On completion of this chapter, you should be able to:
Discuss normal anatomic location, function, and sonographic appearance of urinary system organs
Discuss normal physiology of the urinary system
Describe the sonographic scanning technique to image the urinary system
Define and discuss the pathologies discussed in this chapter
Identify and define the sonographic appearance of pathologies included in this chapter
Discuss the role and limitations of sonography in post–renal transplant patients
Describe the clinical signs and symptoms of urinary tract problems and the laboratory tests that are used to evaluate them
The urinary system has two principal functions: excreting wastes and regulating the composition of blood. Blood composition must not be allowed to vary beyond tolerable limits, or the conditions in tissue necessary for cellular life will be lost. Regulating blood composition involves not only removing harmful wastes but also conserving water and metabolites in the body.
Anatomy of the urinary system
The urinary system is located posterior to the peritoneum lining the abdominal cavity in an area called the retroperitoneum. The kidneys lie in the retroperitoneal cavity near the posterior body wall, just below the diaphragm ( Figure 15-1 ). The lower ribs protect both kidneys. The right kidney lies slightly inferior to the left kidney because the large right lobe of the liver pushes it inferiorly. The kidneys move readily with respiration; on deep inspiration, both kidneys move downward approximately 1 inch.
The kidneys are dark red, bean-shaped organs that measure 9 to 12 cm long, 5 cm wide, and 2.5 cm thick. The outer cortex of the kidney is darker than the inner medulla because of the increased perfusion of blood. The inner surface of the medulla is folded into projections called renal pyramids, which empty into the renal pelvis. The arcuate arteries are located at the base of the pyramids and separate the medulla from the cortex. Numerous collecting tubules bring the urine from its sites of formation in the cortex to the pyramids. The renal tubules, or nephrons, are the functional units of the kidney.
On the medial surface of each kidney is a vertical indentation called the renal hilum, where the renal vessels and ureter enter and exit. Within the hilus of the kidney are other vascular structures, a ureter, and the lymphatics. The renal artery is posterior and superior to the renal veins. The two branches of the renal vein are anterior to the renal artery ( Figure 15-2 ). The ureter is located slightly inferior to the renal artery. When present, the third branch of the renal artery may be seen to arise from the hilus. The lymph vessels and sympathetic fibers also are found within the renal hilus.
Three layers of tissue surround and protect the kidneys. The inner layer that surrounds the kidney is a fibrous capsule called the true capsule. Outside of this fibrous capsule is a covering of perinephric fat. The perinephric fascia surrounds the perinephric fat and encloses the kidneys and adrenal glands. The perinephric fascia is a condensation of areolar tissue that is continuous laterally with the fascia transversalis. The renal fascia, known as Gerota’s fascia, surrounds the true capsule and perinephric fat.
Anterior to the right kidney are the right adrenal gland, liver, Morison’s pouch, second part of the duodenum, and right colic flexure ( Figure 15-3 ). Anterior to the left kidney are the left adrenal gland, spleen, stomach, pancreas, left colic flexure, and coils of jejunum.
Posterior to the right kidney are the diaphragm, costodiaphragmatic recess of the pleura, twelfth rib, psoas muscle, quadratus lumborum, and transversus abdominis muscles. The subcostal (T12), iliohypogastric, and ilioinguinal (L1) nerves run downward and laterally. Posterior to the left kidney are the diaphragm, costodiaphragmatic recess of the pleura, eleventh and twelve ribs, psoas muscle, quadratus lumborum, and transversus abdominis muscles. The same nerves are seen near the left kidney as in the right.
Within the kidney, the upper expanded end of the ureter, known as the renal pelvis of the ureter, divides into two or three major calyces, each of which divides further into two or three minor calyces (see Figure 15-2 ). The apex of a medullary pyramid, called the renal papilla, indents each minor calyx. The kidney consists of an internal medullary portion and an external cortical substance. The medullary substance consists of a series of striated conical masses, called the renal pyramids. The pyramids vary from 8 to 18 in number, and their bases are directed toward the outer circumference of the kidney. Their apices converge toward the renal sinus, where their prominent papillae project into the lumina of the minor calyces. Spirally arranged muscles surround the calyces and may exert a milking action on these tubes, aiding in the flow of urine into the renal pelvis. As the pelvis leaves the renal sinus, it rapidly becomes smaller and ultimately merges with the ureter.
The nephrons are located in the renal parenchyma and consists of two main structures—a renal corpuscle and a renal tubule. Nephrons filter the blood and produce urine. Blood is filtered in the renal corpuscle. The filtered fluid passes through the renal tubule. As the filtrate moves through the tubule, substances needed by the body are returned to the blood. Waste products, excess water, and other substances not needed by the body pass into the collecting ducts as urine.
