Chapter 6. Radiology of the Urinary Tract

The pace of innovation in diagnostic radiology has increased exponentially, in tandem with computer advances and the rapid evolution of microprocessing power. Imaging of the urinary tract, as a result, has become more flexible and precise, with new procedures offering a great selection of options, and new imaging algorithms being implemented. Ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) provide higher soft-tissue contrast resolution than conventional radiography, as well as multiplanar imaging capability, resulting in significant advances in almost all areas of uroradiology. In academic centers, metabolic and molecular imaging techniques have become the focus of new research, and have begun to enter the realm of daily clinical practice. While imaging advances have produced new algorithms for approaching diagnostic evaluation, appropriate use of imaging in each particular case also depends greatly on the equipment and professional talent available. One imaging modality or protocol may offer specific advantages over another depending on the clinical question, and the importance of a collaborative approach from the medical team cannot be overemphasized.

In summary, ever changing uroradiology remains indispensable in the diagnosis and treatment of patients with urologic disorders. This chapter will discuss the imaging techniques used in uroradiology, with summaries of the advantages and disadvantages of the various techniques, and will end with a brief discussion comparing imaging methods.

X-rays are electromagnetic waves with photon energies that typically fall between those of gamma rays and ultraviolet radiation. Radiography is possible because tissues differ in their ability to absorb x-rays. A radiopaque contrast medium is frequently employed to enhance soft-tissue contrast.

Although newer imaging techniques have largely replaced conventional radiography for diagnosis of many urologic problems, general radiography remains useful for some urologic disorders; therefore, the urologist should be familiar with x-ray equipment and uroradiologic techniques. The basic types of uroradiologic studies are plain (conventional) abdominal films, (also known as KUB, which stands for kidney, ureter, bladder) intravenous urograms (IVUs), cystourethrograms, urethrograms, and angiograms. These studies are described separately in sections that follow.

Basic Equipment and Techniques

Radiography Fluoroscopy

Many conventional x-ray units contain both radiographic and fluoroscopic capabilities. These require a high voltage power supply, an x-ray tube, a collimating device, and an x-ray detector or film. Fluoroscopic units also use an electronic image intensifier and an image display system. Today, more radiology departments have become completely “filmless” as digital recording, displaying, and archiving of images are replacing film-based techniques.

Image Intensification

Image intensifiers, coupled to video cameras, electronically augment the ordinary dim fluoroscopic image.

Image Recording

Conventional recording of an x-ray image uses film and intensifying screens. The image intensifier and camera can be used to capture dynamic and static images. Real-time images are now typically recorded using conventional or digital video. Conventional spot or cine images may be acquired on x-ray film or digitally recorded.

Contrast Media

Radiographic contrast media used in uroradiography are water-soluble iodinated compounds that are radiopaque. Similar compounds are used for basic radiographic techniques and CT, though iodine concentrations will differ depending on preference and route of administration. In general, intravenous administration for CT or IVU is performed with iodine 200 mg/lb body weight in adults, and direct instillation to the collecting system or bladders uses similar media diluted to 15–45% concentration. The extracellular distribution of these agents results in improved contrast resolution and conspicuity of various structures.

Significant advances in water-soluble contrast media occurred with the introduction of low-osmolality, nonionic organic iodine-containing compounds. These nonionic agents significantly improve patient tolerance and decrease the incidence of adverse reactions, and at many institutions the use of nonionic agents is standard. Whether they reduce the mortality associated with the use of contrast media has not been proven. The major obstacle to their use is higher cost.

Adverse Reactions

All procedures using intravascular contrast media carry a small but definite risk of adverse reactions. The overall incidence of adverse reactions is about 5%. Reactions in nonintravenous use (ie, cystograms) are extremely rare but have been reported.

