The meaning and significance of the equilibrium phase in enhanced computed tomography of the liver

The "equilibrium phase" (EP) in liver contrast enhancement in CT is generally held to be a potential pitfall by way of loss of intrinsic lesion conspicuity. Unfortunately, the definitions extant in the literature are not firmly rooted in a pharmacokinetic basis. Such a basis is developed in this article and a method is described for estimating the time of onset of the EP. It is shown that this may be, effectively, as early as 120 s or so after the beginning of a typical infusion of contrast agent. This means that incremental scanning protocols are likely to result in later slices being acquired in the EP. It is further shown that the pharmacokinetic model does predict a danger of loss of lesion conspicuity in EP scanning. Fortunately, rapid helical/spiral scanning makes it a danger that is easily avoided.