Imaging of the Male Pelvis

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What is the normal anatomy and imaging appearance of the prostate gland and seminal tract on ultrasonography (US), computed tomography (CT), and magnetic resonance imaging (MRI)?

The prostate gland is an extraperitoneal fibromuscular gland surrounding the prostatic urethra at the bladder base. It is separated into a peripheral zone posteriorly, and a central zone and transitional zone more anteriorly. These zones constitute 70%, 25%, and 5% of the prostate gland, respectively. The transitional zone surrounds the proximal prostatic urethra, whereas the central zone surrounds the transitional zone and ejaculatory ducts. The prostate gland is also divided craniocaudally into the prostate base (superiorly), mid gland, and apex (inferiorly). The anterior fibromuscular stroma is a thick layer of nonglandular tissue that forms the anterior surface of the prostate gland, and the paired neurovascular bundles, responsible for erectile function, are located within the periprostatic fat posterolateral to the prostate gland at the 5 o'clock and 7 o'clock positions.

The central and transitional zones are not well demarcated on cross-sectional imaging and are collectively referred to as the central gland. On US, the peripheral zone is homogeneous and hyperechoic, whereas the central gland is more heterogeneous and hypoechoic, a differentiation that becomes more marked as the prostate ages and undergoes changes of benign prostatic hyperplasia (BPH). On CT, the peripheral zone is homogeneous and hypoattenuating relative to the more heterogeneously enhancing central gland. However, CT is not generally used for primary evaluation of the prostate gland and seminal tract, given its suboptimal soft contrast resolution. On T1-weighted MR images, the peripheral zone and central gland have low-intermediate signal intensity relative to skeletal muscle and are not well demarcated. On T2-weighted MR images, the peripheral zone has predominantly high signal intensity with thin linear low signal intensity fibrous septa, whereas the central gland has low-intermediate signal intensity relative to skeletal muscle. The anterior fibromuscular stroma has low T1-weighted and T2-weighted signal intensity ( Figure 39-1 ).

The seminal vesicles are extraperitoneal paired accessory sex glands that are located superior and posterior to the prostate gland. They are essentially long tubes that are convoluted to become approximately 3 cm in length and 1.5 cm in width. The seminal vesicles narrow inferomedially to form the seminal vesicle ducts. On US, the seminal vesicles appear as elongated hypoechoic septated cystic structures. On CT, they are seen as fluid-containing structures with a bow tie configuration. On T1-weighted MR images, the seminal vesicles have intermediate signal intensity. On T2-weighted images, the multiple tubular convolutions are well seen and contain very high signal intensity fluid surrounded by low signal intensity walls (see Figure 39-1 ).

The paired vasa deferentia are continuations of the epididymal tails, which ascend from the scrotum within the spermatic cords and into the pelvis via the deep inguinal rings, course posteriorly along the lateral pelvic walls, cross over the ureters, and then curve along the superomedial aspect of the seminal vesicles, where they become more dilated with thicker walls to form the ampullary portions. These then join with the paired seminal vesicle ducts to form the ejaculatory ducts, which are 1 to 2 mm in width, and extend inferiorly through the central zone of the prostate gland to drain into the prostatic urethra on both sides of the verumontanum.

The paired Cowper (bulbourethral) glands are pea-sized structures that are located within the urogenital diaphragm on either side of the membranous urethra. They each have a single duct that extends through the corpus spongiosum to drain into the bulbar urethra. Given their small size, they are not well visualized on cross-sectional imaging studies.

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What are the clinical indications for cross-sectional imaging of the prostate gland and seminal tract?

The most common clinical indication is staging of known prostate cancer diagnosed by previous biopsy, where prostate MRI is used most frequently for this purpose. Other indications include evaluation of patients who have hematospermia or male infertility.

Transrectal ultrasonography (TRUS) and MRI are most often utilized for evaluation of the prostate gland and seminal tract. CT is not very useful for this purpose given its inferior soft tissue contrast resolution compared to TRUS and MRI.

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What is the normal size of the prostate gland?

Prostate gland volume is estimated by multiplying the anteroposterior, transverse, and craniocaudal prostate gland dimensions and 0.52 (an approximation for π/6). Prostate gland volume normally increases with increasing age, and the prostate gland is generally considered to be enlarged when its volume is >40 ml.

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What is hematospermia, and what are its major causes?

