Urinary System

Anatomy and Physiology

The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra (Fig. 7-1). The urinary system forms urine to remove waste from the bloodstream for excretion. The kidneys are the site where urine is formed and excreted through the remarkable processes of filtration and reabsorption, involving up to 180 liters (L) of blood per day. Urine formed by this process amounts to approximately 1 to 1.5 L per day and passes from the kidneys to the bladder through the ureters. Stored in the bladder, it is eventually excreted through the urethra.

The kidneys are retroperitoneal, normally located between the twelfth thoracic vertebra and the third lumbar vertebra. The right kidney lies slightly lower because of the presence of the liver superiorly. The notch located on the medial surface of each kidney is the hilus, the area where structures enter and leave the kidney. These structures include the renal artery and vein, lymphatics, and a nerve plexus. Microscopically, the nephron is the functional unit of the kidney responsible for forming and excreting urine (Fig. 7-2). The nephron unit is composed of the glomerulus, Bowman capsule, and numerous convoluted tubules. Blood flowing through the glomerulus, a ball-like cluster of specialized capillaries, is filtered and cleaned of impurities. Fluid moves out of the glomerulus into Bowman capsules and through the various convoluted tubules, resulting in the production of urine. The nephron unit terminates into a collecting tubule, which forms a tube opening at the renal papilla into a minor calyx. Minor calyces terminate in the major calyces, which, in turn, terminate at the renal pelvis (Fig. 7-3).

FIGURE 7-2 The microscopic structure of a nephron.

The ureters extend from the kidneys to the urinary bladder and are approximately 10 inches in length (Fig. 7-4). They normally enter the bladder obliquely in the posterolateral portion of the bladder, equidistant from the urethral orifice in a triangular fashion. A number of variations of this exist. The function of the ureters is to drain the urine from the kidneys to the bladder.

FIGURE 7-4 An anterior cutaway view of the bladder.

The bladder is located posterior to the symphysis pubis. It serves as a reservoir for urine before urine is expelled from the body. The bladder is very muscular and capable of distension. Valves located at the junction of the ureters and bladder prevent the backflow of urine.

The urethra is a tube leading from the urinary bladder to the exterior of the body. The female urethra is approximately 1 to inches in length, whereas the male urethra is approximately 8 inches in length. In men, the urethra passes through the prostate gland and also serves as a part of the reproductive system by receiving seminal fluid via the ejaculatory ducts, which open into the urethra from the prostate. The male urethra is classified by three separate portions: (1) the prostatic portion, (2) the membranous portion, and (3) the cavernous portion. The urethra opens to the exterior of the body via the urinary meatus.

Imaging Considerations

Urinary disorders may be suggested by abnormal laboratory or clinical findings. Clinical findings include frequent urination, polyuria, oliguria, dysuria, or obstructive symptoms. The urine may also have an abnormal color, resulting from a variety of factors. Kidney pain is generally located in the flank or back around the level of the twelfth thoracic vertebra, whereas bladder pain resulting from cystitis is usually limited to the urinary bladder. Patient renal function should be assessed before administering intravenous contrast agents in radiology. The most common laboratory tests conducted include serum creatinine, blood urea nitrogen (BUN), and glomerular filtration rate (GFR). In a normal adult, serum creatinine production and excretion are constant. Creatinine is a waste product derived from a breakdown of a compound normally found in muscle tissue. BUN levels are influenced by urine flow and the production and metabolism of urea. BUN designates the ability of the urinary system to break down nitrogenous compounds from proteins to produce urea nitrogen. Individuals with significant kidney function impairment often have raised blood levels of creatinine, urea nitrogen, or both because the glomerulus cannot adequately filter substances, the tubular system is not functioning properly, or both. The GFR may be estimated (eGFR) by using the serum creatinine value in combination with the patient’s age, race, and gender. Normally, the GFR should be 90 milliliters per minute per 1.73 meters cubed (mL/min/1.73 m²) or greater. Intravenous contrast agents should not be used in patients with a BUN greater than 50 milligrams per deciliter (mg/dL) or a serum creatinine greater than 3 mg/dL. The exact GFR threshold contraindicating the administration of intravenous (IV) contrast medium has not been established at this time.

KUB Radiography

KUB (kidney, ureter, bladder) radiography is useful in demonstrating the size and location of the kidneys. These organs may be visible radiographically because of the perirenal fat capsule that surrounds them. The kidneys are generally well fixed to the abdominal wall and are seen to move with respiratory effort. As mentioned earlier, the right kidney is usually located inferior to the left kidney because of the presence of the liver. Men’s kidneys are generally larger than those of women. The kidneys lie in an oblique plane within the abdomen and tend to parallel the borders of the psoas muscle shadows. Evaluation of the kidneys using only a KUB image is limited because the kidney shadows may often be obscured by bowel content and are difficult to visualize because of the inherent low subject contrast in the abdomen. However, KUB radiography is the usual beginning for intravenous urography (IVU), sometimes referred to as intravenous pyelography (IVP) (Fig. 7-5). In this case, its primary purposes are to (1) determine if adequate bowel preparation has been accomplished and (2) visualize radiopaque calculi of the KUB that may otherwise be hidden by the presence of contrast media. The radiologist also examines areas unrelated to the urinary tract because they may hold clues to the diagnosis and may also assist in differentiating between gastrointestinal (GI) and genitourinary disorders.

