T1 shading sign

Definition

• MRI finding in endometriomas (ovarian "chocolate cysts").
• High T1 signal (blood products) that shows loss of signal on T2 (shading).

Pathophysiology

• Repeated cyclic haemorrhage → accumulation of blood + high protein/iron.
• Leads to:
  • T1 hyperintensity (methemoglobin, proteinaceous fluid).
  • Progressive T2 hypointensity (deoxyhemoglobin, hemosiderin, high viscosity).

Imaging features

• T1WI: Hyperintense cyst content.
• T2WI: Signal loss ("shading").
• Fat-sat T1: Remains bright (distinguishes from fat-containing dermoid).
• Often multiple, bilateral, "kissing ovaries".

Pearls

• T1 shading sign is highly specific for endometrioma.
• Always look for associated deep infiltrating endometriosis (DIE).
• Distinguish from:
  • Haemorrhagic cyst (T1 bright, but T2 usually not shaded + resolves on follow-up).
  • Dermoid cyst (fat suppression knocks signal out).
  • Proteinaceous cyst (may mimic but lacks clinical context of endometriosis).