Mapping of the cerebral response to hypoxia measured using graded asymmetric spin echo EPI

Graded asymmetric spin echo-echo planar imaging (ASE-EPI) was used to measu re transient alterations in cerebral oxygenation resulting from 60 seconds of anoxia in alpha -chloralose anaesthetised rats. The anoxic period induce d a transient fall (similar to1 min) in signal intensity followed by a prol onged signal overshoot consistent with an autoregulatory response to oxygen deprivation. The magnitude of signal response, integrated over the entire brain, increased linearly with the echo asymmetry (t(ge)). However, that in crease in sensitivity was offset by a reduced signal to noise ratio and qua lity of the image data. The responses of four regions of interest within th e brain to the anoxic stimulus, and the effect of increasing the echo asymm etry, were compared. A comparable magnitude of signal decrease was observed in all brain regions except the superficial cortex that included pial vess els. As t(ge) was incremented differences in signal attenuation between reg ions became more pronounced. The signal overshoot observed upon restoration of normal breathing gases showed similar trends, producing similar normali sed vascular responses for all regions of interest studied. Different regio ns of interest showed comparable time courses of the signal overshoot sugge sting that similar autoregulatory vascular mechanisms operate in all brain regions. These findings additionally show that the use of graded ASE-EPI pr oduced a characteristic profile of maximum signal change measured during an d following the anoxic period for each brain region. They suggest that the shape of this profile was determined by the local vasculature within each r egion of interest; this feature could be exploited in activation studies to eliminate regions with significant signal changes originating from large d raining vessels. Finally, the consistent physiological response observed, w hen the overshoot was compared to the magnitude of the signal drop, demonst rated that modification of the spin echo offset parameter did not mask or d etrimentally alter the signal change resulting from the underlying physiolo gical perturbation. (C) 2000 Elsevier Science Inc. All rights reserved.