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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.
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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.
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Basic Equipment and Techniques
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Radiography Fluoroscopy
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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.
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Image Intensification
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Image intensifiers, coupled to video cameras, electronically augment the ordinary dim fluoroscopic image.
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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.
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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.
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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.
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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.
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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.
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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.
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Advantages and Disadvantages
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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.
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Plain Film of the Abdomen (Figures 6–1, 6–2, and 6–3)
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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.
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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.
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The collecting structures of the kidneys, ureters, and bladder can be demonstrated radiologically with contrast media by the following methods:
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Intravenous Urography
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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.
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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).
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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.
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It is controversial whether preliminary bowel cleansing is beneficial. The choice may be made according to individual preference.
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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.
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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.
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Technique Modifications
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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.
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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.
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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.
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Percutaneous Urograms
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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).
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Cystography, Voiding, Cystourethrography, and Urodynamics (Figures 6–9, 6–10, 6–11, and 6–12)
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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.
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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).
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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.
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Lymphangiography has been largely abandoned and replaced by CT and MRI.
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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.
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Aortorenal and Selective Renal Arteriography (Figure 6–17)
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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.
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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.
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Inferior Venacavography and Selective Venography (Figures 6–18 and 6–19)
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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.
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Miscellaneous Urologic Angiography
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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.
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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.