The renal corpuscle consists of a network of capillaries called the glomerulus, which is surrounded by a cuplike structure known as Bowman’s capsule. Blood flows into the glomerulus through a small afferent arteriole and leaves the glomerulus through an efferent arteriole. This arteriole conducts blood to a second set of capillaries, the peritubular capillaries, which surround the renal tubule.
Filtrate passes into the renal tubule through an opening in the bottom of Bowman’s capsule. The first part of the renal tubule is the coiled proximal convoluted tubule. After passing through the proximal convoluted tubule, filtrate flows into the loop of Henle and then into the distal convoluted tubule. Urine from the distal convoluted tubules of several nephrons drains into a collecting duct. A portion of the distal convoluted tubule curves upward and contacts the afferent and efferent arterioles. Some cells of the distal convoluted tubule and some cells of the afferent arteriole are modified to form the juxtaglomerular apparatus, a structure that helps regulate blood pressure in the kidney.
The renal corpuscle, the proximal convoluted tubule, and the distal convoluted tubule of each nephron are located within the renal cortex. The loops of Henle dip down into the medulla.
The ureter is a 25-cm tubular structure whose proximal end is expanded and continuous with the funnel shape of the renal pelvis. The renal pelvis lies within the hilus of the kidney and receives major calyces. The ureter emerges from the hilus of the kidney and runs vertically downward behind the parietal peritoneum along the psoas muscle, which separates it from the tips of the transverse processes of the lumbar vertebrae. It enters the pelvis by crossing the bifurcation of the common iliac artery anterior to the sacroiliac joint. The ureter courses along the lateral wall of the pelvis to the region of the ischial spine and turns forward to enter the lateral angle of the bladder. The ureter from the ureteropelvic junction to the bladder is not routinely visualized on a sonogram. The ureters are located in the retroperitoneal cavity with the superior and distal ends of the ureters more readily visualized than the midsection due to overlying bowel gas.
Three constrictions occur along the ureter’s course: (1) where the ureter leaves the renal pelvis, (2) where it is kinked as it crosses the pelvic brim, and (3) where it pierces the bladder wall.
The urinary bladder is a large muscular bag located above and behind the pubic bone. It has a posterior and lateral opening for the ureters and an anterior opening for the urethra. The interior of the bladder is lined with highly elastic transitional epithelium. When the bladder is full, the lining is smooth and stretched; when it is empty, the lining is a series of folds. In the middle layer, a series of smooth muscle coats distend as urine collects and contract to expel urine through the urethra. Urine is produced almost continuously and accumulates in the bladder until the increased pressure stimulates the organ’s nervous receptors to relax the urethra’s sphincter and urine is released from the urinary bladder. The urinary bladder is visualized sonographically when it is distended with fluid.
The urethra is a membranous tube that passes from the anterior part of the urinary bladder to the outside of the body. It includes two sphincters: the internal sphincter and the external sphincter. The urethra is not routinely visualized sonographically.
The main renal artery supplies blood to the kidney. When a person is at rest, approximately 1.2 liters of blood per minute is pumped to the kidneys. The renal arteries are lateral branches of the aorta that are located just inferior to the superior mesenteric artery ( Figure 15-4 ). The branches of the renal artery may vary in size and number. In most cases, the renal artery is divided into two primary branches: a larger anterior and a smaller posterior. These arteries break down into smaller segmental arteries, then into interlobar arteries, and finally into tiny arcuate arteries.
Five to six veins join to form the main renal vein. This vein emerges from the renal hilus anterior to the renal artery. The renal vein drains into the lateral walls of the inferior vena cava (see Figure 15-4 ). The left renal vein courses transversely across the body going anterior to the aorta and posterior to the superior mesenteric artery.
The lymphatic vessels follow the renal artery to the lateral aortic lymph nodes near the origin of the renal artery. Nerves originate in the renal sympathetic plexus and are distributed along the branches of the renal vessels.
Blood supply to nephrons begins at the renal artery. The artery subdivides within the kidneys. A small vessel (afferent arteriole) enters Bowman’s capsule, where it forms a tuft of capillaries, the glomerulus, which entirely fills the concavity of the capsule. Blood leaves the glomerulus via the efferent arteriole, which subdivides into a network of capillaries that surround the proximal and distal tubules and eventually unite as veins, which become the renal vein.
The renal vein returns the cleansed blood to the general circulation. Movements of substances between the nephron and the capillaries of the tubules change the composition of the blood filtrate moving along in the tubules. From the nephrons, the fluid moves to collecting tubules and into the ureter, leading to the bladder, where urine is stored.
The arterial supply to the ureter is provided by the following three sources: the renal artery, the testicular or ovarian artery, and the superior vesical artery.
Physiology and laboratory data of the urinary system
The urinary system consists of two kidneys, which remove wastes from the blood and produce urine, and two ureters, which act as tubal ducts leading from the hilus of the kidneys and drain into the urinary bladder. The bladder collects and stores urine, which is eventually discharged through the urethra. The urinary system is located posterior to the peritoneum lining the abdominal cavity in an area called the retroperitoneum.
