The medial posterior choroidal artery is a small branch (often multiple - 40% of hemispheres) usually arising from the P2 segment of the PCA. It may also arise from one of the PCA branches, e.g. parieto-occiptal, calcarine, splenial artery.
It ascends deep to the rest of the PCA and supplies small branches to the tegmentum, midbrain, posterior thalamus and pineal gland as the cisternal segment. It then penetrates the velum interpositum, becoming the plexal segment. It then runs in the roof of the third ventricle supplying the ipsilateral choroid plexus. As it reaches the foramen of Monro it passes through it to anastamose with branches of the lateral posterior choroidal artery.