Most reactions are minor, for example, nausea, vomiting, hives, rash, or flushing, and usually require only reassurance. Cardiopulmonary and anaphylactoid reactions can occur with little warning and can be life threatening or fatal. In a large meta-analysis, the incidence of death due to intravascular injection of contrast media was 0.9 deaths/100,000 injections. There are no reliable methods for pretesting patients for possible adverse reactions. The risks and benefits of contrast use should be carefully evaluated for each patient before the procedure is initiated. Treatment of adverse reactions involves the use of antihistamines, epinephrine, vascular volume expanders, bronchodilators, and other cardiopulmonary drugs as well as ancillary procedures indicated by the nature and severity of the reaction. In some cases a radiographic examination using intravascular contrast media is critical even if the patient has had a prior moderate or severe reaction. Such patients are given nonionic contrast agents and pretreated with corticosteroids, sometimes in combination with antihistamines, in an effort to prevent recurrence. This preventive treatment is not always successful, so any decision to administer contrast under these circumstances should be carefully weighed against the risks.

Nephrotoxicity caused by intravascular contrast agents is another concern. The pathogenesis of contrast nephropathy (CN) likely involves medullary ischemia due to contrast-induced vasoconstriction and direct tubular injury. Patients at higher risk are those with preexisting renal insufficiency, diabetes, dehydration, or patients who receive higher volumes of contrast material. Alternative procedures can be selected in high-risk patients. If contrast use is deemed necessary in a high-risk individual, CN can be minimized through attention to proper hydration, discontinuation of drugs that may exacerbate toxic effects, adequate hydration in the 24 hours prior to scanning, reduction of contrast volume, and possibly administration of oral N-acetylcysteine.

Advantages and Disadvantages

Radiography produces anatomic images of almost any body part. Costs are moderate compared with cross-sectional imaging systems. Space requirements are modest, and portable equipment is available for use in hospital wards, operating rooms, and intensive care units. Because there are a great many specialists trained in radiography, its use is not confined to large medical centers. The major disadvantage of radiographic imaging is the use of ionizing radiation and relatively poor soft-tissue contrast. The evaluation of the urinary tract almost always requires opacification by iodine contrast media.

Plain Film of the Abdomen (Figures 6–1, 6–2, and 6–3)

A plain film of the abdomen, frequently called a KUB film, is the simplest uroradiologic examination. It is generally the preliminary radiograph in extended radiologic examinations, such as intravenous urography, and is usually taken with the patient supine. It may demonstrate osseous abnormalities, abnormal calcifications, or large soft-tissue masses.

Kidney outlines usually can be seen on the plain film, so that their size, number, shape, and position can be assessed. The size of normal adult kidneys varies widely. The long diameter (the length) of the kidney is the most widely used and most convenient radiographic measurement. The average adult kidney is about 12–14 cm long. In children older than 2 years of age, the length of a normal kidney is approximately equal to the distance from the top of the first to the bottom of the fourth lumbar vertebral body. Patterns of calcification in the urinary tract (Figures 6–1 and 6–2) may help to identify specific diseases.

Urography (Figures 6–4, 6–5, 6–6, 6–7, and 6–8)

The collecting structures of the kidneys, ureters, and bladder can be demonstrated radiologically with contrast media by the following methods:

Intravenous Urography

The IVU, also known as excretory urography (EU) (Figure 6–4), or intravenous pyelography (IVP), can demonstrate a wide variety of urinary tract lesions (Figures 6–4 and 6–5), is simple to perform, and is well tolerated by most patients.

CT, sonography and MRI have replaced urography in the vast majority of cases. Nevertheless, urography is occasionally used and is useful for demonstrating small lesions in the urinary tract (eg, papillary necrosis, medullary sponge kidney, uroepithelial tumors, pyeloureteritis cystica).

Patient Preparation

At one time dehydration was advocated as optimal preparation for intravenous urography (EU). This is no longer required. Furthermore, dehydration is to be avoided in infants, debilitated and elderly patients, and patients with diabetes mellitus, renal failure, multiple myeloma, or hyperuricemia.

It is controversial whether preliminary bowel cleansing is beneficial. The choice may be made according to individual preference.

Standard Technique

Following a preliminary plain film of the abdomen, additional radiographs are taken at timed intervals after the intravenous injection of iodine-containing contrast medium. Normal kidneys promptly excrete contrast agents, almost entirely by glomerular filtration.