The function of the kidneys is to excrete urine. More than any other organ, the kidneys regulate the amounts of water and electrolytes leaving the body so that these equal the amounts of substances entering the body. The formation of urine involves the following three processes: glomerular filtration, tubular reabsorption, and tubular secretion.
Cells in the body continually carry on metabolic activities that produce waste products. If permitted to accumulate, metabolic wastes eventually reach toxic concentrations and threaten homeostasis. To prevent this, metabolic wastes must be quickly excreted. The process of excretion entails separating and removing substances harmful to the body. The skin, lungs, liver, large intestine, and kidneys carry out excretion.
The principal metabolic waste products are water, carbon dioxide, and nitrogenous wastes, including urea, uric acid, and creatinine (Cr). Nitrogen is derived from amino acids and nucleic acids. Amino acids break down in the liver, and the nitrogen-containing amino group is removed. The amino group is then converted to ammonia, which is chemically converted to urea. Uric acid is formed from the breakdown of nucleic acids. Both urea and uric acid are carried away from the liver into the kidneys by the vascular system. Creatinine is nitrogenous waste produced from phosphocreatine in the muscles .
Laboratory tests for renal disease
The clinical symptoms of a patient with specific renal pathology may be nonspecific. Therefore a patient with symptoms of renal infection, renal insufficiency, or disease may undergo a number of laboratory tests to help the clinician determine the cause of the problem.
A patient’s history of infection, previous urinary tract problems (renal stones), or hypertension or family history of renal cystic disease is useful information. A patient with a renal infection or disease process may have any of the following symptoms: flank pain, hematuria, polyuria, oliguria, fever, urgency, weight loss, or general edema.
Urinalysis is essential to detect urinary tract disorders in patients whose renal function is impaired or absent. Most renal inflammatory processes introduce a characteristic exudate for a specific type of inflammation into the urine. The presence of an acute infection causes hematuria, or red blood cells in the urine; pyuria is pus in the urine.
Urine pH is very important in managing diseases such as bacteriuria and renal calculi. The pH refers to the strength of the urine as a partly acidic or alkaline solution. The abundance of hydrogen ions in a solution is called pH. If urine contains an increased concentration of hydrogen ions, the urine is acidic. The formation of renal calculi depends in part on the pH of urine. Other conditions, such as renal tubular acidosis and chronic renal failure, are associated with alkaline urine.
The specific gravity is the measurement of the kidney’s ability to concentrate urine. The concentration factor depends on the quantity of dissolved waste products. Excessive intake of fluids or decreased perspiration may cause a large output of urine and a decrease in the specific gravity. Low fluid intake, excessive perspiration, or diarrhea can cause the output of urine to be low and the specific gravity to increase. The specific gravity is especially low in cases of renal failure, glomerular nephritis, and pyelonephritis. These diseases cause renal tubular damage, which affects the ability of the kidneys to concentrate urine.
Hematuria is the appearance of blood cells in the urine; it can be associated with early renal disease. An abundance of red blood cells in the urine may suggest renal trauma, neoplasm, calculi, pyelonephritis, or glomerular or vascular inflammatory processes, such as acute glomerulonephritis and renal infarction.
Leukocytes may be present whenever inflammation, infection, or tissue necrosis originates from anywhere in the urinary tract.
The hematocrit is the relative ratio of plasma to packed cell volume in the blood. Decreased hematocrit occurs with acute hemorrhagic processes secondary to disease or blunt trauma.
Hemoglobin is present in urine whenever extensive damage or destruction of the functioning erythrocytes occurs. This condition injures the kidney and can cause acute renal failure.
When glomerular damage is evident, albumin and other plasma proteins may be filtered in excess, allowing the overflow to enter the urine, which lowers the blood serum albumin concentration. Albuminuria is commonly found with benign and malignant neoplasms, calculi, chronic infection, and pyelonephritis.
Specific measurements of creatinine concentrations in urine and blood serum are considered an accurate index for determining the glomerular filtration rate. Creatinine is a by-product of muscle energy metabolism; it is normally produced at a constant rate as long as the body muscle mass remains relatively constant. Creatinine normally goes through complete glomerular filtration without being reabsorbed by the renal tubules. Decreased urinary creatinine clearance indicates renal dysfunction because creatinine blood levels are constant, and only decreased renal function prevents the normal excretion of creatinine.
Blood urea nitrogen.
The blood urea nitrogen (BUN) is the concentration of urea nitrogen in blood and is the end product of cellular metabolism. Urea is formed in the liver and is carried to the kidneys through the blood to be excreted in urine. Impairment of renal function and increased protein catabolism result in BUN elevation that is relative to the degree of renal impairment and the rate of urea nitrogen excretion by the kidneys.
Renal dysfunction also results in serum creatinine elevation. Blood serum creatinine levels are said to be more specific and more sensitive in determining renal impairment than BUN.
