Computed Radiography Cr Digital Radiography Dr3148
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Transcript of Computed Radiography Cr Digital Radiography Dr3148
George DavidAssociate Professor of Radiology
Medical College of Georgia
Computed Radiography (CR)Computed Radiography (CR)
• Re-usable metal imaging plates replace film & cassette
• Uses conventional bucky & x-ray equipment
CR Exposure & ReadoutCR Exposure & Readout
CR ReadoutCR Readout
Another View: CR OperationAnother View: CR Operation
Computer Radiography (CR)Computer Radiography (CR)
• plate is photostimulable phosphor
• radiation traps electrons in high energy states
• higher statesform latent image
Higher EnergyElectronState
Lower EnergyElectronState
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X-RayPhoton
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Photon pumpselectron tohigher energy state
Reading Imaging PlateReading Imaging Plate• reader scans plate with laser• laser releases
electrons trapped inhigh energystates
• electrons fall to lowenergy states
• electrons give upenergy as visible light
• light intensity ismeasure of incident radiation
Laser Beam
Higher EnergyElectronState
Lower EnergyElectronState
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Lower Energy Electron State
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 byPM tube &recorded by computer
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
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
CR Phosphor LayerCR Phosphor Layer• Phosphor balanced for
x-ray absorption characteristics light output laser light scatterscreen thickness
• Above variables affectelectronic noise image resolution propertiesspeed of imaging system
• Overcoat protects plate from physical damage
CR ResolutionCR Resolution
• Small cassettes have better spatial resolutionSmaller pixelsMore pixels / mm
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
CR LatitudeCR Latitude
• Much greater latitude than screen/film
• Plate responds to many decades of input exposureunder / overexposures
unlikely
• Computer scale inputs exposure to viewable densitiesUnlike film, receptor
separate from viewer
Film Screen vs. CR LatitudeFilm Screen vs. CR Latitude
CR Latitude: .01 –
100 mR
100
CR Very Sensitive to ScatterCR Very Sensitive to Scatter
Digital Radiography (DR)Digital Radiography (DR)
• Digital bucky
• Incorporated into x-ray equipment
Digital Radiography (DR)
Digital Radiography (DR)
• Receptor provides direct digital output
• No processor / reader requiredImages available in < 15 secondsMuch less work for technologist
Direct vs. IndirectDirect vs. Indirect
TFT = THIN-FILM TRANSISTOR ARRAY
“Direct” DR“Direct” DR• X-ray energy
converted directly to electrical signal
• X-rays interact with semiconductor materialAmorphous selenium
• X-rays converted directly into electrical chargeNo intermediate
steps
“Indirect” DR“Indirect” DR
• X-ray strike scintillator producing light
• Photodiode array converts light to electrons
Light
Indirect DRIndirect DR
• Light spreads can limit spatial resolution
• Can be controlled by “channeling”
• Winning in the marketplace
Digital Radiography (DR)
Digital Radiography (DR)
• Potentially lower patient dose than CR
• High latitude as for CR
• Digital bucky fragileFirst DR portables coming
to market
SummarySummary
• DR becoming industry leader in radiographic imaging
• DR images displayed & stored in about 8 seconds
• DR has faster throughputUp to 2-4 times faster than traditional
screen-film-darkroom technology
Raw Data ImageRaw 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-
processingSpecific software algorithms must be
applied to image prior to presenting it as finished radiograph
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
*
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
Image SegmentationImage Segmentation
• Computer must establish location of collimated border of image
• Computer then defines anatomic region
• Finished image produced by tone scalingRequires histogram analysis of
anatomic region
HistogramHistogram
• Graph showing how much of image is exposed at various levels
Tone ScalingPost-Processing
Tone ScalingPost-Processing
• Body part & projection-specific algorithms determine average exposureMust correctly identify anatomical region
• LUT computed to display image with properDensityContrast
LUT can Simulate Appearance of FilmLUT can Simulate
Appearance of Film
LUT SelectionLUT Selection
• LUT calculated by algorithm depends onBody partprojection
• User can also alter LUT manually
LUT SelectionLUT Selection
• Monitors on CR reader or DR console compared to reading workstations havelower resolutionpoorer qualityRecommended that
LUT not be manually modified
Film/Screen Limited LatitudeFilm/Screen Limited Latitude
• Film use has little ambiguity about proper radiation exposure
Should I Worry?Should I Worry?
