Testis and Epididymis

The testes develop in the retroperitoneum during early fetal development. Each testis is attached to the inferior scrotal wall by the gubernaculum testis, which does not grow significantly in length as the rest of the fetus develops. As a result, the testes are tethered to the scrotum. As the torso grows, the spermatic cord stretches in length, maintaining the vascular supply in the form of the testicular artery and vein, and maintaining the reproductive pathway through the vas deferens. Thus, although this process is often described as the testes “descending” into the scrotum during the second month of gestation, it may be more accurate to say the fetal abdomen grows away from the testis. In adulthood, the gubernaculum remains as a part of the mesentery-like structure that maintains appropriate orientation of the testis within the scrotum. Redundancy of this mesorchium results in the “bell-clapper” deformity that increases the risk for testicular torsion. If there is more complete failure of scrotal attachment, the result is an “undescended testis,” which is more appropriately termed cryptorchidism.

A portion of the peritoneal space is trapped by the testis and gubernaculum. This pocket of peritoneum is called the processus vaginalis and typically obliterates late in development. Persistent communication of the processus vaginalis with the peritoneal space provides a pathway for peritoneal fluid and disease to descend into the scrotum. Discontinuous areas that fail to obliterate result in noncommunicating hydrocele.

Each testis receives arterial supply from a testicular artery that arises from the abdominal aorta. The pampiniform plexus is a network of veins that provide venous outflow along the spermatic cord, ultimately uniting to form a single testicular vein on each side. The left testicular vein courses along the retroperitoneum from the inguinal canal to the left renal vein. The right testicular vein usually joins the inferior vena cava at the level of the inferior pole of the right kidney, although it does occasionally join the right renal vein.

Testicular parenchyma consists primarily of seminiferous tubules where spermatogenesis takes place. These are soft, convoluted tubular structures that resemble strands of spaghetti several microns in diameter. The tubules are compacted together and are encapsulated within the tunica albuginea, giving the testis its ovoid shape (Fig. 20-1). A small incision or laceration in the tunica allows the seminiferous parenchyma to spill out like spaghetti from a plastic bag. The mediastinum testis is a septum that runs within the testicular parenchyma, providing a lattice for the vascular structures and efferent ductules that carry sperm from the seminiferous tubules to the epididymis.

Figure 20-1 Schematic illustration of anatomy of the testis and epididymis.

Sperm travels through the mediastinum testis to the head of the epididymis, which also consists of multiple tubular structures. These tubules begin to coalesce into more organized, straightened tubules in the epididymal tail until they finally channel into a single tube, the vas deferens. The vas deferens courses along the spermatic cord to the internal inguinal ring. Once inside the pelvis, it deviates away from the gonadal vessels, and travels inferior and medial to insert into the ejaculatory ducts near the ostium of the ipsilateral seminal vesicle.

Prostate and Seminal Vesicles

The prostate has an inverted cone shape. As such, the base of the prostate is cranial and the apex is caudal. The bladder sits atop the prostatic base, with the seminal vesicles situated between the posteroinferior bladder wall and rectum. The urogenital diaphragm delineates the caudal-most border of the prostatic apex. The prostate gland is separated from the rectum posteriorly by Denonvilliers (rectovesical) fascia. Santorini’s plexus of veins (a venous plexus that surrounds the ventral and lateral prostate gland) and the pubic symphysis border the prostate anteriorly. The levator ani complex borders the lateral walls of the prostate caudally; the obturator internis muscles abut the lateral margin more cranially.

The prostate gland can be divided into one-third nonglandular and two-thirds glandular elements. The nonglandular components of the gland consist primarily of the anterior fibromuscular stroma and prostatic urethra. The fibromuscular stroma is located directly anterior to the urethra and includes the majority of the anterior lobe of the prostate (Fig. 20-2).

Figure 20-2 Schematic illustration of prostatic anatomy in the sagittal plane.

The bladder neck enters the base of the prostate centrally to join the proximal prostatic urethra. The ejaculatory ducts enter the prostatic urethra at a posterior mound called the verumontanum, defining the beginning of the distal prostatic urethra.

