Liver metastases: optimizing detection with multislice CT (MSCT)

Conventional, single-slice helical computed tomography (CT) allows for scanning the majority of the liver during the critical portal venous phase. This is often referred to as the ‘optimal temporal window’. In general, it occurs following a 70-s scan delay and is coincidental with the maximal delivery of contrast via the portal vein that provides 80% of the hepatic blood supply. This yields maximal conspicuity between low attenuation lesions and the enhanced normal liver parenchyma. This provides optimal imaging for the vast majority of hepatic metastases. Most importantly, these scanners, when compared to conventional non-helical scanners, avoid impinging upon the ‘equilibrium’ phase where tumors can become isodense/invisible. Helical CT also allows scanning during the arterial phase for detection of hypervascular lesions but was limited in its ability to scan effectively in multiple phases necessary for detection of hypervascular metastases. With the introduction of multislice CT, imaging speed has increased significantly especially with the introduction of 8- and 16-detector systems and will continue to increase in the future volumetric CT. This provides a number of important gains that are discussed in detail.