What is the most likely diagnosis? What are the two differentials?
Dermoid is by far the most likely diagnosis. Two lesions to be considered are mature teratoma and a lipoma.
Which sequence is key in identifying the reason for T1 hyperintensity?
The two fat saturated sequences (T1 C+ fat sat and gradient echo (MERGE)) demonstrate loss of signal from the mass consistent with fat.
What makes a lipoma less likely?
Lipomas usually demonstrate more homogeneous signal intensity (they are composed just of adipose tissue rather than a mixture of cellular debris, hair etc...).
True or false: the fat signal of a dermoid cyst is due to adipose tissue (lipocytes).
False. This is a common misconception which leads to quite a lot of confusion. Dermoids contain only ectodermal structures including skin appendages such as glands and hair follicles. The fat intensity / density components are due to the mixture of sebum and desquamated cells. Adipose tissue on the other hand is of mesodermal origin and is only found in teratomas.
What is a common complication of intracranial dermoid cysts, not present in this case?
Rupture (spontaneous, traumatic, or iatrogenic (at resection)) : leakage of sebum into the subarachnoid space results in an aseptic chemical meningitis. Presentation is variable, ranging form headache, to seizures, vasospasm and even death.
MRI of the brain demonstrates an extra-axial mass with intrinsic high T1 which suppresses on fat suppressed sequences. It has heterogeneous signal on both T1 an T2 weighted images. It is separate from the pituitary and is intimately associated with the left A1 segment of the anterior cerebral artery.
The mass straddles the left optic nerve which is pushed inferiorly.