The volume and speed of injection of the contrast medium, as well as the number and type of films taken, vary by preference, patient tolerance, and the particular clinical scenario.

Technique Modifications

Radiographic tomography, x-ray imaging of a selected plane in the body, permits recognition of kidney structures that otherwise are obscured on standard radiograms by extrarenal shadows, for example, those due to bone or feces (Figure 6–6). Image-intensified fluoroscopy permits study of urinary tract dynamics. “Immediate” films, which are taken immediately after the rapid (bolus) injection of contrast, typically show a dense nephrogram and permit better visualization of renal outlines. Abdominal (ureteral) compression devices temporarily obstruct the upper urinary tract during EU and improve the filling of renal collecting structures. “Delayed” films, taken hours later or on the following day, can contribute useful information. “Upright” films, taken with the patient standing or partially erect, reveal the degree of mobility and drainage of the kidneys and, if taken immediately after the patient has voided (“postvoiding” film), show any residual urine in the bladder.

Retrograde Urograms

Retrograde urography is a minimally invasive procedure that requires cystoscopy and the placement of catheters in the ureters. A radiopaque contrast medium is introduced into the ureters or renal collecting structures through the ureteral catheters (Figures 6–7 and 6–8), and radiographs of the abdomen are taken. This study must be performed by a urologist or experienced interventional uroradiologist. Some type of local or general anesthesia should be used, and the procedure occasionally causes later morbidity or urinary tract infection.

Retrograde urograms may be necessary if excretory urograms or CT urogram (CTU) are unsatisfactory, if the patient has a history of adverse reaction to intravenous contrast media, or if other methods of imaging are unavailable or inappropriate.

Percutaneous Urograms

Outlining the renal collecting structures and ureters by percutaneous catheter is occasionally done when excretory or retrograde urography has failed or is contraindicated, or when there is a nephrostomy tube in place and delineation of the collecting system is desired. For antegrade studies, contrast medium is introduced either through nephrostomy tubes (nephrostogram) or by direct injection into the renal collecting structures via a percutaneous puncture through the patient's back. Percutaneous retrograde urograms of the upper urinary tract are made by retrograde injection of contrast medium through the opening of a skin ureterostomy or pyelostomy (skin ureterogram, skin urogram) or through the ostium of an interposed conduit, usually a segment of small bowel (loopogram).

Cystography, Voiding, Cystourethrography, and Urodynamics (Figures 6–9, 6–10, 6–11, and 6–12)

Direct instillation of contrast media into the urinary bladder (cystography) allows a more focused examination of the bladder. Contrast is usually instilled via a transurethral catheter, but when necessary can be administered via percutaneous suprapubic bladder puncture. For urodynamic studies, pressure transducers are used within the bladder lumen and rectum for dynamic measurement of intraluminal and intra-abdominal pressures, respectively. Radiographs can be taken using standard overhead x-rays, or during fluoroscopy. Voiding cystourethrograms are radiographs of the bladder and urethra obtained during micturition. Cystography and cystourethrography are important radiologic techniques for detecting vesicoureteral reflux and may be used in the workup of patients with urinary stress incontinence. CT cystography (CT of the pelvis after the instillation of dilute contrast medium into the bladder) has been shown useful in the evaluation of traumatic bladder rupture.

Urethrography (Figures 6–13, 6–14, and 6–15)

The urethra can be imaged radiographically by retrograde injection of radiopaque fluid or in antegrade fashion with voiding cystourethrography, or with voiding following EU. The antegrade technique is required when lesions of the posterior urethra, for example, posterior urethral valves, are suspected; the retrograde technique is more useful for examining the anterior (penile) urethra. The urethra may also be evaluated by tailored MRI examinations using thin-section imaging with a small field of view. Urethral tumor or diverticula, for example, may be readily demonstrated with MRI (Ryu and Kim, 2001).

