Functional imaging in computed tomography. The use of contrast-enhanced computed tomography for the study of renal...

Computed tomography (CT) numbers may provide the raw material for calculations of important physiologic parameters, such as organ regional perfusion and glomerular filtration rate. Because, except in cases of severe renal dysfunction, iodinated intravascular contrast agents are excreted primarily by passive glomerular filtration, the decline in their concentration in blood or tissues will reflect renal function. The time evolution of contrast agent distribution within the kidney, from blood to tubules, can also be seen as a measure of renal function. The authors illustrate how CT numbers may be employed to obtain useful data regarding renal function.

Two CT methods were used to yield information on renal function. Delayed CT: 2 hours after administration of 200 mL of a contrast agent (iohexol) for a routine CT, a patient with normal renal function was brought back to the scanner, and a single fixed section through the abdomen, including the psoas muscles, was imaged; the scan was repeated at 2, 3, and 4 hours. Large regions of interest were selected in soft tissue, and six measurements of CT numbers were taken and averaged for each time; after taking logs, these were plotted against time. Dynamic CT: A rapid bolus of 40 mL nonionic contrast agent was injected into an antecubital vein at 5 mL/sec. At a fixed level, images were taken through the center of the kidneys at 5-second intervals for 2 minutes after injection. The patient was allowed to breathe gently throughout the examination. Regions of interest were selected over the kidney and the aorta, and curves of CT numbers against time were generated; a Patlak plot was performed using the parameters measured. Using the renal CT numbers collected during the first 2 minutes after injection, it was possible to analyze the handling of the contrast agent and to obtain a measure of the glomerular filtration rate.

Delayed CT: Data yielded good values for glomerular filtration rate; the typical patient illustrated had a glomerular filtration rate per unit extracellular fluid volume of 0.005 min-1. Dynamic CT: The correlation coefficient for the line fit was 0.97, a value typically found in these studies, indicating the efficacy and ruggedness of this method in spite of the patient's shallow breathing throughout the examination.

Computed tomography offers the potential for quantitative and physiologic measurement that has not been widely explored. The problem with the delayed CT technique is that since measurements are made at 2 hours and onward, it is applicable only to patients who have received larger doses of contrast. Otherwise, the CT numbers are low and the measurement error, large. Also, bringing the patient back to the scanner 2 hours later may be inconvenient to both patient and radiologist, and may disrupt case scheduling. This method is, however, less inconvenient than the established blood-level measurement. The dynamic CT method yields a new measure of renal clearance, namely, clearance per unit volume of kidney. The disadvantages of this method are the radiation burden involved in repeated rapid CT scans and the fact that, if renal disease is uneven, a nonrepresentative slice could be chosen. Its advantages are its simplicity and that it can be completed within the first 2 minutes of a CT examination. It may be expected to have an interesting application to situations in which there is known renal disease. Although difficulties are associated with both of these techniques that could prevent their general adoption as clinical routines, they nevertheless should find application in selected patients and should prove useful research tools in the study of renal disease.