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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 l.jpg
Computed Radiography (CR)

  • Re-usable metal imaging plates replace film & cassette

  • Uses conventional bucky & x-ray equipment





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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 l.jpg
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


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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


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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


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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


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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


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CR Resolution

  • Small cassettes have better spatial resolution

    • Smaller pixels

    • More pixels / mm


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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


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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


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Film Screen vs. CR Latitude

CR Latitude: .01 – 100 mR

100



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Digital Radiography (DR)

  • Digital bucky

  • Incorporated into x-ray equipment


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Digital Radiography (DR)

  • Receptor provides direct digital output

  • No processor / reader required

    • Images available in < 15 seconds

    • Much less work for technologist


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Direct vs. Indirect

TFT = THIN-FILM TRANSISTOR ARRAY


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“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


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Light

“Indirect” DR

  • X-ray strike scintillator producing light

  • Photodiode array converts light to electrons


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Indirect DR

  • Light spreads can limit spatial resolution

  • Can be controlled by “channeling”

  • Winning in the marketplace


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Digital Radiography (DR)

  • Potentially lower patient dose than CR

  • High latitude as for CR

  • Digital bucky fragile

    • First DR portables comingto market


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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


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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


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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


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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


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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


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Histogram

  • Graph showing how much of image is exposed at various levels


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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



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LUT Selection

  • LUT calculated by algorithm depends on

    • Body part

    • projection

  • User can also alter LUT manually


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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


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Film/Screen Limited Latitude

  • Film use has little ambiguity about proper radiation exposure


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Should I Worry?

  • In CR & DR, image density is no longer a reliable indicator of exposure factor control.


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CR / DR Latitude

DANGER Will Robinson!!!

  • Almost impossible to under or overexpose CR / DR

  • Underexposures look noisy

  • Overexposures look GOOD!!!


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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




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Exposure Index image?

  • Each manufacturer provides feedback to technologist on exposure to digital receptor

  • Displayed on CR reader monitor

  • Displayed on workstations


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Exposure Index image?

  • Measure of radiation received by receptor below anatomy

  • Not a direct measure of patient exposure

  • If exposure index higher than recommended range, patient overexposed


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Exposure Indication Varies between Manufacturers image?

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


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Exposure Index image?

  • 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!


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Calculated Exposure Index Affected by image?

  • 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


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Phototimed Phantom Image image?

  • 75 kVp

  • 88 mAs

  • 2460 EI


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Let’s Approximately Double mAs image?

  • 75 kVp

  • 88 mAs

  • 2460 EI

  • 75 kVp

  • 160 mAs

  • 2680 EI


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Let’s Go Crazy image?

  • 75 kVp

  • 88 mAs

  • 2460 EI

  • 75 kVp

  • 640 mAs

  • 3300 EI


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How Low Can You Go? Cut mAs in Half! image?

  • 75 kVp

  • 88 mAs

  • 2460 EI

  • 75 kVp

  • 40 mAs

  • 2060 EI


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Let’s Go Crazy Low image?

  • 75 kVp

  • 8 mAs

  • 1380 EI

  • 75 kVp

  • 1 mAs

  • 550 EI


Cr artifacts l.jpg
CR Artifacts image?

  • Physical damage to imaging plates

    • Cracks, scuffs, scratches

    • Contamination

    • Dust / dirt

  • Dirt in reader

  • Highly sensitive to scatter radiation


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CR Grid Interference image?

  • 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.


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DR Artifacts image?

  • Dead detector elements

  • Spatial variations in background signal & gain

  • Grid interference

  • Software can help correct for above


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Shifting Gears: image?Fluoroscopy Issues


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Digital Video Sources

  • DR type image receptor

  • Conventional Image Intensifier with Video Signal Digitized (“Frame Grabber”)


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Digital Spot Film image?

  • Frame grabber digitizes image

  • Digital image saved by computer

  • Radiographic Technique used

    • required to control quantum noise


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Last Image Hold image?

  • 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


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Fluoro Frame Averaging image?

  • Conventional fluoro only displays current frame

  • Frame averaging allows computer to average current with user-selectable number of previous frames

    • Averages current frame & history


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Fluoro Frame Averaging Tradeoff image?

  • Advantage:

    • Reduces quantum noise

  • Disadvantage

    • Because history frames are averaged with current frame, any motion can result in lag


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Other Fluoro Features image?

  • 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



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DR Mobile Units image?

  • See image immediately

  • Wireless transmission of images

GE Definium AMX 700


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Other Possibilities image?

  • Tomosynthesis

    • Multi-slice linear tomography from one exposure series

  • Histogram Equalization

    • Use computer to provide approximately equal density to various areas of image.


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DR & Energy Subtraction image?

  • 2 images taken milliseconds apart at 2 different kVp’s

  • Combine / subtract images

Soft Tissue Image

Bone Image


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The End image?

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