Analysis of surface dose variation in CT procedures

An experimental and theoretical study has been made of the variations in air kerma- length product (AKLP) at the surface of a phantom exposed in a CT scanner, using clinical parameters. For the theoretical part, a computer simulation was developed, based on a simple analytical model, requiring information generic to the scanner model. The effects of patient size, position within the gantry plane and beam-shaping filter type were studied using three different elliptical phantoms. A dose reduction technique based on a sinusoidal tube current modulation system was also investigated. The surface AKLP was shown to be independent of phantom size (within experimental error) but decreases as the surface moves vertically away from the isocentre. The major contributor to this variation is the beam-shaping filter. A maximum difference of 19% between the values of surface AKLP was calculated for the two beam-shaping filters available. When the tube current modulation system was used, the maximum surface dose reduction was 18%. A maximum difference of 2% was found between measurements and computer simulations. It is therefore possible to predict the behaviour of AKLP using the analytical model. As the dose measured on the surface of a patient is simply related to AKLP, the model can be applied to data obtained from patient surface dose surveys and can be helpful in interpreting the sources of variation in the latter. CT is a high dose diagnostic technique, esti- mated to be responsible for 40% of the total collective dose from all diagnostic procedures in the UK (1-3). State-of-the-art CT scanners (multi- slice CT scanners) now allow a wider choice of slice thickness and a decrease in exposure time. Such scanners have the potential to improve visualization of small structures and so examina- tion frequency is expected to further increase (4). Consequently, there is a need to evaluate patient doses and the risk associated with these proce- dures. It is very important that CT examination exposure parameters are optimized and patient dose is kept as low as reasonably achievable consistent with the need to obtain adequate diagnostic information. This is in accordance with a recent European Directive (5). Therefore, measurement of patient dose forms an essential part of the CT quality assurance programme. Patient dose can be estimated from the CT dose index (CTDI) (6), which for this purpose is defined as CTDI~ 1