The glandular components of the prostate consist of the peripheral zone, central zone, transitional zone, and periurethral glandular tissues. The peripheral zone is localized around the posterior periphery of the gland and extends anterolaterally toward the anterior fibromuscular stroma but does not surround it. It contains and surrounds the entire posterolateral aspect of the gland, similar to a baseball mitt holding a baseball. In young men, the peripheral zone comprises approximately 70% of the prostate and envelops the entire central gland, which consists of the transitional zone, central zone, and periurethral glandular tissue. The transitional and central zones comprise approximately 5% and 25% of the gland, respectively, in the young male individual. As men age, this ratio inverts with the transitional zone taking up the majority of the central glandular tissue as it undergoes significant proliferative changes resulting in benign prostatic hyperplasia (BPH). Hyperplastic changes in the transitional zone and periurethral glandular tissues compress the central zone against the surrounding peripheral zone into a thin rim resulting in the surgical pseudocapsule. This pseudocapsule is separate and different from the true fibrous capsule, which is a 2- to 3-mm fibromuscular layer that encapsulates the external surface of the prostate. The surgical pseudocapsule serves as the surgical plane for transurethral prostatectomy often used for treatment of BPH.

Scrotal Contents

Not only does the design of the scrotum provide a climate conducive to spermatogenesis, but it also places the testes and paratesticular tissues in a location convenient to physical inspection and evaluation by ultrasound (US). Because of wide availability of high-resolution transducers with excellent near-field resolution and color, spectral, and power Doppler, nearly all scrotal imaging is performed by sonography. Because no ionizing radiation is involved, serial examinations can be performed to evaluate disease progression or regression without concern for patient safety.

By US, each testis is an ovoid structure that is homogenous and mildly hypoechoic compared with the sparse adjacent soft tissues (Fig. 20-3). Because it is not possible to compare the echogenicity of the testes with other organs, it may be difficult to identify diffuse bilateral abnormalities of echogenicity. Fortunately, most testicular disease processes result in focal, multifocal, or diffuse unilateral abnormalities.

Figure 20-3 Normal “buddy shot.” Transverse ultrasound image includes both testes, allowing comparison of echogenicity between the two.

An echogenic line (the mediastinum testis) can usually be identified with US, bisecting the testis along its long axis asymmetrically (Fig. 20-4). A mediastinal artery is visible about 50% of the time and can mimic a small cyst or mass when viewed in cross section. Elongating the vessel and use of color Doppler imaging confirm the vascular nature of this finding (Fig. 20-5). Capsular arteries underlie the tunica albuginea and give rise to centripetal branches that radiate inward. Spectral Doppler demonstrates a low-resistance arterial waveform, with resistive indices typically between 0.50 and 0.75.

Figure 20-4 Mediastinum testis. A, Sagittal ultrasound image of the testis demonstrates an echogenic line representing the fibrofatty tissue of the mediastinum testis. B, Transverse image demonstrates a typical eccentric location. This example is slightly more pronounced than typical but is not abnormal.

Figure 20-5 Ultrasound evaluation of arterial flow in the testis. A, Transverse ultrasound image with color Doppler imaging demonstrates mediastinal artery and vein. B, Color Doppler imaging demonstrates capsular artery and centripetal branch. C, Spectral Doppler imaging demonstrates a normal low-resistance waveform.

The head of the epididymis is seen on sagittal US images as a triangular, solid structure adjacent to the superior pole of the testis. Although the epididymal head is usually mildly echogenic compared with the testis, the tubules become more organized in the tail, which can render the tail hypoechoic. This can result in a “two-toned” appearance at the junction of the epididymal head and body (Fig. 20-6).

Figure 20-6 Sagittal ultrasound image demonstrates the head of the epididymis. This “two-toned” appearance from a mildly echogenic head to a hypoechoic tail is a normal finding and should not be mistaken for a mass.

Prostate and Seminal Vesicles

Typical appearance of the prostate on CT is an ovular soft-tissue density, often difficult to separate from the surrounding fascia and musculature. Large BPH nodules and global prostatic enlargement are sometimes evident as irregular soft-tissue nodules seen encroaching into the bladder base, often with associated bladder wall thickening. Crude zonal anatomy can be appreciated on contrast-enhanced images in patients with BPH, as the central gland enhances heterogeneously, delineating it from the surrounding hypovascular peripheral zone (Fig. 20-7). Calcification within the prostate gland is common and usually asymptomatic, although it is more common in men with recurrent prostatitis, BPH, and a variety of metabolic disorders. Calcification of the seminal vesicles and vas deferens are also common and are associated with diabetes mellitus.

