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Epididymis

The epididymis is a long, coiled tubular structure that lies along the posterior border of the testis. It serves as a storage, maturation, and transport site for spermatozoa. The epididymis is divided into three parts — head (caput), body (corpus), and tail (cauda) — each with specific anatomic and functional roles. It is of clinical significance in urology and radiology, as it is commonly involved in infections (epididymitis), cysts, tumors, and post-surgical changes.

Synonyms

  • Ductus epididymidis

  • Testicular appendage (historical mislabel)

  • Testicular collecting duct

Location and Parts

  • Location:

    • Lies on the posterolateral aspect of the testis within the scrotum

    • Connected to the testis via efferent ductules from the rete testis

  • Parts:

    • Head (caput): Broad superior part, receives efferent ductules from testis

    • Body (corpus): Narrow middle portion, highly coiled duct

    • Tail (cauda): Inferior part that continues as the vas deferens (ductus deferens)

Relations

  • Anteriorly: Tunica vaginalis (parietal and visceral layers)

  • Posteriorly: Tunica albuginea of testis

  • Superiorly: Spermatic cord structures at epididymal head

  • Inferiorly: Continuous with vas deferens at epididymal tail

  • Medially: Testicular parenchyma

Function

  • Storage and maturation of spermatozoa

  • Absorption of testicular fluid and concentration of sperm

  • Transport of sperm from rete testis to ductus deferens

  • Provides functional environment for sperm motility development

Clinical Significance

  • Epididymitis: Common infection, causes scrotal pain and swelling

  • Spermatoceles/epididymal cysts: Benign cystic dilatations

  • Tumors: Rare, but include adenomatoid tumors or sarcomas

  • Post-vasectomy changes: Cystic dilatation and tubular ectasia

  • Torsion of epididymal appendix: Causes acute scrotal pain in children

  • Imaging relevance: Important to differentiate epididymal lesions from intratesticular pathology

MRI Appearance

T1-weighted images:

  • Epididymis shows low-to-intermediate signal intensity

  • Fat in surrounding scrotum appears bright

  • Hemorrhagic or proteinaceous cysts may appear bright

T2-weighted images:

  • Epididymis appears with intermediate-to-high signal intensity

  • Simple cysts appear very bright

  • Inflammation (epididymitis) shows increased signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal epididymis appears intermediate-to-high signal intensity

  • Pathology (infection, edema, inflammation) shows bright hyperintensity

  • Fat signal suppressed, enhancing epididymal visibility

T1 Fat-Sat Post-Contrast:

  • Normal epididymis enhances mildly and uniformly

  • Epididymitis: diffuse, intense enhancement

  • Abscess: rim enhancement with central non-enhancing core

  • Tumors: heterogeneous enhancement

3D T2 SPACE / CISS:

  • Epididymal ducts appear as intermediate to mildly hyperintense tubular structures compared to muscle

  • Surrounded by bright fluid/fat, providing clear visualization of cysts or dilatation

CT Appearance

Non-Contrast CT:

  • Epididymis appears as a soft tissue density structure posterior to the testis

  • Difficult to distinguish unless enlarged or cystic

  • Calcifications may be seen in chronic infection or post-surgery

Post-Contrast CT:

  • Normal epididymis enhances mildly and homogenously

  • Epididymitis shows diffuse, prominent enhancement

  • Abscess shows rim enhancement with central low density

  • Tumors may show irregular or heterogeneous enhancement

MRI image