Radionuclide Renal Imaging

Radionuclide renal imaging, which involves I.V. injection of a radionuclide followed by scintigraphy, provides a wealth of information for evaluating the kidneys. Observing the uptake concentration and transit of the radionuclide during this test allows assessment of renal blood flow, renal structure, and nephron and collecting system function. Depending on the patient's clinical presentation, this procedure may include dynamic scans to assess renal perfusion and function or static scans to assess structure.

The radioisotope injected depends on the specific information required and the examiner's preference. However, this procedure often includes double isotope technique to obtain a sequence of perfusion and function studies, followed by static images. This test may also be substituted for excretory urography in patients with hypersensitivity to contrast agents.

Purpose

• To detect and assess functional and structural renal abnormalities (such as lesions)
• To detect renovascular hypertension and acute and chronic renal disease (such as pyelonephritis and glomerulonephritis)
• To assess renal transplantation or renal injury due to trauma and obstruction of the urinary tract

Patient preparation

• Explain to the patient that this test permits evaluation of the structure, blood flow, and function of the kidneys.
• Tell him who will perform the test and where, and that it takes about 90 minutes. (If static scans are ordered, there will be a delay of several hours before the images are taken.)
• Inform him that he'll receive an injection of a radionuclide and that he may experience transient flushing and nausea.
• Emphasize that only a small amount of radionuclide is administered and that it's usually excreted within 24 hours.
• Tell him several series of films will be taken of his bladder.
• Make sure the patient or a responsible family member has signed a consent form.
• Make sure the patient isn't scheduled for other radionuclide scans on the same day as this test.
• If the patient receives antihypertensive medication, it may be with held before the test.
• Women who are pregnant and young children may receive supersaturated solution of potassium iodide (SSKI) 1 to 3 hours before the test to block thyroid uptake of iodine.

Equipment

Computerized gamma scintillation camera, 99m Tc-DTPA (technetium and diethylenetriaminepentaacetic acid) for perfusion study, 131 I-orthoiodohippurate (Hippuran) for function study, oscilloscope, magnetic tape, I.V. equipment

Procedure and posttest care

• The patient is commonly placed in a prone position so that posterior views may be obtained. If the test is being performed to evaluate transplantation, the patient is positioned supine for anterior views.
• Instruct the patient not change his position.
• A perfusion study (radionuclide angiography) is performed first to evaluate renal blood flow. The 99mTc-DTPA is administered I.V., and rapidsequence photographs (one per second) are taken for 1 minute.
• Next, a function study is performed to measure the transit time of the radionuclide through the kidneys' functional units. After 131 I-orthoiodohippurate is administered I.V., images are obtained at a rate of one per minute for 20 minutes. Alternatively, this entire procedure can be recorded on computer-compatible magnetic tape and concurrent renogram curves plotted.
• Lasix 40 mg may be injected I.V. to evaluate an obstruction.
• Finally, static images are obtained 4 or more hours later, after the radionuclide has drained through the pelvicalyceal system.
• Instruct the patient to flush the toilet immediately after each voiding for 24 hours as a radiation precaution.

Normal Findings

Because 25% of cardiac output goes directly to the kidneys, renal perfusion should be evident immediately following uptake of the 99mTc-DTPA in the abdominal aorta. Within 1 to 2 minutes, a normal pattern of renal circulation should appear. The radionuclide should delineate the kidneys simultaneously, symmetrically, and with equal intensity.

The 131I-orthoiodohippurate administered for the function study rapidly outlines the kidneys - which should be normal in size, shape, and position - and also defines the collecting system and bladder. Maximum counts of the radionuclide in the kidneys occur within 5 minutes after injection (and within 1 minute of each other) and should fall to approximately one­third or less of the maximum counts in the same kidney within 25 minutes. Within this time, the function of both kidneys can be compared as the concentration of radionuclide shifts from the cortex to the pelvis and, finally, to the bladder.

Renal function is best evaluated by comparing these images to the renogram curves. Total function is considered normal when the effective renal plasma flow is 420 ml/minute or greater and the percentage of the dose excreted in the urine at 30 to 35 minutes is greater than 66%.

Abnormal findings

Images from the perfusion study can identify impeded renal circulation, such as that caused by trauma and renal artery stenosis or renal infarction. These conditions may occur in patients with renovascular hypertension and abdominal aortic disease. Because malignant renal tumors are usually vascular, these images can help differentiate tumors from cysts.

In evaluating a kidney transplant, abnormal perfusion may indicate obstruction of the vascular grafts. The function study can detect abnormalities of the collecting system and extravasation of the urine. Markedly decreased tubular function causes reduced radionuclide activity in the collecting system; outflow obstruction causes decreased radionuclide activity in the tubules, with increased activity in the collecting system. This test can also define the level of ureteral obstruction.

Static images can demonstrate lesions, congenital abnormalities, and traumatic injury. These images also detect space-occupying lesions within or surrounding the kidney, such as tumors, infarcts, and inflammatory masses (abscesses, for example); they can also identify congenital disorders, such as horseshoe kidney and polycystic kidney disease. They can define regions of infarction, rupture, or hemorrhage after trauma.

A lower-than-normal total concentration of the radionuclide, as opposed to focal defects, suggests a diffuse renal disorder, such as acute tubular necrosis, severe infection, or ischemia. In a patient who has had a kidney transplant, decreased radionuclide uptake generally indicates organ rejection. Failure of visualization may indicate congenital ectopia or aplasia.

Definitive diagnosis usually requires the combined analysis of static images, perfusion studies, and function studies.

Interfering factors

• Antihypertensives (possible masking of abnormalities)
• Scans of different organs performed on the same day (possible poor imaging)