Calculating effective dose on a cone beam computed tomography device:: 3D Accuitomo and 3D Accuitomo FPD

Objectives: This study evaluates two methods for calculating effective dose, CT dose index (CTDI) and dose-area product (DAP) for a cone beam CT (CBCT) device: 3D Accuitomo at field size 30 x 40 mm and 3D Accuitomo FPD at field sizes 40 x 40 mm and 60 x 60 mm. Furthermore, the effective dose of three commonly used examinations in dental radiology was determined. Methods: CTDI100 measurements were performed in a CT head dose phantom with a pencil ionization chamber connected to an electrometer. The rotation centre was placed in the centre of the phantom and also, to simulate a patient examination, in the upper left cuspid region. The DAP value was determined with a plane-parallel transmission ionization chamber connected to an electrometer. A conversion factor of 0.08 mSv per Gy cm(2) was used to determine the effective dose from DAP values. Based on data from 90 patient examinations, DAP and effective dose were determined. Results: CTDI100 measurements showed an asymmetric dose distribution in the phantom when simulating a patient examination. Hence a correct value of CTDIW could not be calculated. The DAP value increased with higher tube current and tube voltage values. The DAP value was also proportional to the field size. The effective dose was found to be 11-77 mu Sv for the specific examinations. Conclusions: DAP measurement was found to be the best method for determining effective dose for the Accuitomo. Determination of specific conversion factors in dental radiology must, however, be further developed.