Ultrasonography was initially developed as a military tool and was adopted by the medical world after World War II. In 1961, Schlegel et al first reported the usefulness of sonography in urology, for the detection of renal calculi.

Ultrasonography is mainly performed by radiologists. However, nonradiologist clinicians commonly perform and interpret specific types of ultrasound (eg, obstetricians, fetal ultrasound; urologists, transrectal ultrasound [TRUS] of the prostate). The ideal ultrasound examination is one in which the clinician interprets it in real time while it is being performed. Therefore, physicians should observe ultrasound studies during the examination.

Ultrasound Equipment and Physics

An ultrasound probe is a housing structure for an ultrasound transducer and the associated wiring for connection to a console with a computer. The probe is shaped for the desired application, eg, cylindrical for endorectal use. The transducer generates high-frequency sound waves (typically 5-10 MHz) and directs them through body tissues via a probe held against the skin. Various probes and transducers are available for examination of different organs and body parts. The probe also contains a receiver to detect sound waves (called echoes) reflected from tissues.^[2]

Through a process called acoustic-electric conversion, the transducer transforms the sound energy into electrical energy. The electrical energy is processed by the computer in the ultrasound console to generate an image of minute white dots (pixels) corresponding to the returning signals. Displayed on a black background, the white pixels produce an image of assorted shades of gray. When the sound waves travel easily through uniform substances (eg, water, oil, urine), no echoes are generated. The ultrasound image seen on the screen is therefore black; no echoes are present. When the sound waves encounter tissues of different densities, the sound waves are absorbed, reflected back to the probe, or transmitted through the tissue at different velocities. When this happens, the ultrasound image is white or gray depending on the intensity of the reflection.

Unlike radiography or CT scans, ultrasound does not reveal tissue density. Rather, it shows sonotransmission (the passage or reflection of sound). Highly dense tissues, such as bone or kidney stones, readily reflect echoes and, therefore, appear bright white on an ultrasound image. Air, such as in the bowel, also readily reflects echoes, so the edge of the bowel appears white on an ultrasound image. Thus, substances with widely differing densities (eg, air, bone) may appear bright white on an ultrasound image. The range of gray shades generated lends this imaging technique the alternative label "gray-scale imaging," which distinguishes it from color Doppler ultrasonography.

Sound waves are emitted and received by transducers in a single, narrow band. In order to produce a recognizable image, the transducer must be swept across the area of interest, producing multiple bands that are combined to form an image. The system for moving the transducer is called the scanner. As the speed at which the scanner sweeps across the imaged area decreases, resolution increases.

Coupling medium

The acoustic properties of soft tissue are very similar to those of water, but air is distinctly different; the presence of air between the probe and the tissue of interest can distort or obscure the image. For this reason, a water-density substance, termed a coupling medium, is used for transmission of the ultrasound image. This coupling medium is usually a sonographic jelly or lubricant and should be placed between the probe and the skin surface.

Artifacts

The computer within the ultrasound console is designed with the assumption that ultrasound signals propagate through tissue at a constant velocity and reflect back to the transducer in a narrow, straight line. In fact, the velocity and angle of ultrasound wave propagation is affected by different tissue density, the rate of change in these tissue densities (abrupt vs gradual), and the dimensions and configuration of the transducer. Such variations can lead to deviation of the ultrasound signals from the assumed direction of propagation, creating artifacts. Kossoff cautions that a feature should not be considered by the examiner to be real simply because it is displayed sonographically until it is appropriately evaluated from various angles. Conversely, a feature that is not displayed is not necessarily absent. ^[3]

Shadowing can occur due to intense reflectors such as calcifications or air, and increased through-enhancement is exhibited with fluid-filled structures such as cysts.

Reverberation is an artifact caused by the ultrasound signal striking a very echogenic surface near the ultrasound transducer. This signal is ricocheted back and forth between the transducer and the reflector. An image representing the echogenic structure is accurately produced on the monitor, but each subsequent reflection of the sound wave back and forth, which takes twice as long to reach the transducer as the prior reflection, is interpreted by the transducer as another structure. This results in artifactual images, equally spaced, distal to the original reflector, and of decreasing intensity. In TRUS imaging, this effect is usually produced by the rectal wall and condom covering the probe and results in multiple hyperechoic arches evenly spaced between the rectal wall and the anterior aspect of the image. Such an effect can be minimized by using copious amounts of coupling medium and ensuring that no air is between the probe and the rectum.

Phase cancellation of the ultrasound signal can occur when the signal laterally strikes a curved structure, reflecting the signal laterally away from the transducer. The transducer interprets this absence of signal as an absence of tissue, which results in a lack of echoes on the image. In TRUS imaging, this artifact is often encountered during transverse sector scanning of large prostates. The sound waves striking the curve of the posterolateral margin of the prostate are scattered, resulting in a hypoechoic shadow extending anteriorly and laterally from this edge of the prostate. Similar shadows can also be generated by the posterolateral margin of the transition zone (TZ), resulting in additional shadows. These shadows can be minimized by centering the probe under the lateral portions of the gland when inspecting this area of the prostate.

Lateral and anterior refraction, also called dispersion or scatter, of the ultrasound signals is spread in a fanlike configuration away from the central portion of the image. Anterior refraction is not a significant problem in TRUS imaging of the prostate because the gland is relatively close to the transducer. Lateral refraction during sector scanning results in elongation of the lateral aspect of the image, creating a more bean-shaped image of the prostate than the actual spherical configuration of the gland.

Renal Ultrasound

Normal appearance

The cortex (the periphery of the kidney tissue) is seen as gray with some darker circles spaced uniformly around the edge. These darker circles correspond to the renal pyramids. A dromedary hump, the incidental finding of an extra mass of normal renal cortex tissue only on the lateral portion of the left kidney, is a normal variant.

Relative to the liver parenchyma, the kidney is isoechoic (the same shade of gray) or slightly hypoechoic (a darker shade of gray). The liver is superior to the kidney and superficial (towards the top of the image). It is rather homogeneous (with a fairly regular gray pattern). The kidney is not as homogeneous. These 2 organs can be compared to determine if renal medical disease is present.

The center, or hilum, of the kidney contains multiple structures, such as the renal pelvis, blood vessels, nerves, fat, and lymphatics. The fat of the renal sinus is particularly echogenic (bright white). These structures transmit sound differently, and, as the sound waves hit these interfaces between 2 such structures, an echo is generated. Because of this, the renal hilum has increased echogenicity.

A prominent column of Bertin is partial hypertrophy of the renal cortex protruding into the renal sinus, and this is another normal variant. The upper pole of the kidney, particularly on the left, can sometimes be hidden behind rib shadows. This fact makes it very important for the sonographer to use breathing techniques and multiple windows to visualize the entire kidney.

Doppler ultrasound examination is a useful adjunct to renal sonography. Color Doppler imaging can reveal whether blood flow to an area of tissue is increased, decreased, or normal, narrowing the differential diagnosis.

In addition to the presence of blood flow, a waveform tracing can be generated to show the nature of the flow throughout the cardiac cycle. Normally, a renal waveform tracing shows a rapid rise to a peak during systole and a slow fall to a plateau during diastole. The tracing remains above the baseline throughout, indicating flow throughout systole and diastole. The resistive index is a ratio and is essentially a measure of the end-diastolic flow in the arterial system (resistive index = [peak systolic - end diastolic]/peak systolic). Typical resistive index is 70%. The higher the resistive index, the higher the downstream resistance. A resistive index higher than 90% is definitely abnormal and suggests the possibility of renal artery stenosis. A high resistive index can also indicate ureteral obstruction and may serve as an indirect sign for a ureteral stone. Power Doppler is significantly more sensitive to flow than standard color Doppler and can demonstrate blood flow on the arteriolar level.

Obstruction

Ultrasound is the initial imaging modality of choice in any patient with an unexplained elevation of the creatinine level or recent onset of renal dysfunction. In most instances, the screening ultrasound images in the setting of acute renal failure are normal. Certain risk factors, such as prior obstruction, recent surgery, and pelvic neoplasms, can increase the likelihood of finding an obstructed kidney.

The crucial finding of obstruction in renal ultrasound is renal pelvis and calyceal dilation characterized by effacement of the renal sinus fat by an anechoic-branched structure with posterior enhancement and through-transmission. This dilation involving the calyces and renal pelvis is termed hydronephrosis or pelvocaliectasis. Dilation of the ureter is referred to as hydroureter. Combined ureteral and renal pelvic dilation can be called hydroureteronephrosis. The degree of collecting system dilation is graded subjectively based on ultrasound findings; however, these terms are most useful in the serial follow-up care of a given patient. Mild dilation can be associated with complete obstruction, and marked dilation can be present without any accompanying obstruction.

The size of the kidney and ureteral findings on ultrasound may aid in diagnosing the etiology of hydronephrosis. Hydronephrosis may be caused by obstruction (eg, ureteropelvic junction obstruction, stones) or by other causes (vesicoureteral reflux). Renal size is important because significant chronic obstruction can result in postobstructive renal atrophy. The presence of a dilated ureter may help to identify the location of any obstruction. Assess the kidney and visualized ureter for possible calculi.

