高速二重螺旋スキャンにおけるCT-AECの応答特性評価

摘要

X線 computed tomography (CT)検査は，従来のX線撮影と比較して肺がんによる死亡率を低減させるなど，現在の医療において重要な検査の一つとなっている.しかし，X線CT検査における被ばく線量は，他の検査よりも比較的多いためその管理は重要である.CT検査における線量管理に関しては，International Commission on Radiological Protection (ICRP)Publication 87 において，automatic exposure control (AEC)が最も有用であると述べている. CT-AECは，被写体サイズ，体軸方向に対するX線吸収の変化，X線管の回転角度ごとにおけるX線吸収の変化に対応して管電流を変調する機構であり，これを用いることによって被ばく線量と画質の最適化を図ることができる.%The performance of individual computed tomography automatic exposure control (CT-AEC) is very important for radiation dose reduction and image quality equalization in CT examinations. The purpose of this study was to evaluate the performance of CT-AEC in conventional pitch mode (Normal spiral) and fast dual spiral scan (Flash spiral) in a 128-slice dual-source CT scanner. To evaluate the response properties of CT-AEC in the 128-slice DSCT scanner, a chest phantom was placed on the patient table and was fixed at the center of the field of view (FOV). The phantom scan was performed using Normal spiral and Flash spiral scanning. We measured the effective tube current time product (Eff. mAs) of simulated organs in the chest phantom along the longitudinal(z) direction, and the dose dependence (distribution) of in-plane locations for the respective scan modes was also evaluated by using a 100-mm-long pencil-type ionization chamber. The dose length product (DLP) was evaluated using the value displayed on the console after scanning. It was revealed that the response properties of CT-AEC in Normal spiral scanning depend on the respective pitches and Flash spiral scanning is independent of the respective pitches. In-plane radiation dose of Flash spiral was lower than that of Normal spiral. The DLP values showed a difference of approximately 1.7 times at the maximum. The results of our experiments provide information for adjustments for appropriate scanning parameters using CT-AEC in a 128-slice DSCT scanner.