Quantitation of Renal Function With Static Imaging Agents

Abstract

DMSA is an excellent static imaging agent. Approximately 50% of the injected dose remains bound to the cortical tubules of the kidney up to 24 hr after injection. During this time, 30% of the dose is excreted in the urine, background activity drops significantly, and kidney-to-background ratio is enhanced markedly. A 5 mCi dose is adequate for static imaging even out to 24 hr and the 5 mCi dose can be reduced for 1–4 hr post injection images. Patients should be adequately hydrated prior to a DMSA study since adequate hydration may enhance the excretion of DMSA, decrease background activity, improve the kidney-to-background ratio, and lower the radiation dose to the patient. A number of studies have shown that the relative uptake of DMSA correlates well with the relative ERPF and GFR in patients with both normal and asymmetrically functioning kidneys however, there are only a limited number of comparison studies in patients with severely compromised renal function. In this group of patients, comparison and validation studies are complicated by the fact that measuring the relative renal uptake of hippurate or DTPA during the first 1–3 min post-injection can be statistically less accuarate than determining the relative uptake of DMSA. A few investigators have reported DMSA scanning may be misleading in patients with obstruction due to retention of DMSA in the renal pelvis. This conclusion is based on a discrepancy in the relative renal function determined using DMSA and 131 I-hippurate or DTPA. Since obstructed kidneys generally have diminished function, this discrepancy may be related to the problem of region of interest assignment and background substraction during the first 1–3 min after hippurate or DTPA injection. While other investigators have not observed obstruction to be a problem, it is probably safer to confirm DMSA results in patients with obstruction with an additional study using hippurate or DTPA until more comparison data become available. After reconstitution with 99m Tc pertechnetate, the DMSA preparation is quite sensitive to oxidation and air should not be introduced into the reaction vial. If more than one dose is to be obtained from a single vial, the two or more doses should be drawn immediately after each other and administered as soon as possible. Oxidation will degrade the DMSA complex, decrease uptake in the kidney and increase liver background activity. It also is important to note that acidification of the urine in rats with ammonium chloride doubled the liver uptake of DMSA and decreased the renal uptake by 50%. Acidification of the urine in patients may also alter the biodistribution and, in theory, may also alter the relative distribution of DMSA between asymmetrically functioning kidneys. These possibilities need to be further evaluated. Finally, since a number of variables can affect the total renal accumulation of DMSA, DMSA should not be quantitated as an absolute index of renal function unless it is done by experienced investigators under carefully controlled conditions.