Technetium-99m diethylenetriaminepentaacetic acid (^99mTc-DTPA) is the common agent for assessing glomerular filtration rate (GFR). The ideal agent for measuring GFR is cleared only by glomerular filtration and is not secreted or reabsorbed. ^99mTc-DTPA satisfies the first requirement but has variable degrees of protein binding which deviate its kinetics from the ideal agent such as inulin. For a 20 mCi (740 MBq) dose, the radiation exposures to the kidneys and urinary bladder are 1.8 and 2.3 rads, respectively (1).

The mechanism of iodine-131 orthoiodohippurate (¹³¹I-OIH) renal clearance is about 20% by GFR and 80% by tubular secretion. ¹³¹I-OIH is an acceptable alternative to para-aminohippuric acid (PAH) for determining renal plasma flow although its clearance is 15% lower than that of PAH. PAH is not entirely cleared by the kidneys with about 10% of arterial PAH remaining in the renal venous blood. Therefore, ¹³¹I-OIH measures effective renal plasma flow (ERPF). The tubular extraction efficiency of ¹³¹I-OIH is 90% and there is no hepatobiliary excretion. OIH may also be labeled with ¹²³I that not only provides equivalent urinary kinetics as that for ¹³¹I label but also offers improved image quality due to typically larger administered dose in view of its more favorable radiation exposure. For a 300-uCi (11 MBq) dose of ¹³¹I-OIH, the radiation exposures to the kidneys and urinary bladder are 0.02 and 1.4 rads, respectively. Few drops of nonradioactive iodine (e.g., saturated solution of potassium iodide) orally minimize the thyroid uptake of free ¹³¹I (1).

Technetium-99m mercaptoacetyltriglycine (^99mTc-MAG3) has similar properties to OIH but has significant advantages of better image quality and less radiation exposure. The tubular extraction fraction of MAG3 is lower than OIH at about 60% to 70%. There is also about 3% hepatobiliary excretion which increases with renal insufficiency. Despite these features, however, MAG3 is a common agent used in the scintigraphic evaluation of renal function. For a 10-mCi (370 MBq) dose, the radiation exposures to the kidneys and urinary bladder are 0.15 and 4.4 rads, respectively (1).

Technetium-99m dimercaptosuccinic acid (^99mTc-DMSA) localizes to renal cortex at high concentration and has slow urinary excretion rate. About 50% of the injected dose accumulates in the renal cortex at 1 h. The tracer is bound to the renal proximal tubular cells. Because of the high retention of DMSA in the renal cortex, it has become useful for the imaging of renal parenchyma. For a 6-mCi (11 MBq) dose, the radiation exposures to the kidneys and urinary balder are 3.78 and 0.42 rads, respectively (1).

Fluorodeoxyglucose (FDG) is the most common positron-labeled radiotracer in PET. ¹⁸F-labeled deoxyglucose is a modified form of glucose in which the hydroxyl group in the 2-position is replaced by the ¹⁸F positron emitter. FDG accumulates in cells in proportion to glucose metabolism. Cell membrane glucose transporters facilitate the transport of glucose and FDG across the cell membrane. Both glucose and FDG are phosphorylated in the 6-position by the hexokinase. The conversion of glucose-6-phosphate or FDG-6-phosphate back to glucose or FDG, respectively, is affected by the enzyme phosphatase. In most tissues, including cancer, there is little phosphatase activity. FDG-6-phosphate cannot undergo further conversions and, therefore, is trapped in the cell. FDG is excreted in the urine. The typical FDG dose is 0.144 mCi/kg (minimum 1 mCi and maximum 20 mCi). The urinary bladder wall receives the highest radiation dose from FDG (2,3). The radiation dose depends on the excretion rate, the varying size of the bladder, the bladder volume at the time of FDG administration, and an estimated bladder time-activity curve. For a typical 15 mCi FDG dose and voiding at 1 h after tracer injection, the average estimated absorbed radiation dose to the adult bladder wall is 3.3 rads (0.22 rads/mCi) (4). The doses to other organs are between 0.75 and 1.28 rads (0.050-0.085 rads/mCi) for an average organ dose of 1.0 rad (4).

In renal failure, glomerular and tubular dysfunctions are reflected by abnormal renal scans and renograms. Renal uptake of MAG3 is prolonged with tubular tracer stasis and little or no excretion. It has been shown that in patients with acute renal failure, the demonstration of MAG3 renal activity more than hepatic activity at 1 to 3 min indicates likely recovery while when renal uptake is less than the hepatic uptake, dialysis may be needed (7). In chronic renal failure, there is diminished renal perfusion, cortical tracer extraction, and excretion. However, this imaging pattern is nonspecific and needs to be interpreted in the clinical context (1).

Acute pyelonephritis is associated with fever, flank pain, leukocytosis, and pyuria. Radiolabeled leukocyte (e.g., ¹¹¹In-WBC) and ⁶⁷Ga-citrate scans can help identify acute pyelonephritis. However, these methods have the drawbacks of extended imaging time (more than 24 h) and higher radiation exposure. Cortical imaging with DMSA is highly sensitive for detecting acute
pyelonephritis in the appropriate clinical setting (8). Acute pyelonephritis demonstrates segmental regions of decreased tracer uptake in oval, round, or wedge pattern. There may also be diffuse generalized decrease in renal uptake which in association with normal or slightly enlarged kidney is suspicious for an acute infectious process. The pathophysiologic basis for decline in DMSA cortical uptake in infection is related to diminished tracer delivery to the infected area and to direct infectious injury to the tubular cells compromising their function and tracer uptake. A wedge-shaped cortical defect with regional decrease in renal size is compatible with post-infectious scarring. Renal infarcts may also have a similar appearance (1,5).