0918. Grey matter perfusion is preserved during normotensive hypovolaemia, despite reductions in total cerebral blood flow: evidence of local, pressure independent, intra-cerebral vascular autoregulatory response

Methods Eleven healthy volunteers (20-31 years; 8-male 3-female) underwent MRI studies with a lower body negative pressure (LBNP) chamber. Imaging included quantitative phase contrast angiographic (PCA) measurements of blood volume flow in the carotid and vertebral arteries and ASL of the whole brain. An ASL sequence followed by PCA acquisition was performed at rest (control) and under -20mmHg LBNP. ASL imaging used STAR labeling collected at 4 inversion times: 800ms, 1200ms, 1600ms and 2000ms. PCA acquisition was collected using 2D cine phase-contrast images. Throughout scanning the subjects pulse and blood pressure were monitored. ASL images were analysed using in-house code assuming a single blood compartment model. Control and labelled images were subtracted and a two-parameter fit for bolus arrival time (BAT) and perfusion was performed on a voxel by voxel basis, producing perfusion and BAT maps. Perfusion was calculated with units ml/100ml/min. Automated tissue segmentation masks were created from aligned T1 images applied to co-registered perfusion and BAT maps for both cortical and subcortical grey matter structures.