腎囊胞のCT画像に生じるpseudoenhancement effectの 抑制に関わる撮影条件の検討：連続X線と仮想単色X線， 画像再構成法，およびビーム幅の違いにおけるフアントム検証

摘要

Purpose: The purpose of this study was to evaluate the effect of the virtual monochromatic spectral images (VMSI) and the model-based iterative reconstruction (MBIR) images, to evaluate the influence of the aperture size (40- and 20-mm beam) on renal pseudoenhancement (PE) compared with the filtered back projection (FBP) images. Methods: The renal compartment-CT phantom was filled with iodinated contrast material diluted to the attenuation of 180 Hounsfield units (HU) at 120 kV. The water-filled spherical structures, which simulate cyst, were inserted into the renal compartment. Those diameters were 7,15 and 25 mm. These were scanned by conventional mode (helical scan,120 kV-FBP) and dual energy mode. 70 keV-VMSI were reconstructed from the dual energy mode, and MBIR images were reconstructed from conventional mode at 40- and 20-mm aperture. Additionally, the phantom was scanned using non-helical mode with 20-mm aperture, and FBP images were reconstructed. The CT value of the PE for cyst areas was measured for these images. Results: The CT values of the cysts were 20.0-14.3 HU on the FBP images, 12.8-12.7 HU on the 70 keV-VMSI (PE-inhibition ratio was 36.0-11.2%) and 16.2-14.0 HU on the MBIR images (19.0-2.1%), respectively, at 40-mm aperture. The PE-inhibition ratio scanned by 20-mm aperture was improved by 28.0% with FBP, 32.8% with 70 keV-VMSI and 29.6% with MBIR compared with 40-mm aperture. One of the FBP images with non-helical mode was 11.6 HU. Conclusions: The best CT technique to minimize PE was the combination of 70 keV-VMSI and 20-mm aperture.%腎臓腫瘤のcomputed tomography (CT)を用いた造 影検査を行う際に，不均一な内部構造を呈する腫瘤性病変に対し，充実性腫瘤か囊胞性腫瘤かを鑑別するこ とは臨床的に非常に重要である.その基準として，造 影画像における腎腫瘤性病変のCT値が非造影画像と比べ，10 Hounsfield unit(HU)以上の造影効果をもっ て充実性腫瘤とみなすという報告がなされている. しかしながら，従来の造影CT検査では，腎囊胞の CT値が造影効果の高い腎実質の影響により高値にシ フトしてしまい，腎腫瘤の鑑別に支障をきたすことが ある.この現象をpseudoenhancement effect(PE) と呼び，考えられる要因としてpartial volume effect による影響,およびbeam-hardening(BH)の影響が 挙げられる.