure
No post processing
Improperly exposed films lose contrast

56. Should I Worry?
In CR & DR, image density is no longer a reliable indicator of exposure factor control.

58. CR / DR Latitude DANGER Will Robinson!!!
Almost impossible to under or overexpose CR / DR
Underexposures look noisy
Overexposures look GOOD!!!

59. How to Determine Optimum Radiation Dose?

60. Carestream Exposure Index
Each manufacturer provides feedback to technologist on exposure to digital receptor
Displayed on CR reader monitor
Displayed on workstations
Fuji S-Number
Carestream Exposure Index
Deviation Index

61. Exposure Index Indexes differ by manufacturer Form
Linear
Logrithmic
Some indexes go up with radiation, some go down

62. Calculated Exposure Index Affected by
X-Ray technique selection
Improper centering
Improper selection of study or projection
Placing two or more views on same CR cassette

63. Phototimed CR Phantom Image
75 kVp
88 mAs
2460 EI

64. Let’s Approximately Double mAs
75 kVp
88 mAs
2460 EI
75 kVp
160 mAs
2680 EI

65. Let’s Go Crazy
75 kVp
88 mAs
2460 EI
75 kVp
640 mAs
3300 EI

66. How Low Can You Go? Cut mAs in Half!
75 kVp
88 mAs
2460 EI
75 kVp
40 mAs
2060 EI

67. Let’s Go Crazy Low
75 kVp
8 mAs
1380 EI
75 kVp
1 mAs
550 EI

68. CR Artifacts Physical damage to imaging plates Dirt in reader
Cracks, scuffs, scratches
Contamination
Dust / dirt
Dirt in reader
Highly sensitive to scatter radiation

69. DR Artifacts Dead detector elements
Spatial variations in background signal & gain
Grid interference
Software can help correct for above

70. Shifting Gears: Fluoroscopy Issues

71. Digital Video Sources DR type image receptor
Conventional Image Intensifier with Video Signal Digitized (“Frame Grabber”) I m a g e T u b
X-Ray
Input
Image
Tube TV
Amplfier
Analog to
Digital
Converter
Memory
(Computer)
Lens System

72. Digital Spot Film Frame grabber digitizes image
Digital image saved by computer
Radiographic Technique used
required to control quantum noise

73. Last Image Hold
Computer displays last fluoro image before radiation shut off.
Image noisier than for digital spot
Image made at fluoroscopic technique / intensity
Allows operator to review static processes without keeping beam on
ideal for teaching environments
ideal for orthopedic applications such as hip pinning
Less radiation than digital spot

74. Fluoro Frame Averaging
Conventional fluoro only displays current frame
Frame averaging allows computer to average current with user-selectable number of previous frames
Averages current frame & history

75. Fluoro Frame Averaging Tradeoff
Advantage:
Reduces quantum noise
Disadvantage
Because history frames are averaged with current frame, any motion can result in lag

76. Other Fluoro Features Real-time Edge Enhancement / Image Filtering
Option of using lower frame rates (15, 7.5, 3.75 fps rather than 30)
computer displays last frame until next one
reduces flicker
Lowers patient and scatter exposure
Exposure proportional to frame rate
dynamic studies may be jumpy

77. Digital Subtraction Immediate replay of run Free selection of mask
before or after bolus
>1 frame may be averaged for mask
Note
subtraction adds noise

78. Digital Image Manipulations
on-screen measurements
distances
angles
volumes/areas
stenosis
image annotation
peak opacification / roadmapping
peak opacification displays vessels after a test injection
allows visualization of live catheter on top to saved image of test injection

79. Digital Applications
Multi-modality imaging / Image fusion
PET/CT

80. DR & Energy Subtraction
2 images taken milliseconds apart at 2 kVp’s
Combine / subtract images
Soft Tissue Image
Bone Image

81. Other Possibilities Tomosynthesis Histogram Equalization
Multi-slice linear tomography from one exposure series
Histogram Equalization
Use computer to provide approximately equal density to various areas of image.

82. The End ?