Hematospermia is the presence of blood in the seminal fluid. The majority of cases of transient hematospermia in men <40 years of age are benign and self-limited and are not due to significant underlying disease processes. These patients are treated with conservative management. In men ≥40 years of age, or in those with other associated symptoms or signs of disease such as persistent hematospermia, TRUS or MRI can be used to evaluate for underlying pathologies including prostate cancer.

Major causes of hematospermia include:

• Idiopathic (the most common cause).

• Prior biopsy.

• Cysts, calculi, or calcification in the prostate gland or ejaculatory tract.

• Calculi or calcification.

• Benign prostatic hyperplasia (BPH).

• Inflammation or infection.

• Prostate cancer.

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What does hemorrhage in the prostate gland and seminal tract typically look like on MRI?

Hemorrhage in the prostate gland and seminal tract typically has high T1-weighted signal intensity, along with variably low T2-weighted signal intensity on MRI ( Figure 39-2 ). Such hemorrhage is very commonly seen on staging MRI following prostatic biopsy.

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What is the significance of calcification of the vas deferens?

Calcification of the vas deferens usually indicates the presence of diabetes mellitus and tends to be bilateral, symmetric, and in the walls of the vasa deferentia.

Chronic inflammation or prior infection may also lead to dystrophic calcification in the vas deferens; in these situations, the calcification is more often unilateral, segmental, and intraluminal in location.

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What kinds of cysts occur in the prostate gland and seminal tract?

• Utricular cysts are congenital and endodermal in origin and arise from the utricle. These are seen within the midline of the prostate gland, do not extend above the superior aspect of the prostate gland, and may communicate with the posterior urethra or ejaculatory duct.

• Müllerian duct cysts are congenital and mesodermal in origin and arise from the caudal ends of the fused müllerian ducts. These are seen within the midline of the prostate gland, often have a teardrop shape, and may extend above the superior aspect of the prostate gland but do not communicate with the urethra or ejaculatory duct.

• Ejaculatory duct cysts are usually acquired secondary to partial obstruction of the ejaculatory duct and are seen in a paramedian location along the course of the ejaculatory duct.

• Seminal vesicle cysts are typically located laterally in one or both seminal vesicles. They are associated with ipsilateral genitourinary anomalies such as renal agenesis, congenital absence of the vas deferens, and ectopic ureteral insertion into mesonephric duct derivatives such as the seminal vesicle or ejaculatory duct. Seminal vesicle cysts may also occur in patients with autosomal dominant polycystic kidney disease (ADPKD).

• Cowper's gland cysts are usually encountered in children and rarely in adults. These are typically located posterior or posterolateral to the bulbomembranous portion of the posterior urethra within the urogenital diaphragm.

• Retention cysts are usually secondary to obstruction of glandular components in the prostate gland and may occur anywhere in the prostate gland, usually off midline.

• Although cysts in the prostate gland and seminal tract are often asymptomatic, they may sometimes cause irritative or obstructive lower urinary tract symptoms secondary to compression/obstruction of the prostatic urethra or ducts and may result in hematospermia. Cysts may also get infected, and calculi or calcifications may form in the cysts.

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What is prostatitis, and what are its cross-sectional imaging features?

Prostatitis is inflammation or infection of the prostate gland. It is one of the most common urologic diseases in the United States and may be acute or chronic in clinical presentation. In men <35 years old, acute bacterial prostatitis (most often by Gram negative organisms) is most common, whereas in older men, nonbacterial prostatitis is most common.

On TRUS, focal, multifocal, or diffuse areas of decreased echogenicity may be seen. On MRI, both T1-weighted and T2-weighted images demonstrate low signal intensity foci in the peripheral zone, usually without associated contour deformity. There may be associated findings in acute prostatitis such as enlargement of the prostate gland, increased enhancement on CT and MRI, increased blood flow on Doppler US, and periprostatic inflammatory changes.

Granulomatous infections such as tuberculosis may lead to seminal vesiculitis, which manifests on imaging as wall thickening, loss of convolutions, or atrophy of the seminal vesicles. Calcifications may be seen in the prostate gland, vasa deferentia, and seminal vesicles in patients with granulomatous infection.

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Which patients are at risk for developing a prostatic abscess, and what are its cross-sectional imaging features?

Prostatic abscesses are encountered in up to 2.5% of patients who are hospitalized for prostatitis and are secondary to Gram negative organisms in 90% to 95% of cases, most often Escherichia coli . Risk factors include diabetes mellitus, chronic dialysis, immunosuppression, chronic urinary catheterization, and recent urethral manipulation. Treatment is typically with antimicrobial drugs and percutaneous or transurethral drainage.

