How do you measure the epididymal head using ultrasound in an adult male patient?

Measuring the Epididymal Head on Ultrasound

I cannot provide an actual image, but I can describe the precise measurement technique used in clinical practice.

Standard Measurement Technique

The epididymal head should be measured in two perpendicular planes (anteroposterior and transverse dimensions) using high-frequency ultrasound (typically 7.5-12 MHz linear transducer). 1

Step-by-Step Measurement Protocol

Patient Positioning and Probe Placement

• Position the patient supine with the scrotum supported on a towel between the thighs 1
• Apply generous ultrasound gel for optimal acoustic coupling 1
• Use a high-frequency linear transducer (minimum 7.5 MHz, preferably 10-12 MHz for optimal resolution) 2, 1

Locating the Epididymal Head

• In 88-98% of cases, the epididymal head is located above (superior to) the upper pole of the testis 1
• The epididymal head appears isoechoic (same echogenicity) compared to the testicular parenchyma 1
• In approximately 2-12% of cases, anatomic variants exist where the epididymis may be inverted with the head located below the lower pole 1

Measurement Technique

• Obtain measurements in two orthogonal planes:

  • Longitudinal (long-axis) view of the testis to visualize the epididymal head at the superior pole 1
  • Transverse (short-axis) view perpendicular to the first measurement 1

• Normal epididymal head dimensions: 7.6 ± 1.6 mm (mean ± standard deviation) 1

• Measure the maximum diameter in each plane using electronic calipers 1

• Compare measurements to the contralateral side, as bilateral comparison serves as an internal control 2

Key Sonographic Features to Document

Gray-Scale Characteristics

• The epididymal head should appear homogeneous and isoechoic relative to testicular tissue 1
• Any heterogeneity, hypoechoic areas, or mass-like enlargement warrants further evaluation 2, 3

Doppler Assessment

• Always include color or power Doppler evaluation when measuring the epididymal head 2, 4
• Normal epididymal head shows minimal to mild vascularity 2
• Markedly increased blood flow suggests epididymitis (sensitivity near 100% for inflammation) 2

Critical Measurement Pitfalls

Common Errors to Avoid

• Failing to identify anatomic variants: In 2-12% of cases, the epididymal head may be inverted or in an atypical location 1
• Inadequate transducer frequency: Using frequencies below 7.5 MHz results in suboptimal resolution for accurate epididymal assessment 1, 3
• Measuring the wrong structure: The pampiniform plexus appears as a dome-shaped structure that can mask the upper part of the epididymal head 5
• Not comparing to the contralateral side: Bilateral comparison is essential for detecting subtle abnormalities 2

When Measurements Indicate Pathology

• Enlargement >2 standard deviations above normal (>10.8 mm) suggests pathology 6
• Solid masses within the epididymal head require differentiation between benign lesions (adenomatoid tumor, cystadenoma) and malignancy 6, 3
• Any solid intratesticular mass detected during epididymal evaluation must be considered malignant until proven otherwise 7, 8
• Ultrasound has 98-100% sensitivity for detecting testicular tumors that may present as epididymal disease 7, 8

Additional Structures to Evaluate

Complete Epididymal Assessment

• Epididymal body: Located lateral to the testis, measures 3.2 ± 0.8 mm, appears hypoechoic compared to testicular parenchyma 1
• Epididymal tail: Always clearly visible, less echoic than testicular parenchyma with heterogeneous structure 1, 5
• Vas deferens: Measures 1.9 ± 0.2 mm, appears anechoic (fluid-filled tubular structure) 1

Documentation Requirements

• Record measurements in millimeters for both dimensions 1
• Document echogenicity relative to testicular parenchyma 1
• Note any asymmetry compared to the contralateral side 2
• Include Doppler findings regarding vascularity 2