WE-D-332-03: How Well Does CTDI Estimate Organ Dose to Patients From Multidetector (MDCT) Imaging?

Abstract

Purpose: Although CTDI was designed to characterize CT scanner performance, it is often used to estimate patient dose. This study uses voxelized patient models based on actual patient anatomy and Monte Carlo based MDCT simulations to answer the question: how well does CTDI predict patient organdose?Method and Materials: 26 voxelized models of female anatomy of various sizes were created from clinical CT data. Breast tissue was contoured by a radiologist and glandular and lungtissues automatically segmented. A previously validated Monte Carlo model of an MDCT scanner was used taking into account source spectra, filtration, collimation, and geometry. Two fixed mA clinical acquisition protocols were simulated: Chest CT and retrospectively gated coronary CTA. For each exam, doses were calculated for breast and lungtissues, and compared to CTDIvol (=CTDIw/pitch), CTDI100center/pitch, and CTDI100periphery/pitch. Percent difference between each CTDI value and organdose was calculated and the mean of these differences was reported. Results: For the Chest CT Protocol: CTDIvol measurements differ from breast and lungdose by an average of 84% and 52% respectively. CTDIperiphery/pitch differs from breast and lungdose by an average of 117% and 80% respectively; CTDIcenter/pitch differs from breast and lungdose by an average of 18% and 3% respectively. For the CTA Protocol: CTDIvol measurements differ from breast and lungdose by an average of 34% and 16% respectively. CTDIperiphery/pitch differs from breast dose and lungdose by an average of 58% and 37% respectively; CTDIcenter/pitch differs from breast and lungdose by an average of 15% and 26% respectively. Conclusion:CTDIvol generally overestimates organdose, while CTDIcenter /pitch provides better estimates of organdose;Organdose is affected by patient size (i.e. perimeter); therefore, future work will look into the possibility of scaling CTDI measurements to estimate organdose based on patient size.