What features confirm the mass to be composed of fat (there are 5 - depending on how you count them)?
1 and 2: follows fat on T1 and T2 weighted images. 3 and 4: signal fully attenuated on STIR and T1 fat sat sequences. 5: chemical shift artefact visible on sagittal T2.
What is the likely diagnosis? Why?
This lesion almost certainly represents a lipoma as the mass is entirely composed of fat. In most cases dermoid cysts have at least a small component of solid non-fatty tissue.
What traumatic injury is visible? On which sequence is this best appreciated?
Compression fractures (micro-trabecular) are seen as linear regions of bone marrow oedema in T5 and T8, best seen on STIR.
MRI demonstrates an elliptical intradural extramedullary fairly homogenous fat-containing lesion on the posterior aspect of cord, compressing and displacing the cord anteriorly. Tiny internal nodules are seen, with equivocal enhancement in few of them. No extradural extension. 9 x 4mm (axial). Rim chemical shift artefact.
Bone marrow oedema is confirmed in T8 as well as T5 consistent with compression fractures.