Ultrasound study of Sebaceous cyst:

This patient had a large cystic mass on the anterior abdominal wall. Sonography of the lesion showed a hypoechoic, inhomogenous mass with no flow on color Doppler imaging. The above ultrasound images show the "mass" suggestive of a jelly like material within the lesion; these appearances are diagnostic of sebaecous cyst (epidermoid cyst). The neck or outlet of the sebaceous cyst is also visible in these ultrasound images. Obstruction of the outlet near the neck of the cyst due to dirt or keratinous material is the cause of the obstruction and formation of the epidermoid cyst. (These ultrasound images are courtesy of Mr. Shlomo Gobi, Israel.)
References: http://www.ultrasound-images.com/musculoskeletal.htm (free ultrasound images/ case study of epidermoid cysts).