Optimizing CT Image Quality: CT Dose

November 2011 Review of RT X-RAY OPTIMIZING CT IMAGE QUALITY RADIOLOGIC PHYSICS PHYSICS REVIEW Six Parts (30 minutes each) Walter Huda Kerry Greene-Donnelly Dose Walter Huda PhD Image Quality Professor of Radiology CARE Dose4D MUSC CARE kV MEDICAL UNIVERSITY of SOUTH CAROLINA Clinical Techniques Clinical Practice ASRT PART 1: CT DOSE Continuing Education (CE) Credits CT Dose Index [CTDI] asrt. CTDI & X-ray Techniques American Society of Radiologic Technologists CTDI & Patient Dose 40 Questions ≥ 75% (30/40) Up to Three Attempts CT DOSE INDEX [CTDI] Volume CTDI in mGy (CTDIvol) X-ray tube voltage (kV) X-ray tube current (mA) X-ray tube rotation time (s) CT Pitch (P) CTDIvol & DLP Phantom Size (S or L) Part 1 (Dose) Huda 1 November 2011 CTDIvol 15 mGy Dose-length product (DLP) DLP = CTDIvol x Scan Length mGy-cm CTDI Independent of Dose Length Product Scan Length CTDIvol is Radiation Intensity kV mAs/ref CTDIvol DLP mGy mGy-cm DLP is Total Radiation Used 120 390/390 60S 900 1 09 20-17:01:33-DST-1.3.1 .. 11.09.20-17:01:33-DS Hospital *20-Sep-1956, M, 55Y SOMATOM Definition Flash CT 2011A.1_SP1.5_P20 20-Sep-2011 17:01 UMDS Physician Operator: Total mas 2541 Total DLP 1298 mGycm Scan KV MAS / ref. CTDIVol* DLP mGy mGycm TI mm Patient Position H-SP Topogram Head 2 138 35 mA 09 / 390 20.29 8 123.69 S 291 27 0.6 Head kV mAs/ref CTDIvol DLP mGy mGy-cm CTDI & X-RAY TECHNIQUES 120 90/90 5.1L 125 Phantom Size mAs (axial) Effective mAs (helical) kV Abdomen Part 1 (Dose) Huda 2 November 2011 CTDI Phantoms CTDIvol [S] CTDIvol [S] 16 cm (S) 22 mGy 60 mGy [Head] 32 cm (L) [Body] CTDIvol [L] L CTDI is 11 mGy 0.5 x S CTDI ~ Axial CT Helical CT CTDIvol is directly proportional to: CTDIvol is directly proportional to: Tube current (mA) mA Tube rotation time (s) s 1/Pitch mAs Helical CT CTDIvol ∞ kV 2.6 CTDIvol is directly proportional to: Effective mAs Head phantom. CTDIvol Body phantom 100 120 Tube Voltage (kV) Effective mAs is mAs/Pitch 80  140 kV Independent of Pitch CTDI  > 400% Part 1 (Dose) Huda 3 November 2011 CTDIvol versus mAs CTDIvol  “Universal” 60 mGy @ 120 kV CTDIvol >>> mAs! 123RF 21 mGy @ 80 kV kV X-ray filter 400 mAs Model Vendor CTDIvol / DLP & PATIENT DOSES CTDIvol Radiation Intensity “Incident On Patient” CTDIvol = 20 mGy Not The Patient Dose Absorbed Dose Organ Dose  Organ Risk Total CTDIvol (i.e., Incident Radiation) x Patient Size Factor x Scan Length Factor Part 1 (Dose) Huda 4 November 2011 Embryo Dose Estimates in Body CT Abdomen/Pelvic Scan (Pregnant) + Chest Huda et al. American Journal Roentgenology CTDIvol 194 (2010) 874-880 20 mGy AIR ....... Absorbed Dose to Fetus = 28 mGy Effective Dose (mSv) Effective Dose (E) → Total Radiation Received E = DLP x k Natural Background ~ 3 mSv/Year CT effective dose per dose length product using ICRP 103 weighting factors Walter Huda and Dennise Magill Department of Radiology and Radiological Science, Medical University of South Carolina, Charleston. South Carolina 29425 Cosmic [12%] Medical Physics Terrestial [12%] Volume 38 March 2011 Internal [16%] pp 1261-1265 Radon [60%] Adult Head 1 year old Head CT CTDIvol: 30 mGy CTDIvol: 60 mGy DLP: 1000 mGy-cm DLP: 360 mGy-cm k = 0.002 k = 0.005 mSv/mGy-cm mSv/mGy-cm E = 2 mSv E = 1.8 mSv Part 1 (Dose) Huda 5 November 2011 Adult Chest Adult Body Size Correction CTDIvol: 7.5 mGy 1.4 1.2 DLP: 150 mGy-cm 1.0 0.8 Relative dose k = 0.02 0.6 mSv/mGy-cm 0.4 15 20 25 30 35 40 Anterior-Posterior dimension (cm) E = 3 mSv Fixed CTDIvol and DLP CT DOSE REVIEW CTDIvol is “Radiation Intensity” DLP is “Total Amount of Radiation” Used to Perform CT Scan CTDIvol directly proportional to: Patient Doses mAs (axial) Total CTDIvol  Organ Dose Total DLP  Effective Dose Effective mAs (helical) kV2.6 Part 1 (Dose) Huda 6 November 2011 Next IMAGE QUALITY X-RAY RADIATION Part 1 (Dose) Huda 7