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Reproductive

Male Reproductive Anatomy

The male reproductive system spans the pelvic prostate and seminal vesicles and the scrotal testes, epididymes, and spermatic cords, evaluated principally with multiparametric prostate MRI and high-resolution scrotal ultrasound. The zonal anatomy of the prostate is central to cancer detection and PI-RADS interpretation, while ultrasound is definitive for scrotal emergencies. Knowledge of ductal pathways and vascular supply informs assessment of infertility, infection, and malignancy.

Key Structures

  • Prostate zones: peripheral zone (high T2, origin of ~70% of cancers), central zone, transition zone (site of BPH), and anterior fibromuscular stroma
  • Seminal vesicles: paired grape-like, T2-hyperintense structures posterosuperior to the prostate; join the vas to form ejaculatory ducts
  • Testes: homogeneous mid-level echotexture on US, intermediate-high T2 on MRI; mediastinum testis as an echogenic band
  • Epididymis: head (globus major) at the superior pole, body, and tail; slightly more heterogeneous than the testis
  • Vas deferens and spermatic cord: cord contains the vas, testicular artery, and pampiniform venous plexus traversing the inguinal canal
  • Penis: paired corpora cavernosa and the corpus spongiosum enclosing the urethra, bound by the tunica albuginea and Buck fascia
  • Neurovascular bundles posterolateral to the prostate—a route of extracapsular tumor spread

Imaging Planes & Modalities

  • Multiparametric prostate MRI (high-resolution T2, DWI with high b-value/ADC, and dynamic contrast) detects and stages cancer using PI-RADS; T2 and DWI are dominant sequences
  • High-frequency scrotal ultrasound with color and spectral Doppler is first-line for pain, masses, and torsion, assessing echotexture and vascularity
  • Axial, coronal, and sagittal T2 aligned to the prostate optimize zonal and capsular assessment and seminal-vesicle invasion
  • Transrectal ultrasound guides prostate biopsy (often MRI/US fusion) and assesses prostate volume
  • MRI of the penis/scrotum characterizes tumors, trauma (tunica rupture), and Peyronie disease
  • PET/CT (PSMA) stages nodal and metastatic prostate cancer beyond the local MRI assessment

Landmarks & Normal Values

  • PI-RADS assessment category 1-5 based on the dominant sequence (DWI for peripheral zone, T2 for transition zone)
  • Normal prostate volume ~20-30 mL in young men; PSA density (PSA/volume) aids risk stratification
  • Normal adult testis ~3-5 cm long, 15-25 mL volume, symmetric echotexture
  • Testicular artery low-resistance waveform on Doppler; asymmetric absence of flow indicates torsion
  • Seminal vesicles and the neurovascular bundles define extraprostatic extension landmarks for staging

Anatomical Variants

  • Undescended testis (cryptorchidism), commonly in the inguinal canal—raises malignancy risk
  • Testicular microlithiasis—multiple echogenic foci, associated with germ cell tumor risk in select populations
  • Congenital absence of the vas deferens (associated with CFTR mutations) and seminal vesicle agenesis/cysts
  • Persistent Müllerian remnants (prostatic utricle/Müllerian duct cyst) in the midline
  • Polyorchidism and epididymal/tunica albuginea cysts
  • Variant prostatic median lobe hypertrophy projecting into the bladder

Pathology Correlation

  • Prostate cancer: focal restricted diffusion and low T2 in the peripheral zone; extracapsular extension, seminal-vesicle invasion, and neurovascular bundle involvement determine staging
  • BPH: transition-zone nodular enlargement; encapsulated nodules can mimic or obscure cancer on MRI
  • Testicular torsion: enlarged, heterogeneous testis with absent intratesticular Doppler flow—a surgical emergency
  • Epididymo-orchitis: hyperemia with an enlarged, hypoechoic epididymis and reactive hydrocele
  • Testicular tumors: intratesticular solid masses (seminoma homogeneous; non-seminomatous heterogeneous)—any solid intratesticular lesion is malignant until proven otherwise
  • Varicocele: dilated pampiniform veins (>2-3 mm) with reflux on Valsalva, more common on the left and a cause of infertility

Pitfalls & Mimics

  • Central-zone and BPH nodules can mimic transition-zone cancer—apply PI-RADS T2 criteria carefully
  • Post-biopsy hemorrhage causes T1 hyperintensity that confounds MRI—image before biopsy or allow an interval
  • An extratesticular mass is usually benign, whereas an intratesticular solid lesion is malignant until proven otherwise—precisely localize
  • Intermittent or early torsion may retain some flow; a heterogeneous testis with clinical suspicion warrants surgery regardless
  • Do not mistake a hydrocele, spermatocele, or epididymal cyst for a solid mass
  • Anterior fibromuscular stroma and periprostatic structures can be mistaken for tumor on T2

References

  • ACR PI-RADS v2.1 Prostate Imaging Reporting and Data System
  • Dogra VS, et al. Sonography of the scrotum. Radiology / scrotal ultrasound literature
  • Hricak H, Scardino PT. Prostate Cancer (Contemporary Issues in Cancer Imaging). Cambridge
  • Rumack CM, Levine D. Diagnostic Ultrasound. Elsevier

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.