Sonographic evaluation of the urinary system
Sonographic evaluation of the kidneys is a noninvasive, relatively inexpensive, reproducible diagnostic test used to evaluate renal anatomy and pathology. In patients with renal colic without a history of renal stones, a noncontrast computed tomography (NCCT) is typically performed. NCCT requires no patient preparation and is not operator or patient dependent. The main disadvantages of NCCT are cost and the use of ionizing radiation. Patients with a history of renal stones require a plain film x-ray, and a renal sonogram with Doppler is usually the first diagnostic test performed.
Magnetic resonance imaging (MRI) using magnetic resonance urography (MRU) is currently being investigated for diagnosing renal disease. MRU can assess renal function, in addition to diagnosing obstructive uropathy. MRI can assess other abdominal organs for disease.
A renal sonogram is able to identify the presence and location of both kidneys, image renal congenital anomalies, determine renal size, show parenchymal detail, and delineate an abnormal lie of a kidney resulting from an extrarenal mass. In addition, sonography can demonstrate the acoustic properties of a mass, or determine whether hydronephrosis is secondary to renal stones. Sonography can also define perirenal fluid collections, such as a hematoma or an abscess, and detect dilated ureters and hydronephrosis.
Normal texture and patterns.
The kidneys are imaged by sonography as organs with smooth, thin outer contours surrounded by reflected echoes of perirenal fat. The renal parenchyma surrounds the fatty central renal sinus, which contains the calyces, infundibula, pelvis, vessels, and lymphatics ( Figure 15-5 ). Because of the fat interface, the renal sinus is imaged as an area of intense echoes with variable contours. If two separate collections of renal sinus fat are identified, a double collecting system should be suspected.
Generally, patients are given nothing by mouth before a sonogram or other imaging examinations are performed. This state of dehydration causes the infundibula and renal pelvis to be collapsed and thus indistinguishable from the echo-dense renal sinus fat. If, on the other hand, the bladder is distended from rehydration, the intrarenal collecting system also will become distended. An extrarenal pelvis may be seen as a fluid-filled structure medial to the kidney on transverse scans. The normal variant from obstruction is differentiated by noting the absence of a distended intrasinus portion of the renal pelvis and infundibula. Dilation of the collecting system has also been noted in pregnant patients. (The right kidney is generally involved with a mild degree of hydronephrosis. This distention returns to normal shortly after delivery.)
Patient position and technique.
The patient should be in a supine and/or decubitus position using the liver as a window to image the right kidney ( Figures 15-6 and 15-7 ) or through the spleen for the left kidney ( Figure 15-8 ). Several alternative scanning windows can be used to image the kidney. These include the right posterior oblique, right lateral decubitus, and left lateral decubitus views. Having the patient take in a deep breath will move the liver and spleen distally, which may create a better window to enhance visualization of the kidneys. A subcostal or intercostal transducer approach may be used for visualization of the upper and lower poles of the kidneys.
Proper adjustment of time gain compensation (TGC) with adequate sensitivity settings allows a uniform acoustic pattern throughout the image. The renal cortical echo amplitude should be compared with the normal liver parenchymal echo amplitude at the same depth to effectively set the TGC and sensitivity.
If the patient has a substantial amount of perirenal fat, a high-frequency transducer may not provide the penetration necessary to optimally visualize the area. The deeper areas of the kidney may appear hypoechoic. Renal detail may also be obscured if the patient has hepatocellular disease, gallstones, rib interference ( Figures 15-9 and 15-10 ), or other abnormal collections between the liver and kidney. The use of harmonic imaging or tissue contrast enhancement technology ( Figure 15-11 ) may help to optimize visualization of the kidneys.
The parenchyma is the area from the renal sinus to the outer renal surface ( Figure 15-12 ). The arcuate arteries and interlobar vessels are found within and are best demonstrated as intense specular echoes in cross section or oblique section at the corticomedullary junction.
The cortex generally is mid-level echo producing ( Figure 15-13 ) (although its echoes are less echogenic than those from normal liver), whereas the medullary pyramids are hypoechoic ( Figure 15-14 ). The two are separated from each other by bands of cortical tissue, called columns of Bertin, which extend inward to the renal sinus.
Diseases of the renal parenchyma are those that accentuate cortical echoes but preserve or exaggerate the corticomedullary junction (type I) and those that distort the normal anatomy, obliterating the corticomedullary differentiation in a focal or diffuse manner (type II).
Criteria for type I changes include the following: (1) The echo intensity in the cortex must be equal to or greater than that in the adjacent liver or spleen, and (2) the echo intensity in the cortex must be equal to that in the adjacent renal sinus. Minor signs would include the loss of identifiable arcuate vessels and the accentuation of corticomedullary definition.
Type II changes can be seen in focal disruption of normal anatomy with any mass lesion, including cysts, tumors, abscesses, and hematomas.
