Normal Epididymis Resistance Index on Ultrasound
The epididymis is a coiled tubular structure posterior to the testis responsible for sperm maturation and transport. Doppler ultrasound allows quantitative assessment of epididymal blood flow through the resistance index (RI), calculated as (peak systolic velocity − end-diastolic velocity) / peak systolic velocity. Accurate RI measurement is clinically important in distinguishing inflammatory, obstructive, and ischemic scrotal conditions.
Normal Reference Values
| Measurement |
|---|
| 0.46-0.68 |
Clinical Significance
A normal epididymal RI of 0.46–0.68 reflects low-resistance arterial flow consistent with healthy perfusion. Values falling outside this range can indicate underlying pathology and should prompt clinical correlation with symptoms and grayscale findings.
Elevated RI values (above 0.68) may suggest increased vascular resistance, which can be seen in epididymal obstruction, chronic epididymitis, or venous congestion. Conversely, markedly reduced RI may reflect hypervascular states such as acute epididymo-orchitis, where diffuse hyperemia is the hallmark finding. It is important to note that RI values must be interpreted alongside grayscale morphology, clinical presentation, and contralateral comparison, as significant overlap exists between entities.
- Acute epididymitis: Decreased RI with markedly increased flow
- Chronic epididymitis: Possible elevated RI with fibrotic change
- Epididymal obstruction: Elevated RI due to downstream resistance
- Torsion of the appendix epididymis: Focal hyperemia adjacent to normal epididymal flow
- Epididymal spermatocele/cyst: Typically normal surrounding RI
Reference: Dogra VS, Gottlieb RH, Oka M et al. Sonography of the scrotum. Radiology. 2003;227(1):18-36.
Imaging Notes
Epididymal RI is obtained using color and spectral Doppler ultrasound with a high-frequency linear transducer (10–15 MHz). The patient should be supine with the scrotum supported on a towel between the thighs. Doppler gate placement should be within the epididymal head or body, with the sample volume kept small (1–2 mm) and the angle of insonation ideally below 60°. Multiple waveforms should be averaged to obtain a reliable RI value.
Technique pitfalls include excessive transducer pressure, which can artifactually reduce diastolic flow and falsely elevate the RI. Patient discomfort or cremasteric contraction may also alter measurements. Bilateral comparison is essential, as asymmetry in RI between sides may be more diagnostically useful than absolute values alone.