FIG 1. Noncontrast head CT. Two contiguous slices at the level of the basal ganglia 6 days prior to left M2 segment balloon angioplasty demonstrate left frontal and subinsular small vessel disease.FIG 2. Left ICA angiogram (anterior projection) at the time of initial evaluation reveals a severe, approximately 95%, focal stenosis of the proximal M2 segment (arrow) of the left MCA.FIG 3. Xe-CT with axial CT and blood flow map images on two contiguous slices before (A) and after (B) acetazolamide challenge 1 day prior to balloon angioplasty. Loss of cerebrovascular autoregulation is most notable in the left frontal lobe, as demonstrated by paradoxically low cerebral blood flow after acetazolamide administration. Note arrows between areas on the postacetazolamide scans. The color coding scale is defined in units of cc/100 g tissue/min