The arteries are best seen with the supine and left lateral decubitus views (right side up). The right renal artery extends from the lateral wall of the aorta to enter the central renal sinus ( Figure 15-15 ). On the longitudinal scan, the right renal artery can be seen as a round anechoic structure posterior to the inferior vena cava ( Figure 15-16 ). The right renal vein extends from the central renal sinus directly into the inferior vena cava ( Figure 15-17 ). Both vessels appear as tubular structures in the transverse plane.
The renal arteries have an echo-free central lumen with highly echogenic borders that consist of a vessel wall and surrounding retroperitoneal fat and connective tissue. They lie posterior to the veins and can be demonstrated with certainty if their junction with the aorta is seen.
The left renal artery flows from the lateral wall of the aorta to the central renal sinus ( Figure 15-18 ). The left renal vein flows from the central renal sinus, anterior to the aorta and posterior to the superior mesenteric artery, to join the inferior vena cava ( Figure 15-19 ). It is seen as a tubular structure on the transverse scan.
The diaphragmatic crura run transversely in the para-aortic region. The crura lie posterior to the renal arteries and should be identified by their lack of pulsations and absence of Doppler flow ( Figure 15-20 ). They vary in echogenicity, depending on the amount of surrounding retroperitoneal fat. They may appear hypoechoic, as lymph nodes do.
The renal medulla consists of hypoechoic pyramids dispersed in a uniform distribution, separated by bands of intervening parenchyma that extend toward the renal sinus. The pyramids are uniform in size, shape (triangular), and distribution. The apex of the pyramid points toward the sinus, and the base lies adjacent to the renal cortex. The interlobar arteries lie alongside the pyramids, and arcuate vessels lie at the base of the pyramids (see Figures 15-2 and 15-10 ).
Renal variants include slight alterations in anatomy that may lead the sonographer to suspect an abnormality is present when it really is a normal variation. See Table 15-1 for a description of renal variants and anomalies.
|Type||Location||Sonographic Appearance||Differential Considerations||Distinguishing Characteristics|
|Column of Bertin||Medulla||Indentation of the renal sinus||Renal mass effect||Similar to renal parenchyma; contiguous with cortex|
|Dromedary hump||Lateral border of the kidney||Identical to the renal cortex||Mass effect||Usually seen on the left kidney|
|Junctional parenchymal defect||Upper pole of renal parenchyma||Echogenic triangular area||Mass effect||Best seen on sagittal scans|
|Fetal lobulation||Surface of the kidney||Indentations between the calyces||Mass effect||Best seen on sagittal scans|
|Lobar dysmorphism||Middle and upper calyces||Elongation of upper and middle calyces||Column of Bertin||Best seen on sagittal scans|
|Duplex collecting (complete) system||Central renal sinus||Two echogenic regions separated by moderately echogenic parenchymal tissue||Mass effect||“Faceless”; no echogenic renal pelvis seen on transverse view at the level of the midpole|
|Bifid renal pelvis (incomplete duplication)||Central renal sinus||Middle calyces, two echogenic regions||Pseudomass effect||One ureter entering the bladder on each side of the bladder|
|Extrarenal pelvis||Long renal pelvis that extends outside the renal border||Central cystic region that extends beyond the medial renal border||Renal aneurysm, dilated proximal ureter||Best seen on a transverse view at the level of the midpole|
|Horseshoe kidney||Kidneys seen more medial and anterior to the spine||Fusion of the polar region, usually the lower poles||Inferior poles lie more medial, associated with pyelocaliectasis, anomalous extrarenal pelvis, urinary calculi|
Columns of bertin.
The columns of Bertin are prominent invaginations of the cortex located at varying depths within the medullary substance of the kidneys. Hypertrophied columns of Bertin contain renal pyramids and may be difficult to differentiate from an avascular renal neoplasm. The columns are most exaggerated in patients with complete or partial duplication ( Figure 15-21 ).
Sonographic features of a renal mass effect produced by a hypertrophied column of Bertin include the following: a lateral indentation of the renal sinus, a clear definition from the renal sinus, or a maximum dimension that does not exceed 3 cm. Contiguity with the renal cortex is evident, and overall echogenicity is similar to that of the renal parenchyma.
A dromedary hump is a bulge of cortical tissue on the lateral surface of a kidney (usually the left), resembling the hump of a dromedary camel. It is seen in persons whose spleen or liver presses down. It is a normal variant but may resemble a renal neoplasm.
On sonography, the echogenicity is identical to the rest of the renal cortex, and a renal pseudotumor needs to be considered ( Figure 15-22 ).
Junctional parenchymal defect.
A junctional parenchymal defect is a triangular, echogenic area typically located anteriorly and superiorly. It is a result of partial fusion of two embryonic parenchymal masses called renunculi during normal development ( Figure 15-23 ).
