A renal scan (renal scintigraphy) is a nuclear medicine imaging test that provides insight into how the kidneys are functioning. Unlike standard imaging techniques that primarily show anatomy, this procedure uses a small amount of radioactive material to track the movement of substances through the kidney tissue and the urinary system. This allows physicians to evaluate not only the size and shape of the kidneys but, more importantly, their physiological performance in real-time.
How Nuclear Medicine Works in the Kidneys
The basis of a renal scan relies on introducing a specialized radioactive tracer, or radiopharmaceutical, into the patient’s bloodstream. This tracer is chemically designed to be handled by the kidneys in the same way certain naturally occurring substances are filtered or secreted. Once injected, typically through an intravenous line in the arm, the tracer travels quickly to the kidneys.
The radiopharmaceutical contains a radioisotope that emits gamma rays as it moves through the body. A specialized external gamma camera detects these emissions, converting the energy signals into detailed images on a computer. Tracers are chosen based on the specific function being assessed; for instance, agents like technetium-99m (Tc-99m) MAG3 or DTPA are used as functional tracers to monitor blood flow, filtration, and excretion.
Other tracers, such as Tc-99m DMSA, are considered structural tracers because they bind directly to the renal tubules (cortex). This binding allows for a static image to be taken several hours after injection, which is used to assess the kidney’s anatomy and look for areas of scarring or damage. By monitoring the tracer’s concentration over time, the scan can generate dynamic curves that precisely quantify the kidney’s ability to extract the substance from the blood and pass it into the urine.
Clinical Reasons for a Renal Scan
A renal scan offers unique functional data that complements anatomical information provided by other imaging methods, like ultrasound or CT scans. A primary application is determining the differential function, which measures the precise percentage contribution of each kidney to the total kidney function. This measurement is especially important before any surgical procedure that might affect one kidney.
The scan is also frequently used to assess renal blood flow (perfusion), which is the rate at which blood enters the kidneys. This is particularly relevant in cases of renovascular hypertension, where narrowing of the renal arteries may cause high blood pressure. To investigate this condition, the scan may be performed before and after the patient takes an angiotensin-converting enzyme inhibitor (ACE inhibitor) medication, in a procedure known as ACE inhibitor renal scintigraphy.
Another reason for the scan is to identify urinary tract obstruction, a condition where urine flow is blocked. In a dynamic scan, a diuretic agent like furosemide (Lasix) may be administered to stimulate urine production and test whether the collecting system can clear the tracer efficiently. If the tracer remains in the kidney despite the diuretic, it suggests a true blockage, a procedure known as diuretic renography.
Other Applications
The test is also valuable for:
- Evaluating a transplanted kidney to check for rejection or vascular complications.
- Locating areas of damage or scarring caused by chronic infections, such as pyelonephritis.
What to Expect During the Procedure
The renal scan is typically performed on an outpatient basis and requires preparation, including ensuring the patient is well-hydrated before and during the test. Hydration promotes adequate urine flow, which helps optimize image quality and minimize the radiation dose to the bladder. Patients may also be instructed to temporarily stop certain blood pressure medications, particularly ACE inhibitors, if the scan is intended to investigate renovascular hypertension.
Once prepared, the patient lies on an examination table, and a nuclear medicine technologist inserts an intravenous line into a vein in the hand or arm. The radioactive tracer is then injected through this line, and the gamma camera is positioned over the back or abdomen to begin capturing images. The camera itself does not emit radiation; it only detects the gamma rays coming from the patient’s kidneys.
The total time for the procedure can vary significantly, ranging from 30 minutes to over three hours, depending on the specific type of scan being performed. During imaging, it is important to remain still to prevent blurring of the images, which are often taken in rapid succession to capture the flow of the tracer. As part of the dynamic study, an adjunct medication like a diuretic may be administered midway through the scan to assess the kidney’s drainage capacity.
Safety and Understanding the Results
The radioactive tracer used in a renal scan exposes the patient to a minimal amount of radiation, often less than that received from a standard X-ray or CT scan. The isotopes used are short-lived, and the body naturally eliminates the remaining tracer, mainly through the urine, typically within 24 hours. To assist with clearance and reduce the radiation dose to the bladder, patients are advised to drink plenty of fluids and urinate frequently after the scan is complete.
While allergic reactions to the radiotracer are extremely rare, patients are asked to disclose any known allergies beforehand. Because of the minor radiation dose, women who are pregnant or breastfeeding should inform their physician, as precautions may be necessary. The images are interpreted by a nuclear medicine physician who looks for patterns of tracer uptake, transit time, and drainage.
A normal result shows a symmetrical uptake of the tracer in both kidneys and a prompt, clear excretion into the ureters and bladder. Abnormal results may indicate a range of issues, such as delayed tracer uptake suggesting poor blood flow, or retention of the tracer indicating an obstruction or poor function. The physician compiles a comprehensive report detailing these findings, which is sent to the ordering doctor for discussion and treatment planning.