The quantification of renal function with enhanced computed tomography

The changes with time in renal parenchymal and vascular attenuation following injection of an intravascular contrast medium can be exploited to study renal physiology in several ways. Shortly after injection, both contrast medium clearance and fractional vascular volume per unit volume of kidney can be measured on a regional basis by obtaining sequential CT data from one anatomical level and the application of a graphical analysis to the baseline subtracted CT numbers for tissue and vascular regions of interest: c(t) and b(t), respectively. By plotting the ratio c(t)/b(t) against integral b(t)dt/b(t), both fractional vascular volume and contrast medium clearance can be estimated graphically. At later time points, a semilogarithmic plot against time of the declining attenuation of vascular or parenchymal regions of interest gives information about overall, as opposed to regional, renal function: the gradient estimates the glomerular filtration rate per unit volume of extracellular fluid. Absolute glomerular filtration rates can also be calculated with blood sampling. Although further experimental evaluation is needed, these techniques have a strong theoretical basis, and experimental work performed so far is encouraging. Possible applications of these techniques are discussed.