George DavidAssociate Professor of Radiology

Medical College of Georgia

Quick Review of Technology

Computed Radiography (CR)Re-usable metal imaging plates

replace film & cassette

                                                                 

CR Exposure

Reading Imaging PlateReader scans

plate with laser light using rotating mirror

Plate pulled through scanner by rollers

Light given off by plate measured byPM tube &recorded by computer

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 LatitudeVery largePlate responds to

decades of input exposure

Computer scale inputs exposure to viewable densities

               

CR Very Sensitive to Scatter

Digital Radiography (DR)Digital bucky incorporated into

equipmentDirect digital outputHigh Lattitude

Raw Data ImageUnprocessed image as read from receptor

Not a readable diagnostic image

Requires computer processing before presentation as finished radiograph

Enhancing Raw Image (Image Segmentation)

1. Identify collimated image border

2. Separate raw radiation from anatomy

3. Apply appropriate tone-scale to image

Create look-up table (LUTLUT) Maps pixel value to display gray

shade

This process is specific to a

particular body part and

projection

*

Image SegmentationEstablish location of

collimated border

• Define anatomic region

• Produce look-up table based on histogram from anatomical area• Body part & projection specific

Gross Overexposure

Properly exposed

• 8X increase in mAs

Gross Underexposure

Properly exposed

• mAs reduced by ~ 100

Artifacts in Digital Radiography

Artifact CategoriesNon-digital-related technical errorsLook-up table / image processing errorsExposure artifactsCR artifactsDR artifactsInterference artifactsDisplay artifacts

Radiography ArtifactsDigital or Not

Mis-positioningMotionIncorrect patient IDDouble exposuresGrid cut-off

No grid102 cm SID.33R ESE

GridGrid122 cm SID122 cm SID2.2 R ESE2.2 R ESE

http://http://radiographics.rsnajnls.org/radiographics.rsnajnls.org/cgi/reprint/11/2/307?cgi/reprint/11/2/307?maxtoshow=&HITS=10&hitmaxtoshow=&HITS=10&hits=10&RESULTFORMAT=s=10&RESULTFORMAT=&fulltext=grid+artifacts&and&fulltext=grid+artifacts&andorexactfulltext=and&searchorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortid=1&FIRSTINDEX=0&sortspec=relevance&resourcetspec=relevance&resourcetype=HWCIType=HWCIT

References

“Digital Radiography Artifacts”from Veterinary Radiology & UltrasoundWm Tod Drost, David J. Reese, William J. Hornof

“Artifacts in Digital Radiography” from Veterinary Radiology & UltrasoundDavid A. Jimenez, Lauara J. Armbrust, Robert T.

O’Brien, David S. BillerRadiographicshttp://www.slideshare.net/ricksw78/digital-

radiography

Look-up Table / Image Processing Errors

•Diagonal collimation caused algorithm to improperly recognize collimated area•Gonadal shields can fool algorithm•2 exposures on one plate can fool algorithm

http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=amaxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resortifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCITurcetype=HWCIT

Chest incorrectly specified by technologist to CR reader as lumbar spine

http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWsearchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCITCIT

Uberschwinger Artifact

Causes appearance of fine black line halo

Result of implementation of edge enhancement

Can be mistaken for evidence of loosening of orthpedic device

Can mimic pneumothorax

More Uberschwinger

ClippingOccurs because of applying LUT during pre-

processingKodak recommends technologists do not

window/level on qc workstation.Raw data: 12-14 bit imageLUT during pre-processing: Only 10-12 bits

savedInformation may be lost

Low Exposure Results in Image Noise

Increasing mAs

Exposure ArtifactVERY high exposures can result in saturationMaximum detector response reachedNo response to increased doseUniformly darkCannot be windowed/leveled

CR ArtifactsFadingLight leakPhysical damage to imaging plates

Cracks, scuffs, scratchesContaminationDust / dirt

Dirt in readerHighly sensitive to scatter radiationUpside-down insertion into bucky

CR Artifact: FadingCR latent image consists of excited electrons

stuck in high energy state orbitsOver time if image not read then electrons

fall back to ground energy stateSeveral days required before significant

fading occurs

CR Artifact: Light Leak in CassetteCR plates erased by exposure to bright lightLight leaks in cassette can cause partial

premature erasure of plate

•On imaging plate during reading•In reader

•Can get on plate•Can be blocked in light guide of reader

•White line in direction of plate movement

•CR readers require periodic preventive maintenance & cleaning http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=amaxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resortifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCITurcetype=HWCIT

Dirt on CR plate. Right image repeated on screen-film

http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searmaxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCITchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCIT

What are white lines?

Cracks on image plate due to mechanical wear

http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifmaxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCIType=HWCIT

http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=amaxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&resortifact&searchid=1&FIRSTINDEX=10&sortspec=relevance&reso

urcetype=HWCITurcetype=HWCIT

Transport problems in CR reader

Laser

RotatingMirror

Beam scannedacross film

PM Tube

Analog/Digital

Computer

Light

Plate Travel

CR Plates are Very Susceptible to Scatter

Do not keep plates in room during other exposures

Do not store where likely to receive scatter

Erase plate before use if it has been sitting a long time

DR ArtifactsDead detector elements

Software correction to a point…

R/F interferenceDetector shielded to

prevent thisPeriodic pattern

DR Artifacts: Spatial Variations in Background Signal & Gain

Addressed by acquiring flood and building calibration maskSimilar to uniformity correction in nuclear

medicineArtifact results if attenuator in beam during

calibration, then removedContrastTape

Tape artifact seen after tape removed

Interferance

CR Grid Interference103 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.

Moire effect because of interference between scan frequency of matrix and spiral

DisplayPoor calibrationDead pixelsBacklight issues

The End

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