[Determination of computer tomography dose index by means of ion chamber measurements]

The effective dose (ED) and the organ dose given with Computer Tomography (CT) scanners can be determined by means of a computing method suggested by the English National Radiological Protection Board (NRPB). This method uses the integral air-dose profile as input. Due to the non-rectangular shape of the CT profiles, a Computer Tomography Dose Index (CTDI) has been defined, as the integral dose profile divided by nominal slice thickness. The experimental determination of the CTDI can be carried out with thermoluminescent dosimeters. However, the measurement can be simplified by using a cylindrical ion chamber. This paper reports the calibration of a PMO5 ion chamber (5 cm3 in volume and 5 cm in length) in terms of sensitivity value (4.45 pC mGy-1 mm-1). CTDI measurements by means of a PMO5 ion chamber were used to determine the effective doses and lung doses for the CT scanners used at the Institute of Radiology of the Catholic University in Rome. The dose levels thus obtained are in agreement with other national and international data. Moreover, the method allows the differences in the doses absorbed by the patients submitted to CT with different technical approaches to be calculated.