There are numerous different types of radionuclide imaging which are used in renal imaging--more commonly in pediatric than in adult patients, and as such my knowledge of these procedures as it pertains to adult nephrology patients has always been a bit hazy.
Fortunately there is an excellent review of this topic in this month's Nephsap from ASN featuring Renal Imaging.
One of the tests often ordered in pediatric populations is that so-called "Mag-3 Renal Scan". This stands for technitium-99m-mercapto acetyl tri glycine-3, the radiotracer used in this form of imaging termed "dynamic renal scintigraphy." The radiotracer is injected intravenously and then dynamic images of the collecting system are obtained over a 30-minute period, over which time the tracer should be excreted and appear in the bladder. This technique is useful for demonstrating functional obstruction--for example, due to a kidney stone or other anatomic abnormality--based on the rate of Mag-3 washout from the collecting system, particularly in instances when other imaging modalities (e.g., ultrasound, CT urography) fail to demonstrate a definitive obstruction but there is still a high suspicion that one exists.
Other commonly used nuclear medicine renal imaging tests include "renal cortical scintigraphy" (in which the radiotracer Tc-99m-DMSA is used to demonstrate acute or chronic pyelonephritis/scarring, usually in pediatric populations) and "radionuclide cytography" (which assesses for vesico-urethral reflux by injection of a technicium radiotracer in either an anterograde or retrograde fashion.)