How to Measure the Epididymal Head Using Ultrasound
The epididymal head should be measured in two perpendicular planes using high-frequency ultrasound (typically ≥7.5 MHz linear array transducer), with normal size being 7.6 ± 1.6 mm in adult males. 1
Scanning Technique
Equipment and Patient Positioning
- Use a high-frequency linear array transducer (7.5-10 MHz or higher) for optimal resolution of scrotal structures 2
- Position the patient supine with the scrotum supported on a towel to stabilize the structures 1
Measurement Protocol
- Obtain measurements in two perpendicular planes (typically anteroposterior and transverse dimensions) to accurately assess epididymal head size 1, 3
- The epididymal head appears isoechoic compared to the testicular parenchyma on grayscale imaging, which helps distinguish it from other structures 1
- Normal epididymal head size: 7.6 ± 1.6 mm (mean ± standard deviation) 1
Anatomic Localization
Standard Anatomy (Most Common)
- In 88-98% of cases, the epididymal head is located above the upper pole of the testis 1
- The epididymal body runs lateral to the testis, and the epididymo-deferential loop sits below the lower pole 1
Anatomic Variants to Recognize
- In 6-9% of cases, the epididymal body is located posterior to the testicular body with an inverted epididymo-deferential loop 1
- In 2-12% of cases, complete epididymal inversion occurs with the epididymal head located below the lower pole of the testis 1
- Failure to recognize these variants can lead to measurement errors or missed pathology, so systematic scanning of all scrotal regions is essential 1
Key Technical Considerations
Distinguishing Normal Structures
- The pampiniform plexus appears as a dome-shaped structure that may partially mask the upper portion of the epididymal head 4
- The epididymal body appears hypoechoic relative to testicular parenchyma 1
- The vas deferens appears anechoic and measures approximately 1.9 ± 0.2 mm 1
When Enlargement is Significant
- Epididymal head enlargement more than two standard deviations above normal (>10.8 mm) should be considered abnormal 3
- Bilateral enlargement may suggest systemic conditions like von Hippel-Lindau disease, where epididymal cystadenomas commonly present as 15-20 mm solid masses with small cystic components 3
Clinical Pitfalls to Avoid
- Always use both grayscale and color Doppler imaging to assess for inflammatory conditions that may enlarge the epididymal head, as grayscale alone has limited sensitivity 2
- Do not rely solely on clinical examination to exclude testicular pathology presenting as epididymal disease—ultrasound has 98% sensitivity and 99.8% specificity for detecting occult testicular tumors in patients clinically diagnosed with epididymal disease 5
- Measure consistently in the same planes across examinations to allow accurate comparison over time, as measurement variability can affect clinical decision-making 1