6. Computer Radiography (CR)Computer Radiography (CR)
• plate is
photostimulable
phosphor
• radiation traps
electrons in
high energy
states
• higher states
form latent
image
Higher Energy
Electron
State
Lower Energy
Electron
State
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X-Ray
Photon
-
Photon pumps
electron to
higher energy state
7. Reading Imaging PlateReading 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
Laser Beam
Higher Energy
Elect ron
St at e
Lower Energy
Elect ron
St at e
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Lower Energy
Electron State
8. Reading Imaging PlateReading 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
9. Laser & Emitted Light are Different ColorsLaser & 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
10. CR OperationCR 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
11. CR Phosphor LayerCR Phosphor Layer
• Phosphor balanced for
x-ray absorption characteristics
light output
laser light scatter
screen thickness
• Above variables affect
electronic noise
image resolution properties
speed of imaging system
• Overcoat protects plate from
physical damage
13. CR ThroughputCR 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
14. CR LatitudeCR 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
15. Film Screen vs. CR LatitudeFilm Screen vs. CR Latitude
CR
Latitude: .01
– 100 mR
100
20. “Direct” DR“Direct” DR
• X-ray energy
converted directly
to electrical signal
• X-rays interact
with
semiconductor
material
Amorphous selenium
• X-rays converted
directly into
electrical charge
No intermediate
steps
21. “Indirect” DR“Indirect” DR
• X-ray strike scintillator producing
light
• Photodiode array converts light to
electrons
Light
22. Indirect DRIndirect DR
• Light spreads can limit spatial resolution
• Can be controlled by “channeling”
• Winning in the marketplace
24. SummarySummary
• DR becoming industry leader in
radiographic imaging
• DR images displayed & stored in
about 8 seconds
• DR has faster throughput
Up to 2-4 times faster than traditional
screen-film-darkroom technology
25. Raw Data ImageRaw Data Image
• Unprocessed image as read from
receptor
CR
» 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
26. Enhancing Raw Image
(Image Segmentation)
Enhancing Raw Image
(Image Segmentation)
1. Identify collimated image
border
2. Separate raw radiation
from anatomy
3. Apply appropriate tone-
scale to image
Done with look-up table (LUTLUT)
This process is
specific to a
particular body
part and
projection
*
27. Look Up Table (LUT)Look Up Table (LUT)
• Converts a raw
data pixel value to
a processed pixel
value
• “Original” raw
data pixel value
indicates amount
of radiation falling
on pixel
28. Image SegmentationImage Segmentation
• 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
32. LUT SelectionLUT Selection
• LUT
calculated
by algorithm
depends on
Body part
projection
• User can
also alter
LUT
manually
33. LUT SelectionLUT Selection
• Monitors on CR
reader or DR
console compared
to reading
workstations have
lower resolution
poorer quality
Recommended that
LUT not be manually
modified
35. Should I Worry?Should I Worry?
In CR & DR,
image density is
no longer a
reliable indicator
of exposure factor
control.
36.
37. • Almost impossible to
under or overexpose CR /
DR
• Underexposures look
noisy
• Overexposures look
GOOD!!!
CR / DR LatitudeCR / DR Latitude
DANGER
Will
Robinson!!!
38. Exposure CreepExposure Creep:
Tendency of radiographs toward
higher-then-necessary exposures
Exposure CreepExposure Creep:
Tendency of radiographs toward
higher-then-necessary exposures
• No detrimental effect on image quality
• Desire to see less noise on radiographs
• Increased exposure latitude
• No one complains
39. So how do I know if exposure is
optimum by looking at my image?
So how do I know if exposure is
optimum by looking at my image?
40.
41. Exposure IndexExposure Index
• Each manufacturer provides feedback to
technologist on exposure to digital
receptor
• Displayed on CR reader monitor
• Displayed on workstations
42. Exposure IndexExposure Index
• Measure of radiation received by receptor
below anatomy
• Not a direct measure of patient exposure
• If exposure index higher than
recommended range, patient
overexposed
43. Exposure Indication Varies
between Manufacturers
Exposure Indication Varies
between Manufacturers
Receptor
Exposure
Kodak
EI
Fuji S
Number
0.5 1700 400
1 2000 200
2 2300 100
4 2600 50
Fuji
“S” number goes down
as exposure goes up!
S is half when
exposure doubled
Kodak
Logarithmic scale
EI goes up 300 when
exposure doubled
44. Exposure IndexExposure Index
• Technologist should strive to keep
exposure index consistent
• Kodak recommendation for exposure
index
1800 – 2200
• George’s recommendation
“Maximum tolerable noise”
As low as possible while providing
tolerable noise
This is not a beauty contest!
45. Calculated Exposure
Index Affected by
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
47. Let’s Approximately Double mAsLet’s Approximately Double mAs
• 75 kVp
• 88 mAs
• 2460 EI
• 75 kVp
• 160 mAs
• 2680 EI
48. Let’s Go CrazyLet’s Go Crazy
• 75 kVp
• 88 mAs
• 2460 EI
• 75 kVp
• 640 mAs
• 3300 EI
49. How Low Can You Go? Cut mAs in Half!How Low Can You Go? Cut mAs in Half!
• 75 kVp
• 88 mAs
• 2460 EI
• 75 kVp
• 40 mAs
• 2060 EI
50. Let’s Go Crazy LowLet’s Go Crazy Low
• 75 kVp
• 8 mAs
• 1380 EI
• 75 kVp
• 1 mAs
• 550 EI
51. CR ArtifactsCR Artifacts
• Physical damage to imaging
plates
Cracks, scuffs, scratches
Contamination
Dust / dirt
• Dirt in reader
• Highly sensitive to scatter
radiation
52. CR Grid InterferenceCR Grid Interference
• 103 lines / inch grids have same frequency
as CR laser scanner. This can cause
“Moire” pattern artifact
• Align grid lines perpendicular to scan
orientation whenever possible
Reduces chances of artifacts caused by laser scanner.
53. DR ArtifactsDR Artifacts
• Dead detector elements
• Spatial variations in
background signal & gain
• Grid interference
• Software can help correct for
above
55. Digital Video SourcesDigital Video Sources
• DR type image receptor
• Conventional Image Intensifier with Video
Signal Digitized (“Frame Grabber”)
I
m
a
g
e
T
u
b
e
X-Ray
Input
Image
Tube TV
Amplfier
Analog
to
Digital
Convert
er
Digital
Memory
(Computer)
Lens System
56. Digital Spot FilmDigital Spot Film
• Frame grabber digitizes image
• Digital image saved by computer
• Radiographic Technique used
required to control quantum noise
57. Last Image HoldLast 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
58. Fluoro Frame
Averaging
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
59. Fluoro Frame
Averaging Tradeoff
Fluoro Frame
Averaging Tradeoff
• Advantage:
Reduces quantum noise
• Disadvantage
Because history frames are averaged with
current frame, any motion can result in lag
60. Other Fluoro
Features
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
62. DR Mobile UnitsDR Mobile Units
• See image
immediately
• Wireless
transmission of
images
63. Other PossibilitiesOther Possibilities
• Tomosynthesis
Multi-slice linear tomography from
one exposure series
• Histogram Equalization
Use computer to provide
approximately equal density to
various areas of image.
64. DR & Energy SubtractionDR & Energy Subtraction
• 2 images taken milliseconds
apart at 2 different kVp’s
• Combine / subtract images
Soft Tissue Image Bone Image