Junctional parenchymal defects are best demonstrated on sagittal scans and must not be confused with pathologic processes such as parenchymal renal scars and angiomyolipoma. A lobar dysmorphism is a lobar fusion variant in which malrotation of the renal lobe occurs. The middle and upper calyces may be splayed and displaced, and the lower calyx is deviated posteriorly. The dysmorphic lobe may resemble a mass or prominent column of Bertin on a sonogram ( Figure 15-24 ).
Fetal lobulation is developmental variation that is usually present in children up to 5 years old, and may be persistent in up to 51% of adults. The surfaces of the kidneys are generally indented in between the calyces, giving the kidneys a slightly lobulated appearance ( Figure 15-25 ).
Sinus lipomatosis is a condition characterized by deposition of a moderate amount of fat in the renal sinus with parenchymal atrophy ( Figure 15-26 ). In sinus lipomatosis, the abundant fibrous tissue may cause enlargement of the sinus region with increased echogenicity and regression toward the center of the parenchymal. Occasionally, a fatty mass is localized in only one area; this is called lipomatosis circumscripta.
The normal renal pelvis is a triangular structure. Its axis points inferiorly and medially. The intrarenal pelvis lies almost completely within the confines of the central renal sinus. This is usually small and foreshortened. The extrarenal pelvis tends to be larger with long major calyces.
On sonography, the pelvis appears as a central cystic area that may be partially or entirely beyond the confines of the bulk of the renal substance. Transverse views are best for viewing continuity with the renal sinus. The dilated extrarenal pelvis will usually decompress when the patient is placed in the prone position ( Figure 15-27 ).
Renal anomalies comprise abnormalities in number, size, position, structure, or form ( Figures 15-28 and 15-29 ) (see Table 15-1 ). Anomalies in number include agenesis, dysgenesis (defective embryonic development of the kidney), and supernumerary kidney. Supernumerary kidney is an additional kidney to the number usually present, which is two. In some cases, separation of the reduplicated organ is incomplete (fused supernumerary kidney). Bifid means cleft, or split into two parts. Bifid renal pelvis is a common anomaly and is considered a normal variant. The renal pelvis may appear to be more prominent on sonography. A pseudotumor is an overgrowth of cortical tissue that indents the echogenic renal sinus and may be mistaken for a renal tumor on sonography.
Renal agenesis is absence of the kidney or failure of the kidney to form; it may be bilateral or unilateral. Bilateral renal agenesis is very rare and is incompatible with life. Unilateral renal agenesis results in a solitary kidney. Congenital absence of one kidney is rare, and is commonly associated with other congenital anomalies such as seminal vesical cyst, vaginal agenesis, or bicorn uterus. Renal compensatory hypertrophy (enlargement) generally occurs with a solitary kidney ( Figure 15-30 ).
Renal hypoplasia is incomplete development of the kidney, usually with fewer than five calyces. Functionally and morphologically, the kidney is normal and should be differentiated from an atrophy kidney secondary to pyelonephrosis or renal artery stenosis. Usually, the pyelonephritic kidney is scarred and echogenic, and the small kidney that results from renal artery stenosis has abnormal Doppler parameters (tardus and parvus waveform).
Bifid renal pelvis.
A common renal anomaly with a duplication of the renal pelvis and one ureter is considered a normal variant.
Incomplete, or partial, duplication is the most frequently occurring congenital anomaly in the neonate. Duplication consists of two collecting systems and two ureters, with a single ureter entering into the urinary bladder. The two ureters join and form a single ureter anywhere between the kidney and the bladder.
Complete duplication is the rare condition of a duplex collecting system. This anomaly results in two separate collecting systems, each with its own ureter that enters the bladder. In cases of double ureter, the ureter from the upper pole of the kidney usually opens below and medial to the one from the lower pole (rule of Weigert-Meyer). The ureter of the lower calyx inserts into the bladder more superiorly and laterally to the normal location of the vesicoureteral orifice, with a short intramural portion. This short intramural portion of the ureter increases the chance of a prevesicoureteral reflux. The ureter from the upper pole calyx inserts into the bladder medially and distally to the normal location of the vesicoureteral orifice. The low insertion of the ureter into the bladder causes an ectopic posterior insertion of the urethra with posterior displacement of the vagina, which increases the chance of urethral obstruction by a stricture or ureterocele, vesicoureteral reflux, or both.
The way to confirm a complete collecting system is to demonstrate two ureteral jets entering the bladder on the same side. The duplex kidney is usually enlarged with smooth margins. The central renal sinus appears as two echogenic regions separated by a cleft of moderately echogenic tissue similar in appearance to the normal renal parenchyma. On the transverse view, the area separating the renal pelvis is called “faceless” because the tissue is homogenous, with no central echogenic renal pelvis. Hydronephrosis of the upper pole with a ureterocele, or hydronephrosis of the upper pole and lower pole calyces, may be present ( Figures 15-31 and 15-32 ).
