Week 9: Penis Anatomy and Ultrasound

Anatomy of the Penis

  • The penis comprises three cylindrical structures:

    • Two corpus cavernosa, located on the dorsal surface.

    • One corpus spongiosum, located on the ventral surface.

  • Corpus Cavernosa:

    • Paired dorsal structures with low reflectivity on ultrasound.

    • Main erectile structures containing distensible sinusoidal tissue.

    • Surrounded by a thin, highly reflective tunica albuginea.

  • Corpus Spongiosum:

    • Ventral structure with high reflectivity.

    • Expands distally to form the glans penis.

    • The urethra passes through its center.

  • Both Corpus Cavernosa and Corpus Spongiosum:

    • Composed of sinusoidal spaces lined by smooth muscle and endothelium.

    • These spaces fill with blood during an erection.

    • The corpus spongiosum fills with blood but contributes minimally to the erectile state.

  • Tunica Albuginea:

    • A thick fascial sheath encasing both the corpus cavernosum and corpus spongiosum.

Blood Supply of the Penis

  • Primarily from the right and left internal pudendal arteries, originating from the right and left iliac arteries.

  • Each internal pudendal artery divides into:

    • Dorsal penile artery.

    • Bulbar artery.

    • Cavernosal arteries.

  • Cavernosal Arteries:

    • Primary source of blood flow for erectile function.

  • Dorsal Arteries:

    • Paired arteries supplying blood primarily to the skin and glans of the penis.

Venous Drainage of the Penis

  • Occurs via superficial and deep dorsal veins.

  • Superficial emissary veins perforate the tunica albuginea.

  • Drain into the deep dorsal vein via the cavernosal, spongiosal, and circumflex veins (visible on ultrasound).

  • The skin and glans are drained through the superficial dorsal veins.

Embryology

  • At the end of the first month of pregnancy:

    • The posterior intestine and the developing urogenital system migrate to the embryo's surface in the cloacal membrane region ventrally.

  • The cloacal membrane divides into:

    • A posterior (anal) half.

    • An anterior (urogenital) half.

  • Around the urogenital membrane, three bulges form:

    • Cephalic bulge: urogenital tubercle.

    • Two genital bulges: located on either side of the urogenital membrane.

  • At this stage, male and female embryos appear similar.

  • Male external genitalia development:

    • Triggered by testosterone release in response to luteinizing hormone from the pituitary gland.

    • The distance between the anus and genital structures increases.

    • The penis elongates.

    • The penile urethra forms from the penile groove.

    • The foreskin develops.

  • Foreskin development:

    • Starts in the 13th week of pregnancy via circular invagination of the ectoderm.

    • By the 20th week, the ectoderm covers the entire glans.

Physiology

  • Penile Erection:

    • Results from smooth muscle relaxation in the walls of the sinusoids, helicine, and cavernosal arteries of each corpus cavernosum.

    • As sinusoidal muscle tone decreases and sinusoids distend with blood, the small emissary veins are compressed between sinusoids and the peripheral tunica albuginea.

  • Veno-occlusive Mechanism:

    • Activated to maintain sinusoidal distension.

    • Limits venous outflow from sinusoidal spaces.

    • Results in arterial inflow and limited venous outflow.

    • Sinusoidal spaces distend, creating rigidity in cavernosal tissue, leading to penile erection.

Scanning Technique

  • Patient Preparation:

    • Maintain patient privacy (use a "do not disturb" sign).

    • Patient undresses from the waist down, with the gown open at the back.

    • Patient lies supine, with the penis positioned on the anterior abdominal wall.

      • Use the gown or a towel to hold the penis in place at the distal end.

    • Preserve patient dignity and comfort by covering areas not being scanned (e.g., the scrotal area).

    • Use warm gel if possible.

  • Transducer:

    • High-frequency (7.5-18MHz) linear transducer for high-resolution images.

    • Clean the transducer with appropriate disinfectant wipes after each scan.

    • Leave the patient in the room to dress in privacy.

Imaging (Transverse)

  • Technique:

    • Place the transducer on the ventral surface at the level of the glans.

    • Move down to the base of the penis to complete a survey in the transverse position.

    • Take images at various intervals from the base, mid-shaft, and distal regions of the penis.

  • Corpus spongiosum is easily compressed.

    • Use extra gel to alleviate compression.

  • Colour Doppler evaluation:

    • Performed in the transverse position to visualize the cavernosal arteries.

Imaging (Longitudinal)

  • Technique:

    • Obtain longitudinal evaluations of each corporal body through the ventral surface.

    • Cavernosal arteries appear as tubular structures with echogenic walls in the center of the corpora cavernosa.

    • Image the corpus spongiosum and urethra from the ventral aspect.

    • Avoid unnecessary compression of the penis by using extra gel or a stand-off acoustic pad.

Imaging – Colour Doppler

  • Used to demonstrate the cavernosal arteries in longitudinal views.

  • Duplex Doppler exam of cavernosal arteries:

    • A non-invasive test to assess impotence.

    • Color Doppler in an erect state is needed when assessing erectile dysfunction.

When Imaging the Penis, Look For:

  • Plaque placement

  • Calcifications

  • Septal defects

  • Corporal fibrosis