Liver cyst. Coloured ultrasound image of a cyst (dark, centre-left) in the liver of a patient. This cyst is a benign tumour which is filled with fluid and therefore transparent on an ultrasound scan. Such cysts are classified as abnormal lumps or swellings that can occur in any body organ or tissue. They are usually harmless but may grow so large as to disrupt the normal functioning of the liver. In such a case, surgical removal may be required. Ultrasound uses high-frequency sound waves to create images of the body.