Vasography (Figure 6–16)

Vasoseminal vesiculography is most often used in the investigation of male sterility. The radiopaque contrast medium is introduced into the ductal system by direct injection into an ejaculatory duct following panendoscopy or, more commonly, by injection into the vas deferens after it has been surgically exposed through a small incision in the scrotal neck.

Lymphangiography

Lymphangiography has been largely abandoned and replaced by CT and MRI.

Angiography

Nearly 50 years after Seldinger described techniques for percutaneous arteriography, catheter angiography maintains a limited role in treatment of some urologic disorders but is being replaced by CT or MRI for diagnostic examinations. Although an established imaging technique with proven value and an acceptable incidence of complications and morbidity, angiography is moderately invasive and relatively expensive.

Aortorenal and Selective Renal Arteriography (Figure 6–17)

Conventional arteriographic studies are performed almost exclusively by percutaneous needle puncture and catheterization of the common femoral arteries. Rapid sequence images are obtained during injection of nonionic contrast. Aortograms at the level of the renal vessels are followed by selective catheterization of renal arteries. CT and MR angiography involve peripheral injection of contrast media with breath hold rapid image acquisition through the targeted region of interest. CT angiography offers higher spatial resolution than magnetic resonance angiography (MRA), but carries the risks of radiation exposure and iodinated contrast usage.

Indications for renal arteriography include suspected renal artery stenosis (renovascular hypertension), vascular malformations, tumor embolization to minimize surgical blood loss or treat bleeding tumors, and trauma. Diagnostic renal angiography to demonstrate renal vascular anatomy is uncommon, as this information may be obtained noninvasively. Complications from conventional catheter angiography include bleeding at the puncture site, contrast allergy or nephrotoxicity, and renal or distal emboli.

Inferior Venacavography and Selective Venography (Figures 6–18 and 6–19)

The common femoral veins, or less commonly the internal jugular veins, are catheterized for angiography of the inferior vena cava, renal, and adrenal veins. Venography is rarely used today since the information can be obtained at cross-sectional imaging (CT or MRI) in almost all cases. Adrenal and renal venography is performed occasionally for venous sampling to localize hormone secretion in patients with indeterminate noninvasive imaging studies.

Miscellaneous Urologic Angiography

Although angiography has little or no value in examination of the ureter, bladder, adrenals, and prostate, angiograms of these structures may be indicated in particular clinical situations, in which case the studies are usually “tailored” to the clinical problem. In this era of multiple cross-sectional methods, these procedures are rarely used.

Although uncommon, corpus cavernosograms are made by direct injection of suitable contrast material into the corpora cavernosa of the penis. They can be useful in examining for Peyronie's disease or fibrosis, impotence, priapism, and traumatic penile lesions.

See Figures 6–20, 6–21, 6–22, 6–23, 6–24, 6–25, and 6–26

Basic Principles

Sound is the mechanical propagation of pressure changes, or waves, through a deformable medium. A wave frequency of 1 cycle/s (cps) is called a hertz (Hz). Sound frequencies greater than 20 kHz are beyond the range of human hearing and are called ultrasound. Medical sonography uses ultrasound to produce images. The frequencies commonly used in medical sonography are between 3.5 and 15 MHz.

Ultrasound waves for imaging are generated by transducers, devices that convert electrical energy to sound energy and vice versa. These transducers act as both sonic transmitters and detectors. Ultrasound images are reflection images formed when part of the sound that was emitted by the transducer bounces back from tissue interfaces to the transducer. The sound reflected by stationary tissues forms anatomic gray-scale images. The sound reflected by moving structures has an altered frequency due to the Doppler effect. By determining the Doppler shift, vascular flow direction and velocity can be encoded graphically (spectral Doppler) or by color (color Doppler). A more sensitive method of detecting flow, called power mode Doppler, is available on modern equipment. This technique displays the integrated power of the Doppler signal rather than the mean Doppler frequency shift. Direction or velocity of flow is not displayed in power mode. Newer intravascular microbubble contrast agents provide more exquisite imaging of vascularity, and their use will likely increase in coming years (Ascenti et al, 2007, Fan et al, 2008).