Transrectal or transvaginal scanning may aid in the identification of distal ureteral calculi. The ultrasonographer should look for and identify the presence of ureteral jets in the bladder to rule out obstruction. Further studies may be required to determine whether the dilation is functionally significant; a diuretic nuclear renal scan is the most common functional study used to make this distinction.

Findings that can be confused with hydronephrosis due to obstruction include an extrarenal pelvis, prominent renal vasculature, residual dilation from previous obstruction, dilation as a result of ureterovesical reflux, congenital megacalices, papillary necrosis, pyelonephritis, distended urinary bladder, diabetes insipidus, and other, less-common causes. Pregnant women, patients who have undergone surgical repair of anatomic obstruction, and patients with urinary diversion or urinary stents normally display some degree of dilation of the collecting system, making the diagnosis of hydronephrosis due to obstruction particularly difficult in these patients. In contrast, obstruction can be present in the absence of hydronephrosis, in conditions such as early acute obstruction, hypovolemia, retroperitoneal metastases, and retroperitoneal fibrosis.

Medical renal disease

Renal disease caused by glomerulonephritis or systemic illnesses, such as diabetes, hypertension, arteriosclerosis, or autoimmune diseases, results in kidneys that are hyperechoic (brighter gray) compared to the adjacent liver parenchyma. Also, the kidneys are often smaller than normal.

Pyelonephritis sonography

In most cases of acute pyelonephritis, ultrasound images of the kidneys appear normal, with sensitivity for detecting cortical inflammation that is lower than CT scan or radionucleotide imaging. However, sonography can be very useful for detecting inciting or exacerbating abnormalities such as stones, hydronephrosis, or cysts. Mild hydronephrosis may be associated with uncomplicated pyelonephritis due to diminished ureteral peristalsis. Ultrasound is the imaging modality of choice for acute pyelonephritis in pregnant women.

Renal enlargement and compression of the renal sinuses can be detected sonographically in severe cases of pyelonephritis. Ill-defined areas of hypoechogenicity may be seen, due to edema, or areas of hyperechogenicity may be present, due to hemorrhage. These changes may be accompanied by effacement of the corticomedullary boundaries. Ultrasound can be used to detect and monitor the progress of focal inflammatory masses and to evaluate complications of pyelonephritis, such as renal abscesses or pyohydronephrosis.

Emphysematous pyelonephritis is characterized by gas within the renal parenchyma due to infection with gas-forming bacteria. Sonography shows hyperechoic foci with vague shadowing and distortion of the renal sinus due to the presence of air in the system. Ultrasound often underestimates the extension of the infection because structures deep to the gas collections are obliterated. As a result, CT scan should be performed immediately for further assessment if emphysematous pyelonephritis is suggested.

Pyonephrosis

Please see the Medscape Reference article Pyonephrosis for details on this entity.

Nephrocalcinosis

Medullary nephrocalcinosis is usually associated with hypercalcemic states. Hyperparathyroidism and distal renal tubular acidosis account for more than 60% of cases. The remaining 40% are due to a variety of other causes of hypercalcemia and hypercalciuria, such as Cushing syndrome, sarcoidosis, bone metastasis, furosemide therapy, or excessive dietary calcium and/or oxalate. Both kidneys reveal multiple areas of increased echogenicity involving all the renal pyramids. The renal cortex is normal, and no evidence of hydronephrosis is present.

Renal hemorrhage

Renal parenchymal bleeding due to trauma or surgery is usually present as a subcapsular hematoma that is echogenic immediately after the hemorrhage but becomes progressively less echogenic as the hematoma resolves. Within days, the appearance is anechoic with a fluid-debris level. Blood within the renal collecting system and hemorrhagic cysts are discussed below.

Collecting system masses

The differential diagnosis of masses within the renal collecting system includes soft tissue masses such as transitional cell carcinoma (TCC); blood clots; fungus balls; sloughed papilla; fibroepithelial polyps; invasion by renal cell carcinoma, vessel impressions, or metastatic tumors. Other masses include stones and the presence of air.

• Soft tissue: On static gray-scale imaging, blood clots, fungus balls, and sloughed papillae are difficult to distinguish from abnormal growths that involve the renal collecting system. During real-time imaging, these masses can often be appreciated as floating freely in the renal pelvis and, on Doppler imaging, are devoid of blood flow. TCC, fibroepithelial polyps, intrapelvic extension of renal parenchymal tumors, and metastatic lesions show normal-to-increased flow on power Doppler images. An adjacent blood vessel compressing the renal pelvis to create the appearance of an intraluminal lesion shows the typical dramatic intravascular blood flow on Doppler images. Intravenous pyelogram, CT scanning, and/or ureteroscopic examination is often necessary to confirm the diagnosis.
• Renal calculi: Renal calculi are depicted sonographically as intensely hyperechoic linear or arching structures that conform to the surface shape of the stone that is closest to the ultrasound transducer. Dramatic posterior shadowing is seen, which can obscure any findings on the opposite side of the stone, including hydronephrosis, additional stones, and tumors.

  • Because of this, the finding of calcifications requires the sonographer to scan from several different angles to get a clear picture of the location and number of stones. Identification is highly dependent on the skill of the sonographer, and stones smaller than 5 mm in diameter are difficult to identify.
  • An intravenous pyelogram, noncontrast CT scan, and/or plain film of the abdomen can provide more information than ultrasound images for quantification and treatment planning for urinary stones. In a study comparing ultrasound to noncontrast CT scan by Fowler et al, the sensitivity of ultrasound for any calculi was 44% and, for individual calculi, was 24%. Of calculi not visualized on ultrasound images, 73% were smaller than 3 mm. ^[4]
• Air: Air in the renal collecting system has similar echogenicity to stones, but, rather than occurring in the more dependent portions of the collecting system, air rises to the most superior aspect of the collecting system and is more mobile than stones. Repositioning of the patient to take advantage of this principle aids in the diagnosis. Causes of air within the collecting system include recent open or endoscopic surgery, fistula of the renal collecting system to the intestinal or female reproductive tract, or emphysematous pyelitis or pyelonephritis.

Cysts are the most common masses found in the kidney. They can occur singly or in multiple numbers and are thought to arise from epithelial overgrowth of tubules or collecting ducts, with resulting distention of the nephrons. The chance of developing renal cysts increases with age. They are very rare in children and uncommon in individuals younger than 40 years, but they are present in 50% of the population older than 55 years. Cysts also tend to enlarge over time and develop thin septations. Ultrasound is the modality of choice when imaging renal cysts. The initial assessment of a possible cyst is a determination of whether it is a fluid-filled, solid, or semisolid mass. A fluid-filled cyst appears anechoic, while a solid cyst or a cyst containing debris appears much more echogenic or shows low-level echoes within the lumen. Also, strong posterior enhancement with an anechoic lumen almost certainly excludes a solid mass.

Cysts in the lower pole of the kidney are seen more easily because the ribs are not superimposed. Anterior artifacts within the cyst can be caused by reverberation from the skin-transducer interface. Transducer interface can be suppressed by scanning from opposite approaches.

• Simple cysts: The most common type of renal cyst is a simple cyst. Simple cysts are collections of serous fluid, usually originating in the renal cortex, which are predominantly asymptomatic.

  • Cysts must meet several criteria before they can be considered simple. They must be spherical, have an anechoic lumen without internal echoes, have a well-defined back wall or clear wall demarcation, have no measurable wall thickness, and show acoustic enhancement posterior to the cyst.
  • Ultrasonically, the cyst must show a narrow band of acoustic edge-shadowing bordering the acoustic enhancement posterior to each lateral border of the cyst. This shadowing is due to the refraction and deflection of echoes around the curved surface of a cystic mass. Three percent of simple cysts show simple curvilinear calcifications in association with the cyst wall. Usually, such calcifications are the result of prior hemorrhage, infection, or ischemia. If calcifications are thick and globular, the cyst can no longer be considered simple and this finding may indicate malignancy.
  • Occasionally, a renal cyst that is not distended will be collapsible, showing an irregular border of the portion of the cyst that is in contact with the renal parenchyma. This border irregularity is considered an artifact and does not exclude a diagnosis of simple cyst.
  • The differential diagnosis of simple renal cysts includes caliceal diverticula, aneurysms, pseudoaneurysms, arteriovenous malformations, papillary necrosis, obstructed upper pole duplications, and lymphoma.
  • True simple cysts require no form of intervention, especially if they are asymptomatic. Diagnosis of a cyst based on ultrasound findings is 95-98% accurate. Incorrect diagnosis of a simple cyst can occur if the lesion is smaller than 2 cm in diameter. Another reason for incorrect diagnosis is the occurrence of hematomas, which can mimic cysts. Doppler ultrasound can be useful in differentiating simple cysts from other types of cysts. If a cyst does not meet all the criteria for a simple cyst, then a CT scan is indicated.
• Complex cysts: Renal cysts are usually unilocular, ie, they have one chamber inside. Septated cysts are multilocular, ie, with multiple chambers.

