Renal Scintigraphy

Introduction

Renal evaluation using radionuclides can provide information on renal function and anatomy; however the primary use is to assess function as there are more specific techniques available to assess anatomy.

For functional renal imaging radionuclide studies are an excellent alternative to intravenous urography(IVU). They carry a much smaller risk of allergic reaction and complications because unlike the contrast agents used to visualize the urinary system in an IVU, the pharmaceuticals used are chosen because of their minimal effect on function and low occurrence of allergic reaction. The strength of the IVU lies in its ability to provide excellent information on the anatomy of the entire urinary system (kidneys, ureters and bladder). As far as anatomy is concerned, an IVU is far superior to renal scintigraphy with the only exception being renal cortical imaging*.

The ability to generate a renogram separates renal scintigraphy from IVU. A renogram is a time-activity curve of the kidneys; it shows the rate of build up and excretion of the radiopharmaceutical in the kidneys. A renogram cannot be generated from an IVU because imaging is discrete and the mechanism by which these images are produced makes it impossible to acquire the quantitative information required to generate a graph. Because radionuclide renal studies are quantitative (counts) and involve dynamic imaging (continuous), graphs showing rates of uptake and excretion of the kidneys are possible (Urodynamics). A flow phase, 1 to 5 second images, acquired immediately after the IV administration of the radiopharmaceutical allows for the generation of an angiogram. Figure 1.1 shows sequential images of renal blood flow. Figure 1.2 shows a typical renogram curve (counts vs. time).

Figure 1.1 Nuclear renal angiogram. Inspection of the images shows two equally perfused kidneys. This is confirmed by outlining each kidney as a ROI and instructing the computer to tally the number of counts for each image.

Figure 1.2 Time activity curves of a typical renogram. Note the time taken to reach maximum uptake; ~ 4 min. This patient had two good functioning kidneys and a percentage uptake ratio of approximately 50:50

GFR and RPF

As mentioned in the previous paragraph radionuclide renal studies provide quantitative information not available or possible with other imaging modalities. The glomerular filtration rate (GFR) and the renal plasma flow (RPF) are two quantities used to assess renal function. The GFR and RPF are important because abnormalities in their values are evident before the onset of abnormal serum creatinine levels. Pharmaceuticals used in dynamic renal scintigraphy are either completely, filtered by the glomerulus or secreted by the tubules, or partially filtered and secreted. The classic method of determining GFR and RPF is to measure the rates of clearance of inulin and para-aminohippurate (PAH) as plasma passes through the kidneys. In renal scintigraphy GFR and RPF can be accurately estimated using computer based methods. For GFR the radiopharmaceutical analog to inulin is technetium 99m (Tc-99m) labeled diethylenetriaminepenta-acetic acid (DTPA) and for RPF the analog to PAH is Tc-99m labeled mercaptylacetyltriglycine (MAG3).

Clinical Applications of renal scintigraphy

In a clinical setting renal scintigraphy may be used solely or in conjunction with other methods when diagnosing patients with suspected renal problems. Knowledge of the strengths and weaknesses of, and alternatives to a particular method are keys to choosing the most suitable technique(s). In this section the clinical applications of renal scintigraphy are outlined.

Urinary Tract Obstruction

When presented with a patient showing symptoms of urinary tract obstruction, it is important to differentiate between mechanical obstructions (obstructive hydronephrosis) and non-obstructive dilation due to infection, congenital malformations, previous obstruction, a non-compliant bladder or vesicoureteral reflux (non-obstructive hydronephrosis). A modification of conventional renal scintigraphy to include a diuretic is an effective method for diagnosing patients with urinary tract obstruction. Provided there is sufficient renal function to promote diuresis, the increase in urine flow caused by the administration of a diuretic will produce a prompt washout in a dilated non-obstructed system but will have an insignificant effect when the obstruction is mechanical. (Figure 1.2 non-obstructive hydronephrosis, 1.3 obstructive hydronephrosis)

Figure 1.3 Renogram of patient with non-obstructive hydronephrosis.The administration of the diuretic at ~ 15 min induced excretion, indicating the absence of a mechanical obstruction.

Figure 1.4 Renogram of patient with obstructive hydronephrosis of the right kidney.There is no excretory phase for the right kidney even after the administration of the diuretic at 15 minutes. The cause of this patient’s hydronephrosis is mechanical.

Renovascular Hypertension

Renovascular hypertension refers to hypertension caused by renal arterial hypoperfusion secondary to vascular stenosis of the renal artery or one of its major branches (Thrall and Zeissman 2001). Atherosclerosis and fibromuscular dysplasia are the two main causes of renovascular hypertension. Non-invasive renal scintigraphy can be used to test whether renovascular hypertension is caused by renal artery stenosis. This may eliminate the need for angiography which is an invasive procedure. In patients with significant renal artery stenosis, pressure at the renal glomerulus drops and renal filtration as measured by GFR also decreases. To compensate renin is secreted by the juxtaglomerular apparatus. Renin effects the production of angiotensin I, which in turn is converted, to angiotensin II, in the lungs, by angiotensin-converting enzyme (ACE). Angiotensin II is a vasoconstrictor, it produces constriction of the glomerulus arterioles, thus raising the filtration pressure and maintaining GFR at normal levels. By modifying the standard renal scintigraphy exam to include an ACE inhibitor it is possible to diagnose the significance of renal artery stenosis and whether it is the cause of renovascular hypertension. The ACE inhibitor prevents the conversion of angiotensin I to angiotensin II, effectively stopping the vasoconstriction of the arterioles. If renal scintigraphy exams are performed pre and post administration of an ACE inhibitor it can be determined whether a patient' s hypertension is renin dependent. ACE inhibited scintigraphy is not intended to be used as a screening process for hypertension because it is not cost effective and it would lead to unnecessary exposure to radiation of patients. This exam should only be used on patients with a moderate to high probability of renovascular hypertension.

Renal Transplant Evaluation

Renal scintigraphy is an important tool for both post and pre transplant evaluation. In the clinical workup before transplant, renal scintigraphy can be used to query renal function and the overall suitability of the donor. After transplant medical and surgical complications are possibilities; renal scintigraphy has been used extensively because it is easily to perform and repeat and it can detect a number of these complications. Potential medical complications are: Acute tubular necrosis (ATN), hyperacute rejection, acute rejection and chronic rejection. Surgical complications are: Arterial stenosis, urinary leak and ureteral obstruction.

Figure 1.5 Clinical summary of Tc-99m MAG-3 renal scan of a potential kidney donor. Good perfusion but both kidneys exhibit a delayed excretion phase and hence average function. A patient with two kidneys of only average function is unlikely to be a suitable donor.