Direct measurements of cerebral blood flow (CBF) by 133xenon or stable xenon computerized tomography (CT) or other methods appear to be the most reliable measure of the adequacy of flow. Other techniques including transcranial Doppler (TCD), cerebral angiography, and various types of magnetic resonance imaging (MRI) are proxy measures for this value. Positron emission tomography (PET) measures CBF and several variables that are interrelated in a consistent and predictable manner to decreasing perfusion pressure. J. P. Witt analyzed quantitative CBF data obtained with Xenon CT CBF technology prior to and during BTO, and classified 11 separate response patterns based on symmetry and degree of CBF change after balloon test occlusion (BTO). Metabolic response of the brain in symptomatic carotid occlusion has been characterized by MR spectroscopy. The hemodynamic physiology of carotid occlusion has been extensively studied in many disparate clinical situations.