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Next Generation Diagnostic Reference Levels Keyed to Patient Size Aaron Sodickson MD, PhD Disclosure Research grant from Siemens on Dual Energy CT [email protected] Section Chief, Emergency Radiology Director, Brigham NightWatch Program Medical Director of CT, Brigham Radiology Network Associate Professor, Harvard Medical School NIH - National Library of Medicine: R01LM010679-01: Automated Radiation Monitoring and Decision Support to Reduce Cumulative Exposure Brigham and Women’s Hospital Harvard Medical School [email protected] Diagnostic Reference Levels • A DRL is intended to represent: “An investigational level used to identify unusually high radiation doses for common diagnostic medical X-ray imaging procedures. DRLs are suggested action levels above which a facility should review its methods and determine if acceptable image quality can be achieved at lower doses. DRLs are based on standard phantom or patient measurements under specific conditions at a number of representative clinical facilities. DRLs have been set at approximately the 75th percentile of measured patient or phantom data. This means that procedures performed at 75% of the institutions surveyed have exposure levels at or below the DRL.” ACR–AAPM PRACTICE GUIDELINE FOR DIAGNOSTIC REFERENCE LEVELS AND ACHIEVABLE DOSES IN MEDICAL XRAY IMAGING, Rev 2013, at: http://www.acr.org/Quality-Safety/Standards-Guidelines/Practice-Guidelines-by-Modality/~/media/ACR/Documents/PGTS/ guidelines/Reference_Levels.pdf [email protected] NEMA XR-25 Dose Check • Derived from 1st 3 years of ACR accreditation data (2002-2005 ), 2005 NEXT data and NCRP Report 172 • “LAT dimensions are for average patients of the specified age.” • “Individual patients should not be compared against these values.” ACR–AAPM PRACTICE GUIDELINE FOR DIAGNOSTIC REFERENCE LEVELS AND ACHIEVABLE DOSES IN MEDICAL XRAY IMAGING, Rev 2013, at: http://www.acr.org/Quality-Safety/Standards-Guidelines/Practice-Guidelines-by-Modality/~/media/ACR/Documents/PGTS/ guidelines/Reference_Levels.pdf [email protected] AAPM recommended Dose Notification Values • 2010 standard of the National Electrical Manufacturers Association, from collaboration of the CT manufacturers • Allows scanners to be configured for: - Dose Notification: User warning if configured CTDIvol or DLP may be exceeded for any scan part - Dose Alert: Warning if cumulative CTDIvol or DLP may be exceeded during entire scan at any anatomic location http://www.nema.org/Standards/Pages/Computed-Tomography-Dose-Check.aspx [email protected] • Intentionally higher than ACR DRLs • “The XR 25 standard does not provide a mechanism to take patient size into account when evaluating dose indices. Hence, for obese patients, use of CTDIvol or DLP values that exceed the notification levels is likely to occur but may be completely appropriate.” • Intent of the alerts is to “present a ‘time-out’ opportunity for the user to confirm scan settings.” http://www.aapm.org/pubs/CTProtocols/documents/NotificationLevelsStatement.pdf [email protected] 1 • Helpful for high level screening of institutional techniques • But CT technique and patient dose depend heavily on patient size • Meaningful protocol surveillance / optimization must address this size dependence • Outlier detection in the torso is severely limited without considering patient size [email protected] Mean organ dose / CTDIvol across scanners How then can DRLs and dose alerts be used? Patient Perimeter (cm) Turner, Zhang, Khatonabadi et al. The feasibility of patient size-corrected, scannerindependent organ dose estimates for abdominal CT exams. Med Phys 2011;38(2):820-829 CTDIvol from PE CT Tube Current Modulation Appropriate 8-fold variation in CTDIvol Modified from: Kalender WA. Computed Tomography, 3rd ed. Publicis Publishing, Erlangen Germany, 2011 Implications of Size Dependence Size-agnostic alert values do not make sense for all patient sizes Sodickson, Weiss. Effects of Patient Size on Radiation Dose Reduction and Image Quality in low-kVp CT Pulmonary Angiography Performed with Reduced IV Contrast Dose. Emergency Radiology. 2012 CT Protocol Quality Control CTDIvol of PE CT, ER scanner, by Patient Weight – Too low for large patients – Too high for small patients – Potential to expose small patients to FAR more radiation than needed – Small patients imaged at the notification value receive better than needed image quality, at higher organ dose per CTDIvol – False +’ve alerts in large patients – False –’ve alerts in small patients [email protected] [email protected] 2 50 Outlier Detection AAPM adult torso recommended dose notification value Size-Dependent Notification Values CTDIvol of PE CT, ER scanner, by Patient Weight 21 CTDIvol of PE CT, ER scanner, by Patient Weight Select size-appropriate values based on any convenient threshold: - 75th percentile - 1.5 stddev above mean ACR adult chest DRL Outliers have higher than expected CTDIvol relative to