Ultrasound images are rapidly updated on a video display, giving an integrated cross-sectional anatomic depiction of the site studied. Individual frames may be frozen during an examination for motion-free analysis and recording, or cine images may be rapidly recorded as digital video.

Clinical Applications

Ultrasound is commonly used for the evaluation of the kidney, urinary bladder, prostate, testis, and penis.

Ultrasound is useful for assessing renal size and growth. It is also helpful in triaging patients with renal failure. For example, small echogenic kidneys suggest renal parenchymal (medical) disease, whereas a dilated pelvocaliceal system indicates an obstructive, and potentially reversible, cause of renal failure.

Renal ultrasound is useful in detection and characterization of renal masses. Ultrasound provides an effective method of distinguishing benign cortical cysts from potentially malignant solid renal lesions. Since the most common renal lesion is a simple cortical cyst, ultrasound is a cost-effective method to confirm this diagnosis. Ultra-sound may also be used to follow up mildly complicated cysts detected on CT, for example, hyperdense cysts or cysts with thin septations.

The differential diagnosis for echogenic renal masses includes renal stones, angiomyolipomas, renal cortical neoplasms (including carcinoma), and, less commonly, abscesses and hematomas. All echogenic renal masses should be correlated with clinical history and, if necessary, confirmed with another imaging modality or follow-up ultrasound. Thin-section CT showing fat within the renal lesion characterizes it as a benign angiomyolipoma, and no further investigation is required. Echogenic lesions smaller than 1 cm are more difficult to characterize by CT owing to partial volume averaging; in the correct clinical setting, follow-up ultrasound rather than repeat CT may be more useful.

Doppler sonography is useful for the evaluation of renal vessels, vascularity of renal masses, and complications following renal transplant. It can detect renal vein thrombosis, renal artery stenosis, and ureteral obstruction prior to the development of hydronephrosis, arteriovenous fistulas, and pseudoaneurysms. Perinephric fluid collections following renal transplantation, extracorporeal shockwave lithotripsy, or acute obstructions are reliably detected by ultrasound.

Developments in other imaging modalities have decreased the use of ultrasound in several clinical scenarios. Most patients with suspected renovascular hypertension are evaluated with CTA or MRA rather than Doppler ultrasonography. Unenhanced helical CT is now the initial procedure of choice for the evaluation of the patient with acute flank pain and suspected urolithiasis. Helical CT also has the potential for identifying other causes of flank pain such as appendicitis and diverticulitis. For the evaluation of hematuria, recent studies indicate that CT (or CTU) is the preferred modality. A balance between optimizing imaging quality and minimizing radiation exposure is advocated.

Applications of bladder sonography include assessment of bladder volume, wall thickness, and detection of bladder calculi and tumors. The suprapubic transabdominal approach is most commonly used. The transurethral approach during cystoscopy has been recommended for tumor detection and staging.

Ultrasound examination of the testis and scrotum is performed with a high-frequency transducer (10–15 MHz) for excellent spatial resolution. The addition of color and spectral Doppler sonography provides simultaneous display of morphology and blood flow. Sonography is highly accurate in differentiating intratesticular from extratesticular disease, and in the detection of intratesticular pathology. Ultrasound is commonly used to evaluate acute conditions of the scrotum. It can distinguish between inflammatory processes, inguinal hernias, and acute testicular torsion (Hart et al, 2008; Sparano et al, 2008). Epididymitis not responding to antibiotics within 2 weeks should be investigated further with scrotal ultrasonography.

Advantages and Disadvantages

The main advantages of ultrasound are ease of use, high patient tolerance, noninvasiveness, lack of ionizing radiation, low relative cost, and wide availability. Disadvantages include a relatively low signal-to-noise level, tissue nonspecificity, limited field of view, and dependence on the operator's skill and the patient's habitus.

See Figures 6–27, 6–28, 6–29, 6–30, 6–31, 6–32, and 6–33

Basic Principles

In CT scanning, a thin, collimated beam of x-rays is passed through the patient and captured by an array of solid-state or gas detectors.

