Ultrasound- Spermatoceles versus epididymal cysts:

Left Epididymal cyst





























Right spermatocele:




























Spermatoceles are the commonest cystic lesions of the scrotum. Both spermatocele and simple cysts of the epididymis are often difficult to distinguish from each other purely on ultrasound. The main difference is usually presence of echogenic debris or particles in the fluid within the cystic mass; particles (inactive sperms) are present in spermatocele (see images above) whilst clear fluid is seen in simple cyst of the epididymis (top most images).
See: http://www.ultrasound-images.com/scrotum.htm
for more on this topic.