What is the main differential of this lesion and how would you exclude it?
A small amount of fat could have very similar appearances (e.g. lipoma or dermoid or teratoma). The easiest way to prove that his is not fat is to examine a CT (if already performed) or perform a fat saturated sequence; fat will drop in signal, the posterior pituitary tissue will not. Chemical shift artefact can also be useful.
If symptomatic, how do these patients usually present?
Patients with ectopic posterior pituitary glands often present with features of growth hormone deficiency (pituitary dwarfism). Alternatively they may present due to symptoms / signs referable any one of numerous associated malformations (see related article).
No normal posterior pituitary bright spot is seen in the pituitary fossa, but rather a small region of high T1 signal (yellow arrow) is seen high up at the top of the infundibulum (which appears small).