What is the normal anatomy and imaging appearance of the penis on US CT and MRI?
The penis is composed of three cylindrical cavernous structures: the paired dorsal corpora cavernosa and the single ventral corpus spongiosum. The crura of the corpora cavernosa are the most proximal portions, which diverge laterally to attach to the ischiopubic rami. The corpus spongiosum extends and enlarges distally to form the glans penis and proximally to form the bulb of the penis, and it contains the anterior portion of the urethra. The tunica albuginea is a thin layer of fibrous tissue that surrounds each of the corpora cavernosa and the corpus spongiosum. The deep (Buck’s) fascia is just superficial to the tunica albuginea and surrounds the corpora cavernosa in a dorsal compartment and the corpus spongiosum in a separate ventral compartment.
The cavernosal arteries are terminal branches of the internal pudendal arteries located within the paired corpora cavernosa. They give off the helicine arteries, which provide the primary source of blood for erectile function. The paired dorsal arteries and deep dorsal vein are located deep to the deep (Buck’s) fascia and superficial to the tunica albuginea along the dorsal aspect of the penis. The deep dorsal vein provides the primary venous drainage for the corpora cavernosa, whereas the paired dorsal arteries supply blood flow to the glans penis and penile skin.
On US, the tunica albuginea and deep (Buck’s) fascia together appear as a thin echogenic border surrounding the homogeneous and hypoechoic corpora. In addition, the cavernosal arteries appear as punctate central foci of echogenicity within the corpora cavernosa. On unenhanced CT, the tunica albuginea and deep (Buck’s) fascia appear as higher soft tissue attenuation curvilinear foci surrounding the relatively lower soft tissue attenuation corpora. On MRI, the tunica albuginea and deep (Buck’s) fascia are inseparable and appear as a single low signal intensity border surrounding the intermediate T1-weighted and high T2-weighted signal intensity corpora. In addition, the cavernosal arteries appear as punctate central foci of low signal intensity within the corpora cavernosa, and the urethra appears as a flat band of low signal intensity within the corpus spongiosum when viewed in cross-section on T2-weighted images. On both CT and MRI, following intravenous contrast administration, the corpus spongiosum enhances early whereas the corpora cavernosa enhance gradually in a centrifugal fashion given the central location of the cavernosal arteries