Renal ectopia, or ectopic kidney, describes a kidney that is not located in its usual position, the renal fascia. It results when the kidney fails to ascend from its origin in the true pelvis or from a superiorly ascended kidney located in the thorax. Pelvic kidney, also called sacral kidney, is the most common renal ectopia and should not be misdiagnosed as a primary pelvic tumor. It is almost always malrotated; the renal pelvis faces anteriorly and is predisposed to reflux, infection, ureteropelvic junction (UPJ) obstruction, and stone formation ( Figure 15-33 ). Pelvic kidney may be bilateral, but this is very rare. A thoracic kidney migrates through the diaphragm into the thoracic cavity. It is a rare finding and is not easily diagnosed with ultrasound. Other renal ectopias include intrathoracic kidney and abdominal (iliac crest) kidney.
Two types of crossed renal ectopia can occur: fused and nonfused. Both are associated with malrotation. Fused crossed renal ectopia occurs more frequently than nonfused and most often on the right side. In most cases of crossed renal ectopia, the ureters are not ectopic. Cystoscopy reveals a normal trigone, and the incidence of associated congenital anomalies is low. Renal calculi are the most common complication. Sonography shows both kidneys located on the same side, with most demonstrating fusion ( Figure 15-34 ).
Horseshoe kidney is the most common anomaly of renal fusion. Fusion of the lower poles occurs in 96% of cases, with ureters passing anterior to the renal parenchyma and variation of arterial land venous blood supply. The isthmus, or connecting bridge, typically consists of renal parenchymal tissue; rarely is it fibrotic tissue. The most common complications associated with horseshoe kidney are kidney malrotation, urolithiasis, UPJ obstruction, and infection. The isthmus of the kidney lies anterior to the spine and may simulate a solid pelvic mass or enlarged lymph nodes ( Figure 15-35 ).
Evaluation of a renal mass
Before starting the sonographic examination for the evaluation of a renal mass, the sonographer should review the patient’s chart, including the laboratory findings and previous diagnostic examinations, which may include a plain radiograph of the abdomen, computed tomography (CT), or MRI. Whenever possible, these films should be obtained before the sonogram is done, so the examination can be tailored to address the clinical problem. The sonographer should evaluate the sonographic images to determine the shape and size of the kidney and the location of the mass lesion, to observe distortion of the renal or ureter structure, and to look for calcium stones or gas within the kidney.
Renal masses are categorized as cystic, solid, or complex by a sonographic evaluation. A cystic mass sonographically displays several characteristic features: (1) smooth, thin, well-defined border; (2) round or oval shape; (3) sharp interface between the cyst and the renal parenchyma; (4) no internal echoes (anechoic); and (5) increased posterior acoustic enhancement.
A solid lesion projects as a nongeometric shape with irregular borders, a poorly defined interface between the mass and the kidney, low-level internal echoes, a weak posterior border caused by increased attenuation of the mass, and poor through-transmission.
Areas of necrosis, hemorrhage, abscess, or calcification within the mass may alter the classification and cause the lesion to fall into the complex category. This means the mass shows characteristics associated with both cystic and solid lesions.
Sonography allows the sonographer to carefully evaluate the renal parenchyma in many stages of respiration. If the mass is very small, respiratory motion may cause it to move in and out of the field of view. Careful evaluation of the best respiratory phase combined with use of the cine-loop feature will allow the sonographer to adequately image most renal masses to determine their characteristic composition.
Aspiration of renal masses
Most renal masses that have met the criteria for a simple cystic mass do not require needle aspiration. The Bosniak classification of cysts is used to determine the appropriate workup for a cystic mass ( Table 15-2 ). A needle aspiration may be recommended to obtain fluid from the lesion to evaluate its internal composition.
|Simple cyst (I)||Thin, smooth wall, anechoic, round or oval in shape; increased through-transmission||None|
|Mildly complex cyst (II)||Thin septation or calcified wall||2–3 month follow-up with CT or sonogram|
|Mildly complex (IIF)||Atypical features; does not fall into category II||6–12 month follow-up|
|Indeterminate lesion (III)||Multiple septa, thickened septa, internal echoes||Biopsy or partial nephrectomy—increased risk for malignancy|
|Malignant lesion (IV)||Solid component, irregular walls||Nephrectomy|
The patient should be placed in a prone position with sandbags or rolled sheets under the abdomen to help push the kidneys toward the posterior abdominal wall and provide a flat scanning surface. Sterile technique is used for aspiration and biopsy procedures. The transducer must be gas sterilized. Sterile lubricant is used to couple the transducer to the patient’s skin.
The renal mass should be located in the transverse and longitudinal planes, with scans performed at midinspiration. Hold the transducer lightly over the scanning surface so as not to compress the subcutaneous tissue. The depth of the mass should be noted from its posterior to anterior borders, so the exact depth can be given to aid in placement of the needle. Compression of the subcutaneous tissue results in an inaccurate depth measurement. When the area of aspiration is outlined on the patient’s back, the distance is measured from the posterior surface to the middle of the lesion.
