Imaging tests are often used to evaluate patients with renal and urologic disorders.
Plain Radiographs Without Contrast
Abdominal radiographs without radiopaque contrast agents may be done to check for positioning of ureteral stents or to monitor position and growth of kidney stones. However, for initial diagnosis of urolithiasis, plain radiographs are less sensitive and less specific than CT and lack anatomic detail, so they are not the study of choice.
Radiographs 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 widely used; they have fewer adverse effects than older hyperosmolal agents but still pose a risk of acute kidney injury (Images taken after administration of water-soluble contrast agents highlight the kidneys and urinary collecting system. Nonionic iso-osmolal agents (eg, iohexol, iopamidol) are widely used; they have fewer adverse effects than older hyperosmolal agents but still pose a risk of acute kidney injury (contrast nephropathy).
In urography, a radiograph 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.
IV urography (IVU)
IVU (sometimes called IVP, for intravenous pyelography) has been largely superseded by rapid multidimensional CT and MRI with or without a contrast agent. When IVU is performed, abdominal radiographs are specifically timed to capture radiopaque contrast agents (administered intravenously) as they pass through the kidneys, ureter, and bladder. Abdominal compression may improve visualization of the renal pelvis and proximal ureters (with application) and distal ureters (after release). Additional radiographs at 12 and 24 hours after contrast administration may be indicated for detection of postrenal obstruction or hydronephrosis.
Percutaneous antegrade urography
For percutaneous antegrade 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. Antegrade urography is used in the following circumstances:
When retrograde urography is unsuccessful (eg, because of tumor obstruction at the bladder level)
When large kidney calculi requiring percutaneous surgery must be evaluated
When transitional cell carcinoma of the upper collecting system is suspected
When patients cannot tolerate general anesthesia or the degree of sedation required for retrograde urography
Complications relate to puncture and placement of the catheter in the genitourinary tract and include bleeding, infection, injury to the lungs or colon, hematuria, pain, and prolonged urinary extravasation.
Retrograde urography
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 can be used when CT and MRI with IV contrast agents are contraindicated (eg, in chronic kidney disease) or unavailable or when results are equivocal (eg, in end-stage renal disease).
It is also useful for detailed examination of the pelvicaliceal collecting system and ureters to check for injury, stricture, or fistula. Overdistention and backflow from a kidney into the venous system creates back pressure that may distort calyces and obscure details. Risk of infection is higher than with other types of urography. Acute ureteral edema and secondary stricture formation are rare complications.
Cystography and cystourethrography
In cystography and cystourethrography, a radiopaque contrast agent is introduced directly into the urethra and bladder through a catheter. In cystography, full and post-void images are obtained and provide information about structural abnormalities in the bladder and bladder leaks. Cystourethrography evaluates the bladder and the urethra and provides more details than other imaging studies for evaluation of the following:
Recurrent urinary tract infections (UTIs)
Suspected urethral or bladder trauma (eg, perforation)
Voiding cystourethrography is done during urination and is primarily used to image the posterior urethra (eg, for strictures or valves). Adverse effects include UTIs and urosepsis.
Angiography
Conventional catheter angiography has been largely replaced by noninvasive vascular imaging (eg, magnetic resonance angiography, CT angiography, ultrasound, radionuclide scanning). Remaining indications include renal vein renin imaging and, among patients with renal artery stenosis, angioplasty and stent placement. Arteriography is also rarely used for evaluation and treatment of renal hemorrhage and before kidney-sparing surgery. Digital subtraction angiography is not used when rapid-sequence multidimensional CT or helical (spiral) CT is available.
Ultrasound
Ultrasound can provide useful images of many genitourinary structures without exposing patients to ionizing radiation. Images are interpreted as they are acquired, so the technician can focus on concerning areas and obtain additional information if necessary. Its main disadvantages are the need for a skilled operator and the time required. A full bladder helps provide better images of certain structures, but no other preparation is needed.