  • Septated cysts contain linear internal echoes on ultrasound images, which represent the septations. The septa are usually incomplete, allowing free communication between the different areas of the cyst. The septa should appear thin. Thick septations, if seen, may indicate a cystic neoplasm. Septations are seen in only 5% of all benign renal cysts. The sonographer must make a determination on septation thickness, remembering that this is an important distinction, ie, a distinction between benign and possibly malignant cysts.
  • Bilobed renal cysts are not characteristically oval or circular. Cysts contain no septations within but have 2 obvious compartments as seen by the shape of the cyst. These cysts are unusual and do not account for a large percentage of the total occurrence of renal cysts.
• Classification of renal cysts: Most renal cysts are benign. Sonography alone cannot distinguish between benign and malignant cysts, but the use of the Bosniak classification can help categorize the probability for malignancy.

  • Bosniak classification of renal cysts is as follows:
    • Type I - Simple cysts that are anechoic with good through-transmission, posterior wall acoustic enhancement, no internal echoes, sharply marginated, and a smooth wall
    • Type II - Mildly complicated cysts that have thin septa, have small areas of calcification, and are hyperdense
    • Type IIf - Mildly complicated cysts that have thicker septa and small areas of calcification or are hyperdense with heterogenous areas (They are designated type IIf to denote the need for serial follow-up involving repeat imaging to ensure no changes suggest malignant degeneration.)
    • Type III - Indeterminate cysts with large, irregular calcification; thickened, prominent septa; and thick walls
    • Type IV - Malignant cysts that have a large solid component with cystic areas having irregular margins, increased vascularity, very thick, irregular septations, and large/multiple dystrophic calcifications
  • If growth of a complex cyst is seen on subsequent examinations, consider malignancy and perform further testing. A cyst with thin septations can be considered benign, but a cyst with thick septations or thick globular calcifications may indicate malignancy.
• Parapelvic cysts: Parapelvic cysts account for 6% of renal cysts. They are usually lymphatic in origin or arise from other nonparenchymal tissues. Sonographically, they are visible within the fat of the renal sinus and are often bilateral. Parapelvic cysts can be confused with hydronephrosis; the use of real-time ultrasound is important to demonstrate that these cysts do not communicate with the collecting system of the kidney. Parapelvic cysts have an irregular shape, conforming to the contents of the renal sinus, and may envelop other components of the renal hilum, producing apparent septa.
• Hemorrhagic cysts: Hemorrhagic cysts are usually simple cysts that have bled, and they account for up to 11.5% of all renal cysts. An intraluminal hemorrhage occurs in approximately 5% of normal simple cysts, and the chance of simple cysts hemorrhaging is greater in persons with polycystic kidney disease. The blood of an acute hemorrhage is very echogenic and appears bright on ultrasound images, conforming to the shape of the cyst. Resolving hemorrhagic cysts show internal echoes that may lie in the dependent portion of the cyst, producing a fluid-debris level or fibrinous membrane and lack of posterior acoustic enhancement. The reported incidence of neoplasms associated with hemorrhagic cysts is 31%. Therefore, repeat imaging in 2-3 months is indicated for follow-up and accurate diagnosis.
• Infected cysts: Infected cysts are simple cysts that have become infected and contain debris or purulent material. The walls appear thicker on the sonogram, with complex internal echo patterns. These cysts can also contain necrotic exudate; this occurs when particulate matter floats in the fluid of the cyst. This diagnosis can be bolstered by the occurrence of related symptoms in the patient, such as fever or other infections. The combination of the clinical findings, CT imaging, and cyst aspiration confirms the diagnoses.
• Milk of calcium cyst: The milk of calcium cyst is a unique renal cyst. Calyceal diverticula that have either partial or complete loss of communication with the collecting system are subject to stasis of urine, with resulting calcium carbonate crystallization. These minute crystals are partially suspended in the cyst fluid. With repositioning, the crystal suspension flows to more dependent areas of the cyst in a fluid fashion, giving the milk appearance on plain films or fluoroscopy. This crystal-containing fluid can be visualized ultrasonically and causes acoustic shadowing within the cyst, excluding the diagnosis of simple cyst.
• Calcified cysts: Calcified cysts are simple cysts in which calcifications have formed. This occurs in only 1-2% of all simple cysts. These cysts are usually hemorrhagic or infected, and, after a certain period, the internal debris has calcified. Calcifications can diminish sound transmissions and cause the cyst to appear solid. Many calcifications require the sonographer to scan from several different planes to get a clear picture of the location and number of calcifications. Thick globular calcifications are associated with malignancy.
• Multiple cysts and polycystic kidney disease: Ultrasound can reveal adult or autosomal dominant polycystic kidney disease in patients in their second and third decades of life. Ultrasound demonstrates multiple cysts of varying sizes bilaterally. Hemorrhagic cysts may have internal echoes, fluid-debris levels, and thick walls. Differentiation between hemorrhage and infection is impossible based on ultrasound findings alone. Dystrophic calcification secondary to hemorrhage may be curvilinear or with scattered plaques. Associated cysts of the liver, pancreas, or spleen appear as echo-free masses with increased through-transmission.
• Recessive or infantile polycystic kidney disease: This condition demonstrates a combination of renal and hepatic manifestations and manifests in infancy. Sonographically, the kidneys are enlarged bilaterally and diffuse and increased renal parenchymal echogenicity is seen. No distinct corticomedullary junction is present, and no hydronephrosis is identified. Pathologically, the kidney has innumerable small cysts that represent dilation of the collecting tubules. Liver involvement results in proliferation of bile ducts and mild periportal fibrosis. Consider this diagnosis when the kidneys are enlarged and hyperechoic and the bladder is small, especially in the prenatal setting of oligohydramnios.
• Multicystic kidneys: The multicystic kidney is characterized by multiple simple cysts that increase in number and size as patients age. Medium-to-large cysts can be visualized easily, but small cysts may be difficult to clearly define. Clusters of small cysts appear as a highly echogenic region.
• Multicystic dysplastic kidneys: Twenty years ago, most infants with a multicystic dysplastic kidney (MCDK) presented with a large palpable flank mass. Today, most are diagnosed in utero with prenatal ultrasound. The majority of MCDKs are unilateral and represent the most common cause of abdominal masses in infancy. Approximately 15% of patients have some abnormality of the contralateral kidney. Bilateral MCDK is lethal. The differentiation of MCDK from hydronephrosis is essential because treatment options vary. Perform nuclear medicine studies to assess function. Ultrasound over the palpable mass reveals multiple noncommunicating cysts that occupy the entire renal fossa, with no evidence of renal parenchyma. The cysts are variable in size, with the largest cysts located in the periphery. Typically, upon follow-up ultrasound, the MCDK shrinks and may eventually disappear. However, malignant degeneration has been reported.

Kidney tumors appear as heterogeneous echogenic masses and may be accompanied by slightly dilated adjacent renal calyces, due to compression of infundibuli. Consider any solid renal mass malignant until it has been proven otherwise. Eighty-five percent of solid renal masses are renal cell carcinoma, while another 10% are due to other malignancies such as renal sarcoma, lymphoma, TCC, or metastases. The remaining 5% are benign and include oncocytoma, angiomyolipoma, and fibroma. Of these, only angiomyolipoma is distinguishable from the others sonographically, due to the presence of the highly echogenic fat within these tumors. Intraoperative renal ultrasound is now becoming commonplace for locating tumors during partial nephrectomy.

Renal transplant sonography

The superficial placement of a renal graft in the iliac fossa makes sonographic imaging of this structure easier than imaging native kidneys. Most patients are referred for ultrasound because of deteriorating renal function, as demonstrated by a rise in serum creatinine levels and/or decreased urine output. In the immediate postoperative period, renal vein thrombosis is a major complication, resulting in graft failure. As might be expected, the vascular resistance of the kidney is extremely high, and spectral Doppler tracing shows a rapid, transient peak systolic spike followed by reversed diastolic flow. The primary role of ultrasound in kidneys that have survived the immediate postoperative dangers is to differentiate between ureteral obstruction and systemic causes of reduced function, such as rejection or acute tubular necrosis.

In severe rejection, the transplant acutely becomes more echogenic. Swelling is present, hypoechogenicity of the medullary pyramids is seen, the corticomedullary junction is indistinct, and the globular size of the kidney is increased. The size increase is not specific for rejection; it may be seen in persons with pyelonephritis and transplant vein thrombosis. Color Doppler is used to identify the interlobar and arcuate vessels of the transplanted kidney. Indications for Doppler ultrasound in renal transplant recipients include measurement of the resistive index in intrarenal arteries, possible renal vein thrombosis, renal artery stenosis, and renal cortical perfusion.

