1. Fundamentals of Digital Radiology
(year 1)
George David, MS, FAAPM, FACR
Medical College of Georgia

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

3. What we really mean by Digital
No more File Room!!!

4. Digital Image Formation
Place mesh over image

5. Digital Image Formation
Assign each square (pixel) a number based on density
Numbers form the digital image
194 73 22

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

7. Numbers / Gray Shades
Each number of a digital image corresponds to a gray shade for one picture element or pixel

8. Computer Storage 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
Storage = 262,144 pixels X 1 byte / pixel = 262,144 bytes = 256 KBytes = .25 MBytes

9. Image Size Related to both matrix size & bit depth
higher (finer) matrix requires more storage
doubling matrix size quadruples image size
higher bit depth requires more storage
doubling bit depth theoretically doubles image size
Computer may require storage in multiples of 8 bits (bytes)
10 or 12 bits stored in 16 bit slot
alters image size requirements 1 2 3 4 5 6 7 8 9

10. 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

11. 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

12. 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

13. 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

14. So what is a digital image?
Image stored as 2D array of #’s representing some image attribute such as
optical density
x-ray attenuation
echo intensity
magnetization
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

15. Computer Storage
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
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

16. = Digital Copies Digital copies are identical
All digital images are originals =

17. Image Matrix 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

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

19. The Bit
Fundamental unit of computer storage

20. # of unique values which can be represented by 1 bit
2 unique combinations / values 1 2

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. Bit Depth & Contrast Resolution
bit depth indicates # of possible brightness levels for a pixel
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

24. Computer Storage 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
Storage = 262,144 pixels X 1 byte / pixel = 262,144 bytes = 256 KBytes = .25 MBytes

25. Image Size Related to both matrix size & bit depth
higher (finer) matrix requires more storage
doubling matrix size quadruples image size
higher bit depth requires more storage
doubling bit depth theoretically doubles image size
Computer may require storage in multiples of 8 bits (bytes)
10 or 12 bits stored in 16 bit slot
alters image size requirements 1 2 3 4 5 6 7 8 9

26. 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

27. 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

28. 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

29. 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

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

31. CR Exposure & Readout

32. CR Readout

33. CR Operation after read-out, plate erased using a bright light
plate can be erased virtually without limit
Plate life defined not by erasure cycles but by physical wear

34. Another View: CR Operation

35. Computer Radiography (CR)
plate is photostimulable phosphor
radiation traps electrons in high energy states
higher 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 - - - - - - - - -

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

37. Reading Imaging Plate
Reader scans plate with laser light using rotating mirror
Film pulled through scanner by rollers
Light given off by plate measured by PM tube & recorded by computer

38. 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 color of light emitted by phosphor measured by PMT

39. CR Operation after read-out, plate erased using a bright light
plate can be erased virtually without limit
Plate life defined not by erasure cycles but by physical wear

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

41. 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
               

42. Film Screen vs. CR Latitude
CR Latitude: .01 – 100 mR
100

43. Digital Radiography (DR)
Digital bucky
Electronic
Incorporated into x-ray equipment

44. Digital Radiography (DR)
Receptor provides direct digital output
No processor / reader required
Images available in ~ 5 seconds
Much faster throughput
Eliminates many steps for technologist

45. Raw Data Image Unprocessed image as read from receptor
Not a readable diagnostic image
Requires computer post-processing
Specific software algorithms must be applied to image prior to presenting it as finished radiograph

46. 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

47. Image Segmentation Computer defines anatomic region
Computer establishes collimated border of image
Computer defines anatomic region
Finished image produced by tone scaling
Requires histogram analysis of anatomic region

48. Image Post-Processing
Histogram
Graph showing how much of image is exposed at various levels
Tone scaling
Body part & projection-specific algorithms
Must correctly identify anatomical region
Histogram used to display image with proper
Density
Contrast

49. Film/Screen Limited Latitude
Film use has little ambiguity about proper radiation exposure

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

52. So how do I know if exposure is optimum by looking at my image?

53. Exposure Index
Each manufacturer provides feedback to technologist on exposure to digital receptor
Displayed on PACS monitor

54. Calculated Exposure Index Affected by
X-Ray technique selection
Improper centering of image on cassette
Improper selection of study or projection
Placing two or more views on same cassette
Can cause image to appear dark

55. Phototimed Phantom Image
75 kVp
88 mAs
2460 EI

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

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

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

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

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

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

62. Shifting Gears: Fluoroscopy Issues

63. 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

64. Digital Spot Film Frame grabber digitizes image
Digital image saved by computer

65. Last Image Hold Computer displays last fluoro image
Allows operator to review static processes without keeping beam on

66. 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

67. 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

68. 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

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

70. 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

71. Digital Possibilities
Multi-modality imaging / Image fusion
PET/CT

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

73. DR Mobile Units GE Definium AMX 700 See image immediately
Wireless transmission of images
GE Definium AMX 700

74. The End ?