The interconnected x-ray source and detector system are rapidly rotated in the gantry around the recumbent patient. Computers integrate the collected x-ray transmission data to reconstruct a cross-sectional image (tomogram).

Spiral (or helical) CT uses a slip-ring gantry that rotates continuously while the patient moves constantly through the collimated x-ray beam. Spiral CT technology affords the ability to image during specific phases of contrast bolus enhancement, including the ability to perform CT angiography, and allows improved image reformations. Multidetector, or multislice, helical CT scanners have an array of multiple rows of detectors in a helical scanner such that multiple scan images can be acquired per gantry rotation, and as a by-product thinner sections and higher resolution achieved. Such systems are optimally paired with powerful computer workstations so that high-quality three-dimensional (3D) and multiplanar reformations can be quickly generated and analyzed.

Clinical Applications

Renal CT is most commonly used in the evaluation of acute flank pain, hematuria, renal infection (search for abscess) or trauma, and in the characterization and staging of renal neoplasm. CT evaluation of renal anatomy and pathology generally requires intravenous injection of iodinated contrast media; noncontrast scans are needed, however, when renal or perirenal calcification, hemorrhage, or urine extravasation is suspected, since scans obtained after the administration of contrast media may mask these abnormalities. Also, pre- and postcontrast scans are required to determine whether a mass is solid or cystic.

Contrast media is usually administered as a rapid intravenous bolus for assessment of renal anatomy or measurement of aortorenal transit time. Using a bolus injection and rapid sequence scanning, renal arterial opacification is followed immediately by enhancement of the cortex. A nephrogram phase with medullary enhancement is reached within 60 seconds. Excretion of contrast material into the collecting structures can be expected within 2–3 minutes after initiation of contrast administration.

Although CT can detect ureteral tumors, the current role of CT in the evaluation of the ureters is predominantly for tumor staging and evaluation of the cause and level of obstruction. Helical CT without oral or intravenous contrast is the preferred imaging modality for patients with renal colic or suspected urolithiasis (Figure 6–32).

In the evaluation of the urinary bladder, CT is used primarily in staging bladder tumors and in diagnosing bladder rupture following trauma (Shin et al, 2007). Performing CT after filling the bladder with dilute contrast medium (CT cystography) improves the sensitivity of this modality for detecting tumors and bladder rupture. For prostate diseases, CT is used for detection of lymphadenopathy, gross extraprostatic tumor extension, and to delineate prostatic abscesses. Newer, multidetector CT angiography may also have a role in prostate cancer localization. CT is used for detection of the abdominal location of suspected undescended testes, for staging of testicular tumors, and in the search for nodal or distant metastasis.

The addition of delayed CT imaging 10–15 minutes after intravenous contrast-enhanced CT shows high sensitivity and specificity in characterizing adrenal lesions. Benign adenomas, including lipid poor adenomas, show brisk contrast washout. CTA or MRA are replacing conventional angiography for diagnostic examinations. New, dual-energy CT using dual or alternating sources of scan energy offers added benefits for postcontrast evaluation and allows virtual noncontrast imaging. Dual-energy CT will likely play an increasing role in evaluation for incidental renal (Brown et al, 2009; Graser et al, 2009) and adrenal masses, as well as in evaluation for urolithiasis, including stone composition determination (e.g., for urate stones) (Boll et al, 2009; Graser et al, 2008; Primak et al, 2007).

Advantages and Disadvantages

The main advantages of CT include a wide field of view, the ability to detect subtle differences in the x-ray attenuation properties of various tissues, good spatial resolution, anatomical cross-sectional images, and relative operator independence. Carefully tailored examinations are essential. Reformatted helical image data in different planes and in 3D has made renal CT imaging, with renal angiography and urography, valuable in preoperative planning, such as for partial nephrectomy. Limitations of CT include restriction to the transaxial plane for direct imaging, tissue nonspecificity, low soft-tissue contrast resolution, and the need for contrast media (both oral and intravenous). Even with careful use of contrast media, tissue contrast is sometimes unsatisfactory. Finally, radiation exposure is a consideration with multisequence CT imaging (Newfal et al, 2004). Ongoing studies evaluating reduced exposure, and modifying protocols are under way.

