Half value layer measurements to facilitate patient dose assessment for newer CT scanners using published normalized...

The National Radiological Protection Board (NRPB) method of calculating patient doses from CT can be applied to scanners not represented in normalized dose data sets, published in NRPB Report R250, if an appropriate existing data set is selected. A method of matching scanner models to data sets is currently being developed in a national scanner survey, organized by the Department of Health ImPACT CT evaluation group. This paper demonstrates the use of an alternative method of data set selection to that employed in the national study, by application to the Picker PQCT and Siemens +4 scanners. Half value layers (HVLs) evaluated at the isocentre and at intervals within the fan angle of the X-ray beam of either the Picker PQCT or Siemens +4 were compared with HVLs for three scanner models represented in the NRPB data sets. A good match indicates the suitability of a normalized dose data set, since data sets are dependent on the quality of the X-ray beam incident on a patient. The results demonstrate that Picker 1200SX data sets could be used with the Picker PQCT scanner. The most likely match for the Siemens +4, from the limited number of scanners investigated, was identified as being the Philips Tomoscan LX.