Variation of surface dose in CT

An experimental study has been made of the variations of the air kerma-length product at the surface of a phantom exposed in a CT scanner using clinical parameters. the effect of patient size, position within the gantry plane and beam-shaping filter type was studied using three different elliptical phantoms. A dose reduction technique based on a sinusoidal tube current modulation system was also investigated. the result of the variation of the surface dose was analysed for the case of CT fluoroscopy procedures. The surface air kerma-length product was shown to be independent of phantom size (within experimental error) but decreases as the surface moves vertically away from the iso centre. The major contributor to this variation the beam-shaping filter. The possibility of underestimating the surface dose if an AAPM body phantom is used (a 36% increase for the head phantom) is an indirect result of the previous conclusion when applied to CT fluoroscopy. This highlights the importance of considering the size and position of the patient when measuring the does. Finally, when the tube current modulation system was used, the maximum surface dose reduction was 18%. (Author) 23 refs