A beveled needle causes multiple echoes within the walls of the lesion. If the needle is slightly bent, many echoes appear until the bent needle is completely out of the transducer’s path. The larger the needle gauge, the stronger the reflection.
The patient’s skin is painted with tincture of benzalkonium (Zephiran), and sterile drapes are applied. A local anesthetic agent is administered over the area of interest, and the sterile transducer is used to relocate the lesion. The needle is inserted into the central core of the cyst. The needle stop helps ensure that the needle does not go through the cyst. The fluid is then withdrawn according to volume calculations. The volume of the cyst may be determined by measuring the radius of the mass and using the following formula: V = 4/3πr 3 .
The diameter of the mass can be applied to this formula:
V = d 3 /2
Lower urinary tract
Ureteral narrowing due to fibrosis is a common form of ureteral stricture. Ureteral strictures may also result from inflammatory disease, tuberculosis, localized periureteral fibrosis, impacted ureteral stone, schistostomiasis, iatrogenic ureteral injury, or radiation therapy. Other causes include amyloidosis, adjacent malignancies, metastases, extrinsic compression due to primary retroperitoneal tumors, enlarged lymph nodes, and medial lower pole renal masses ( Box 15-1 ).
Localized periureteral fibrosis
Impacted ureteral stone
Iatrogenic ureteral injury
Primary retroperitoneal tumors
Enlarged lymph nodes
Medial lower renal pole mass
A ureterocele is a cystlike enlargement of the lower end of the ureter ( Figure 15-36 ) caused by congenital or acquired stenosis of the distal end of the ureter. Ureteroceles are usually small and asymptomatic, although they may cause obstruction and infection of the upper urinary system. If large, a ureterocele may cause bladder outlet obstruction. Ureteroceles are found more often in adults than in children and may be unilateral or bilateral. On sonography, a cobra head appearance is seen in sagittal view.
A large ureterocele may fill the urinary bladder and have the same sonographic appearance as diverticula. If the patient can partially empty the bladder, a better diagnostic-quality image will be produced, as the ureterocele will be empty. One of the advantages of ultrasound is dynamic imaging; alternate filling and emptying of the ureterocele as the result of peristalsis may be demonstrated. Calculi may also be present.
Ectopic ureteroceles are rare and are found more commonly in children and young adults, especially in females. They usually are associated with complete ureteral duplication. The ureter, which empties the upper pole, inserts low in the bladder by the bladder neck, urethra, or lower genital tract. The ectopic ureter may become stenotic and cause ureteral obstruction, which is associated with hydroureter and hydronephrosis. The ureterocele sac may obstruct the bladder outlet or may prolapse through the urethra.
An ectopic ureterocele appears on sonography as a round, thin-walled cystic structure that may contain debris protruding into the bladder.
Ultrasound is not the imaging modality of choice to examine the bladder. Cystoscopy is usually used to examine the bladder because of its ability to diagnose early neoplasms. Transabdominal sonography will allow visualization of most lesions greater than 5 mm. A transurethral intravesicular sonographic approach has been used to evaluate bladder tumors.
The urinary bladder should be examined at the same time as the upper urinary tract. A complete review of the patient’s chart, including previous diagnostic imaging procedures, should be conducted before a sonographic examination of the bladder is begun.
A sonogram of the bladder is obtained with a distended bladder. The patient lies in a supine position. A right or left decubitus position may be used to demonstrate movement of calculi. Proper adjustment of the TGC allows for minimization of anterior wall reverberations and anechoic bladder, with posterior acoustic enhancement. The depth of the image should be set to visualize any structure that may lie posterior or caudal to the bladder. A 3.5-MHz transducer is usually used. In very thin patients, a 5-MHz transducer may be used. If evaluation of the anterior bladder wall is indicated, a high-frequency, curved linear-array transducer or linear transducer will give a larger anterior field if view.
The transducer should be placed in the middle of the filled urinary bladder and angled laterally, inferiorly, and superiorly. The bladder walls should be smooth and thin (3 to 6 mm). The bladder should be midline and should not be deviated to either side or have any irregular or asymmetric indentations.
Sonography is used to evaluate residual bladder volume in patients with outflow obstruction. The postvoid bladder is scanned in two planes: anteroposterior and transverse. Measurements are obtained in three planes: anteroposterior, transverse, and longitudinal. Images and measurements are obtained at the largest dimensions. Because bladder shape varies, any volume measurement can be used to approximate volume. A residue of less than 20 ml of urine is considered normal in an adult.
Ureteral jets should be identified as flashes of Doppler color entering the bladder from the lateral posterior border of the bladder and coursing superior and medial.
An enlarged prostate, enlarged uterus, pelvic mass, or filled loop of bowel may indent and displace the urinary bladder. Box 15-2 lists the conditions under which the bladder may not empty completely.