Iterative Reconstruction Techniques (ASIR, MBIR, IRIS): New
Transcription
Iterative Reconstruction Techniques (ASIR, MBIR, IRIS): New
Iterative Reconstruction Techniques (ASIR, MBIR, IRIS): New Opportunities to Lower Dose and Improve Image Quality Dushyant Sahani MD Director of CT Associate Professor of Radiology MGH email: dsahani@partners.org HARVARD MEDICAL SCHOOL Disclosures Research Grant Support from GE Healthcare Steps for Lowering Dose DECREASE IN • Tube current • Tube potential • Gantry rotation time * • Scan length • Overlap scanning • Number of scan phases INCREASE IN • Pitch (Table speed) * •Beam collimation *Provided other scanning factors are kept constant MGH Webster Center for Radiation Dose Research and Education Image Quality (IQ) and Dose Optimum Reduced Low dose scan Lower radiation dose reduced image quality MGH Webster Center for Radiation Dose Research and Education Low Dose CT Challenges: Image Quality Scan Parameters Diagnostic Performance Safety (dose) Image Quality NI-15 NI-15 NI-35 NI-35 Low Dose Imaging • Standard Filtered back projection (FBP) treats projection data as if coming form an idealized system • Efficient but require more dose for IQ • Excessive image noise at low dose • Artifacts in large patients and dense structures • Reduced CNR How Does IR helps ? Optimum Reduced IR removes the noise and return the image quality MGH Webster Center for Radiation Dose Research and Education Chest CT at 25% Dose Low-Dose Imaging FBP In x-ray CT, lowering radiation dosage increases noise Fewer photon penetration to detector Iterative reconstruction algorithms can increase image signal-to-noise ratio Can potentially produce same diagnostic accuracy / confidence as full-dose CT, but at a fraction of the radiation Courtesy: H Pien, MGH Iter. Recon Iterative Recon – Low Dose Phantom Filtered Backprojection Reconstructions 100% Dose 75% Dose 50% Dose 25% Dose IRT Courtesy: H Pien, MGH Resolution Improvement 10 lp/cm FBP46f(100%D) FBP46f(50%D) IRT(50%D) Contrast window [500,2000] Courtesy: H Pien, MGH Standard FBP Statistical IR (ASiR) Fast Image Data Space IRIS Fast Image Data Space Fast Image Data Space Basic image correction Image data recon Exact image correction Compare Raw data recon Raw data recon Basic raw data projection Compare Slow Raw Data Space Slow Raw Data Space Slow Raw Data Space FBP Simple VS. Comprehensive Simple Fast High noise VS. CPU Better IQ Low dose Low noise Full IR Advanced Few Iterations Ideal System CPU Partial IR Advanced model CPU MGH Webster Center for Radiation Dose Research and Education Optimal IQ Low noise Better resolution Iterative Reconstruction System Model Raw Data MBIR Image Cost Function Data Statistics Nonlinear Smoothing Optimization (Recon) • Optimal use of spatiallyvarying statistical model for stable, low noise, and high resolution IQ Courtesy: Bob Becket, GE Healthcare NO ASIR 30 ASIR 60 ASIR 10 ASIR 40 ASIR 70 ASIR 20 ASIR 50 ASIR 100 ASIR Jan 10 2008 Crohn’s Disease Dec 20 2008 23 yrs old with Crohn’s Disease. Baseline and FU CT exam 40% ASIR 55% Dose Reduction Radiation Dose – 11.6mSv Radiation Dose – 5.1 mSv July 2 2007 Kidney Stone Feb 26 2009 60% ASIR 263 lbs DLP 744 11.16mSv 81% DLP 138 2.07mSv NI = 28.7 kV= 120, 0.5s rotation, 1.375 pitch, 55mm/rot CTDIVol = 9.72mGy DLP = 456.73 mGy-cm FBP FBP + 60% ASIR MBIR Coronal Reformat at 0.625mm thickness FBP no ASIR FBP w/ 50% ASIR MBIR NI=24.2, kV= 120, 0.5s rotation, 1.375 pitch, 55mm/rot CTDIVol = 12.44mGy DLP = 609.26 mGy-cm Summary • Commercially available Iterative techniques (ASiR/IRIS/MBIR) can improve image quality of low dose exams • Dual benefits of improved Image Quality and dose for patients with large body habitus • Enough experience with ASiR and IRIS to justify their role in clinical use • Higher cost and available only for high end scanners (64-MDCT) • Potential work flow limitations (slower data processing) with MBIR/Veo
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