DETERMINATION OF C.T. DOSES- USE OF THE NRPB SOFTWARE SR250

wide variation in patient dose for patients undergoing CT examinations. The NRPB’s conclusions were that this dose variation depended largely on the type of scanner and clinical practice. There was some interest among users to know how doses from local scanners compared against those measured nationally. The NRPB’s software(2) for calculating organ doses was available and work was under taken to determine local doses. The NRPB’s Software SR250 The software contains normalised organ dose data which was produced for a number of different CT scanner types using Monte Carlo calculations. The Monte Carlo calculations were based on a hermaphrodite, mathematical phantom consisting of 208 slabs each 5 mm thick. Each data set relates to a specific scanner type at a single operating kV. Dose values are given for each organ within a particular 5 mm thick slab. The organ doses are normalised and expressed relative to the dose on the central axis of the scanner in the absence of the phantom. Organ doses are hence in the form of, for example, normalised dose for the lung in slab 125 is 5.59 x 10-6 mGy/mGy. Each data set contains hundreds of normalised dose data values making a computerised method necessary for handling the data. To obtain organ doses for a particular examination it was necessary know details of the clinical scanning protocol (length of scan, mAs value and slice overlap or spacing) and to measure CT dose index. CT dose index (CTDI) was normalised and therefore expressed as mGy/mAs. CTDI was initially measured using TLDs but later measured using a pencil ion chamber. (3) TLDs collimated X-ray beam Hollow perspex ro