Imaging tests are often used to evaluate patients with renal and urologic disorders.
Plain X-Rays Without Contrast
Abdominal x-rays without radiopaque contrast agents are virtually useless in evaluation of renal and urologic disorders. These x-rays are not sensitive, showing only about 50 to 60% of renal calculi (Ca oxalate calculi and rarely staghorn calculi); calcifications consistent with calculi are also nonspecific.
X-Rays With Use of Contrast
Images taken after administration of water-soluble contrast agents highlight the kidneys and urinary collecting system. Nonionic iso-osmolal agents (eg, iohexol, iopamidol) are now widely used; they have fewer adverse effects than older hyperosmolal agents but still pose a risk of acute renal injury (contrast nephropathy—see Principles of Radiologic Imaging: Contrast nephropathy).
In urography, an x-ray is taken after IV, percutaneous antegrade or retrograde, or cystoscopic retrograde administration of a radiopaque contrast agent. Primary contraindications for all patients are iodine allergy and risk factors for contrast nephropathy.
IVU (IV urography or pyelography):
IVU has been largely superseded by rapid multidimensional CT and MRI with or without a contrast agent. When IVU is done, abdominal compression may improve visualization of the renal pelvis and proximal ureters (with application) and distal ureters (after release). Additional x-rays at 12 and 24 h after contrast administration may be indicated for detection of postrenal obstruction or hydronephrosis.
Percutaneous anterograde urography:
For percutaneous anterograde urography, a radiopaque contrast agent is introduced through an existing nephrostomy tube or, less commonly, through percutaneous puncture of the renal pelvis guided by fluoroscopy. Occasionally, a ureterostomy or an ileal conduit can be used. Anterograde urography is used in the following circumstances:
Complications relate to puncture and placement of the catheter in the GU tract and include bleeding, sepsis, injury to adjacent organs, microscopic hematuria, pain, and urinary extravasation.
Retrograde urography uses cystoscopy and ureteral catheterization to introduce a radiopaque contrast agent directly into the ureters and renal collecting system. Sedation or general anesthesia is required. This technique is used when CT or MRI is required (eg, to identify the exact location or cause of obstruction) but is unsuccessful.
It is also useful for detailed examination of the pelvicaliceal collecting system, ureters (eg, to check for ureterovaginal fistulas), and bladder. However, overdistention and backflow may distort calyces and obscure details. Risk of infection is higher than that with other types of urography. Acute ureteral edema and secondary stricture formation are rare complications.
For cystourethrography, the radiopaque contrast agent is introduced directly into the urethra and bladder. This technique provides more details than other imaging studies for evaluation of the following:
Voiding cystourethrograms are taken during urination and are used to identify posterior urethral valves. No patient preparation is necessary. Adverse effects include UTIs and urosepsis. Severe urethral stricture is a relative contraindication.
Conventional catheter angiography has been largely replaced by noninvasive vascular imaging (eg, magnetic resonance angiography, CT angiography, ultrasonography, radionuclide scanning). Remaining indications include renal vein renin imaging, and, among patients with renal artery stenosis, angioplasty and stenting. Arteriography is also rarely used for evaluation and treatment of renal hemorrhage and before kidney-sparing surgery. Digital subtraction angiography is no longer used when rapid-sequence multidimensional CT or helical (spiral) CT is available.
Doppler ultrasonography is widely used to image the renal arteries, kidneys, bladder, prostate, testes, and penis. The test is safe but provides no information about renal function, and renal images may be difficult to obtain in overweight or obese patients. Also, there is no means to improve distinction between types of tissues, and image quality is operator-dependent. No patient preparation is necessary, but a full bladder facilitates its imaging. Ultrasonography can show urine volume after micturition (postvoiding residual). Doppler ultrasonography in patients with testicular pain helps distinguish torsion from other causes by assessing testicular blood flow.
CT provides a broad view of the urinary tract and surrounding structures. Conventional or helical scanners are used for most purposes with or without IV contrast agents. Use of contrast agents with either technique resembles IVU but provides additional detail. Helical CT without contrast agents is the study of choice for imaging of calculi. Radiopaque contrast agents are also best avoided for CT evaluation of trauma and other disorders that may involve acute hemorrhage (which appears bright white and can be confused with contrast agents) or urine extravasation. CT angiography is a less invasive alternative to conventional angiography (see Principles of Radiologic Imaging: Computed Tomography).
Magnetic Resonance Imaging
MRI is safer than CT for patients at risk of contrast nephropathy and exposes patients to no ionizing radiation. Uses include all of the following:
Magnetic resonance angiography, used to enhance images of blood vessels, has virtually replaced conventional angiography for evaluating renal artery stenosis and renal vein thrombosis in patients with normal renal function. However, nephrogenic systemic fibrosis is a risk from gadolinium-containing contrast agents, particularly when GFR is < 30 mL/min/1.73 m2 BSA. MRI defines intrarenal calcifications poorly because they have few mobile protons. MRI with IV lymphotropic superparamagnetic nanoparticles (eg, monocrystalline iron oxide) can identify lymph node metastases in prostate cancer but is not widely available.
Cortical tracers that bind to proximal tubular cells (eg, technetium-99m dimercaptosuccinic acid [99mTc DMSA]) are used to image the renal parenchyma. Excretory tracers that are rapidly filtered and excreted into urine (eg, iodine-125 iothalmate, 99mTc diethylenetriamine pentaacetic acid [DTPA], 99mTc mercaptoacetyltriglycine-3 [MAG3]) are used to assess GFR and overall renal perfusion. Radionuclide scanning can be used to evaluate renal function when use of IV contrast is undesirable. Radionuclide scanning also provides more information than does IVU or cross-sectional imaging about the following:
99mTc pertechnetate can be used to image blood flow to the testes and to distinguish torsion from epididymitis in patients with acute testicular pain, although Doppler ultrasonography is used more commonly because it is quicker. No patient preparation is necessary for radionuclide scanning, but patients should be asked about known allergies to the tracer.
Last full review/revision September 2009 by Seyed-Ali Sadjadi, MD
Content last modified April 2012