In CR & DR, image density is no longer a reliable indicator of exposure factor control.
• Almost impossible to under or overexpose CR / DR
• Underexposures look noisy
• Overexposures look GOOD!!!
CR / DR LatitudeCR / DR Latitude
DANGER Will
Robinson!!!
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
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?
Exposure IndexExposure Index
• Each manufacturer provides feedback to technologist on exposure to digital receptor
• Displayed on CR reader monitor
• Displayed on workstations
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
Exposure Indication Varies between Manufacturers
Exposure Indication Varies between Manufacturers
Receptor Exposure
Kodak EI
Fuji S Number
0.5 1700 4001 2000 2002 2300 1004 2600 50
Fuji“S” number goes down
as exposure goes up!S is half when
exposure doubled
KodakLogarithmic scaleEI goes up 300 when
exposure doubled
Exposure IndexExposure Index• Technologist should strive to keep
exposure index consistent• Kodak recommendation for exposure
index1800 – 2200
• George’s recommendation“Maximum tolerable noise”As low as possible while providing
tolerable noiseThis is not a beauty contest!
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 cassetteCan cause image to appear dark
Phototimed Phantom ImagePhototimed Phantom Image
• 75 kVp
• 88 mAs
• 2460 EI
Let’s Approximately Double mAsLet’s Approximately Double mAs
• 75 kVp• 88 mAs• 2460 EI
• 75 kVp• 160 mAs• 2680 EI
Let’s Go CrazyLet’s Go Crazy
• 75 kVp• 88 mAs• 2460 EI
• 75 kVp• 640 mAs• 3300 EI
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
Let’s Go Crazy LowLet’s Go Crazy Low
• 75 kVp• 8 mAs• 1380 EI
• 75 kVp• 1 mAs• 550 EI
CR ArtifactsCR Artifacts
• Physical damage to imaging platesCracks, scuffs, scratchesContaminationDust / dirt
• Dirt in reader• Highly sensitive to scatter
radiation
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 possibleReduces chances of artifacts caused by laser scanner.
DR ArtifactsDR Artifacts
• Dead detector elements
• Spatial variations in background signal & gain
• Grid interference
• Software can help correct for above
Shifting Gears:Fluoroscopy Issues
Shifting Gears:Fluoroscopy Issues
Digital Video SourcesDigital Video Sources• DR type image receptor• Conventional Image Intensifier with Video
Signal Digitized (“Frame Grabber”)
Image
Tube
X-RayInput
ImageTube TV
Amplfier
Analogto
DigitalConvert
er
DigitalMemory
(Computer)
Lens System
Digital Spot FilmDigital Spot Film
• Frame grabber digitizes image• Digital image saved by computer• Radiographic Technique used
required to control quantum noise
Last Image HoldLast Image Hold
• Computer displays last fluoro image before radiation shut off.
• Image noisier than for digital spotImage 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
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 framesAverages current frame & history
Fluoro Frame Averaging Tradeoff
Fluoro Frame Averaging Tradeoff
• Advantage:Reduces quantum noise
• DisadvantageBecause history frames are averaged with
current frame, any motion can result in lag
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
The Future of DigitalThe Future of Digital
DR Mobile Units DR Mobile Units
• See image immediately
• Wireless transmission of images
Other PossibilitiesOther Possibilities
• TomosynthesisMulti-slice linear tomography from
one exposure series
• Histogram EqualizationUse computer to provide
approximately equal density to various areas of image.
DR & Energy SubtractionDR & Energy Subtraction
• 2 images taken milliseconds apart at 2 different kVp’s
• Combine / subtract images
Soft Tissue Image Bone Image
The EndThe End
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