MRI of the Prostate and Male Genitourinary System

Anatomy

The prostate gland is a walnut-shaped gland that sits in the base of the pelvis. The base of the gland is the broad superior portion and the apex is the narrow inferior portion ( Fig. 11.1 ). The prostate gland has zonal anatomy ( Fig. 11.2 ). The peripheral gland is made up of the peripheral zone, which accounts for approximately 70% of the gland volume in young men. The peripheral zone contains the majority of the prostatic glandular tissue. The central gland is made up of the central and transition zones. The transition zone accounts for approximately 5% of the volume of the prostate gland in young men. The transition zone is made up of the glandular tissue, which encircles the proximal prostatic urethra. The central zone accounts for approximately 25% of the gland volume in young men. As men age there is nodular enlargement of the transition zone (benign prostatic hyperplasia, a.k.a. BPH) and compression of the central zone ( Fig. 11.3 ). There is fibrous stroma at the anterior aspect of the gland, aptly termed the “anterior fibrous stroma.”

The true prostatic capsule is a thin fibromuscular layer, which separates the gland from the periprostatic tissues. The periprostatic tissues are made up of fat and paired neurovascular bundles. Additionally, superior to the prostate gland are paired seminal vesicles.

Normal Appearance

In a normal prostate the peripheral zone demonstrates high T2-weighted signal intensity secondary to the high volume of glandular tissue. Whereas the central gland (central and transition zones) demonstrates low T2-weighted signal intensity. The T1-weighted appearance of the prostate gland is isointense to muscle. The prostate capsule demonstrates low T2-weighted signal secondary to its fibrous composition ( Fig. 11.4 ).

Field Strength

Increased signal to noise at higher field strengths can be used to increase spatial or temporal resolution, or both. At higher field strengths it is necessary to ensure that artifacts and signal heterogeneity are mitigated.

Endorectal Coil

The use of an endorectal coil can aid in increasing signal to noise when integrated with a surface coil. This may be especially true in larger patients where it is difficult to get adequate signal or penetration. However, use of an endorectal coil may increase the cost or length of the examination, cause deformation of the gland, and introduce artifacts. Some patients have also anecdotally reported discomfort, which could decrease compliance with the examination ( Fig. 11.5 ).

T2-Weighted Imaging

Axial T2-weighted imaging provides relatively high spatial resolution, and therefore the most critical information needed to differentiate the normal zonal anatomy of the prostate gland. Sagittal imaging is useful in distinguishing cancers of the base of the prostate gland from high benign prostatic hyperplasia. Furthermore, this may be useful in evaluating for seminal vesicle invasion. Coronal imaging allows for evaluation of apical lesions.

T1-Weighted Imaging

Axial T1-weighted imaging provides excellent tissue contrast for determining when fat planes are obliterated, as in the case of extracapsular extension. Additionally, T1-weighted imaging is useful in the evaluation of postbiopsy hemorrhage.

Diffusion-Weighted Imaging

Diffusion-weighted imaging depends on the microscopic mobility of water (Brownian motion). Brownian motion occurs as a result of thermal agitation and is dependent on the cellular environment of water. In tissues, water diffusivity is restricted by tissue cellularity, tissue organization, extracellular space tortuosity, and the integrity of cell membranes. Increased cellularity of neoplastic lesions manifests as restricted diffusion secondary to a decrease in Brownian motion. During prostate imaging, the apparent diffusion coefficient (ADC) maps are more reliable given the inherently increased T2-weighted signal intensity of the prostate gland ( Fig. 11.5 ). High b-value diffusion imaging should either be acquired or synthesized (extrapolated from lower b-value data).

Dynamic Contrast Enhancement

Dynamic contrast enhancement should be acquired in all prostate MR examinations so as not to miss small foci of early enhancement, which can then be thoroughly interrogated by T2-weighted and diffusion-weighted imaging.

Prostatic and Ejaculatory Duct Cysts

Prostatic cysts are generally divided into midline and paramedian. Midline cysts are either utricle cysts or Müllerian duct cysts. Utricle cysts are generally small and do not extend above the gland ( Fig. 11.4 and Fig. 11.6 ). In general these midline cysts tend to be asymptomatic, but, if needed, can be aspirated or resected. Paramedian cysts can cause obstruction of the ejaculatory ducts and potentially infertility, for which resection is curative.

Seminal Vesicle Cysts

Seminal vesicle cysts are uncommon benign lesions that can be associated with ipsilateral renal agenesis and autosomal dominant polycystic kidney disease. Symptomatic cysts can be marsupialized or resected ( Fig. 11.7 ).