FIGURE 7-5 A preliminary or scout image before injection of intravenous contrast for an intravenous urogram. The image demonstrates the renal and psoas major muscle shadows.

Intravenous Urography

One procedure used to assess the urinary system is the IVU (or IVP). The indications for performing IVU include suspected urinary tract obstruction, abnormal urinary sediment (especially hematuria), systemic hypertension, or, frequently in men, symptoms of prostatism. Although few serious adverse effects typically accompany the injection of urographic contrast agents, current research indicates an increased risk of mortality in white female older adults because of renal failure and anaphylaxis. The risk of adverse reactions to an iodinated contrast agent increases because of a variety of factors, including a history of previous contrast reactions; asthma or other allergies; heart disease; dehydration; preexisting kidney disease; treatment with β-blockers, NSAIDs, or interleukin-2 (IL-2); a history of other pathologic diseases such as sickle cell anemia, polycythemia, and myeloma; or all of these factors. The use of nonionic, low-osmolar contrast agents significantly reduces minor and moderate reactions. These contrast agents still contain iodine, but the molecular makeup prevents them from disassociating into ions (nonionic) in the bloodstream, thus reducing the risk of an anaphylactic reaction. Visualization of the urinary system depends on the concentration of contrast material filtered by the kidneys and present in the collecting system; therefore, the patient must have fairly normal physiologic function for diagnostic images to be obtained. Other imaging techniques such as sonography and computed tomography (CT) should be considered in patients with compromised renal function.

Many IVU routines allow for an image to be taken within 30 seconds to 1 minute after contrast medium injection. Because the contrast agents for most IVU examinations are injected by hand, the timing generally begins on completion of the bolus injection and will vary from institution to institution. This is termed the nephrogram phase and may be used to demonstrate the contrast agent in the nephrons before it reaches the renal calyces. Ready visualization of the renal parenchyma allows for an inspection of the renal outline. Indentations or bulges may indicate the presence of disease. The nephrogram image is also used to check for normal kidney position, which may be altered by congenital malposition, ptosis, or the presence of a retroperitoneal mass.

Although the numbers and types of images obtained may vary from one institution to another, a series of collecting system sequence images are the final part of IVU (Fig. 7-6). The renal pelvis, calyces, ureters, and bladder are examined for any abnormalities. The calyces should be evenly distributed and reasonably symmetric. Usually, they appear as buttercup-shaped projections surrounding the renal papillae. Calyceal dilatation may be demonstrated as a result of acute or chronic urinary tract obstruction, obstructive uropathy, or reflux. Dilatation secondary to destruction of the renal pyramids is less common.

FIGURE 7-6 A 15-minute postinjection image during intravenous urography demonstrating the normal collecting system.

Because of the peristaltic activity of ureters, only part of their length in a collecting system sequence may be demonstrated (Fig. 7-7). Nonopaque ureteral calculi sometimes cause filling defects and an obstructive dilatation of the ureter. The majority of all urinary tract calculi are found at the vesicoureteral junction. Any pronounced deviation of the ureter suggests the presence of a retroperitoneal mass. Various filling defects may be demonstrated in the contrast agent–filled ureter during IVU, including tumors, blood clots, and nonopaque calculi. Common bladder defects visualized during IVU include urinary catheter balloons, normal uterus and colon, and extrinsic deformities such as uterine or sigmoid colon tumors. A “postvoid” image usually completes an IVU procedure and allows assessment of the bladder function (Fig. 7-8).

FIGURE 7-7 A right posterior oblique projection after contrast injection for intravenous urography demonstrating the correct entrance of the ureters into the posterior bladder wall.

Cystography

Cystography is a common radiographic examination for studying the lower urinary tract. This involves insertion of a urinary catheter into the urethra and retrograde filling of the bladder with iodinated water-soluble contrast material (Fig. 7-9). A frequent indication for this procedure is to identify vesicoureteral reflux (VUR). In the normal bladder, increased pressure as the bladder fills effectively shuts down any chance of reflux. Bladder infection, however, may render the ureteral “valve” incompetent, refluxing the infection into the kidney. Cystography may also be used to study congenital bladder anomalies, tumors, diverticula (Fig. 7-10), calculi, bladder rupture, or neurogenic bladder. Voiding (micturition) cystography is sometimes used in conjunction with retrograde cystography to allow study of the urethra on voiding. Urethrography may be accomplished using the antegrade approach, as with voiding cystourethrography, or retrograde when cystography is not necessary. The antegrade approach is used to study the posterior urethra, especially in the male patient, and the retrograde approach is helpful in studying the anterior urethra (Fig. 7-11). The usual intent of voiding cystography is to allow study of a urethral stricture (Fig. 7-12).

