Tl-weighted, Optic Chiasm. The intracavernous ICA is seen both in longitudinal and in cross sections because of its S-shaped course. ICA is also seen in the supra sellar cistern, inferolateral to OC. CN V-3 is seen as it exits MC and passes into the fora men ovale. The su praoptic recess of V3 is seen above OC. Bitemporal hemianopia is the hallmark of chias mal dysfunction. If compression of OC comes from below, the field defect begins superi orly and progresses in a c1ock- wise direction in the right vi sual field and in a counter- clockwise direction in the left. PiG lies approximately 10 mm below the chiasm. Before visual field defects become ap parent, pituitary adenomas ust be large enough to break through the diaphragma sellae and extend into the suprasellar cistern, where they compress the visual pathway. If OC is compressed from above, as in a craniopharyngioma, the inferior temporal fields may be the first affected. CC = Corpus Callosum, CN III = Oculomotor Nerve, CN IV = Trochlear Nerve, CN V•3 = Trigeminal Nerve-3rd Division, ICA = Internal Carotid Artery, Man = Mandible, MC = Meckel's Cave, OC = Optic Chiasm, PIG = Pituitary Gland, PS = Pituitary Stalk, V3 = Third Ventricle.