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Contrast-enhanced computed tomography using iodinated contrast media is useful for diagnosis of gastrointestinal diseases. However, contrast-induced nephropathy remains problematic for kidney diseases patients. Although current guidelines recommended the use of a minimal dose of contrast media necessary to obtain adequate images for diagnosis, obtaining adequate images with sufficient contrast enhancement is difficult with conventional computed tomography using reduced contrast media. Dual-layer spectral detector computed tomography enables the simultaneous acquisition of low- and high-energy data and the reconstruction of virtual monochromatic images ranging from 40 to 200 keV, retrospectively. Low-energy virtual monochromatic images can enhance the contrast of images, thereby facilitating reduced contrast media. In case 1, abdominal computed tomography angiography at 50 keV using 40% of the conventional dose of contrast media revealed the artery that was the source of diverticular bleeding in the ascending colon. In case 2, ischemia of the transverse colon was diagnosed by contrast-enhanced computed tomography and iodine-selective imaging using 40% of the conventional dose of contrast media. In case 3, advanced esophagogastric junctional cancer was staged and preoperative abdominal computed tomography angiography could be obtained with 30% of the conventional dose of contrast media. However, the texture of virtual monochromatic images may be a limitation at low energy. Keywords: Virtual monochromatic images, Contrast-induced nephropathy

Computed tomography (CT) is useful for diagnosing biliary stones. However, the presence of stones not detected by conventional CT, such as iso-dense stones with CT numbers similar to those of bile or small stones, is problematic. Although conventional CT provides only 120-kVp images corresponding to CT numbers at approximately 70 keV, dual-layer spectral detector CT uses one X-ray source and dual-layer detectors to collect low- and high-energy data simultaneously; retrospective spectral analysis, including virtual monochromatic images with photon energy levels of 40-200 keV, material decomposition images, and spectral curves, can be immediately performed on demand. This technique can immediately discriminate between materials with similar conventional CT numbers. Therefore, prompt and accurate diagnosis of iso-dense stones can be performed. In two out of three of our cases, iso-dense stones were detected in virtual monochromatic images at 40 keV, but in the remaining case a common 4-mm bile duct stone was not detected on 120-kVp and 40-keV images by retrospective spectral analysis. However, this stone was detected by magnetic resonance cholangiopancreatography. Retrospective spectral analysis using dual-layer spectral detector CT was useful for prompt and accurate diagnosis of iso-dense stones, but detection of <5-mm stones may be a limitation of this technique and of conventional CT. [ABSTRACT FROM AUTHOR]