This MRI demonstrates typical characteristics of a third ventricular colloid cyst. Whereas the appearance on CT (not shown) is quite predictable, appearing as hyperdense non-enhancing nodule at the foramen of Monro in the roof of the third ventricle, on MRI signal characteristics are much more variable, depending on the composition of the cyst fluid. 

This example is the most common pattern of signal intensity: 

• hyperintense to adjacent white matter on T1 weighted images
• hypointense to adjacent white matter on T2 weighted images
• no central enhancement: thin peripheral enhancement is attributed to stretched septal veins
• no calcification

The lack of enhancement and calcification makes subependymal giant cell astrocytomas (SGCA) and subependymal nodules of tuberous sclerosis most unlikely (especially in the absence of other stigmata of the syndrome). 

It is important to assess for the presence of hydrocephalus which is the most dreaded complication. In this case, the ventricles are unremarkable, the sulci not effaced and there is no evidence of transependymal edema. 

This patient went on to have an endoscopic resection which confirmed the diagnosis.