Genitourinary Imaging Tests
Imaging tests are often used to evaluate patients with renal and urologic disorders.
Abdominal x-rays 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 x-rays are less sensitive than CT and lack anatomic detail, so they are not the study of choice.
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).
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IVU has been largely superseded by rapid multidimensional CT and MRI with or without a contrast agent.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.
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 GU tract and include bleeding, infection, injury to the lungs or colon, hematuria, pain, and prolonged 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 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 renal insufficiency).
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 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:
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). No patient preparation is necessary. Adverse effects include UTIs and urosepsis.
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 stent placement. 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.
Ultrasonography can provide useful images of many GU 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
Urethra: To measure length and caliber of urethral strictures
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 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 with modern imaging techniques and protocols, this distinction can be made. 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 (see Computed Tomography).
Compared with CT, MRI is safer for patients at risk of contrast nephropathy, does not expose patients to ionizing radiation, and provides superior soft-tissue detail (but images bones and calculi poorly). Magnetic resonance imaging is contraindicated in patients with ferromagnetic metal (ie, containing iron) implants and magnetically activated or electronically controlled devices (eg, cardiac pacemakers). See also the MRI safety web site.
Also, due to the risk of nephrogenic systemic fibrosis, MRI with gadolinium contrast is contraindicated in patients with GFR < 30 mL/min.
The most common urologic application of MRI is evaluation of renal cysts and small renal masses. Multiparametric MRI is now the imaging study of choice for staging prostate cancer and for serial imaging of patients with prostate cancer on active surveillance. MRI is also helpful in imaging blood vessels (eg, for renal artery stenosis and renal vein thrombosis), and its use is increasing as MRI becomes more 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.
Newly available nuclear PET agents can detect metastatic prostate cancer.