Part 3
In case 3.1 there is complete loss of signal on FLAIR, similar to that seen in the CSF. This indicates that the lesion contains pure fluid and is in keeping with an arachnoid cyst.
In case 3.2, however, there remains high signal on FLAIR indicating the mass contains something other than fluid. In this case it is an epidermoid cyst.
We can see differences that confirm this on the other sequences. In case 3.1 the signal of the lesion follows that of CSF on all sequences. In case 3.2 the signal of the lesion does not follow CSF.
N.B. a characteristic of epidermoid cysts is that they restrict. This is thought to be due to desquamation. On pathology they contain very thick material which could, very well, restrict the diffusion of water.
Tip:
The signal of the CSF in the ventricles and subarachnoid space is a very helpful pointer. Not only will it help you to identify which sequence you are looking at but it will also help you to figure out whether a lesion contains fluid or not.