Color Doppler ultrasound is the most useful imaging modality for diagnosing acute epididymitis because it allows for real-time assessment of blood flow within the scrotal contents. In acute epididymitis, increased blood flow (hyperemia) in the epididymis and testis is characteristic and helps differentiate it from other causes of scrotal pain such as testicular torsion, which shows decreased or absent flow. This modality is non-invasive, readily available, and highly sensitive for detecting inflammation and ruling out surgical emergencies. Plain X-ray, CT, and MRI provide less specific information and are not first-line for scrotal pathology.

Reference: Campbell-Walsh Urology, 12th Edition.