Usefulness and limitations of dual-layer spectral detector computed tomography for diagnosing biliary stones not detected by conventional computed tomography: a report of three cases.

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.