Natalia Saltybaeva, Presenter: Nothing to Disclose

Mary Ellen Jafari MS, Abstract Co-Author: Nothing to Disclose

Martin Hupfer PhD, Abstract Co-Author: Employee, CT Imaging GmbH

Willi A. Kalender PhD, Abstract Co-Author: Consultant, Siemens AG Consultant, Bayer AG Founder, CT Imaging GmbH Scientific Advisor, CT Imaging GmbH CEO, CT Imaging GmbH

Computed tomography (CT) examinations of the lower extremities are widely used for musculoskeletal conditions as well as for CT angiography (CTA). Existing dose-length product (DLP) to effective dose (E) conversion coefficient tables do not include the lower extremities and hence do not allow calculating effective dose in this region. The purpose of this study was to provide DLP-to-E coefficients for fast and accurate effective dose calculation in order to comply with requirements regarding patient radiation dose recording.

Dose simulations were performed on standard mathematical phantoms using a validated Monte Carlo calculation tool for the following exams: hip (femur), knee, ankle and CTA of the lower limbs. All simulations were performed for scanner geometry, spectra and filtration equivalent to those of a generic clinical CT scanner with tube voltage values from 80 to 140 kV in steps of 20 kV. Effective dose values were calculated as a weighted sum of organ doses with respect to the tissue-weighting factors published in ICRP 103. Values of the dose-length product (DLP) were calculated by multiplying measured CTDI values by the scan length of the corresponding lower extremity CT examinations. The DLP-to-E coefficients were determined as the quotient of E and DLP for a wide range of ages from newborn to adult and for both genders.

Our findings showed that DLP-to-E coefficients for lower extremity examinations differ markedly from the ones published for other body regions. The coefficients depended strongly on the phantom age and size. In the case of a newborn, for example, DLP-to-E values were 0.0612, 0.0046, 0.0014 and 0.047 for hip, knee, ankle and CTA respectively, while in case of adult these values were 0.0110, 0.0004, 0.0002 and 0.0062. Substantial difference of up to 20% between male and female coefficients was observed for CTA examination. Dependence on kV value was found to be negligible with a standard deviation of 5 % on average.

DLP-to-E conversion coefficients were calculated specifically for lower extremity CT examinations and appear suitable for fast and reasonably accurate effective dose calculations.

DLP-to-E conversion coefficients presented in this study allow estimation of effective dose for commonly used clinical musculoskeletal CT and CTA protocols.

Saltybaeva, N, Jafari, M, Hupfer, M, Kalender, W, Estimates of Effective Dose for CT Scans of the Lower Extremities.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14045461.html