REGIONAL DYNAMIC SIGNAL CHANGES DURING CONTROLLED HYPERVENTILATION ASSESSED WITH BLOOD-OXYGEN LEVEL-DEPENDENT FUNCTIONAL MR-IMAGING

PURPOSE: To quantitate the amplitude changes and temporal dynamics of regional functional MR imaging signals during voluntary hyperventilati on using blood oxygen level-dependent contrast echo-planar imaging. ME THODS: Seven male subjects were studied during voluntary hyperventilat ion (Petco(2) = 20 mm Hg) regulated by capnometry. Measurements were m ade on multisection echo-planar MR images obtained with parameters of 1000/66 (repetition time/echo time), flip angle of 30 degrees, and vox el size of 3 x 3 x 5 mm(3). Sensitivity of the functional MR imaging s ignal to changes in Petco(2), time delays in relation to Petco(2) chan ges, and time constants of functional MR imaging signal changes were a ssessed on a region-by-region basis. RESULTS: Within 20 seconds of sta rting hyperventilation, rapid and substantial decreases in the functio nal MR imaging signal (by as much as 10%) were measured in areas of gr ay matter, which were significantly greater than the modest changes ob served in white matter. Regional-specific effects in areas of the fron tal, occipital, and parietooccipital cortex were stronger than in subc ortical regions or in the cerebellum. Signal decreases measured with f unctional MR imaging were significantly delayed with respect to the re duction in Petco(2). Apparent differences between regional time consta nts did not reach statistical significance. CONCLUSION: Regional and g ray-white matter differences in functional MR imaging signal changes d uring controlled hyperventilation may reflect differences in metabolic activity, vascular regulation, and/or capillary density. When measuri ng brain activation with functional MR imaging, arterial Pco(2) differ ences due to unregulated respiration may confound interpretation of ac tivation-related functional MR imaging signal changes.