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

2. So what is “Digital”?

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

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

5. Digital Image Formation
Place mesh over image

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

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

8. What is this?
12 X 9 Matrix

9. Same object, smaller squares
24 X 18 Matrix

10. Same object, smaller squares
48 X 36 Matrix

11. Same object, smaller squares
96 X 72 Matrix

12. Same object, smaller squares
192 X 144 Matrix

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

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 = If you’ve got the same numbers ...
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 ...

17. Digital Copies
then you have an identical copy =

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

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

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

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

22. The Bit Fundamental unit of computer storage Only 2 allowable values1
Computers do all operations with 0’s & 1’s BUT Computers group bits together

23. Special Binary Digit Grouping Terms
Nibble
4 binary bits (0101)
Byte
8 binary bits ( )
Word
16 binary bits ( )
Double Word
32 binary bits ( )

24. 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

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

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

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

28. 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

29. Digital Image Bit Depth
bit depth 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

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

38. CR Exposure & Readout

39. CR Readout

40. Another View: CR Operation

41. 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 - - - - - - - - -

42. 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

43. 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

44. 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

45. 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

46. CR Resolution Small cassettes have better spatial resolution
Smaller pixels
More pixels / mm

47. 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
               

48. 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
               

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

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

51. Digital Radiography (DR)
Receptor provides direct digital output
No processor / reader required
Images available in < 15 seconds
Much less work for technologist

52. TFT = THIN-FILM TRANSISTOR ARRAY
Direct vs. Indirect
TFT = THIN-FILM TRANSISTOR ARRAY

53. Digital Radiography (DR)
Potentially lower patient dose than CR
High latitude as for CR
Digital bucky fragile
First DR portables coming to market

54. Raw Data 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

55. 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

56. Image Segmentation Computer then defines anatomic region
Computer must establish location of collimated border of image
Computer then defines anatomic region
Finished image produced by tone scaling
Requires histogram analysis of anatomic region

57. Histogram
Graph showing how much of image is exposed at various levels

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

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

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

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

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

64. Exposure Index
Each manufacturer provides feedback to technologist on exposure to digital receptor
Displayed on CR reader monitor
Displayed on workstations

65. 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

66. Phototimed Phantom Image
75 kVp
88 mAs
2460 EI

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

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

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

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

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

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

73. Shifting Gears: Fluoroscopy Issues

74. 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

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

76. 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

77. 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

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

79. 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

80. 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

81. 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

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

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

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

85. 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.

86. The End ?