In a man evaluated for reduced ejaculate volume with suspected ejaculatory‑duct obstruction, what findings should be assessed on pelvic MRI?

Pelvic MRI Findings in Suspected Ejaculatory Duct Obstruction

In a man with reduced ejaculate volume and suspected ejaculatory duct obstruction, pelvic MRI should assess for dilated seminal vesicles (anterior-posterior diameter >15 mm), dilated ejaculatory ducts (caliber >2.3 mm), dilated vasal ampullae (>6 mm), and midline or paramedian prostatic cysts that may compress the ejaculatory ducts. 1, 2

Key Anatomical Structures to Evaluate

Seminal Vesicles

• Measure the anterior-posterior diameter of both seminal vesicles—dilatation >15 mm is the most reliable indicator of distal obstruction 1, 2
• Assess for asymmetric dilatation, which may indicate unilateral obstruction 3
• Look for complete absence or aplasia of seminal vesicles, which occurs in congenital bilateral absence of vas deferens (CBAVD) and requires differentiation from true ejaculatory duct obstruction 3, 4

Ejaculatory Ducts

• Measure the caliber of the ejaculatory ducts—dilatation >2.3 mm suggests obstruction 1, 2
• Trace the course of the ejaculatory ducts from the seminal vesicle-vasal junction to their opening at the verumontanum 3
• Identify the precise location and distal extent of any obstruction, as this determines surgical approach 3, 4

Vasal Ampullae

• Assess for dilatation of the vasal ampullae (>6 mm), which indicates obstruction proximal to the ejaculatory duct junction 1, 2
• Confirm the presence of both vasa deferentia to exclude CBAVD, which presents with absent or atrophic seminal vesicles rather than dilated ones 1, 3

Prostatic Cysts and Midline Lesions

• Identify midline prostatic cysts or paramedian/ejaculatory duct cysts that may compress the ejaculatory ducts and cause functional obstruction 1, 2
• Characterize cyst size, location, and relationship to the ejaculatory ducts and prostatic urethra 3, 5
• Assess whether cysts communicate with the ejaculatory ducts or seminal vesicles 3

Relationship to Prostatic Urethra

• Evaluate the spatial relationship between the proximal prostatic urethra and the posterior wall of the ejaculatory ducts—this anatomic detail is critical for surgical planning if transurethral resection of ejaculatory ducts (TURED) is contemplated 3
• MRI provides superior soft-tissue contrast compared to transrectal ultrasound (TRUS) for delineating these relationships 3

When MRI Is Indicated vs. TRUS

• Reserve pelvic MRI for cases where TRUS is negative or inconclusive, as TRUS is the first-line imaging modality (rated 8/9 by ACR) 6
• MRI is particularly useful when surgical planning requires precise anatomic detail or when TRUS cannot adequately visualize the ejaculatory ducts due to body habitus or technical limitations 6, 3
• Do not perform MRI as part of the initial evaluation—it should only be ordered after clinical suspicion is high based on semen parameters (volume <1.4 mL, acidic pH <7.0, azoospermia or severe oligospermia with very low motility, normal testosterone, palpable vas deferens) 1, 2

Patterns of Obstruction on Imaging

Complete Ejaculatory Duct Obstruction

• Bilateral dilated seminal vesicles (>15 mm) with dilated ejaculatory ducts (>2.3 mm) 1, 2, 7
• May be associated with midline prostatic cyst compressing the ducts 7, 5

Partial Ejaculatory Duct Obstruction

• Variable degrees of seminal vesicle dilatation, often asymmetric 7, 5
• Ejaculatory duct caliber may be normal or only mildly increased 5
• Inflammatory stenosis may show thickening or irregularity of the duct walls without marked dilatation 3, 7

Congenital Bilateral Absence of Vas Deferens (CBAVD)

• Absent or severely atrophic seminal vesicles rather than dilated ones 3, 4
• Absent vasa deferentia on imaging (though this is best confirmed by physical examination) 1, 3

Critical Pitfalls to Avoid

• Do not confuse CBAVD with ejaculatory duct obstruction—CBAVD presents with absent/atrophic seminal vesicles, whereas EDO shows dilated seminal vesicles 1, 3, 4
• Do not order MRI routinely for all men with low ejaculate volume—imaging is only indicated when clinical and laboratory findings strongly suggest EDO (acidic semen, azoospermia, normal testosterone, palpable vas) 1, 2
• Ensure the radiologist measures seminal vesicle diameter and ejaculatory duct caliber explicitly—qualitative descriptions like "prominent" or "mildly dilated" are insufficient for surgical decision-making 1, 2
• Recognize that normal imaging does not exclude functional or partial obstruction—in such cases, seminal vesicle aspiration under TRUS guidance may be needed to document sperm presence and confirm obstruction 7, 8