CT画像計測プログラムを用いた円形エッジ法における標的径とFOVに対するMTF測定精度

摘要

X線computed tomography(CT)の画像再構成法には，従来のfiltered back projection(FBP)法に加え，画像雑音やアーチファクト低減を目的として逐次近似法を応用したiterative reconstruction法(IR応用法)が広く使用されている.X線CTの画質評価は，撮影条件の決定や装置の特性を知る上で重要である.X線CT装置における空間分解能の評価では，modula-tion transfer function(MTF)が用いられ，金属ワイヤを用いたwire法が広く普及している.IR応用法におけるMTF評価は，軟部組織を標的とする臨床条件に近い条件で行うことを必要とされるため，低中コントラスト物体を標的とし，臨床条件に近いfield of view(FOV)で再構成された画像を用いることが重要であるとされる.%Purpose: The purpose of this study was to evaluate the effects of target diameter and display-field of view (D-FOV) in modulation transfer function (MTF) by circular edge strategy using the computed tomography (CT) image measurement program "CTmeasure". Methods', We calculated the MTF (MTF_(edge)) using the circular edge strategy applied to cylindrical phantom (200 mmφ)) that inserted with cylinders have10， 20, 30， and 40 mm diameters. The phantom images were reconstructed using filtered back projection method varied with D-FOV (240, 320, 400, and 500 mm). The study compared both MTF_(edge) and MTF_(wire) at MTF_(50%) and MTF_(10%) for target diameter and D-FOV, respectively. Results'. The MTF_(edge) by the different of target diameter indicated in rough compatibility. However, MTF_(edge) of D-FOV diameters (320, 400, and 500 mm) decreased in the high frequency range. Conclusions: The circular edge strategy for MTF depended on the D-FOV, however, it was little dependent on target diameter using the CT image measurement program "CTmeasure".