胸部CT検診専用ファントムによる精度管理

摘要

目的．現在までに，胸部CT検診の実施に対する利益が不利益を上回ることを示す科学的根拠は得られていない．ターゲットを捉える画質と線量のバランスは，特に重要となる．本報告では，胸部CT検診専用ファントムについて概説する．また，ファントムの活用例を提示し，その有効性について述べる．方法．本ファントムシステム（LSCT-001：Kyoto-kagaku）は，胸部ファントムとリニアリティファントムから構成される．挙上位をとる胸部ファントムには，模擬肺と模擬腫瘤が封入されている．模擬腫瘤は，模擬肺に対し，△CT＝100HU（12-4mm￠）と270HU（10-2mm￠）である．ファントムの中心軸に線量測定用ホールが空いている．リニアリティファントムは直径200mm￠，高さ100mmで，8種類のロッドが封入されている．本ファントムを使用し，システム間における濃度曲線の規格化およびスキャン条件の最適化について，胸部CT検診の標準的条件を中心に実験を行った．結果．レーザプリンタのLUT（look up table）の調整により，2機種の濃度曲線はほぼ一致した．呼吸停止時聞（17s）と線量が同等（2.5mGy）のスキャン条件下において，各CT装置により最小識別能は変化した．結論．本ファントムの利用により，各CT装置における最適なスキャン条件を客観的に評価することが可能である．Objective. It has not yet been scientifically demonstrated that the advantages of chest screening CT outweigh its disadvantages. Determining the optimal balance between image quality and exposure dose required to clearly visualize the target is of particular importance. This paper provides a general description of a dedicated phantom for chest screening CT, presents practical examples of the use of this phantom, and discusses its usefulness. Methods. The phantom system used in the present study (LSCT-001, Kyoto-kagaku) comprises a chest phantom and a linearity phantom. The arms-raised chest phantom contains simulated lung tissue and simulated tumors. The ACT values for the simulated tumors relative to the simulated lung tissue are 100 HU (12-4 mm in diameter) and 270 HU (10-2 mm in diameter) . A hole is provided in the central axis of the phantom for dose measurement. The linearity phantom, which is 200 mm in diameter and 100 mm high, has 8 types of rods embedded in it. Using this phantom, experiments were conducted focusing on the standard conditions for chest screening CT examinations, such as the normalization of density curves between systems and the optimization of scanning conditions. Results. The density curves were almost identical between two systems following adjustment of the look-up table (LUT) of the laser printer. Under the same scanning conditions with regard to the breath-holding time (17 s) and dose (2.5mGy), different CT systems were found to exhibit differences in minimum-diameter identification capabilities. Conclusion. The use of the phantom described here makes it possible to objectively determine the optimal scanning conditions for various CT systems.