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The haemosiderin cap sign refers to a rim or focal cap of hemosiderin deposition seen at the superior and/or inferior poles of an intramedullary spinal cord tumor, most classically ependymoma.

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Polar T2 hypointense blooming caps in an intramedullary tumor → think ependymoma

Classical association:

Spinal ependymoma (especially myxopapillary and classic types)
Considered a relatively specific sign for ependymoma among intramedullary tumors

![Sagittal T2WI (A), and sagittal T1WI (B) demonstrate an expansile multisegmental lesion with surrounding edema within the cervical cord. The lesion predominantly appears hyperintense on T2WI, and hypointense on T1WI, suggesting presence of intratumoral cysts. Low-signal intensity foci (demonstrating blooming on T2WI) suggestive of hemosiderin are seen capping this lesion both, cranially and caudally. Contrast-enhanced sagittal T1WI (C) demonstrates heterogenous, nodular peripheral enhancement.

November 29, 2010 - Case of the Week | American Journal of Neuroradiology. https://www.ajnr.org/content/cow/11292010](attachment:5bd6be71-32e9-44db-a480-077249f30b17:11292010img_0.jpg)

Sagittal T2WI (A), and sagittal T1WI (B) demonstrate an expansile multisegmental lesion with surrounding edema within the cervical cord. The lesion predominantly appears hyperintense on T2WI, and hypointense on T1WI, suggesting presence of intratumoral cysts. Low-signal intensity foci (demonstrating blooming on T2WI) suggestive of hemosiderin are seen capping this lesion both, cranially and caudally. Contrast-enhanced sagittal T1WI (C) demonstrates heterogenous, nodular peripheral enhancement.

November 29, 2010 - Case of the Week | American Journal of Neuroradiology. https://www.ajnr.org/content/cow/11292010

Pathophysiology

Results from chronic or repeated microhemorrhages within the tumor
Blood breakdown → hemosiderin deposition at tumor margins
Predominantly accumulates at tumor poles due to gravity and CSF pulsation dynamics

Radiology

MRI (Key modality):

MR sequence	Imaging features
T2	Hypointense rim/cap at tumor poles
T1	Usually iso- to hypointense
GRE / SWI	Blooming artifact → highly sensitive for hemosiderin
Post-contrast	Enhancing intramedullary mass with polar hypointense caps

Differentials

https://doi.org/10.1007/s00234-019-02229-6

Although characteristic, it is not entirely exclusive:

Other hemorrhagic intramedullary tumors:
- Astrocytoma (less common)
- Hemangioblastoma