Estimates of effective dose for CT scans of the lower extremities

To determine the dose-length product (DLP)-effective dose (ED) (DLP/ED) conversion coefficient (k) tables for the lower extremities that can be used for calculating ED. Dose calculations were performed on standard phantoms using a validated Monte Carlo calculation tool. Calculations were performed to obtain ED values for tube voltages from 80 kV to 140 kV in steps of 20 kV for the following examinations: hip (femur), knee, ankle, and computed tomographic (CT) angiography of the lower extremities. Values of the DLP were calculated by multiplying measured CT dose index values by the scan length; k values resulted as the quotients of the ED and DLP values. DLP/ED coefficients averaged over the range of voltage values and their standard deviations were determined for the given lower-extremity CT examinations for all age groups and for both sexes. Coefficients depend strongly on the phantom age and size, but little on the kilovolt value. In the case of the newborn, for example, k values were 0.0612, 0.0046, 0.0014, and 0.047 for hip, knee, ankle, and CT angiography, respectively, while in the case of the adult, these respective values were 0.0110, 0.0004, 0.0002, and 0.0062. A substantial difference up to 20% between coefficients in male and female phantoms was observed for CT angiographic examination. DLP/ED conversion coefficients are provided for lower extremities and allow estimation of ED for commonly used clinical musculoskeletal CT and CT angiographic protocols.