Prostatitis

Imaging is rarely performed for evaluation of prostatitis; however, chronic inflammation of the peripheral zone of the prostate gland can be confused with cancer. Prostatitis often results in a relatively diffuse, low, T2-weighted signal intensity of the peripheral zone without mass effect or capsular irregularity ( Fig. 11.8 ).

Benign Prostatic Hyperplasia

Benign prostatic hyperplasia is characterized by nodular enlargement of the transition zone with heterogeneous T2-weighted signal intensity ( Fig. 11.3 ). The T2-weighted appearance depends on the amount of stromal or glandular elements within the nodule.

Prostate Cancer

Prostate cancer is the second most common cancer among men and the second leading cause of cancer-related death. MRI plays a role in the primary diagnosis, staging, treatment planning, active surveillance, and the diagnosis of recurrent disease following therapy.

Prostate adenocarcinoma is isointense to normal prostate parenchyma in T1-weighted imaging and of lower signal intensity in T2-weighted imaging. The T2-weighted appearance lends itself to the diagnosis of peripheral zone malignancies; however, transition zone cancers remain difficult to diagnose on the basis of T2-weighted imaging alone. The addition of diffusion-weighted imaging and dynamic contrast enhancement (multiparametric MRI) can aid in the diagnosis of transition zone cancers. Currently the American College of Radiology’s Prostate Imaging-Reporting Data System version 2 (PI-RADS v2) depends on T2-weighted and diffusion-weighted imaging features in the peripheral and transition zones, only in certain circumstances does dynamic contrast enhancement influence the determination of the presence of clinically significant cancer.

In the peripheral zone, the likelihood of clinically significant cancer is based primarily on diffusion-weighted imaging assessment and the T2-weighted assessment does not influence the assessment. In the transition zone, the likelihood of clinically significant cancer is based primarily on the T2-weighted assessment, with the diffusion-weighted assessment playing a role in equivocal T2-weighted assessments ( Fig. 11.5 ).

In T2-weighted images, clinically significant peripheral zone cancers are focal round or ill-defined; however, this appearance is not specific and can be seen in a variety of benign conditions including prostatitis, hemorrhage, biopsy-related scarring, and following therapy. Transition zone cancers are generally noncircumscribed, lenticular, homogeneous, and hypointense lesions with speculated margins lacking a continuous capsule.

In diffusion-weighted imaging, clinically significant cancers demonstrate restricted diffusion (increased signal in high b-value diffusion images with corresponding decreased intensity in the ADC map). As stated in the imaging technique section, the ADC appearance of the prostate gland is generally more reliable than the diffusion signal alone secondary to the high T2-weighted signal intensity of the prostate gland, and therefore diffusion assessment should rely on the evaluation of the ADC map along with the diffusion weighted images ( Fig. 11.5 ). Clinically suspicious lesions should have a focal markedly hypointense appearance in the ADC map images with a markedly hyperintense corresponding lesion in the diffusion-weighted images.

Prostate cancers frequently demonstrate early enhancement following the administration of gadolinium contrast agents. However, kinetics of individual tumors can be variable and therefore only wash-in characteristics are considered. Areas of focal rapid enhancement should be correlated with T2-weighted and diffusion-weighted images to assess for suspicious lesions.

To determine whether prostate cancer has spread beyond the prostate capsule, evaluation of the periprostatic fat, seminal vesicles, pelvic lymph nodes, and pelvic bone marrow should be performed as part of a routine evaluation. Involvement of any or all of these structures has implications for therapy.

Metastases

Prostate metastases are rare, but can occur as a result of direct extension from bladder and rectal cancers.

Anatomy

The penis is comprised of three endothelium-lined vascular spaces; two paired corpora cavernosa along the dorsolateral portion and a ventral corpus spongiosum. Posteriorly the corpora cavernosa is attached to the ischial tuberosities. The urethra courses through the corpus spongiosum. Each corpus is surrounded by a tunica albuginea, and all three corporeal bodies are surrounded by the fibrous Buck’s fascia ( Fig. 11.9 ).

Normal Appearance

All of the corporeal bodies are of intermediate T1-weighted signal intensity and high T2-weighted signal intensity, compared with skeletal muscle. The corpus spongiosum has a slightly higher T2-weighted signal intensity, compared with the corpora cavernosa, and this is related to the difference in blood flow rates. The tunica albuginea and Buck’s fascia are inseparable during MR imaging and are of low T1- and T2-weighted signal intensity owing to their fibrous makeup ( Fig. 11.10 ).