Renal vascular disease

The kidneys are highly vascular organs. In their normal state, they show a low resistance to blood flow. Doppler ultrasound has been shown to be useful in hypertension, renal transplantation, and urinary tract obstruction. Primary renal abnormalities are an identifiable cause of hypertension in approximately 4% of patients. Renal artery stenosis occurs in approximately 1-4% of patients with hypertension and is usually due to atheroma or fibromuscular hyperplasia. Doppler ultrasound can help identify renal artery stenosis with examination of the main renal artery or at the intrarenal arterial flow. A spectral Doppler trace from the main renal artery can reveal the typical high-peak velocities of an arterial stenosis. Peak systolic velocities faster than 200 cm/s strongly suggest renal artery stenosis. The upstream narrowing caused by renal artery stenosis results in a damped downstream intrarenal waveform, known as parvus et tardus (small and late).

Bladder Ultrasound

Normal appearance

The full bladder provides a sonographic window for evaluation of the uterus and adnexa and serves to displace bowel gas. On longitudinal view, the full bladder has a teardrop-shaped anechoic appearance, while on the transverse view, it appears rectangular. The thickness of the bladder wall varies with the degree of bladder filling. Masses within the bladder can be diagnosed with relative certainty only if the bladder is full. Infoldings of the wall of an incompletely filled bladder can give the impression of an intraluminal mass.

Bladder diverticula

Most diverticula are acquired secondary to bladder outlet obstruction from neuropathic voiding dysfunction, prostate enlargement, or strictures of the urethra. Acquired diverticula start as small outpouchings of mucosa that evaginate between hypertrophied detrusor muscle bundles and do not extend past the bladder wall. These small outpouchings are termed cellules. With persistent obstruction, progressive herniation of the bladder mucosa occurs through the detrusor muscle to form a diverticulum. The congenital type of diverticulum, a Hutch diverticulum, develops from herniation of the bladder mucosa through a congenital weakness in the detrusor muscle of the bladder slightly superior and lateral to the ureteral orifice. Occasionally, these can cause obstruction or vesicoureteral reflux (VUR).

A bladder diverticulum appears as an anechoic, thin-walled extension from the bladder lumen. The diverticulum can range in shape from teardrop to semicircular, depending on the width of the neck of the diverticulum. Usually, continuity between the bladder and diverticulum can be demonstrated sonographically. If the diverticulum has a narrow neck, infection and stone formation can occur, appearing, respectively, as echogenic debris or a hyperechoic mass with posterior shadowing. Tumors can also occur within the diverticulum because they are lined by uroepithelium. While a small tumor in a diverticulum can occasionally be seen as an irregular mass of medium echogenicity within the diverticular lumen, larger tumors can be difficult to detect sonographically because a diverticulum filled with tumor may be indistinct from the adjacent hypertrophied bladder wall.

Ureterocele

Ureterocele is a prolapse into the bladder of the intravesical portion of the distal ureter with an associated dilation of the distal ureter. Ureteroceles appear as thin-walled curvilinear structures with an anechoic lumen. When the bladder is filled and the ureterocele is extended, the ureterocele is sonographically reminiscent of a thin septation within the bladder.

Orthotopic ureteroceles form in the ureter with a normal insertion into the trigone. They are seen sonographically on the posteroinferior bladder wall. Because of their low posterior location, orthotopic ureteroceles can often be visualized better by TRUS in males or transvaginal ultrasound in females than by transabdominal ultrasound. Orthotopic ureteroceles usually occur in single systems and are usually small, unilateral, and asymptomatic.

Ectopic ureteroceles usually occur in duplicated collecting systems and can cause partial or complete obstruction. These are usually located posteriorly, but closer to the midline and further inferiorly than orthotopic ureteroceles. Because of their inferior location, ectopic ureteroceles may not be visible by transabdominal ultrasound because the pubic bone can interfere with the imaging angle. Infection, stone formation, or both can occur within the lumen of the ureterocele and is seen as debris or as a hyperechoic mass with posterior shadowing within the ureterocele, respectively.

Bladder masses

The classic appearance of TCC within the bladder is of an irregular soft tissue structure of low- to intermediate-echo texture projecting into the bladder lumen from a fixed mural site. Lesions are often branched or frondlike. The attachment to the bladder wall may be either sessile or stalklike. This appearance is indistinguishable sonographically from other neoplasms such as squamous cell carcinoma (SCC) or direct extension into the bladder from prostate, colon, or gynecologic malignancies. Calcification is detected in TCC in fewer than 2% of cases. Bullous edema from inflammation due to cystitis or irritation from a catheter or bladder stone can also appear as an irregular mass but is less echogenic than TCC. Bullous edema may have small anechoic areas, representing edema fluid, within the mass. Carcinoma in situ and squamous metaplasia are generally undetectable by ultrasound but may be seen as focal bladder wall thickening.

Bladder volume measurement

Bladder volume can be calculated by various geometric formulas with commercially available portable bladder scanners. Bladder volume measurements are generally more accurate if the patient is relaxed and supine. A different formula is used for men and women to allow for the different shape of the bladder between sexes. In a woman who has had a hysterectomy, use the male scanner setting.

Prostate Ultrasound

Normal appearance

Understanding the prostate anatomy is essential to understanding ultrasound images of the prostate. The prostate has 3 anatomic areas or zones. The peripheral zone (PZ) is the posterior and distal portion (the apex) of the gland. This zone is palpable upon digital rectal examination, comprises the bulk of the volume of the normal prostate gland, and is the site of origin of the vast majority of prostate cancers. The central zone (CZ) is a small, cone-shaped zone at the cephalad aspect of the prostate (the base) surrounding the ejaculatory ducts; cancers arising from this zone are quite rare. The TZ is composed of 2 lobes of tissue anteriorly, on either side of the urethra; this zone is the site of origin of benign prostatic hyperplasia (BPH) and approximately 10-20% of prostate cancers; the size of this zone is extremely variable, depending on the degree of BPH.

The anatomic distinction between the CZ and PZ is not ordinarily visualized by TRUS, despite the distinctive histologic differences of these zones. The normal CZ and PZ should be a homogeneous light- to medium-gray area occupying the posterior third of the prostate. Relative to these 2 zones, the anteriorly located TZ exhibits moderately heterogeneous hypoechogenicity. BPH nodules in the TZ can be isoechoic or hyperechoic but are most often hypoechoic. This heterogeneity and hypoechogenicity becomes progressively more prominent with increasing volume of BPH and is most likely due to variations in the amount of stromal and glandular elements of the hyperplasia.

The TZ may represent as little as 5% of a normal young man's prostate and more than 90% of a prostate with BPH. With increased size of the TZ, the PZ and CZ become progressively compressed posteriorly. Normally, the TZ does not extend past the verumontanum in the middle portion of the prostate. However, the TZ also expands distally as it enlarges, progressively overhanging the apical PZ. BPH often contains single or numerous cystic structures of various sizes. Cysts appear as anechoic (black) smooth-walled, spherical structures with increased through-enhancement. Increased through-enhancement is caused by the ultrasound waves moving rapidly, without reflection through the low-impedance cyst fluid, then abruptly striking the opposite side of the cyst. The time-gain compensation and higher density of sound waves reaching the opposite wall of the cyst and the tissues beyond it make these areas appear brighter than the surrounding tissues.

The boundary between the TZ and PZ is the surgical capsule for transurethral prostatectomy and is often sharply demarcated by TRUS as a hypoechoic convex line. With increasing BPH, this boundary becomes less convex. This margin is often peppered with corpora amylacea that are markedly hyperechoic. When more calcified and concentrated, these deposits can completely interrupt the ultrasound waves, causing posterior shadowing (a familiar finding with renal calculi on ultrasound) that obscures some or all of the TZ. Such calcifications have been correlated with a history of prostatitis but are often seen in healthy young men with no history of prostatic inflammation.

The preprostatic sphincter, composed of the bladder neck and periurethral tissue, is located between the 2 lobes of the TZ. This structure demonstrates the dramatic hypoechogenicity typical of muscle due to the concentration of smooth muscle fibers. These muscles appear as an inverted Y, which has been termed the Eiffel Tower sign, in the axial plane and as a curved, triangular shape similar to a tornado in the sagittal plane. These muscle fibers often intermingle with the nodules of BPH.

The lumen of the prostatic urethra is not usually visible sonographically unless it has been surgically altered, such as with transurethral prostatectomy, or unless it is distended during sonography. When TRUS is performed in preparation for brachytherapy, the urethra is commonly distended with lubricating jelly or a urethral catheter in order to avoid seed placement near the urethra. In the apex, or distal prostate, in the axial plane, the periurethral tissue appears as a hypoechoic-inverted horseshoe. In the sagittal plane, this structure appears tubular and can be followed along its course to the external sphincter and proximal bulbar urethra.