Figure 20-7 Enhanced axial computed tomography in a 42-year-old man demonstrates some limited zonal anatomy.

MRI provides exquisite anatomic detail of the prostate. Multicoil imaging using the body coil for excitation, and combined endorectal and external surface phased array coils for signal reception have been well established in the literature for 1.5 Tesla (1.5T) systems. However, the optimal technique for 3T imaging of the prostate is still evolving. The theoretical twofold increase in the signal-to-noise ratio achieved on 3T systems relative to 1.5T systems can be utilized to increase spatial resolution, increase temporal resolution for dynamic contrast-enhanced studies, or diminish overall scan time. Now that endorectal coils are commercially available for 3T imaging of the prostate, this technique will likely begin to replace imaging at 1.5T.

T2-weighted MR images provide improved definition of multiplanar prostatic zonal anatomy compared with other imaging modalities (Fig. 20-8). The peripheral zone consists of glandular tissue high in water content, giving it a bright appearance on T2-weighted images. The transitional and central zones of the central gland are difficult to delineate as separate structures in the young male individual, but together appear as intermediate signal intensity on T2-weighted imaging. As the patient ages, the transitional zone and periurethral glandular tissues enlarge, compressing the central zone and creating the surgical pseudocapsule between the hyperplastic transitional zone and the surrounding peripheral zone.

Figure 20-8 Normal zonal anatomy of the adult prostate on high-resolution T2-weighted magnetic resonance images obtained with an endorectal coil in the (A) axial and (B) sagittal planes. Images were obtained on a 3-Tesla magnet. On magnetic resonance images, the transitional zone and central zone are considered together as one region called the central gland. BPH, Benign prostatic hyperplasia.

The seminal vesicles can be best visualized on coronal and axial images as clusters of high-T2 signalintensity tubules just cranial to the posterior prostatic base (Fig. 20-9). The course of the urethra through the prostate is best depicted on sagittal images.

Figure 20-9 Normal appearance of the seminal vesicles on magnetic resonance imaging. Axial T2-weighted image at the level of theprostate base demonstrates the midportion of both seminal vesicles.

The neurovascular bundles can be appreciated on T1- and T2-weighted axial images as small clustered foci of low signal in the rectoprostatic angles, which are the two acute angles formed by the outer rim of the posterior peripheral zone and anterior rectal wall. The site where each neurovascular bundle penetrates the true prostatic capsule provides an avenue for extracapsular spread of prostate cancer; thus, familiarity with this region is critical to MR staging. The true prostatic capsule appears as a region of low signal intensity 1 to 2 mm thick surrounding the exterior surface of the gland on T2-weighted images.

Differential Diagnosis and Triage of Patients with Scrotal Pain

The main goal of imaging for acute scrotal pain is to identify those disease processes that require urgent surgical therapy. Sonographic evaluation usually allows effective triage of patients into one of three categories as shown in Table 20-1.

Table 20-1 Triage of Patients with Scrotal Pain According to Ultrasound Findings

Although these processes can be usually differentiated from one another using US, correct diagnosis often requires “hands-on” sonographic evaluation because correlating physical examination findings such as tenderness with sonographic findings can improve specificity. Because many of these disease processes have helpful clinical discriminators, it can be useful to ask the patient questions regarding onset, location, and duration of pain while scanning. Table 20-2 lists additional clinical discriminators.

Table 20-2 Differentiating Common Causes of Scrotal Pain

Torsion or Infection?

Time is a critical factor in the diagnosis and treatment of testicular torsion. Surgical correction within 6 hours of the onset of pain is associated with salvage rates of greater than 90%, but the likelihood of testicular salvage declines rapidly thereafter. Because physical examination findings cannot always reliably distinguish between epididymitis and testicular torsion, patients with epididymitis are often sent for sonographic evaluation to exclude torsion before initiating medical therapy.

