What are the two main findings?
1) Large heterogeneous mass with areas of haemorrhage; 2) Leptomeningeal enhancement.
What does the restricted diffusion imply? What other sequence would you look at to confirm that this is the case?
Restricted diffusion would imply high cellularity, but you need to ensure that it does not represent T2 shine-through. Looking at the ADC map is thus essential. In this case, the diffusion restriction is real (ADC not shown).
MRI with contrast demonstrates a large heterogeneous mass arising from the left thalamus and growing exophytically into the lateral ventricle. It is associated with dilatation of both lateral ventricles, in keeping with obstruction of their outflow at the foramen of Munro. Areas of the mass are intrinsically high signal on T1 weighted imaging and show signal drop-out on T2 weighted sequences, suggestive of haemorrhage, and on DWI the mass is quite high signal (low on ADC map - not shown), consistent with a degree of restricted diffusion, suggesting high cellularity. Following contrast administration, the mass heterogeneously enhances.
In the posterior fossa, particularly on the ventral surface of the brainstem, there is leptomeningeal enhancement, likely representing CSF seeding.