胸部単純X線画像におけるピクセルマッチング法を用いた経時的差分画像法の開発

摘要

胸部単純X線画像のもつ情報量の多さと検査の簡便性は広く知られ,多くの病院や健康管理施設において,病巣発見のための一次検査として用いられている.また,胸部単純X線画像を読影する際に,その精度向上のために寄与しうる要因として,同一患者の過去画像との比較読影が挙げられる.しかし前述した施設において1日に検査する患者の数は,読影医師にとって大きな負担と感じるほどに多い.そこで,医師の負担を軽減することに加え,読影精度の改善のために胸部のコンピュータ支援診断(computer-aided diagnosis: CAD)への期待が高まっている.%A temporal subtraction image, which is obtained by subtraction of a previous image from a current one, can enhance interval changes on a chest radiograph by removal of most normal structures. However, subtraction artifacts, which tend to reduce its effectiveness in the detection of interval changes, were still included in the conventional method. In this study, we have developed a pixel matching technique to reduce artifacts in the temporal subtraction images. With this technique, the pixel value in a nonlinearly warped previous image is replaced by a pixel value within a kernel, which is closest to the pixel value on a current image. For evaluation of the proposed method, one hundred temporal subtraction images with a simulated nodule were used. When the kernel size of 3×3 was employed in the pixel matching technique, the misregistration artifacts decreased by 72%, and the contrast-to-noise ratio of the simulated lung nodules was increased by 5% in comparison with the conventional method. However, the area of the simulated nodule on the subtraction image decreased by 6%. Our results indicated that the pixel matching technique can enhance simulated nodules, with a substantial reduction of misregistration artifacts in comparison with conventional subtraction images. Therefore, we believe that the temporal subtraction method with the pixel matching technique would assist radiologists' diagnoses for detection of lung nodules in digital chest radiography.