On cross-sectional imaging, a prostatic abscess appears as a heterogeneous multiloculated complex cystic lesion with complex fluid, internal septations, thick rim enhancement on CT and MRI, and peripherally increased blood flow on Doppler US. Ill-defined focal enlargement of the prostate gland may be seen prior to liquefaction into an abscess. The presence of internal gas is highly specific for this diagnosis. Additional findings of coexistent acute prostatitis and seminal vesiculitis may also be seen ( Figure 39-3 ).

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What is benign prostatic hyperplasia (BPH), and what are its cross-sectional imaging features?

BPH is an aging phenomenon in the prostate gland, seen in up to 90% of men of advanced age. It is caused by enlargement of the transitional zone.

On cross-sectional imaging, heterogeneous nodular enlargement of the central gland is seen, leading to overall enlargement of the prostate gland. The enlarged central gland is hypoechoic on US and has increased attenuation and enhancement on CT. On T2-weighted MR images, glandular and cystic foci of BPH have high signal intensity, whereas stromal-dominant foci of BPH have low signal intensity. There may be associated compression of the peripheral zone, extension into the peripheral zone, mass effect upon the bladder base, or findings of chronic bladder outlet obstruction such as bladder wall thickening, trabeculation, or diverticulum formation ( Figure 39-4 ).

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What is the major differential diagnosis for a focal prostatic lesion?

For the answer, see Box 39-1 .

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What is prostate cancer?

Prostate cancer is the most common noncutaneous cancer in men (with a 1 in 6 lifetime risk) and the second most deadly cancer in men (with lung cancer being the deadliest). It most often occurs in older men and is rare before age 40. The tumor biology is quite variable and is often difficult to predict in advance. In many men, the disease behavior is indolent, whereas in others, it is aggressive and sometimes lethal.

Ninety-five percent of prostate cancers are due to adenocarcinomas, 3% are urothelial (transitional cell) carcinomas, and 2% are related to other tumor types such as neuroendocrine tumor, sarcoma, lymphoma, and metastasis.

Treatment may involve radical prostatectomy (involving resection of the prostate gland and seminal vesicles), radiation therapy, androgen deprivation therapy, focal ablative therapy, or active surveillance (involving close monitoring of the disease course in patients with very low risk disease).

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What is the Gleason score?

The Gleason score is the histologic grading system of choice for prostate cancer, which is important for determining patient prognosis. A Gleason score of ≤6 corresponds to a well-differentiated tumor, which portends a good prognosis; a Gleason score of 7 corresponds to a moderately differentiated tumor, which is associated with an intermediate prognosis; and a Gleason score of 8 to 10 corresponds to a poorly differentiated or undifferentiated tumor, which is associated with a worse prognosis.

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What is the role of cross-sectional imaging in the evaluation of patients with prostate adenocarcinoma?

TRUS is most often used for biopsy guidance, usually in the setting of an abnormal digital rectal exam (DRE) or elevated serum prostate specific antigen (PSA) levels. MRI is also used for biopsy guidance but is most commonly used for disease staging and pretreatment planning (particularly in patients with an intermediate-high risk of extracapsular spread of disease), active surveillance, and detection of tumor recurrence following therapeutic intervention. CT is most often utilized for assessment of regional lymphadenopathy (N stage) and distant metastatic disease (M stage) during staging assessment as well as for response assessment following therapeutic intervention.

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What are the cross-sectional imaging features of prostate adenocarcinoma?

Prostate adenocarcinoma arises most commonly from the peripheral zone (70%) and less commonly from the transitional zone (20%) and central zone (10%). It appears as a focal mass in the prostate gland. On US, it is hypoechoic or, less often, isoechoic relative to surrounding prostate gland tissue and sometimes has increased blood flow on Doppler US. On CT, it is usually not well visualized but may sometimes be seen as a focal soft tissue attenuation enhancing mass, particularly when large in size or extending into surrounding structures. On MRI, it typically has low-intermediate T1-weighted and low T2-weighted signal intensity relative to normal peripheral zone (see Figure 39-2 ), restricts diffusion on diffusion-weighted images, and enhances early with subsequent rapid washout.

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What are the cross-sectional imaging features that suggest extracapsular spread of prostate adenocarcinoma?

