Radiology of the Urinary Tract: Technique Selection

TECHNIQUE SELECTION

No one ideal technique is yet available for the comprehensive evaluation of the urinary tract. Each technique has strengths and weaknesses that affect its thoroughness and accuracy in evaluating urinary tract diseases, and also patient com-plaints. Importantly, imaging techniques are not necessarily exclusive and in some circumstances are complementary— taken alone, they may not provide enough information, but together they allow a correct clinical diagnosis. A knowledge of which tests is most appropriate for a given clinical ques-tion is paramount for physicians involved with the treatment of urinary tract disease. Issues of cost, complications, and time are consequences of an injudicious study choice. However, most importantly, the diagnosis of a patient’s condition may not be made unless the appropriate test has been used to eval-uate the condition. This section discusses technique selection.

The plain radiograph (KUB) has fairly limited use for evaluating the genitourinary (GU) tract. Although abnormal “stones, bones, gas, and masses” may be demonstrated by the KUB, the utility of the study is limited by its lack of sensitiv-ity and specificity. The KUB can be used effectively to follow radiographically visible stone disease such as assessing stoneburden or ureteral stone passage; it is also used to assess stent position, especially with ureteral stents.

The conventional radiographic techniques that utilize di-rect contrast injection (retrograde pyelogram, cystogram, and urethrogram) have specific roles. Two particular studies of choice include the voiding cystogram for evaluating ureteral reflux and the retrograde male urethrogram in sus-pected urethral injuries and stricture evaluation.

Nuclear medicine sustains its role in functional evalua-tion of the urinary tract, especially the kidney. Renal scintig-raphy is an important tool in the assessment of renal function and can be useful in evaluating renovascular hyper-tension. The ability to quantitatively assess relative renal function is frequently an important issue for the surgeon, de-termining whether nephrectomy or attempted renal sparing surgery is most appropriate. The nuclear medicine cys-togram, because of its high sensitivity and lower radiation dose, remains a key tool in evaluating and following ureteral reflux, particularly in the young. The MIBG study plays a unique role regarding the pheochromocytoma. The diagno-sis of pheochromocytoma is generally made with a classic clinical history combined with confirmatory biochemical evidence, along with confirmatory imaging, typically CT or MRI demonstrating an adrenal mass. MIBG is most often used in detecting metastatic or recurrent disease, or locating extraadrenal lesions. Although PET/CT is limited in primary evaluation of the urinary tract, it has shown strong utility in evaluating metastatic disease; however, its exact role is yet to be determined.

The safety and availability of ultrasound solidify the utility of this modality. Ultrasound is useful in evaluating the kidney for masses, scarring, and hydronephrosis. Several common in-dications include evaluation of the patient with acute renal fail-ure to exclude postobstructive (hydronephrosis) etiologies, to evaluate for sequelae (scarring) of vesicoureteral reflux in chil-dren, and to diagnose simple renal cysts. Ultrasound is generally the study of choice in evaluating the renal transplant as well. However, the relatively small and sometimes technically limited (in large patients) field of view, lack of visualization of the ureters, and lack of functional assessment limit the use of ultra-sound in some circumstances. For instance, in the setting of ob-struction, ultrasound may demonstrate hydronephrosis, but often the etiology of the obstruction, such as ureteral stone or mass, is not identified. Additionally, solid renal masses are non-specific on ultrasound and require further imaging, usually with CT. Ultrasound has only moderate sensitivity for detecting renal stone disease. Although the retroperitoneal position of the kidneys usually provides an excellent window for ultrasound, patients with a large habitus continue to become more com-mon, and in these patients sensitivity for small masses or calculi may be markedly diminished.

The diagnostic role of conventional angiography has been nearly eliminated with the use of noninvasive CT and MR angiography. However, two main factors allow for a persistent important role for the angiogram. As described in the tech-nique section, conventional angiography still has superior resolution compared to CT and MRI for small-vessel evalua-tion. Thus, diagnostic angiography may play a role in diagno-sis of small-vessel renal disease such as polyarteritis nodosa. More importantly, catheter angiography allows for the ability to simultaneously treat abnormalities diagnosed at the time of angiography. For example, although many modalities are used to evaluate for renovascular hypertension, angiography alone allows for treatment at the time of diagnosis as in the pa-tient with fibromuscular dysplasia whose hypertension may be cured with transluminal angioplasty at the time of arteriogra-phy. The angiogram is similarly used in acute renal hemor-rhage, acute arterial obstruction, and occasional renal mass management.

MR imaging continues to grow in utility for evaluating the urinary tract and is the study of choice in certain instances. Like CT, MR imaging has excellent spatial and contrast reso-lution and can evaluate the renal vasculature and renal and adrenal anatomy, characterize lesions, and evaluate the blad-der and prostate. Fluid/contrast enhancing techniques allow evaluation of the ureters and the remainder of the collecting system. MR imaging is thus an excellent choice to screen for renovascular hypertension and to problem solve difficult renal masses, and it is gaining favor in evaluation of certain ureteral and bladder conditions. However, high cost, limited availability, and contraindications such as pacemakers and claustrophobia limit the use of routine genitourinary MR im-aging. In addition, the advent of NSF as a clinical entity has limited the use of contrast-enhanced MRI in patients with ad-vanced chronic renal disease, a population that previously was specifically indicated for MR to avoid iodinated contrast.

Finally, CT is now the examination of choice for urinary tract imaging. The CT scan is the preferred study for many GU conditions and indications including trauma, flank pain, complicated infections, renal and adrenal masses, neoplastic conditions, retroperitoneal disease, and more. CT may be the sole study needed or may serve as an adjunct to other studies. CT urography has become the first-line procedure for evalu-ation of the urothelium, including hematuria evaluation, re-placing the last indications for the IVP. As discussed previously, CT benefits from wide availability, speed, high contrast and spatial resolution, and patient ease. CT is limited when iodi-nated contrast is contraindicated or radiation exposure is of special concern, such as in pregnancy.

In summary, there is as yet no one comprehensive best imaging examination for the urinary tract; each has its advantages and disadvantages, and their value depends on indications of the study. The physician must combine evidence-based knowledge of the accuracy and utility of various stud-ies with the art of medicine, combining science with finesse to ultimately result in the best possible evaluation and care of the individual patient. Finally, although the requesting physi-cian should be well informed about the utility, accuracy, strengths, and weaknesses of available tests, the best care, espe-cially in the fast-changing field of imaging, is provided by close consultation between the referring physician and radiologist.