Structures that can be imaged and common indications include the following:
Kidneys: For hydronephrosis, stones, and tumors
Bladder: For bladder volumes (eg, postvoid volume, assessed immediately after voiding; in suspected urinary retention due to bladder outlet obstruction), diverticula, and stones
Scrotum: For hydroceles, spermatoceles, testicular tumors, varicoceles, and (with Doppler blood flow measurement) for testicular torsion
Prostate: To measure prostate volume (eg, to help assess benign prostatic hyperplasia or interpret prostate-specific antigen results) and to guide needle biopsy
Penis: To help evaluate Peyronie disease; with Doppler, to assess blood flow (when evaluating erectile dysfunction)
Urethra: To measure length and caliber of urethral strictures
In patients with acute testicular pain, Doppler ultrasound is used more commonly than radionuclide scanning because it is faster.
Computed Tomography
Computed tomography 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 IV urography (IVU) but provides additional detail. Previously, in trauma patients, there was concern that use of contrast would make it difficult to distinguish abdominal hemorrhage from urinary tract disruption, but this distinction can be made with currently available imaging techniques and protocols. Helical CT without a contrast agent is the study of choice for imaging of calculi; dual-energy scanners may provide additional information that can help determine stone composition.
The main disadvantage of CT is that it exposes patients to a relatively large dose of ionizing radiation.
CT angiography is a less invasive alternative to conventional angiography.
Magnetic Resonance Imaging
Compared with CT, magnetic resonance imaging (MRI) procedures are safer for patients at risk of contrast nephropathy, do not expose patients to ionizing radiation, and provide superior soft-tissue detail (but images bones and calculi poorly). MRI is contraindicated in patients with ferromagnetic metal (ie, containing iron) implants and some magnetically activated or electronically controlled devices (eg, cardiac pacemakers). Although some newer devices are considered "MRI-conditional" and allow imaging only under specific conditions, caution is advised. See also the MRI safety web site.
Magnetic resonance imaging with gadolinium contrast is contraindicated in patients with glomerular filtration rate (GFR) < 30 mL/minute due to the risk of nephrogenic systemic fibrosis, a rare but life-threatening disorder that involves fibrosis of the skin, blood vessels, and internal organs.
Urologic application of MRI includes the evaluation of renal cysts and small renal masses. MR urography (MRU) utilizes a specialized protocol to image the collecting system. MR angiography is helpful in imaging blood vessels (eg, for renal artery stenosis and renal vein thrombosis). an
Multiparametric MRI is the imaging study of choice for staging prostate cancer and for serial imaging of patients with prostate cancer on active surveillance. A multiparametric MRI examination includes paramagnetic contrast and consists of 3 separate imaging techniques (parameters): T2-weighted imaging, diffusion-weighted imaging, and dynamic contrast-enhanced imaging. A technique known as MRI fusion biopsy is sometimes used during a prostate biopsy. An MRI of the prostate is obtained prior to ultrasound-guided prostate biopsy, during which the MRI image is digitally combined (fused) with the ultrasound image to better identify lesions that need to be biopsied.
Radionuclide Scanning
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 glomerular filtration rate (GFR) and overall renal perfusion and function. It also has the benefit of not requiring IV contrast and may be used safely in patients at risk for contrast nephropathy. Radionuclide scanning also provides more information than does IV urography or cross-sectional imaging about the following:
Segmental renal emboli
Renal parenchymal scarring due to vesicoureteral reflux
Functional significance of renal artery stenosis
Kidney function in living donors before transplantation
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 ultrasound 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.
Nuclear positron emission tomographic (PET) agents can detect metastatic prostate cancer. Routine FDG PET scans are not very useful for most cases of prostate cancer but can be useful for other genitourinary tumors such as kidney or testicular cancer. PET imaging includes targeting PSMA (prostate-specific membrane antigen) on the surface of prostate cancer cells. These PSMA PET scans can often detect metastatic lesions at very low prostate-specific antigen (PSA) levels, including those that recur locally after surgery, as well as those in lymph nodes, bone, and other metastatic sites.
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