George David Associate 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 Plate Reader scans plate with laser light using rotating mirror Plate pulled through scanner by rollers Light given off by plate measured by PM 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 Very large CR Latitude Plate 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 equipment Direct digital output High Lattitude Raw Data Imageimage as read from Unprocessed receptor Not a readable diagnostic image Requires computer processing before presentation as finished radiograph Enhancing Raw Image (Image Segmentation) * Identify collimated image border Separate raw radiation from anatomy Apply appropriate tone-scale to image 1. 2. 3. Create look-up table (LUT) Maps pixel value to display gray shade This process is specific to a particular body part and projection Image Segmentation Establish 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 Categories Non-digital-related technical errors Look-up table / image processing errors Exposure artifacts CR artifacts DR artifacts Interference artifacts Display artifacts Radiography Artifacts Digital or Not Mis-positioning Motion Incorrect patient ID Double exposures Grid cut-off http://radiographics.rsnajnl s.org/cgi/reprint/11/2/307? maxtoshow=&HITS=10&hit s=10&RESULTFORMAT=& fulltext=grid+artifacts&ando rexactfulltext=and&searchi d=1&FIRSTINDEX=0&sort spec=relevance&resourcet ype=HWCIT No grid 102 cm SID .33R ESE Grid 122 cm SID 2.2 R ESE References “Digital Radiography Artifacts” from Veterinary Radiology & Ultrasound Wm Tod Drost, David J. Reese, William J. Hornof “Artifacts in Digital Radiography” from Veterinary Radiology & Ultrasound David A. Jimenez, Lauara J. Armbrust, Robert T. O’Brien, David S. Biller Radiographics http://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?maxtoshow =&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searc hid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=H WCIT Chest incorrectly specified by technologist to CR reader as lumbar spine http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HIT S=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=1&FIRS TINDEX=10&sortspec=relevance&resourcetype=HWCIT 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 Clipping Occurs because of applying LUT during pre- processing Kodak recommends technologists do not window/level on qc workstation. Raw data: 12-14 bit image LUT during pre-processing: Only 10-12 bits saved Information may be lost Low Exposure Results in Image Noise Increasing mAs Exposure Artifact VERY high exposures can result in saturation Maximum detector response reached No response to increased dose Uniformly dark Cannot be windowed/leveled CR Artifacts Fading Light leak Physical damage to imaging plates Cracks, scuffs, scratches Contamination Dust / dirt Dirt in reader Highly sensitive to scatter radiation Upside-down insertion into bucky CR Artifact: Fading CR latent image consists of excited electrons stuck in high energy state orbits Over time if image not read then electrons fall back to ground energy state Several days required before significant fading occurs CR Artifact: Light Leak in Cassette CR plates erased by exposure to bright light Light 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?maxtoshow =&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searc hid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=H WCIT Dirt on CR plate. Right image repeated on screen-film http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow=&HITS=1 0&hits=10&RESULTFORMAT=&fulltext=artifact&searchid=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?maxtoshow=& HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searchid= 1&FIRSTINDEX=10&sortspec=relevance&resourcetype=HWCIT http://radiographics.rsnajnls.org/cgi/reprint/11/5/795?maxtoshow =&HITS=10&hits=10&RESULTFORMAT=&fulltext=artifact&searc hid=1&FIRSTINDEX=10&sortspec=relevance&resourcetype=H Transport problems in CR reader Rotating Mirror PMTube Light Laser Analog/ Digital Computer Beam scanned across film 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 Artifacts Dead detector elements Software correction to a point… R/F interference Detector shielded to prevent this Periodic pattern DR Artifacts: Spatial Variations in Background Signal & Gain Addressed by acquiring flood and building calibration mask Similar to uniformity correction in nuclear medicine Artifact results if attenuator in beam during calibration, then removed Contrast Tape Tape artifact seen after tape removed Interferance 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. Moire effect because of interference between scan frequency of matrix and spiral Display Poor calibration Dead pixels Backlight issues The End ?