See Figures 6–34, 6–35, 6–36, 6–37, 6–38, 6–39, 6–40, and 6–41

Basic Principles

Clinical MRI has its basis in the nuclear properties of the hydrogen atoms in the body. Hydrogen nuclei, when considered as aggregates, sometimes referred to as “protons,” behave like tiny magnets, with net polarity (positive one direction, negative opposite) oriented along an axis at any given point in space. Ordinarily, the axes of the hydrogen nuclei in the body are randomly oriented. However, if the nuclei are placed in a strong magnetic field (like that produced in an MRI scanner), they precess and wobble like a spinning top around the lines of magnetic force.

When hydrogen nuclei in a strong magnetic field are additionally stimulated by short, pulsed radio waves of appropriate frequency, they absorb energy and invert their orientation with respect to the magnetic field. At the termination of radiofrequency pulses, the hydrogen nuclei return at various rates to their original orientation within the magnetic field, emitting energy in the form of radio waves. This phenomenon is called nuclear magnetic resonance (NMR). The emitted weak radio signals from the resonating hydrogen nuclei are received by sophisticated antenna, or coils, and transformed with various computer programs into cross-sectional images.

Different MR signal intensities reflect different hydrogen densities in body tissues, as well as differing physical, cellular, and chemical microenvironments and also flow (fluid) characteristics. The signals emitted from nuclei contain no innate spatial information. Spatial localization is achieved through varying magnetic field gradients and timing. There are biologically important nuclei other than hydrogen that are MR-sensitive, including those of phosphorus, sodium, and potassium, but these occur in lower physiologic concentrations than hydrogen. Imaging of these nuclei for tissue typing and mapping and as biologic tracers (MR spectroscopy) is undergoing intense research and development.

Clinical Applications

Applications for MR in renal imaging include demonstration of congenital anomalies, diagnosis of renal vein thrombosis, and diagnosis and staging of renal cell carcinoma. MR angiography is useful in evaluating renal transplant vessels, renal vein tumor or thrombosis, and renal artery stenosis.

The use of contrast media in MRI of the kidney has broadened clinical applications. Using bolus injection of gadolinium and rapid sequence imaging, both anatomy and function of the kidney can be assessed. Gadolinium, similar to iodine contrast media, is an extracellular contrast agent primarily excreted by glomerular filtration. Compared to iodinated contrast media, gadolinium has superior renal tolerance in patients with preexisting renal failure. Iodinated contrast agents used in radiography and CT increase attenuation linearly with their concentration. The effect of gadolinium on MR tissue signal intensity is more complex, though in general, at lower concentrations gadolinium causes an increase in signal intensity. Cases of nephrogenic system fibrosis (NSF) have been reported in patients with renal failure who have received gadolinium. Recent studies have suggested that patients on dialysis or with estimated glomerular filtration rates of ≤30 mL/min are at higher risk for developing NSF after gadolinium administration (Wertman et al, 2008).

The use of gadolinium has extended the application of MRI to the evaluation of renal obstruction (MRU may be used when other studies are inconclusive) and the detection and characterization of renal tumors. Although MRI is capable of imaging blood vessels without contrast media, gadolinium bolus followed by rapid imaging is less susceptible to flow direction and overestimation of stenosis, which can be seen in noncontrast MRA imaging. Gadolinium-enhanced MRA is useful for assessing renal artery stenosis and for evaluating potential renal donors (Figure 6–39).

MRI is used primarily to stage bladder tumors and to differentiate between benign bladder wall hypertrophy and infiltrating malignant neoplasm. There may be a potential advantage for combined endorectal and surface coil MR staging for bladder carcinoma as well. In imaging the prostate gland, MRI is principally used to stage patients with prostate cancer (Claus et al, 2004; Coakley et al, 2002; Mazaheri et al, 2008). MR spectroscopy increases specificity and reduces interobserver variability in this setting (Shukla-Dave et al, 2007). MRI of the testis is appropriate when other imaging studies are inconclusive and is applicable to the evaluation of undescended testis, trauma, epididymo-orchitis, and tumors.

