ヘリカルスキャンCTによる大腸の三次元構築画像

Three-Dimensional-CT Imaging of Colorectal Disease with Thin Collimation Helical CT Scanning

要旨　近年関心が高まりつつあるヘリカルスキャンを用いた大腸の三次元構築画像について，当院では3年間にわたって本検査法の研究を行ってきた．大腸の三次元画像の構築には，閾値の設定方法により，バーチャルエンドスコピー法，エアーイメージ法，シュードトラクト法の3種類の方法がある．バーチャルエンドスコピー法は内視鏡に近似した画像を得る方法で，エアーイメージ法は管腔内の空気像を示す．シュードトラクト法は腸管の内側をバーチャルエンドスコピー法，外側をエアーイメージ法で表示し，双方の特徴を兼ね備え，管腔らしく表示する方法である．これらの大腸三次元構築画像は低侵襲的に内視鏡写真や注腸造影像とほぼ同等の画像が得られ，検査時間が短い，検査技術によるばらつきが少ない，被検者の不安が少ないなどの特徴を有する．しかし一方で，計算時間，空気注入量，便との鑑別，スキャン範囲，解像力など解決すべき課題が存在する．

　We have conducted research on three-dimensional (3D)-CT-colonoscopy with thin collimation helical CT scanning over the past three years. This has lately become a subject of special interest. 3D-CT-colonoscopy has three kinds of visualizing methods depending on the threshold setting of CT values. The first one is the virtual endoscopy method which is displayed in a similar fashion to colonoscopic images. The second one is the air image method using the air in the digestive tract as a contrast medium. The third one is the pseudo-tract method which has characteristics of both virtual endoscopy and the air image method and visualizes in a shape of the digestive tract. The image visualized by 3D-CT-colonoscopy is similar to that of conventional colonoscopy and barium enema study, which is obtained with minimal invasion to patients. Obvious advanced carcinomas were easily visualized, and even a small flat polyp measuring 5 mm in size, was able to be observed retrospectively. The characteristics of our method are that we can easily make an examination in a short time and with little dependence on expert technique. Also patients have little discomfort compared to that experienced during colonoscopy and barium enema study. Important features are as follows; 1) long calculation time, 2) insufficient air insufflation, 3) fecal material in the patient's bowel, 4) whole abdominal scan, 5) spatial resolution. In the near future, a multislice CT scanner system will have ability to overcome these problems. Therefore, 3D-CT-colonoscopy might be applied in the future for first line examination as a mass screening for colorectal carcinoma.