Abstract
We present a method allowing determination of resting cerebral oxygen metabolism (CMRO2) from MRI and end-tidal O2 measurements acquired during a pair of respiratory manipulations producing different combinations of hypercapnia and hyperoxia. The approach is based on a recently introduced generalization of calibrated MRI signal models that is valid for arbitrary combinations of blood flow and oxygenation change. Application of this model to MRI and respiratory data during a predominantly hyperoxic gas manipulation yields a specific functional relationship between the resting BOLD signal M and the resting oxygen extraction fraction OEF0. Repeating the procedure using a second, primarily hypercapnic, manipulation provides a different functional form of M vs. OEF0. These two equations can be readily solved for the two unknowns M and OEF0. The procedure also yields the resting arterial O2 content, which when multiplied by resting cerebral blood flow provides the total oxygen delivery in absolute physical units. The resultant map of oxygen delivery can be multiplied by the map of OEF0 to obtain a map of the resting cerebral metabolic rate of oxygen consumption (CMRO2) in absolute physical units.
Application of this procedure in a group of seven human subjects provided average values of 0.35 ± 0.04 and 6.0 ± 0.7% for OEF0 and M, respectively in gray-matter (M valid for 30 ms echo-time at 3 T). Multiplying OEF0 estimates by the individual values of resting gray-matter CBF (mean 52 ± 5 ml/100 g/min) and the measured arterial O2 content gave a group average resting CMRO2 value of 145 ± 30 μmol/100 g/min. The method also allowed the generation of maps depicting resting OEF, BOLD signal, and CMRO2.
