Ovarian cysts. Coloured transvaginal ultrasound scans of cysts (black) in a 26 year old woman's ovary. Cysts, fluid-filled sacs, are quite common in women of child-bearing age, they are rarely malignant (cancerous) and usually shrink and disappear over time. Ultrasound is a diagnostic technique that sends high frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure.