The presence of a fat signal in an ovarian cyst is usually sufficient to make a diagnosis of mature cystic teratoma. The presence of diffusion restriction in cyst content is thought to be due to the presence of keratin which restricts Brownian movement of water molecules 1.
The presence of diffusion restriction in the cyst wall reflects hypercellularity that raises the suspicion of malignant transformation, which occurs in about 1% of cases, usually in elderly patients 1.