For masses in the spinal canal it is critical to determine location; extrathecal, intramedullary or intrathecal extramedullary. Contour of the theca and cord are the two things to assess. Once you know where it is located then the differential.
The haemosiderin cap sign refers to an MR imaging feature in some spinal tumours where a cap of hypointense haemosiderin is above and/or below the tumour due to previous haemorrhage. It is most often associated with spinal cord ependymomas, .