The pubic bone demonstrates a hyperechoic margin with dramatic posterior shadowing. The inner margins of the pubic bone can be recognized and compared to the outer margins of the prostate during evaluation for pubic arch interference during prostate brachytherapy. If present, this interference is usually resolved by reducing the prostate volume with preimplant androgen deprivation therapy. The thick muscular wall and fluid-filled lumens of the seminal vesicles, vas deferens, and ejaculatory ducts lend these structures a dark-gray echo pattern. Anterior to the seminal vesicles, the muscular bladder wall is dark gray and of variable thickness. Urine within the bladder is anechoic and can aid in delineating the anterior extent of the prostate and the presence of any median lobe.

Due to their predominantly adipose composition, the periprostatic tissues are generally quite echogenic, appearing almost entirely white on ultrasound images. The posterolateral aspect of the prostate margin, where the neurovascular structures enter the gland, are generally dark gray, with areas that are completely anechoic due to the fluid-filled, thin-walled veins. With the patient in left lateral decubitus position, the dependent left neurovascular bundle is often more prominent than the right. The anteromedial venous structure of the dorsal vein complex can also be seen as anechoic, somewhat linear structures, within the white periprostatic adipose. Because the scanner rate for prostate sonography is slow, flow in these vascular structures is generally not apparent.

Extending toward the rectum from the dramatic anterior shadowing of the pubic bone, just lateral and distal to the prostate apex, is the levator musculature. This muscle has a distinctive hypoechoic appearance, with hyperechoic parallel streaks representing the adipose-containing fascia separating the muscle bundles.

Anatomic variations

Villers et al evaluated 100 radical prostatectomy specimens, correlating anatomic variations with ultrasound findings. ^[5] In 2% of specimens, the seminal vesicles and vas deferens continued caudally as separate structures for more than 5 mm below the cephalad extent of the CZ. On ultrasound images, this variant gives the appearance of bilateral basilar hypoechogenicity.

In 12% of patients, abnormal penetration of the seminal vesicles and vas deferens was present at the rectal surface of the prostate, characterized by a lack of CZ tissue posterior to these structures, which sonographically appeared as midline hypoechogenicity at the base of the prostate.

In 6% of cases, abnormally large (>2 mm in diameter) smooth muscle bundles were present within the ejaculatory duct sheath, which also produced midline hypoechogenicity at the prostate base on ultrasound imaging.

In 3% of patients, the seminal vesicles extended laterally to the prostate gland. Their distal portion was embedded into the stroma of the CZ without any interposition of fibroadipose tissue. This variant was seen as lateral hypoechogenicity at the prostate base on ultrasound images.

In 2% of patients, ultrasound revealed a cystic utricle, defined as a utricle lumen larger than 4 mm with loss of epithelial papillations, that appeared sonographically as a midline cystic structure at the base with an anechoic lumen and increased through-enhancement.

Prostate volume determination

Determination of prostate volume can be useful in treatment planning of both surgery and radiation therapy. Volume measurements can also be valuable in monitoring the response to hormonal treatment or radiation therapy. Volume measurements yield a calculated volume in cubic centimeters. Because the specific gravity of prostate tissue is 1.050, the volume in cubic centimeters is comparable to the weight of the gland in grams.

Prostate volume is usually calculated with the assumption that the prostate shape is elliptical (eg, a football); the calculation can be performed by either of two methods. In one method, the prostate image at the widest transverse dimension in the axial plane is outlined with the cursor on the ultrasound console, allowing the computer to calculate the surface area of that section of the prostate. The transverse prostate dimension (from the left to the right side of the gland) at the estimated point of widest transverse dimension is then measured. The prostate volume can then be calculated by the following formula, 8 (surface area) squared, divided by 3p (transverse dimension). Most commercially available ultrasound console computers automatically perform this calculation when in the volume measurement mode.

The other, more commonly used method is elliptical volume calculation. This requires measurement of the prostate dimensions. First, dimensions of the prostate are determined in the axial plane by measuring the transverse and anteroposterior dimension of the prostate at the estimated point of widest transverse dimension. The longitudinal dimension is measured in the sagittal plane just off the midline because the bladder neck often obscures the cephalad extent of the gland. The formula, (p/6) (transverse dimension) (anteroposterior dimension) (longitudinal dimension), is then applied to calculate the volume.

Because of the inaccuracy of the longitudinal dimension caused by lateral refraction of the ultrasound image, slightly better prostate volume calculation can be accomplished by eliminating this dimension, using the formula for calculating the volume of a prolate spheroid (eg, an egg), (p/6) (transverse dimension) squared (anteroposterior dimension). The prolate spheroid formula is most accurate for glands less than 80 mL in volume; larger prostates assume a more spherical configuration, and their volume is better calculated by the formula for the volume of a sphere, (p/6) (transverse dimension) cubed. Unfortunately, no prostate gland is a perfect sphere, ellipse, or prolate spheroid, which renders all of these calculations somewhat inaccurate. However, all correlate with prostate weight with correlation coefficients higher than 0.90.

The most accurate means of prostate volume measurement is planimetry, which allows for variations in shape. In this method, the ultrasound probe is mounted on a stepping device, allowing the probe to be marched through the gland in the axial plane at defined intervals. At each interval, the prostate image is outlined with the cursor on the ultrasound console, allowing the computer to calculate the surface area of that section. Prostate volume is calculated by taking the sum of the prostate image surface areas measured at each step and multiplying this value by the stepping interval.

Most stepping devices provide 5- or 2-mm stepping intervals. The shorter the stepping interval, the more accurate the volume calculated. Because of its superior accuracy and reproducibility, this is the method of choice for treatment planning for brachytherapy. The stepping device can also serve to stabilize the ultrasound probe during brachytherapy.

Transrectal ultrasound forinfertility

Transrectal ultrasound is very helpful in the evaluation of the infertile male. It is indicated in men with azoospermia and low ejaculate volume (< 1.5 mL). It is used to evaluate for obstructive causes of infertility by looking for cystic dilation of the ejaculatory ducts and seminal vesicles. The normal seminal vesicle should measure less than 1.5 cm on the anteroposterior view.

Ultrasonography of prostatic malignancy

Originally hailed as a possible diagnostic modality for prostate cancer, transrectal ultrasound is now known to have limited applicability for initial diagnosis. Malignant lesions in the prostate can be hypoechoic, isoechoic, or hyperechoic.

• Hypoechoic tumors: Lee and colleagues were the first to describe the hypoechoic appearance of adenocarcinoma involving the PZ. Approximately 70% of palpable nodules are hypoechoic. In early series by both Carter et al ^[7] and Terris et al, 54% of nonpalpable prostate cancers were hypoechoic. The rate of hypoechogenicity has fallen with the stage migration of prostate cancer.
• Isoechoic tumors: At least 80% of TZ malignancies are isoechoic, as are 30-50% of PZ tumors. Ellis and Brawer showed that the Gleason scores and pathological staging of hypoechoic and isoechoic cancers were similar. ^[8] The elusive nature of these malignancies has stimulated the development of various biopsy schemes. Other physical properties, such as blood flow and bioimpedance, may lead to improvements in the detection of these tumors.
• Hyperechoic tumors: Egawa et al report that 1.3% of tumors in their series were hyperechoic. ^[9] Hyperechoic tumors can be present in either the PZ or the TZ and have been correlated with the ductal variant of prostatic adenocarcinoma. Hyperechogenicity from calcifications can also be associated with prostatic malignancies.

Appearance following treatment

• External irradiation: The calculated volume of the prostate decreases significantly by 6 months after radiotherapy. The rate and degree of reduction correlate significantly with the histologic grade of the tumor (with poorly differentiated tumors shrinking most rapidly) and with the outcome of treatment but not with stage.

  • After treatment, the average number of sites of capsular disruption decreases steadily, reaching 50% of the pretreatment number by 15 months. The entire prostate is more diffusely hypoechoic, and intraprostatic anatomy is poorly defined. Often, an associated thickening of the rectal surface is seen that displaces the prostate anteriorly.
  • Larger hypoechoic cancer foci, particularly those that have not responded well to radiation therapy, show little change in appearance once irradiated, but smaller foci and those responding well to therapy tend to become isoechoic.
  • In general, ultrasound findings are poorly correlated with pathological findings in the irradiated prostate. In the series of postirradiation prostate cancer recurrences reported by Kabalin et al, directed biopsies of hypoechoic areas were positive for cancer in 67% of the cases and isoechoic areas were positive for cancer in 65%. ^[10]
• Brachytherapy: Following brachytherapy, the prostate exhibits many of the same long-term changes in volume and sonographic appearance as with external irradiation. However, within the first few weeks after implantation, one third of patients demonstrate an increase in prostate volume because of postimplant edema. No single parameter, including preimplant prostate volume, preimplant hormonal deprivation, or supplemental external beam radiation therapy, can accurately predict the degree of swelling. The most distinctive characteristic of postbrachytherapy prostate sonography is the appearance of numerous seeds distributed more or less evenly throughout the gland. These seeds are dramatically hyperechoic and may demonstrate posterior shadowing.
• Hormone therapy: A 30% decrease in prostate volume occurs with androgen deprivation in patients with and without prostate cancer. The reduction in volume is greatest in the quartile of men with the largest initial gland volume (60%) and least in the quartile of men with smallest glands (10%). The reduction in volume does not correlate with response of the cancer to therapy. After discontinuation of androgen deprivation, the prostate demonstrates gradual regrowth. Any hypoechoic lesions or sonographically apparent extraprostatic extension progressively diminishes during hormone therapy.
• Radical prostatectomy: TRUS findings following radical prostatectomy are considered normal if the bladder neck tapers smoothly to the urethra. A blunted, nontapered appearance of the vesicourethral anastomosis has been associated with postoperative incontinence. The echogenic retroanastomotic fat plane should be intact. Many patients demonstrate a nodule of tissue anterior to the anastomosis, representing the ligated dorsal vein complex. Any other hyperechoic or hypoechoic lesions or interruption of the retroanastomotic fat plane is considered suggestive. Even without sonographic evidence of cancer recurrence, patients with probable local recurrence should undergo biopsy. Hypoechoic lesions have been reported in 75-95% of patients with locally recurrent cancer. Color Doppler imaging has been used by some authors to improve cancer detection in the prostatic fossa.

