Vascular pathology within the thecal sac includes structural abnormalities such as AVMs and arteriovenous fistulas (AVFs), in addition to ischemic and hemorrhagic disease resulting from these and other etiologies. AVMs are congenital lesions (associated with Osler-Weber-Rendu syndrome) that consist of a nidus of pathologic vessels between enlarged feeding arteries and draining veins. The nidus is typically located within the cord, and high flow into the nidus may result in aneurysms within the feeding spinal arteries. This increased flow may similarly result in ischemia of adjacent cord parenchyma through a steal phenomenon or venous hypertension. On MRI AVMs appear as multiple flow voids representing the nidus along with enlarged, extramedullary (often anterior) feeding vessels. T1WI and T2WI best identify lesions within the cord and CSF, respectively. CSF pulsation artifacts on T2WI may occasionally mimic an AVM, but because the enlarged draining veins of an AVM brightly enhance, contrast administration may aid in this distinction and also in the detection of smaller lesions. High cord SI on T2WI adjacent to an AVM on MRI may represent gliosis or edema resulting from the mechanisms above. Intramedullary hemorrhage may also be present, its MRI appearance varying with the stage of blood products. As opposed to the glomus AVM (intramedullary) discussed above, juvenile AVMs (extradural-intradural) are significantly larger, occupying the entirety of the canal at a given level.