276. A novel methodology for quantification of absolute overscan dose in helical multisection CT

Purpose Recent helical multisection CT provides rapid and wide-coverage scans, but delivers an extra-dose amount at the beginning and at the end of the scan (overscanning) resulting in quite significant contribution to patient exposure. This study aims to quantify the overscan dose contribution for different scan parameters through experimental measurements and simulations. Methods Overscan dose was quantified by means of a previously validated methodology [1] based on novel CT measurement setup and dosimetric formalism. Waveforms, dose-rate versus time, at center and periphery of head and body phantoms were acquired with a 128-detector-row CT-scanner (Siemens SOMATOM Definition Flash) varying protocols, pitches, rotation times and scanning lengths. In order to take into account the random start angle of CT tube, experimental waveforms were used to simulate a simultaneous acquisition at center and periphery of the phantom. The effect of scanning parameters on overscan dose was quantified calculating the DLP due to overscan ( DLP overscan ) as a weighted average of the values at center and periphery. Results The overscan length showed a positive correlation for pitch values ranging from 0.35 to 1.0 (r values, 0.98-1.00); it becomes slightly constant for pitch values higher than 1.0. The contribution of DLP overscan decreases from 36% to 4% increasing the overscan length from 2.5 to 30 cm. The dependence of absolute DLP value due to overscan from pitch was obtained for different rotation times and protocols, showing a positive correlation (r values, 0.99-1.00) in both head and body phantoms. Conclusions This method for quantifying the DLP in overscan range can be used to evaluate the clinical implication of overscanning and to optimize scanning protocols in order to reduce the impact of this extra-dose especially on critical organs.