Appearance of nonadenocarcinoma malignancies

• Transitional cell carcinoma: Prostatic involvement with TCC extending from the bladder is common. Surveillance for prostatic invasion consists primarily of cystoscopic examination of the urethra. Unfortunately, TCC may involve other regions of the prostate that are inaccessible by cystoscopy, in which case TRUS can be useful. Invasion of the prostatic urethra is generally not detectable by TRUS, but 71% of prostatic stromal lesions exhibit hypoechogenicity. TCC-laden ejaculatory ducts are usually prominent and hypoechoic on TRUS images. Periprostatic extension of TCC is also apparent as irregular areas of hypoechogenicity within the hyperechoic periprostatic adipose tissue. Possible TCC involving the prostate detected by TRUS must be confirmed by biopsy findings because bacillus Calmette-Guérin granulomata are very common in patients with TCC and also exhibit hypoechogenicity on TRUS images.
• Squamous cell carcinoma: SCC comprises 0.2-0.3% of prostatic malignancies. Extension from bladder or urethral SCC is much more common than primary prostatic SCC. External beam irradiation of the prostate and androgen deprivation therapy for prostatic adenocarcinoma have been reported as risk factors for primary prostatic SCC. SCC involving the prostate appears as an irregular anterior mass, demonstrating relative hyperechogenicity compared to the normal prostate.
• Adenoid cystic carcinoma: Adenoid cystic carcinoma of the prostate was first reported in 1974. Since then, fewer than 30 cases have been reported. This tumor has also been referred to as an adenoid cystlike carcinoma, basaloid carcinoma, and adenoid basal cell tumor. Pathologically, these tumors resemble adenoid cystic carcinoma of the salivary glands, with multiple large cystic glands. The cystic glands give this tumor an unusual appearance on TRUS images, characterized by multiple, evenly distributed, small anechoic areas with increased through-transmission that are similar in size.
• Sarcoma: Delayed development of prostatic sarcoma is a rare complication of prostatic pelvic irradiation.

  • The TRUS appearance of this lesion is typified by an irregular hypoechoic prostatic mass with an anechoic area consistent with the echogenicity of muscle and/or necrosis. This appearance is distinctly dissimilar to prostatic adenocarcinoma. The sonographic finding of an irregular, hypoechoic, prostatic mass with an anechoic area should raise the suggestion of prostatic sarcoma in patients with a history of pelvic irradiation who develop an abnormal prostate found during a rectal examination and/or who have worsening voiding symptoms despite a normal serum PSA level.
  • Unlike radiation-induced sarcoma involving the prostate, which is predominantly hypoechoic, the echogenicity of rhabdomyosarcoma is similar to that of the normal prostate. TRUS can provide a means of monitoring prostate size and sampling tissue in older patients with prostatic rhabdomyosarcoma but has little value as a diagnostic imaging technique. TRUS is inappropriate in children, who are more commonly affected with prostatic rhabdomyosarcoma.
  • Cystosarcoma phyllodes involving the prostate appear as a large irregular mass containing multiple large anechoic cysts of variable size.
• Hematolymphoid malignancies: Hematolymphoid malignancies involving the prostate are generally inapparent with TRUS imaging. TRUS-guided prostate needle biopsy specimens often demonstrate a lymphocytic infiltrate, but frequently this is attributed to chronic inflammation of the prostate if hematolymphoid malignancy is not a consideration.

Urethral Ultrasound

Sonourethrography has been shown to be accurate, sensitive, and specific for the diagnosis and assessment of penile and bulbar urethral strictures. Compared with conventional retrograde urethrograms, sonourethrograms involve no ionizing radiation and potentially can detect spongiofibrosis and other periurethral abnormalities invisible on radiographic urethrograms. However, the applicability of urethral ultrasound has evolved to include planning for surgical approach to urethral stricture disease. Because ultrasound can be used to identify the extent of spongiofibrosis not seen on retrograde urethrogram, many experts recommend preoperative ultrasound prior to reconstructive surgery. ^[11]

The traditional technique for sonourethrography usually requires repeated instillation of normal saline to distend the urethra and to create contrast with adjacent tissues. Typically, the saline is delivered via either a Foley catheter or a syringe placed directly into the urethral meatus. Although the saline is effective at providing contrast between the urethral lumen and surrounding soft tissues, its low viscosity makes urethral distention difficult to maintain. An alternative to saline is lidocaine hydrochloride jelly.

Scrotal Ultrasound

Ultrasound is the imaging method of choice for evaluation of the scrotal contents. Scrotal imaging is usually performed with a 5- or 7.5-MHz small-parts transducer. A newer, developing technique to increase the diagnostic capabilities of scrotal ultrasound is contrast-enhanced ultrasound (CEUS) with microbubbles. While well-developed and described in other solid organ evaluations with ultrasound, its use in scrotal ultrasonography is currently in its infancy. The contrast consists of hexafluoride microbubbles with a phospholipid shell. The contrast is given intravenously and the ultrasound is performed over a series of minutes after introduction of the agent. The ultrasonographer then makes comparisons of the serial images after introduction of the contrast, looking for enhancement by comparing the brightness of the lesion to surrounding tissue after injection.

In a study of 51 patients, the use of CEUS was shown to allow visualization of testicular microvascularization and possibly help with the assessment of small testicular lesions with hypervascularization. ^[12]Additionally, in a more recent study of 50 patients, CEUS was found to increase accuracy in the final diagnosis compared with routine ultrasound. Specifically, CEUS may help in cases in which the ultrasound diagnosis remains inconclusive, such as in cases of trauma, torsion, or infarction. ^[13]

Testicular imaging

• Normal appearance: Sonographically, the testis is an oval structure displaying a uniform, medium-level, homogeneous echogenicity. The tunica vaginalis and tunica albuginea are not visualized except in the presence of a hydrocele. The mediastinum is identified as an echogenic line running from the superior to the inferior pole of the testis. The rete testis can be seen in some patients. The epididymis has an echogenicity that is isoechoic or slightly hyperechoic compared with the testis. The spermatic cord is not viewed as a separate structure from other components of the inguinal canal; however, blood flow can be detected within it on color Doppler sonography images. The appendix testis is a small oval structure beneath the epididymal head that cannot be discerned from the testis on sonography images except in the case of a hydrocele. ^[14]
• Primary testicular malignancy: High-resolution gray-scale ultrasound has long been the standard for the imaging evaluation of scrotal masses, with a sensitivity of nearly 100%.

  • No reliable sonographic criteria are available to distinguish a malignant lesion from a focal benign intratesticular lesion, such as infarction, hemorrhage, infection, or non–germ-cell tumor. Masses in the epididymis strongly suggest a nonneoplastic process. Consider all intratesticular masses malignant until proven otherwise. Imaging should always include an evaluation of the contralateral testis because 8% of testicular neoplasms are bilateral.
  • Gray-scale sonography depicts most intratesticular malignancies as focal or diffuse, heterogenous, hypoechoic masses in relation to normal testicular echogenicity.
  • Seminoma is typically hypoechoic without any calcification or cystic areas. The margins of seminoma are either smooth or ill defined.
  • Embryonal cell carcinoma is also hypoechoic but is inhomogeneous and less well circumscribed than seminoma. One third of embryonal cell tumors contain cystic areas. Frequently, invasion of the tunica albuginea by embryonal cell carcinoma causes distortion of the testicular contour.
  • Teratoma displays sonolucent and hyperechoic components on gray-scale sonography images. The dense foci, resulting from calcification, cartilage, or fibrosis, produce acoustic shadowing. Teratomas are normally inhomogeneous, with well-defined borders.
  • Choriocarcinoma has mixed echogenicity, owing to areas of hemorrhage, necrosis, and calcification.
• Secondary testicular malignancy: Malignant lymphoma is the most common secondary testicular neoplasm. It comprises 1-7% of all testicular tumors. Testicular lymphoma is the most common primary and secondary testicular neoplasm in men aged 60-80 years. Leukemic involvement of the testis is seen most often in children and is rarely clinically evident in adults. Lymphoma and leukemia generally manifest as testicular enlargement.

