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Brainstem infarcts comprise approximately 10% of all first ischemic brain strokes. The extrinsic vascular supply to the stem is complex. The intrinsic vascularization of the stem may be conceptualized in terms of four relatively constant and distinct vascular territories designated anteromedial, anterolateral, lateral, and dorsal (or dorsolateral). The anatomic structures found within each intrinsic territory determine the symptomatology associated with infarction of that territory. This territorial anatomy permits the knowledgeable physician to plan an MR imaging examination tailored to the patient's history and to predict the patient's neurologic deficits from the MR imaging findings.

We describe a rare case of adult pilocytic astrocytoma in which exophytic growth from the brain stem presented as a right cerebellopontine angle mass. An initial MRI examination using T2- and T1-weighted images without and with contrast suggested the diagnosis of schwannoma. Subsequent use of 3D CISS (three-dimensional constructive interference in steady state) and T1-weighted contrast-enhanced 3D MP-RAGE (three-dimensional magnetization prepared rapid acquisition gradient echo) sequences led to the diagnosis of an exophytic brain stem tumor, documented the precise relationships of the tumor to cranial nerve VIII, revealed encasement of cranial nerves IX-XII (later confirmed intraoperatively), and provided the proper basis for planning surgical management. [ABSTRACT FROM AUTHOR]