Relationships between physical dose quantities and patient dose in CT

Patient dose in CT is usually expressed in terms of organ dose and effective dose. The latter is used as a measure of the stochastic risk. Determination of these doses by measurements or calculations can be time-consuming. We investigated the efficacy of physical dose quantities to describe the organ dose and effective dose. For various CT examinations of the head, neck and trunk, organ doses and effective doses were determined using conversion factors. Dose free-in-air on the axis of rotation (D-air) and weighted computed tomography dose index (CTDIw) were compared with the absorbed doses of organs which are located totally within the body region examined. Dose-length product (DLP) was compared with the effective dose. The ratio of the organ dose to CTDIw was 1.37 (0.87-1.79) mSv mGy(-1). DLP showed a significant correlation with the effective dose (p<0.005). The average ratio of effective dose to DLP was 0.28 x 10(-2) mSv (mGy cm)(-1) for CT of the head, 0.62 x 10(-2) mSv (mGy cm)(-1) for CT of the neck and 1.90 x 10(-2) mSv (mGy cm)(-1) for CT of the trunk. CTDIw and DLP can be used for estimating the organ dose and effective dose associated with CT examinations of the head, neck and trunk.