Imaging appearance of the prostate gland and seminal tract on USG CT MRI
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.
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.
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.