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Computed Radiography (CR), Digital Radiography (DR), & Digital Spots George David Associate Professor of Radiology Medical College of Georgia Computed Radiography (CR) Re-usable metal imaging plates replace film & cassette Uses conventional bucky & x-ray equipment CR Exposure & Readout

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Computed Radiography (CR),

Digital Radiography (DR),

& Digital Spots

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
computer radiography cr

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Computer Radiography (CR)
  • plate is photostimulable phosphor
  • radiation traps electrons in high energy states
  • higher statesform latent image

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reading imaging plate
Reading Imaging Plate
  • reader scans plate with laser
  • laser releaseselectrons trapped inhigh energystates
  • electrons fall to lowenergy states
  • electrons give upenergy as visible light
  • light intensity ismeasure of incident radiation

Lower Energy Electron State

reading imaging plate8
Reading 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 colors
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
cr operation
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
cr phosphor layer
CR 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
cr resolution
CR Resolution
  • Small cassettes have better spatial resolution
    • Smaller pixels
    • More pixels / mm
cr throughput
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
cr latitude
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
film screen vs cr latitude
Film Screen vs. CR Latitude

CR Latitude: .01 – 100 mR

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digital radiography dr
Digital Radiography (DR)
  • Digital bucky
  • Incorporated into x-ray equipment
digital radiography dr18
Digital Radiography (DR)
  • Receptor provides direct digital output
  • No processor / reader required
    • Images available in < 15 seconds
    • Much less work for technologist
direct vs indirect
Direct vs. Indirect

TFT = THIN-FILM TRANSISTOR ARRAY

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
indirect dr

Light

“Indirect” DR
  • X-ray strike scintillator producing light
  • Photodiode array converts light to electrons
indirect dr22
Indirect DR
  • Light spreads can limit spatial resolution
  • Can be controlled by “channeling”
  • Winning in the marketplace
digital radiography dr23
Digital Radiography (DR)
  • Potentially lower patient dose than CR
  • High latitude as for CR
  • Digital bucky fragile
    • First DR portables comingto market
summary
Summary
  • 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
raw data image
Raw 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
enhancing raw image image segmentation
Enhancing Raw Image (Image Segmentation)

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  • 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

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 segmentation
Image 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
histogram
Histogram
  • Graph showing how much of image is exposed at various levels
tone scaling post processing
Tone ScalingPost-Processing
  • Body part & projection-specific algorithms determine average exposure
    • Must correctly identify anatomical region
  • LUT computed to display image with proper
    • Density
    • Contrast
lut selection
LUT Selection
  • LUT calculated by algorithm depends on
    • Body part
    • projection
  • User can also alter LUT manually
lut selection33
LUT Selection
  • Monitors on CR reader or DR console compared to reading workstations have
    • lower resolution
    • poorer quality
    • Recommended that LUT not be manually modified
film screen limited latitude
Film/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.
cr dr latitude
CR / DR Latitude

DANGER Will Robinson!!!

  • Almost impossible to under or overexpose CR / DR
  • Underexposures look noisy
  • Overexposures look GOOD!!!
exposure creep tendency of radiographs toward higher then necessary exposures
Exposure 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
exposure index
Exposure Index
  • Each manufacturer provides feedback to technologist on exposure to digital receptor
  • Displayed on CR reader monitor
  • Displayed on workstations
exposure index42
Exposure 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

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
exposure index44
Exposure 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 providingtolerable noise
    • This 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 cassette
    • Can cause image to appear dark
phototimed phantom image
Phototimed Phantom Image
  • 75 kVp
  • 88 mAs
  • 2460 EI
let s approximately double mas
Let’s Approximately Double mAs
  • 75 kVp
  • 88 mAs
  • 2460 EI
  • 75 kVp
  • 160 mAs
  • 2680 EI
let s go crazy
Let’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 low
Let’s Go Crazy Low
  • 75 kVp
  • 8 mAs
  • 1380 EI
  • 75 kVp
  • 1 mAs
  • 550 EI
cr artifacts
CR Artifacts
  • Physical damage to imaging plates
    • Cracks, scuffs, scratches
    • Contamination
    • Dust / dirt
  • Dirt in reader
  • Highly sensitive to scatter radiation
cr grid interference
CR 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.
dr artifacts
DR Artifacts
  • Dead detector elements
  • Spatial variations in background signal & gain
  • Grid interference
  • Software can help correct for above
digital video sources

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Digital Video Sources
  • DR type image receptor
  • Conventional Image Intensifier with Video Signal Digitized (“Frame Grabber”)
digital spot film
Digital Spot Film
  • Frame grabber digitizes image
  • Digital image saved by computer
  • Radiographic Technique used
    • required to control quantum noise
last image hold
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
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
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
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
dr mobile units
DR Mobile Units
  • See image immediately
  • Wireless transmission of images

GE Definium AMX 700

other possibilities
Other Possibilities
  • Tomosynthesis
    • Multi-slice linear tomography from one exposure series
  • Histogram Equalization
    • Use computer to provide approximately equal density to various areas of image.
dr energy subtraction
DR & Energy Subtraction
  • 2 images taken milliseconds apart at 2 different kVp’s
  • Combine / subtract images

Soft Tissue Image

Bone Image