A T1-weighted sagittal MR image from a control subject, showing the midline structures of the posterior cranial fossa and the brainstem and the cerebellum. d + e = length of clivus; S = sphenooccipital synchondrosis; d = length of basisphenoid between the top of the dorsum sellae and the sphenooccipital synchondrosis of the clivus; e = length of the basiocciput between the synchondrosis and the basion; b = length of the hindbrain between the midbrain-pons junction and the medullocervical junction; a = angle of the cerebellar tentorium to Twining's line; c = length of cerebellar hemisphere; DS = top of the dorsum sellae; IOP = internal occipital protuberance; OP = opisthion; IOP to OP = length of supraocciput; B = basion; TW = Twining's line; McR (B to OP) = McRae's line.