Fortunately, the imaging findings of infection and torsion are also usually quite disparate (Table 20-3). Typical sonographic features of epididymitis include an enlarged epididymis with increased blood flow on color Doppler (Fig. 20-10). In epididymoorchitis, there is also increased blood flow to the testis, which is often enlarged and mildly hypoechoic (Fig. 20-11). Although the testis and epididymis are typically enlarged and hypoechoic with torsion as well, blood flow is decreased or absent (Fig. 20-12). When evaluating blood flow for possible torsion, it is essential to keep in mind that there is often hyperemia surrounding an ischemic or infarcted testicle, so flow within the testicular parenchyma must be present to exclude torsion (Fig. 20-13). A spiral appearance of the spermatic cord vessels has also been described as a sign of torsion. Spectral Doppler evaluation usually demonstrates a low-resistance arterial waveform in orchitis and a high-resistance arterial waveform with decreased, absent, or reversed diastolic flow in testicular torsion. With torsion, loss of venous flow may precede loss of arterial flow on spectral Doppler imaging.

Table 20-3 Ultrasound Findings in Epididymitis and Testicular Torsion

Figure 20-10 Ultrasound findings of epididymitis. Sagittal ultrasound image with color Doppler demonstrates diffuse enlargement and increased vascularity of the epididymis.

Figure 20-11 Epididymoorchitis on ultrasound. A, Color Doppler image demonstrates enlargement of the epididymis with increased blood flow to both the epididymis and testis. B, Transverse midline color Doppler image of both testes allows direct comparison of vascularity, increased on the patient’s right side in this case.

Figure 20-12 Ultrasound findings in testicular torsion. Transverse midline color Doppler ultrasound image in a patient with severe left scrotal pain demonstrates enlargement of the left testis and epididymis compared with the right side. No flow is detected within the left testis, whereas normal flow is demonstrated on the right side.

Figure 20-13 Peripheral hyperemia in testicular torsion. Color Doppler ultrasound image demonstrates increased blood flow surrounding an infarcted testis, but no flow within the testis itself. The presence of peripheral flow does not exclude torsion.

Imaging findings may be confusing, however, when the patient has undergone torsion followed by detorsion. In this scenario, considerable hyperemia to the testis and epididymis can exist, similar to the appearance seen in epididymoorchitis. The clinical history, however, is usually different because torsion/detorsion typically results in marked and abrupt fluctuation in symptoms, unlike the gradual progression usually seen with infectious processes of the scrotum.

A Few Words About Appendages

The appendix epididymis and appendix testis are small (usually ≤5 mm), nodular appendages of benign soft tissue that are often seen protruding from the respective structure. They are usually obscured by adjacent soft tissues during sonography but become more conspicuous when a hydrocele is present (Fig. 20-14). Rarely, one of these appendages may undergo torsion independent of the other scrotal structures, resulting in pain but no risk to the testis. This is more common in boys aged 7 to 14 years but can occur in adults as well. If sonography demonstrates no evidence of infection or torsion, and pain can be directly correlated to an appendix of the testis or epididymis, a diagnosis of appendiceal torsion can be made, resulting in appropriate conservative management.

Figure 20-14 Appendices in the scrotum. A, Sagittal ultrasound (US) image of the epididymal head demonstrates a small appendix epididymis. B, Sagittal US image of the testis in a different patient demonstrates an appendix testis. In both cases, a small amount of hydrocele facilitated visualization.

Complications of Inflammatory Disease in the Scrotum

Several complications can occur when epididymitis, orchitis, or both go untreated or are incompletely treated. The most common of these complications is development of an infected hydrocele called a pyocele. A pyocele consists of infected fluid between the layers of the tunica vaginalis. Unlike simple hydroceles that develop in response to local inflammation, pyocele are usually septated and contain internal echoes (Fig. 20-15).

Figure 20-15 Ultrasound of the scrotum performed after 1 week of swelling and pain demonstrates a pyocele that was subsequently debrided surgically.

