Ultrasound of a 32-year-old woman with pelvis pain shows a 5 cm ovarian cyst with homogeneous low-level internal echoes (ground glass) and no mural nodularity or vascularity. MRI confirms a T1 hyperintense, T2 hypointense (shading) unilocular cyst with no enhancement. This is the 'T2 shading sign'. The most likely diagnosis is:

• A Mature teratoma (dermoid cyst)
• B Mucinous cystadenoma
• C Ovarian endometrioma ('chocolate cyst') ✓
• D Haemorrhagic corpus luteum cyst

Explanation

The T2 shading sign — progressive loss of T2 signal from top to bottom within a cyst that is T1 hyperintense — is characteristic of endometrioma. It results from accumulation of repetitive haemorrhage with breakdown products (heme concentration gradient), producing layered T2 hypointensity. Dermoid cysts show T1 bright fat signal with chemical shift artefact. Haemorrhagic corpus luteum also shows T1 hyperintensity but resolves in 4–8 weeks, lacks T2 shading, and is typically smaller.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.