FIGURE 7-9 A normal cystogram without reflux as seen in this oblique projection of the bladder in a 56-year-old woman.

Retrograde Pyelography

Retrograde pyelography requires the placement of a catheter into the ureteric orifice in a retrograde fashion. This is usually performed by a urologist during cystoscopy to allow injection of contrast medium directly into the urinary tract to outline the renal collecting system. The approach is termed retrograde because the contrast agent is injected through the ureter into the affected kidney, opposite the normal direction of urine flow. Indications for this study may include hematuria of unknown cause, hydronephrosis, and, in cases of a nonfunctioning kidney, the determination of further information about possible obstruction.

Sonography

Sonography is a noninvasive method of imaging both functioning and nonfunctioning kidneys. Because sonography can clearly demonstrate the parenchymal structure of the kidney and the renal pelvis without the use of contrast agents, it is becoming the primary method of visualizing the kidneys and evaluating most renal disorders. It is useful in evaluating kidney stones (Fig. 7-13), calcifications, hydronephrosis (Fig. 7-14), abscesses, renal masses, and renal cysts and to assess renal size, atrophy, or both. Sonography is the modality of choice for evaluating individuals after kidney transplantation. Doppler techniques are helpful in assessing blood flow in the renal arteries and veins for both transplant recipients and individuals with suspected renal artery stenosis. Sonography is also used to visualize abnormalities of the urinary system present in the fetus.

FIGURE 7-13 A sonogram demonstrating a renal stone in the cortex of the kidney.

Computed Tomography

CT is an excellent modality for imaging the kidneys because it can detect small differences in tissue densities within the body. Kidneys can be visualized on CT with or without the use of a contrast agent. Abdominal CT is particularly important in determining the nature of renal masses, either solid or cystic, which may not be visible on a KUB radiograph because of the presence of gas in the bowel. CT evaluation of the urinary system generally requires the use of an IV contrast agent to differentiate renal cysts from solid masses and to evaluate the extent of the lesion (Fig. 7-15). Because most institutions use an automatic injector in CT, scanning may begin when the bolus of contrast medium is injected or shortly after injection, and a delay is programmed into the scanner to allow the contrast medium to reach the bladder before the pelvis is imaged.

CT is also useful for looking for sites of obstruction caused by renal calculi or retroperitoneal masses, which may distort the urinary tract; assessing renal infection or trauma; and staging tumors of the lymph nodes. A CT renal stone study is considered the imaging modality of choice by the American College of Radiology (ACR) when patients present with an acute onset of flank pain or other symptoms suggest the presence of renal calculi. Because CT displays excellent contrast resolution, stones are identified more easily than with conventional radiography, but without the use of an intravenous contrast agent (Fig. 7-16). In addition, pelvic CT is the imaging modality of choice for the evaluation of bladder tumors or masses.

FIGURE 7-16 A computed tomography image demonstrating a calcification in the left kidney indicative of a renal stone without the use of a contrast agent.

Renal Angiography

Renal angiography is one of the most invasive imaging procedures performed on the urinary system. It is usually indicated to further evaluate a renal mass suspected of being malignant, to embolize blood flow to a renal mass, or to assess renal artery stenosis that may cause hypertension, as well as to assess other vascular disorders such as aneurysms or congenital anomalies. It is also performed on kidney donors before surgical removal of the kidney to serve as a “road map” of vascular anatomy for the surgeon. In renal angiography, a catheter is introduced peripherally, most commonly into the femoral artery. The catheter tip may be placed into the specific renal artery of interest or into the abdominal aorta just superior to the renal arteries. The contrast agent is injected via the catheter to image the vasculature of the kidney or kidneys.

Magnetic Resonance Imaging

The role of magnetic resonance imaging (MRI) has greatly improved as a result of breath-hold imaging sequences and bolus injections of gadolinium contrast agents. Abdominal MRI is useful in follow-up studies in patients with known renal cell carcinoma or invasive bladder cancers and adrenal masses. Additionally, magnetic resonance angiography (MRA) is now highly recommended by the ACR in the diagnosis of renovascular hypertension (Fig. 7-17). Contrast-enhanced three-dimensional MRA obtains coronal images of the renal arteries in as little as 20 seconds. The images can then be rotated for better visualization. MRA is also an excellent modality for demonstrating other vascular anomalies such as thrombosis, aneurysms, and arteriovenous malformations (AVMs). Because it allows for imaging of the urinary system in all three planes, it is also used in conjunction with CT for the evaluation of renal masses and their extensions. In cases of renal cyst evaluation, MRI is capable of differentiating between fluid accumulation from hemorrhage and infection. Pelvic MRI is used to readily demonstrate the seminal vesicles and prostate gland in men as well as masses within the urinary bladder. Because of its ability to clearly image soft tissue, pelvic MRI allows thorough evaluation of invasive cancers within the urinary bladder.