RADIATION-DOSIMETRY FOR ELECTRON-BEAM CT

PURPOSE: To measure the radiation dose profile, multiple-scan average dose (MSAD), and computed tomography dose index (CTDI) for electron be am CT and to determine the accuracy of ionization-chamber and manufact urer estimates of patient dose. MATERIALS AND METHODS: High-resolution dose profiles along the longitudinal axis were acquired at several po sitions within the scan plane with use of radiographic film. The full- width-at-half-maximum values, peak radiation dose, CTDI, and MSAD were calculated from the digitized him profiles. CTDI was also measured wi th an ionization chamber. RESULTS: The full-width-half-maximum value o f the radiation profiles were significantly wider than the nominal sca n width for the 6-mm single-section and 8-mm multisection modes. In th e single-section mode, the CTDI underestimated the MSAD by 15%-30%. Th e multisection radiation profile was nonuniform and asymmetric. CONCLU SION: Patient doses in electron beam CT are approximately 125% of the ionization-chamber CTDI measurements in the single-section mode. For t he multisection mode, the average patient dose over the scan volume is approximately 70%-85% of the ionization-chamber CTDI measurements.