Table 2 Main features of endometrioma images on US and MRI examinations [27, 44]

US

Unilocular or multilocular (less than 5 locules) cysts.

Homogenous low-level echogenicity (ground glass echogenicity).

Poor or no vascularization.

Presence of diffuse low-level echoes.

Multilocularity of hyperechoic foci in the wall.

Blot clots or fibrin adjacent to the cyst wall forming papillations (no vascularisation inside).

Thin septa in large endometriomas.

Anechoic thin-walled cyst with echogenic vegetation or focal wall nodularity (blood clots or fibrosis due to recurrent hemorrhage can mimick these findings).

MRI

Specific sign – shading (caused by old blood products containing high levels of iron and protein).

Higher T1, lower T2 signal intensities than in hemorrhagic cysts.

Shortening of T1 and T2 secondary to high protein concentration and increased viscosity.

Bilateral and multifocal lesions.

Cystic mass containing mural nodules and hemorrhagic fluid.

Enhancing mural nodules within endometrioma on T1 W1 is highly suggestive of malignancy.

Absence of characteristic T2-weighted “shading” which disappears in malignant tumor.