A modification of the MRI technique, called chemical shift imaging, can detect microscopic amounts of fat within lesions (Figure 6–35, middle images). This technique is commonly used to characterize adrenal masses. Adrenal masses containing fat are either adrenal adenomas or myelolipomas, so the CT or MRI demonstration of fat in an adrenal lesion characterizes it as a benign lesion, even in the oncologic patient.

MR urography utilizes the sensitivity of MR imaging to demonstrate fluid (urine), producing urogram-like pictures without the need for contrast media. This technique is sensitive in the detection of ureterohydronephrosis and is particularly useful in patients in whom contrast material is contraindicated, such as patients with prior contrast reactions or renal failure. Upper tract tumor can also be demonstrated (Takahashi et al, 2008, 2009).

Diffusion-weighted imaging, a technique under active investigation for urologic imaging with recent clinical implementation, exploits restricted water diffusion in diseased tissue. This technique has shown added benefit and promise, particularly for lesion detection in oncologic imaging (Lim et al, 2009; Mazaheri et al, 2008; Seitz et al, 2009).

Advantages and Disadvantages

Advantages of MRI include direct imaging in any plane desired (though transverse, sagittal, and coronal are most standard), choice of large or small field of view, excellent soft-tissue contrast, imaging without exposure to ionizing radiation, and (as compared to ultrasound) less operator dependence. MRI can image blood vessels and the urinary tract without contrast material. MR scanning, however, is not without drawbacks. The scanning time is relatively slow and as a result image clarity is often inferior compared with CT. Absolute contraindications to MRI include the presence of (1) intracranial aneurysm clips, unless the referring physician is certain that the clip is made of a nonferromagnetic material (such as titanium); (2) intra-orbital metal fragments; and (3) any electrically, magnetically, or mechanically activated implants (including cardiac pacemakers, biostimulators, neurostimulators, cochlear implants, and hearing aids). Relative contraindications such as pregnancy should always be viewed in the light of risk versus benefit of the examination.

See Figures 6–42, 6–43, 6–44, and 6–45

As new imaging methods have been developed, changes have occurred in patterns of use for each type of imaging. For example, increased familiarity with and confidence in sonography and CT scanning have resulted in a substantial decrease in the use of many long-established conventional uroradiologic studies such as EU (Lang et al, 2003).

Several factors are involved in these changes: (1) the increased effectiveness of newer imaging methods over older ones for some aspects of urodiagnosis; (2) the availability of equipment, trained technical personnel to operate it, and physicians to interpret the results; (3) increased awareness of the hazards of ionizing radiation (Nawfel et al, 2004); and (4) the desire to use noninvasive examinations if possible.

Because so many different types of imaging are available, each with different costs, risks, and areas of effectiveness, it may be difficult for the clinician to decide which method will yield the most information with the least cost and risk. A particular study may be critical in one diagnostic situation but useless in another. For example, sonography is an excellent noninvasive, relatively inexpensive method for differentiating simple cysts from other mass lesions in the kidney but is much less effective in imaging the adrenal glands and ureters than is CT scanning. Sonography also relies considerably on the skill of the operator. CT scanning produces excellent images and is currently the preferred imaging method for the examination of the retroperitoneum. MRI rivals CT scanning in imaging capability for some structures, for example, the kidney, but has surpassed CT in imaging the pelvis. With advances in equipment and techniques, the use of MRI in urology will likely grow.

The patient and the clinician both benefit from careful consultation with the radiologist to ensure that the methods of imaging chosen are of value in diagnosis and treatment planning, and do not duplicate or merely confirm established findings, with loss of time and additional expense.

Contrast Agents

Radiography

Ultrasound

Computed Tomography Scanning

Magnetic Resonance Imaging

Comparison of Imaging Methods