tube current modulation output of similarly-sized patients [email protected] [email protected] Size-Dependent Notification Values • Size-dependent DRLs: For accreditation / regulatory purposes, derive from multi-center data collection WITH size info What is the best size metric CTDIvol of PE CT, ER scanner, by Patient Weight • It almost doesn’t matter – use what’s available – Weight, BMI – AP, LAT diameter – Eff diam = sqrt (AP X LAT) – Water equivalent diameter • At institutional level, helpful to gauge overall performance, compare devices and protocols • Size-dependent Dose Notification Values: At scanner level, ideal to derive as protocol-type specific to enable meaningful outlier detection by: – Technologist during scanning – Radiologist during interpretation / technique review – Medical Physicist during regular protocol review [email protected] • Menke J. Comparison of Different Body Size Parameters for Individual Dose Adaptation in Body CT of Adults. Radiology. 2005;236(2):565. • Li B, Behrman RH, Norbash AM. Comparison of topogram-based body size indices for CT dose consideration and scan protocol optimization. Medical Physics. 2012;39:3456. • Wang J, Christner JA, Duan X, Leng S, Yu L, McCollough CH. Attenuation-based estimation of patient size for the purpose of size specific dose estimation in CT. Part II. Implementation on abdomen and thorax phantoms using cross sectional CT images and scanned projection radiograph images. Med Phys. 2012 Nov;39(11):6772–8. • Ikuta, Warden, Andriole, Khorasani, Sodickson. Estimating Patient Dose from X-ray Tube Output Metrics: Automated Measurement of Patient Size from CT Images Enables Large-scale Size-specific Dose Estimates. Radiology. 2013 [email protected] Patient Axial CT Image Cylinder of Water Effective Diameter (Deff) Water-Equivalent Diameter (DW) AP HU HU HU HU HU HU HU HU HU HU HU Lateral HU HU HU DW HU Y X Deff = [email protected] i=1 AP * Lateral DW = ∑ HU +1000 i n 1000 (XY) 4 π Ikuta, Warden, Andriole, Khorasani, Sodickson. Estimating Patient Dose from X-ray Tube Output Metrics: Automated Measurement of Patient Size from CT Images Enables Large-scale Size-specific Dose Estimates. Radiology. 2013 [email protected] 3 Patient size extraction after the scan Patient size extraction after the scan Derived from axial CT images or planning projection radiographs Wang J, Christner JA, Duan X, Leng S, Yu L, McCollough CH. Attenuation-based estimation of patient size for the purpose of size specific dose estimation in CT. Part II. Implementation on abdomen and thorax phantoms using cross sectional CT images and scanned projection radiograph images. Med Phys. 2012 Nov;39(11):6772–8. Deff = AP X LAT [email protected] Patient size extraction after the scan Correcting for Anatomy Truncation • These patients have the same Deff • Dw is substantially smaller (15%) in the chest due to reduced attenuation of the lungs Optimal prospective approach • Water equivalent diameter – Adjusts for patient attenuation – Insensitive to body contours – Mirrors underlying scanner TCM calculations => Tightest distributions of CTDIvol vs size • Based on WED, scanner uses appropriate size-specific dose notification value to help confirm appropriate technique [email protected] Ikuta, Warden, Andriole, Khorasani, Sodickson. Estimating Patient Dose from X-ray Tube Output Metrics: Automated Measurement of Patient Size from CT Images Enables Large-scale Size-specific Dose Estimates. Radiology. 2013 [email protected] Optimal prospective approach • Scanners should output water-equivalent diameter in: – DICOM header (slice by slice & overall scan range average) ⇒ Enables radiologist outlier detection at review – DICOM SR dose reports (average and array by position) to enhance: ⇒ Systematic protocol review ⇒ Patient-specific organ dose calculation [email protected] 4 Acknowledgements CME question NIH - National Library of Medicine: R01LM010679-01: Automated Radiation Monitoring and Decision Support to Reduce Cumulative Exposure BWH - Center for Evidence Based Imaging Ramin Khorasani Graham Warden Tim O’Connell Luciano Prevedello Jenifer Siegelman Matthew Raffol Stacy O’Connor Ichiro Ikuta Dick Hanson Kathy Andriole Bobby Bransfield Ali Raja Eliott Wasser Cami Farkas Creating size-dependent CTDIvol alert values and Diagnostic Reference Levels would most likely: A) Enable better prediction of patient doses B) Improve compliance with accreditation limits C) Decrease variability in CT technique for patients of different sizes D) Reduce the number of alerts in large patients, and increase the number in small patients [email protected] Next Generation Diagnostic Reference Levels Keyed to Patient Size Aaron Sodickson MD, PhD [email protected] Section Chief, Emergency Radiology Director, Brigham NightWatch Program Medical Director of CT, Brigham Radiology Network Associate Professor, Harvard Medical School Brigham and Women’s Hospital Harvard Medical School [email protected] [email protected] [email protected] 5