Relationship between Quantitativeness of Iodine and Accuracy of Virtual Non-contrast Image in Dual-energy Computed Tomography

Abstract

Dual-energy computed tomography(DE-CT)は，物 質弁別や仮想単色画像の生成，実効原子番号の計測な ど，有 益 な 情 報 を 得 る こ と が で き る 画 像 技 術 で あ る 1～4) ．特に物質弁別の技術は低および高電圧で異な る X 線吸収を有する物質の弁別および可視化を可能 にし 5) ，ヨード造影剤と正常組織など原子番号の差の 大きい物質間の弁別に適しているとされている 6) ．こ の DE-CT の物質弁別の技術を応用し，造影画像から ヨード含有量を差分することによって，仮想の非造影 画像である仮想単純(virtual non-contrast: VNC)画像 を得ることができる 7) ． こ れ ま で の single-energy computed tomography (SE-CT)による造影 CT と比較して，VNC 画像と ヨード分布画像，そして仮想単色画像の活用により， DE-CT は病変検出の向上およびその特徴付けに貢献 し得るとの報告がある 8, 9) ．また VNC 画像の取得に よって，真の単純 CT(true non-contrast: TNC)の撮影 Summary Dual-energy computed tomography (DE-CT) is the promising technology, such as enabling material decomposition, generation of the virtual monochromatic image, and measurement of effective atomic numbers. There are reports that utilization of the virtual non-contrast (VNC) image, the iodine map image, and the virtual monochromatic image can contribute to the improvement of lesion detection and its characterization, compared with conventional contrast CT by single-energy computed tomography (SE-CT). In addition, acquisition of the VNC images makes it possible to skip scanning of true non-contrast CT, which is also expected to reduce exposure. However, a reliable evaluation of the accuracy of the VNC image has not been established, and only a few reports have verified their accuracy. In this study, we evaluated the relationship between the quantitativeness of iodine and the CT value of VNC image. As a result of our study, when the iodine volume was overestimated, the CT value of the VNC image was lower than the reference value, and when the iodine volume was underestimated, the CT value was upper than the reference value. Moreover, we clarified that the CT value of the VNC image greatly diverges as the iodine volume increases.