CT QUALITY MANAGEMENT. SPATIAL RESOLUTION CONTRAST RESOLUTION NOISE IMAGE ARTIFACTS RADIATION DOSE.
CR/DR Image Artifacts
Transcript of CR/DR Image Artifacts
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George DavidAssociate Professor of Radiology
Medical College of Georgia
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Quick Review of Technology
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Computed Radiography (CR)Re-usable metal imaging plates
replace film & cassette
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CR Exposure
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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
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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 LatitudeVery largePlate responds to
decades of input exposure
Computer scale inputs exposure to viewable densities
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CR Very Sensitive to Scatter
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Digital Radiography (DR)Digital bucky incorporated into
equipmentDirect digital outputHigh Lattitude
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Raw Data ImageUnprocessed image as read from receptor
Not a readable diagnostic image
Requires computer processing before presentation as finished radiograph
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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
*
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Image SegmentationEstablish location of
collimated border
• Define anatomic region
• Produce look-up table based on histogram from anatomical area• Body part & projection specific
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Gross Overexposure
Properly exposed
• 8X increase in mAs
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Gross Underexposure
Properly exposed
• mAs reduced by ~ 100
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Artifacts in Digital Radiography
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Artifact CategoriesNon-digital-related technical errorsLook-up table / image processing errorsExposure artifactsCR artifactsDR artifactsInterference artifactsDisplay artifacts
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Radiography ArtifactsDigital or Not
Mis-positioningMotionIncorrect patient IDDouble exposuresGrid cut-off
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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
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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
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Look-up Table / Image Processing Errors
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•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
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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
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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
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More Uberschwinger
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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
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Low Exposure Results in Image Noise
Increasing mAs
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Exposure ArtifactVERY high exposures can result in saturationMaximum detector response reachedNo response to increased doseUniformly darkCannot be windowed/leveled
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CR ArtifactsFadingLight leakPhysical damage to imaging plates
Cracks, scuffs, scratchesContaminationDust / dirt
Dirt in readerHighly sensitive to scatter radiationUpside-down insertion into bucky
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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
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CR Artifact: Light Leak in CassetteCR plates erased by exposure to bright lightLight leaks in cassette can cause partial
premature erasure of plate
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•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
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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
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What are white lines?
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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
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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
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Transport problems in CR reader
Laser
RotatingMirror
Beam scannedacross film
PM Tube
Analog/Digital
Computer
Light
Plate Travel
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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
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DR ArtifactsDead detector elements
Software correction to a point…
R/F interferenceDetector shielded to
prevent thisPeriodic pattern
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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
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Tape artifact seen after tape removed
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Interferance
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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.
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Moire effect because of interference between scan frequency of matrix and spiral
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DisplayPoor calibrationDead pixelsBacklight issues
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The End
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