A focal irregular prostatic capsular bulge, obliteration of a rectoprostatic angle, soft tissue within the periprostatic fat contiguous with the prostate gland, asymmetry of the neurovascular bundles, and direct tumor invasion of adjacent organs (e.g., seminal vesicles, bladder, rectum) indicate local extracapsular spread of tumor ( Figure 39-5 ). Presence of regional lymphadenopathy (to internal iliac, external iliac, obturator, or sacral lymph nodes), sclerotic osseous metastases (which are high in attenuation on CT and low in signal intensity on MRI), or uncommonly distant metastases to other sites (most often nonregional lymph nodes, lungs, and liver) are also indicators of extraprostatic spread of tumor.

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Summarize the American Joint Committee on Cancer (AJCC) tumor node metastasis (TNM) staging system for prostate adenocarcinoma.

For the answer, see Box 39-2 .

The management of prostate cancer is in rapid evolution. Any T3 lesion (i.e., primary tumor extending beyond the prostate capsule) indicates at least stage III prostate cancer, and any T4 lesion (i.e., primary tumor invading adjacent structures other than the seminal vesicles), N1 lesion (i.e., presence of regional nodal metastasis), or M1 lesion (i.e., presence of distant metastasis) indicates stage IV prostate cancer. Currently, stage I and II prostate adenocarcinomas are usually treated with local therapy such as radical prostatectomy or radiation therapy, whereas stage III and IV prostate adenocarcinomas are usually treated with nonsurgical multimodal therapy including systemic and radiation therapies.

18

What is the normal anatomy and imaging appearance of the testicles on US, CT, and MRI?

The testicles are composed of densely packed seminiferous tubules that each converge posteriorly into large ducts and drain into the rete testis at the testicular hilum (also known as the mediastinum testis). Testicles are normally 2 to 3 cm wide and 3 to 5 cm long. In the rete testis, 15 to 20 efferent ductules converge to form the epididymal head superiorly and then converge into a single convoluted tubule in the epididymal body and tail inferiorly. The tubules emerge at an acute angle from the epididymal tail to form the vas deferens.

On US, the testicles have a homogeneous echotexture, are surrounded by a thin curvilinear echogenic tunica albuginea, and have symmetric blood flow on Doppler US. A linear band of echogenicity, representing the mediastinum testis, and an anechoic or hypoechoic focus containing small cystic or tubular structures, representing the rete testis, may be seen posteriorly at the testicular hilum. On CT, they have homogeneous soft tissue attenuation and enhancement, although CT is not used for primary evaluation of testicular disorders given the exposure to ionizing radiation and suboptimal soft tissue contrast resolution. On MRI, the testicles have homogeneous intermediate T1-weighted signal intensity and high T2-weighted signal intensity relative to skeletal muscle, are surrounded by a smooth thin curvilinear low signal intensity tunica albuginea, and homogeneously enhance. Thin low signal intensity septa radiating toward the mediastinum testis are often seen within the testicles on T2-weighted images. The mediastinum testis appears as a linear band of low signal intensity at the posteriorly located testicular hilum, whereas the rete testis has high T2-weighted signal intensity due to fluid within the seminiferous tubules ( Figure 39-6 ).

The epididymides are posterolateral to the testicles. On US, they are isoechoic/slightly hyperechoic relative to the testicles and have symmetric blood flow on Doppler US. On CT, they have soft tissue attenuation. On MRI, the epididymides have low-intermediate T1-weighted and low T2-weighted signal intensity relative to the testicles (see Figure 39-6 ).

The testicular arteries arise from the abdominal aorta near the level of the renal vessels and extend to the scrotum to supply the testicles. The right gonadal vein drains into the inferior vena cava, whereas the left gonadal vein drains into the left renal vein. Normal scrotal skin thickness varies between 2 to 8 mm.

19

What is cryptorchidism, and what are its cross-sectional imaging features?

Cryptorchidism occurs when there is incomplete descent of one or both testicles into the scrotum and is seen in 3% of neonates and in 1% of infants (since many testicles descend by age one) and adults. The ectopic testicle is most commonly found in or below the inguinal canal but may less commonly be found elsewhere in the abdomen or pelvis.

MRI is superior to US for localization of a cryptorchid testicle, especially when located in the abdomen. On US, the cryptorchid testicle is usually small and hypoechoic relative to the normal testicle and has an echogenic mediastinum. On MRI, the cryptorchid testicle is usually small relative to the normal testicle and has increased T2-weighted signal intensity relative to skeletal muscle, similar to the normal testicle.

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