관상동맥 CT조영 검사시 선량감소 프로토콜에 관한 연구

AssessmentofDose-reducedScanProtocolsonCTCoronaryAngiography: Evaluation of Image Quality with a Change of Exposure Factor and ECG Gating Method

Purpose To evaluate the feasibility of using relative low tube voltage scan protocols for low-dose about prospective and retrospective ECG gating scan on coronary image quality evaluation with 64-row MDCT. Materials and methods A female ART 300 phantom was used to simulate coronary artery of size(5mm in diameter) with four stenosis degrees(0%, 25%, 50% and 75%) at 55bpm heart rate. Cardiac scans were performed on a 64-row MDCT scanner(GE LightSpeed VCT) with rotation time of 350msec, 350mm scan length with proximal-middle coronary artery and two scan methods of prospective scan(axial) and retrospective scan(helical) of 0.2 pitch under each tens different scan protocols. Tube voltage and current were 100kVp/300~600mA, 120kVp/300~600mA, 140kVp/300~400mA. The simulative coronary arteries were filled with contrast media to reach an approximate value 400HU in the lumen. Background noise was measured to describe the basic image quality accordingly. CNR, SNR and contour sharpness represented in slope of CT density curve was calculated as well. Measured stenosis area and rates, described by the percentage area of stenosis on the cross-section images were also calculated. Comparative analysis standard stenosis rate with measured stenosis rate and the error are measured. Results Dose(CTDIvol) were 7.4~22.3mGy of prospective scan and 24.3~73.1mGy of retrospective scan by the protocols. Prospective scan method was measured less 30.5% radiation dose than retrospective scan. The corresponding image noise levels described in standard deviation of background signals varied with radiation dose, CNR and SNR mainly varied with tube current. The contour sharpness, which can reflect actual spatial resolution, is affected mainly by tube voltage. When comparing the groups with similar radiation dose of prospective(protocol 10) and retrospective(protocol 1) scans, prospective scan was measured obviously steeper mean slope value than retrospective scan. Significant difference stenosis rate error presented between two groups, which is low dose and high dose. Low dose group was overestimated on stenosis rate. When comparing the groups with similar radiation dose in same scan method protocol, protocols with lower tube voltage gained more accuracy in representing stenosis area and rate. Conclusion Prospective scan was estimated similar or higher results than retrospective scan in image quality evaluation as well as lower dose. Dose level and corresponding image quality is relevant to the accuracy of stenosis evaluation on simulated coronary arteries with 64-row MDCT. In this study, we find relative low-dose protocols with acceptable image quality showed a tendency of overestimating stenosis. Furthermore, prospective scan method take an effect reduced dose in condition keep stabile heart rate and using a lower tube voltage and higher tube current to gain accurate imaging result is more applicable than other protocols with the same radiation dose level.