Approach to Urinary Tract Imaging



Approach to Urinary Tract Imaging



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Introduction


The majority of nuclear imaging of the urinary tract focuses on the kidney, designed to assess 1 or more elements of renal blood flow, structure, function, and collecting system drainage. Commonly used radiopharmaceuticals used to achieve this include:




The other commonly performed nuclear imaging study of the urinary tract is radionuclide cystography. Nuclear cystograms are generally performed in children and are designed to evaluate for vesicoureteral reflux (VUR) that might predispose to pyelonephritis and renal scarring.


Most scintigraphic imaging of the native kidneys is accomplished with the camera head positioned posterior to the patient.



Renal Cortex


Renal cortical scanning detects parenchymal defects related to scarring or acute pyelonephritis. Radiopharmaceuticals employed for renal cortical scanning (Tc-99m DMSA, glucoheptonate) concentrate in the renal cortex by binding the proximal convoluted tubules. Pinhole and SPECT imaging allow detection of small parenchymal defects. Both acute pyelonephritis and renal scarring can appear as focal or multifocal, wedge-shaped defects. Extensive scarring may appear as a global decrease in renal size.


Renal cortical scanning has largely been used as a secondary assessment for renal parenchymal damage in the work-up of VUR, which was generally evaluated with a “bottom up” approach aimed at identifying the presence of VUR. More recently, some have been advocating for a “top down” approach in which DMSA is the primary study, arguing that prevention of renal parenchymal injury is the primary aim of VUR therapy.



Renal Obstruction


The assessment of renal obstruction focuses on distinguishing the dilated, nonobstructed collecting system from the obstructed collecting system. Tc-99m MAG3 is the primary radiopharmaceutical used for assessment of renal collecting system drainage. Most examinations aimed at assessing collecting system drainage consist of a set of baseline images in which the radiopharmaceutical is administered, and serial images are obtained as it is extracted from the blood pool, passes through the renal parenchyma, and appears in the collecting system. Split renal function (based on differential perfusion) can also be derived from this baseline set of images. Subsequently, a diuretic is administered, and further serial images are obtained as the collecting system progressively fills and drains. Clearance curves are critical to the interpretation of both phases of the exam with clearance t1/2 derived from the postdiuretic curves used to quantify the efficacy of renal collecting system drainage.



Renal Transplant


There are multiple processes that can compromise transplant function, including acute tubular necrosis (ATN), rejection, drug toxicity, collecting system obstruction, and vascular anastomotic narrowing. Renal scintigraphy allows noninvasive assessment of renal transplant function and can be used to assess perfusion, parenchymal function, and collecting system drainage. Renal scintigraphy can also be helpful in determining the etiology of peritransplant fluid collections.


Tc-99m MAG3 is the most commonly employed radiopharmaceutical in the evaluation of renal transplants, though DMSA may be employed when assessment of renal parenchyma is needed. As with native collecting system obstruction, diuretics are often used when collecting system obstruction is suspected.


Generally, planar imaging is sufficient for the evaluation of a renal transplant. Unlike imaging of the native kidneys, the camera head should be positioned anteriorly, or dual-head imaging should be performed.


SPECT and SPECT/CT may contribute when renal cortical imaging is performed or in the assessment of peritransplant fluid collections.

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May 7, 2023 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Approach to Urinary Tract Imaging
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