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Figure 1. Diagram of the nephron showing GFR agents such as 99mTc-DTPA, 51Cr-EDTA and 125I-iothalamate filtered at the glomerular level. While 99mTc-MAG3 and 123I- or 131I-hippurate are filtered, their predominant method of renal excretion is by tubular secretion at the level of the proximal convoluted tubule. 99mTc-DMSA and 99mTc-glucoheptonate become fixed within the renal tubules. 99mTc-glucoheptonate is also filtered and secreted by the nephron. (Figure provided Ms. L.Ward.)

Figure 1. Diagram of the nephron showing GFR agents such as 99mTc-DTPA, 51Cr-EDTA and 125I-iothalamate filtered at the glomerular level. While 99mTc-MAG3 and 123I- or 131I-hippurate are filtered, their predominant method of renal excretion is by tubular secretion at the level of the proximal convoluted tubule. 99mTc-DMSA and 99mTc-glucoheptonate become fixed within the renal tubules. 99mTc-glucoheptonate is also filtered and secreted by the nephron. (Figure provided Ms. L.Ward.)

and secretion, its clearance is a measure of effective renal plasma flow (ERPF). ERPF is that plasma which flows through the kidney and is available for filtration and/or secretion, excluding capsular and interstitial perfusion.

Technical Considerations

Radiopharmaceuticals

Several tracers are available for renal imaging and functional assessment (Fig. 1). A brief description of some of the more common ones is presented below.

99mTc-Diethylenetriaminepentaacetic Acid (99mTc-DTPA)

This tracer is a nearly ideal agent for assessment of GFR. It is a small chelate with a molecular weight of 500, and its only mode of excretion is glomerular filtration. It is neither secreted nor reabsorbed by the kidneys. Its renal extraction reflects the filtration fraction of plasma (i.e., 20% first pass clearance). GFR determination using 99mTc-DTPA should take into account the possibility of binding of 99mTc-DTPA to plasma proteins. Protein binding is variable between different formulations of 99mTc-DTPA.

131I- or 123I-Orthoiodohippurate (131I- or 123I-OIH)

Hippurate is mainly excreted by tubular secretion (80%) and, to a lesser extent, by filtration (20%). Its first pass clearance is high (about 90%) and therefore it can be used to measure ERPF. 123I-OIH is not available commercially in many areas (including North America), and 131I-OIH can only be used in small amounts because of the high radiation dose that results from the iodine-131 label.

99mTc-Mercaptoacetyltriglycine (99mTc-MAG3)

This is a newer agent that is almost entirely excreted by tubular secretion. Although first pass extraction is only about 50%, it has much higher protein binding than OIH with more staying in plasma and available for uptake by the tubular cells. The appearance of the renogram with 99mTc-MAG3 is nearly identical to that of OIH. It is excreted faster than 99mTc-DTPA at all levels of renal function. Because of its ready availability and excellent imaging characteristics, it is the favored agent for diuretic renography. There is some liver excretion of 99mTc-MAG3, which complicates assessment for urine leaks on delayed views (2-3 hours or more after injection).

99mTc-Ethylenedicysteine (99mTc-EC)

This is another new agent with high extraction efficiency and excretion characteristics similar to 99mTc-MAG3. Its main advantage is easier preparation.

99mTc-Dimercaptosuccinic acid (99mTc-DMSA)

99mTc-DMSA is used for cortical imaging (Fig. 2) because about 50% of the injected dose is retained in the renal cortex, bound to sulfhydryl groups in the proximal convoluted tubules. Peak renal activity is seen at 4-6 hours after injection. It is the favored tracer for assessment of renal morphology and is used in the assessment of pyelonephritis and renal scarring. Its uptake is markedly reduced in conditions that result in acidification of the urine (e.g., renal tubular acidosis).

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