  • Sonographically, these malignancies appear as diffuse or focal regions of decreased echogenicity with maintenance of the normal ovoid testicular shape. Color Doppler sonography increases the sensitivity of lesion detection. Testicular hyperemia without associated epididymal hyperemia is more suggestive of a neoplastic process than orchitis.
  • Metastatic deposits to the testis frequently originate from the prostate and kidney. Other sites of origin include the lungs, pancreas, bladder, thyroid, melanoma, and intestinal tumors. Ultrasound depicts metastatic lesions as hypoechoic, hyperechoic, or a combination of both.
• Benign testicular masses: Non–germ-cell tumors comprise fewer than 5% of testicular neoplasms. They include Leydig cell and Sertoli cell tumors. These sex cord–gonadal tumors are almost always benign and occur in person of any age. The sonographic appearance consists of a solid hypoechoic mass that frequently contains cystic areas. Dilation of the rete testis can appear sonographically as multiple, small spherical or tubular anechoic structures or cystic spaces in the region of the mediastinum testis that can be mistaken for malignant masses. These ultrasound images often exist concomitantly with signs of epididymal obstruction, such as spermatoceles or epididymal hyperemia, on color Doppler sonography images, suggesting epididymitis.
• Microlithiasis: Testicular microlithiasis (TM) is calcification within the testicle. This finding has been associated with an increased risk of testicular malignancy. Microlithiasis appears sonographically as multiple echogenic foci with no acoustic shadowing. When present, careful sonographic examination for the presence of a mass in either testicle is mandatory.

  • Serial ultrasound has been recommended because of the risk of testicular cancer. However, Peterson et al found TM to be present in 5.6% of the population in a screening study of over 1500 asymptomatic men. Further analysis based on the demographics and race distribution of those men suggested that TM was not associated with cancer. They recommend follow-up in men with TM to include serial self-examinations and repeat ultrasound examination only if new masses or changes are perceived on self-examination. ^[15]
  • The same authors followed this cohort of patients for 5 years prospectively. After follow-up, only one of the 84 patients developed testicular cancer. This was a nonpalpable tumor found incidentally on a scrotal ultrasound performed to evaluate a paratesticular mass. They again recommended monitoring this group with serial self–physical examinations without the need for follow-up scrotal imaging. ^[16]
• Orchitis: Orchitis is characterized by focal, peripheral, hypoechoic testicular lesions that are poorly defined, amorphous, or crescent-shaped. Orchitis also exhibits testicular hyperemia on color Doppler sonography images and is usually accompanied by epididymal hyperemia due to concomitant epididymitis. A reactive hydrocele is also frequently associated with epididymoorchitis. Focal testicular infarction can occur as a complication of epididymitis when swelling of the epididymis is severe enough to constrict the testicular blood supply. This appears as a hypoechoic intratesticular mass on ultrasound images; the mass is devoid of blood flow on Doppler imaging.
• Intratesticular cysts: Intratesticular cysts appear sonographically as well-circumscribed anechoic circular structures with imperceptible walls, increased through-transmission, and posterior wall acoustic enhancement. Benign cysts may be intratesticular but are more often extratesticular.
• Cryptorchidism: Cryptorchidism is the complete or partial failure of the testes to descend from the intra-abdominal position to the scrotum during gestation.

  • Cryptorchidism is bilateral in approximately 8% of patients. A palpable testis within the inguinal canal may not require any imaging before proceeding with surgical correction. Despite the fact that completely undescended, intra-abdominal testes are usually not amenable to sonographic imaging, ultrasound should be the initial imaging study for cryptorchidism when a testicle is not palpable in the inguinal canal. Failure to identify a testicle in the inguinal canal with ultrasound is usually followed with laparoscopic examination of the abdomen and surgical correction or removal of the undescended testis. Rarely is additional imaging, such as CT scan or MRI, required.
  • With gray-scale sonography, the undescended testis is oval and homogenous in character but is smaller and slightly less echogenic than the contralateral, descended testis. Rarely, a thin white line or bright echogenic band representing the mediastinum testis can be seen. This can help differentiate the testis from an inguinal lymph node.
• Testicular torsion: Torsion occurs when an abnormally mobile testis twists on the spermatic cord, obstructing its blood supply. Patients present with acute onset of severe testicular pain. The ischemia can lead to testicular necrosis if not corrected within 5-6 hours of the onset of pain. Torsion can be intermittent and can undergo spontaneous detorsion. ^[17]

  • Gray-scale sonography is of limited use in the early stages of torsion because the testicle usually appears sonographically normal. Occasionally, the testicle appears hypoechoic with a prominent echogenic mediastinum.
  • With prolonged torsion, the testis is typically hypoechoic and inhomogeneous and is often accompanied by a surrounding hydrocele. By the time these sonographic findings occur, surgical salvage of the testicle is unlikely.
  • Color Doppler imaging is extremely useful in the evaluation of torsion of the testicle. The complete absence of intratesticular blood flow and normal extratesticular blood flow on color Doppler images is diagnostic if the flow is normal in the contralateral testis. Yet, the presence of flow within the testis does not exclude the presence of torsion because incomplete vascular obstruction can sometimes occur. With absent intratesticular flow on color Doppler images and increased extratesticular flow, chronic ischemia should be considered.
  • Doppler imaging following spontaneous, manual, or surgical detorsion is characterized by slightly increased intratesticular flow and increased extratesticular flow.
  • Color Doppler imaging has limited sensitivity for detecting blood flow in pediatric patients with a testicular volume of less than 1 mL.
  • Thrombosis of the appendix testis (often referred to as torsion of the appendix testis) can be difficult to differentiate clinically from testicular torsion. With ultrasound, torsion of the appendix is seen as a hypoechoic mass between the epididymal head and upper testicular pole surrounded by an echogenic periphery. Upon color Doppler examination, an area of increased perfusion is seen surrounding the hypoechoic mass, and intratesticular flow is normal.
• Testicular trauma: Ultrasound is the imaging method of choice for scrotal trauma. ^{[18, 19]}

  • An acute scrotal hematoma that becomes more sonolucent with time appears as an echogenic collection between the dartos and the tunica vaginalis or in the scrotal septum.
  • A hematocele represents bleeding between the leaves of the tunica vaginalis and appears as a complex fluid collection. With time, this collection can develop loculations, which appear as thick septations.
  • Sonography helps rapidly and noninvasively evaluate testicular rupture as an adjunct to the clinical history and examination.
  • Testicular rupture is seen as focal alterations of testicular echogenicity correlating with areas of intratesticular hemorrhage or infarction.
  • A discrete fracture plane is identified in fewer than 20% of cases, although visible alterations in the testicular contour are a common finding.
  • A hematocele also may be present.
  • Any finding of heterogeneity on testicular ultrasound in a patient with a history of trauma should prompt immediate surgical exploration to repair a likely rupture.

Extratesticular scrotal ultrasound

• Extratesticular cysts: Extratesticular cysts are found in the epididymis and tunica albuginea. Sonographically, spermatoceles and tunica albuginea cysts appear as well-circumscribed anechoic circular structures with thin, smooth walls; increased through-transmission; and posterior wall acoustic enhancement. These are often single cysts but can be multiple and/or multiloculated. Epididymal cysts, or spermatoceles, are a cystic dilation of the tubules of the efferent ductules or aberrant ducts of the epididymis.
• Hydrocele: A hydrocele is a collection of serous fluid in the potential space between the visceral and parietal layers of the tunica vaginalis of the testicle. Hydroceles are typically confined to the anterolateral portions of the scrotum. Acquired hydroceles are usually idiopathic but can be associated with testicular neoplasms, trauma, or inflammation. Thus, sonographic inspection of a hydrocele must include careful imaging of the testicle, particularly because the hydrocele occasionally renders the testis impalpable, concealing any masses from the clinician. Sonographically, a hydrocele is an anechoic, crescentic structure surrounding the anterolateral aspect of the testis. Occasionally, internal echoes can be seen, due to cholesterol crystals, infection, or hemorrhage.
• Hematoceles and pyoceles: Hematoceles and pyoceles are uncommon and are caused by blood and pus, respectively, in the tunica vaginalis cavity. Hematoceles are caused by trauma, surgery, neoplasms, or torsion. Pyoceles form when an epididymal or testicular abscess ruptures into a reactive hydrocele. Pyoceles can also occur in patients with long-term catheterization, due to urethral trauma or erosion in the face of bacterial colonization. On sonography, thick internal septations and loculations can be seen in both hematoceles and pyoceles. A thickened tunica albuginea also may be present.
• Varicocele: A varicocele is an abnormal dilation of the veins of the pampiniform plexus due to either incompetent venous valves or pathologic obstruction of the venous outflow. Most varicoceles occur on the left side. The ultrasound appearance of varicocele is well defined.