Epididymoorchitis can progress to form an intratesticular abscess. The pressure created by an expanding abscess within the confines of the tunica albuginea usually causes severe pain. Increased interstitial pressure can also decrease perfusion to the remainder of the testis, eventually resulting in global or segmental infarction. On US, abscesses are usually hypoechoic compared with adjacent testicular parenchyma, although internal echoes may be sufficient to make the abscess nearly isoechoic. Color Doppler imaging can be useful, demonstrating absent flow within the abscess cavity (Fig. 20-16).

Figure 20-16 Testicular abscess. Color Doppler ultrasound performed for severe testicular pain demonstrates a focal heterogeneous lesion with absent color flow. Enterobacter abscess was confirmed surgically. The absence of color flow within the lesion and clinical presentation help differentiate this lesion from testicular cancer.

Surgical debridement is usually performed for pyocele, and orchiectomy is standard treatment for testicular abscess. If pain and other symptoms of infection are sufficiently mild, intensive antibiotic therapy may be attempted with serial US examinations to look for signs of response or progression.

Fournier Gangrene

The scrotum and perineum are also prone to cellulitis. Left untreated, a life-threatening fasciitis can develop. The gas-producing organisms of Fournier gangrene visibly progress in a matter of hours, and rapid surgical debridement followed by aggressive antibiotic therapy is key to escaping the persistently high mortality rate (30-50%).

Two schools of thought exist regarding the use of imaging in the diagnosis and surgical planning of patients suspected of having Fournier gangrene: (1) Don’t image—just get them to the operating room; and (2) scan quickly with CT to define the expected extent of debridement necessary. If imaging is requested, speed of performance and interpretation are of the utmost importance. Key findings include gas and fat stranding in the subcutaneous soft tissues (Fig. 20-17). Because successful debridement is defined not only by width of resection but also by depth, try to identify areas of gas in adjacent muscles or other deep soft tissues.

Figure 20-17 Axial image from contrast-enhanced computed tomography performed in search of an abscess in a case of severe cellulitis of the perineum. Extensive gas within the soft tissues indicates this is Fournier gangrene, requiring immediate surgical debridement.

Benign or Malignant?

Sonography is the imaging modality of choice when characterizing an intratesticular mass and is nearly 100% sensitive for detection of testicular tumors. Once US has confirmed the presence of a clinically suspected testicular tumor, the next step is differentiating the more common malignant lesions from less common benign ones.

Confident diagnosis of a benign intratesticular lesion can prevent unnecessary orchiectomy. The benign lesions that are easiest to characterize are cystic. These include cysts of the tunica albuginea, simple testicular cysts, and tubular ectasia of the rete testis. A key difference between these benign conditions and a cystic malignant neoplasm is that a solid component is absent in the former conditions. Diagnosis of an intratesticular varicocele can also be made with confidence in most cases using Doppler sonography.

Diagnosis of other benign intratesticular lesions demands more caution. Examples include epidermoid cyst, abscess, hematoma, focal infarction, and granulomatous orchitis. In many cases, the appearances of these lesions overlap that of malignancy; therefore, clinical factors (e.g., a history of trauma or fever) may be crucial in determining initial management. Table 20-4 summarizes US findings that favor a benign diagnosis.

Table 20-4 Focal Testicular Lesions: Ultrasound Findings That Favor Benign Disease

Testicular Cysts

Cysts of the tunica albuginea are usually found in men older than 40 years. They are located at the periphery of the testis, usually at the upper anterior or lateral margins. At grayscale US, they are usually biconvex or lentiform in shape and meet all the criteria of a simple cyst (Fig. 20-18). The cysts are typically unilocular but can be multilocular. Simple intratesticular cysts are similar in appearance to those of the tunica but can be located anywhere in the testis and can be solitary or multiple.

Figure 20-18 Benign testicular cysts. A, Sagittal ultrasound image demonstrates an oval-shaped simple cyst centered on the tunica albuginea. Note that it results in a slight bulge in contour, making it palpable. B, Sagittal image of multiple intratesticular cysts. Although some are adjacent to the tunica, not all of them are. These were not palpable.

Germ cell tumors that undergo necrosis and teratomas with cystic components also can have a cystic appearance. However, cystic malignant lesions have a complex appearance with solid components or thick septations, or both, that distinguish them from simple cysts (Fig. 20-19).

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