  • Sonography displays a varicocele as multiple serpiginous anechoic structures. This can be differentiated from a multiloculated spermatocele by detecting venous flow using color Doppler and the accentuation of the structures by placing patients upright or asking them to perform a Valsalva maneuver.
  • The pampiniform plexus must consist of more than 3 veins that are all larger than 3 mm in diameter. Retrograde flow also must be demonstrated.
  • Sonography can be beneficial in the case of subclinical varicoceles, for which the physical examination findings are normal but the patient has decreased fertility, pain, or other suggestions of a varicocele. Upright positioning and/or Valsalva maneuver are even more important in sonographic examinations when attempting to demonstrate a subclinical varicocele. The true contribution of the subclinical varicocele in fertility and pain is still debated.
• Epididymitis: Epididymitis is the most common inflammatory process involving the scrotal contents. Such infections generally originate in the lower urinary tract from the bladder, urethra, and/or prostate and are typically caused by urinary tract pathogens or sexually transmitted organisms. A sterile chemical epididymitis can result from reflux of sterile urine through the ejaculatory ducts. Concomitant infection of the testis may occur, but orchitis in the absence of epididymitis is rare. Gray-scale sonography shows enlargement with decreased echogenicity of the epididymis. With associated involvement of the testicle, an enlarged, hypoechoic testis is seen. Color Doppler reveals increased perfusion of the epididymis and, if orchitis is present, hyperemia of the testis. When severe, epididymitis can actually constrict the blood flow to the testis, causing ischemia and decreased flow on color Doppler images.

Fetal urologic ultrasound

• Oligohydramnios: Oligohydramnios, ie, diminished volume of amniotic fluid, can be caused by bilateral fetal renal dysfunction, premature rupture of membranes, intrauterine growth restriction, postmaturity, or fetal demise. Oligohydramnios leads to fetal compression, which causes Potter facies, characterized by talipes equinovarus, dehydrated skin, hypertelorism, flat nose, low-set ears, and a small chin. The lack of amniotic fluid can also cause limb defects, intrauterine growth delay, and pulmonary hypoplasia. If due to a fetal abnormality, oligohydramnios usually results from bilateral renal agenesis, urethral atresia, or bilateral nonfunctioning renal dysplasia.
• Renal agenesis: Renal agenesis is the most severe genitourinary anomaly, occurring in 1-3 per 10,000 live births. It results from first-trimester cessation of renal development. Renal development can be an isolated anomaly, or it can be chromosomal, as in Fraser syndrome, which is characterized by bilateral renal agenesis, external genitalia anomalies, and cleft palate. Renal agenesis is fatal, with neonates dying secondary to pulmonary hypoplasia. Fetal sonography reveals oligohydramnios, absent kidneys, and nonvisualization of the fetal bladder after 60-90 minutes of observation and maternal hydration.
• Infantile polycystic kidney disease: This condition is an inherited autosomal recessive condition and therefore has a 25% recurrence rate in successive offspring. It develops during pregnancy or birth because of a defect in collecting tubules resulting in the formation of small cysts. Fetal sonography shows oligohydramnios, nonvisualization of the fetal bladder, and hyperechoic kidneys. Visualizing the small cysts within the kidneys may be difficult. Infantile polycystic kidney disease can be associated with Meckel-Gruber syndrome. In addition to polycystic kidney disease, this lethal syndrome includes occipital encephalocele, polydactyly, and severe oligohydramnios. Any fetal sonographic examinations suggestive of polycystic kidney disease should include assessment for these additional abnormalities.
• MCDK disease: MCDK can be unilateral or bilateral or involve only a portion of one kidney. It results from a malformation of collecting tubules producing dilation and multiple cysts. This developmental anomaly can occur with genetic conditions such as Dandy-Walker syndrome. MCKD can cause severe bilateral renal dysfunction. If bilateral and involving the entire kidney, MCDK is fatal and is often accompanied by hydrocephalus or anencephaly, diaphragmatic hernia, spina bifida, and/or cleft lip or palate. Upon fetal sonography, when cysts are apparent in one or both kidneys, the sonographer should document fluid within the fetal bladder.
• Hydronephrosis: Fetal hydronephrosis is caused by obstruction of the fetal urinary tract and/or VUR. The hydronephrosis can be unilateral or bilateral. The Society for Fetal Urology has well-established grading criteria for hydronephrosis that can be applied after 20 weeks' gestation. ^[20]

  • Grade 1: The pelvic anteroposterior diameter is less than 1 cm and calices are normal.
  • Grade 2: The pelvic anteroposterior diameter is 1-1.5 cm with normal calices.
  • Grade 3: The pelvic anteroposterior diameter is greater than 1.5 cm with slight caliectasis.
  • Grade 4: The pelvic anteroposterior diameter is greater than 1.5 cm with moderate caliectasis.
  • Grade 5: The pelvic anteroposterior diameter is greater than 1.5 cm with severe caliectasis, with thinning of the renal parenchyma to less than 2 mm thick.
• Ureteropelvic junction obstruction: Ureteropelvic junction obstruction is the most common cause of renal dysfunction, occurring in 1 in 1258 newborns. Bilateral ureteropelvic junction obstruction can cause severe renal dysfunction and oligohydramnios. With unilateral ureteropelvic junction obstruction, oligohydramnios is not usually present. Sonographically, fetal hydronephrosis is defined as a dilated renal pelvis and calyces with a diameter larger than 8-10 mm. This size of renal pelvis can be physiologic in the third trimester.
• Posterior urethral valves: Posterior urethral valves are a developmental anomaly caused by the formation of webs of tissue in the urethra distal to the prostate. These valves cause urinary tract obstruction in male fetuses. The obstruction results in severe progressive hydronephrosis, causing severe renal dysplasia and dysfunction. It is often an isolated anomaly but can be associated with trisomy 13 and trisomy 18. Sonography reveals an enlarged proximal urethra and bladder, which may be accompanied by dilated ureters, bilateral hydronephrosis, cortical cysts, and oligohydramnios. A sonographic examination suggestive of posterior urethral valves should include a careful assessment for heart anomalies.
• Prune belly syndrome: Urinary ascites can be caused by severe urinary tract dilation with rupture and spillage of urine. If very severe, this ascites is thought to distend the fetal abdomen, resulting in lax musculature. Prune belly syndrome is observed primarily in males and is characterized by severe urinary tract dilation due to posterior urethral valves and by cryptorchidism. Sonographically, abundant fluid is seen in an abnormally large abdominal cavity. Hydronephrosis and a dilated bladder are additional findings.

Renal neoplasms in the fetus and neonate

• Mesoblastic nephroma: Mesoblastic nephroma (fetal renal hamartoma) is a mesenchymal neoplasm that arises from the metanephric blastema. This is the most common renal neoplasm in fetuses and neonates. A renal mass and polyhydramnios are characteristic sonographic findings.
• Wilms tumor: Wilms tumor, an epithelial neoplasm, is rare in fetuses and neonates. It occurs most commonly in children aged approximately 3 years and is characterized by a large abdominal mass. Sonographically, this is seen as an echogenic unilateral mass.
• Nephroblastomatosis: Nephroblastomatosis, a precursor to Wilms tumor, is caused by an arrest in normal nephrogenesis. Both kidneys are diffusely enlarged by this persistent immature metanephric tissue. With imaging, nephroblastomatosis appears as multifocal subcapsular nodules in the renal cortex. The presence of internal echoes and the absence of increased through-transmission distinguish the nephrogenic rests from cystic structures. The contour of the affected kidneys is generally nodular, a finding also seen with persistent fetal lobulation. Less commonly, nephrogenic rests may appear as isoechoic or hyperechoic-to-normal renal parenchyma. Smaller nephrogenic rests may not be detectable on ultrasound images. If they are detected, also assess the fetus for GI anomalies, hydrocephalus, and polyhydramnios.
• Renal vein thrombosis: This is seen in neonates in cases of traumatic births, septicemia, maternal diabetes, prenatal steroid administration, or congenital renal defects. Clinically, these infants have hematuria and enlarged, palpable kidneys. With sonography, densities may be seen in the enlarged kidneys and intrarenal calcifications.
• Scrotum: Fetal hydroceles are common, resulting from connection with the peritoneal cavity with normal testicular descent. These are usually resorbed within the first 9 months of life. If the connection to the peritoneal cavity persists, it results in an indirect inguinal hernia that may require surgical repair.
• Adrenals: In renal agenesis, the fetal adrenal gland is circular and looks similar to a kidney. In anencephalic individuals, the adrenal glands are small, secondary to the lack of adrenocorticotropic hormone production. Small adrenals are seen in fetuses of preeclamptic mothers and mothers with antepartum hemorrhage. Congenital neoplasms of the adrenal gland, neuroblastomas, are extremely rare.