Field strength dependence of grey matter R₂^* on venous oxygenation

The relationship between venous blood oxygenation and change in transverse relaxation rate (ΔR₂^*) plays a key role in calibrated BOLD fMRI. This relationship, defined by the parameter β, has previously been determined using theoretical simulations and experimental measures. However, these earlier studies have been confounded by the change in venous cerebral blood volume (CBV) in response to functional tasks. This study used a double-echo gradient echo EPI scheme in conjunction with a graded isocapnic hyperoxic sequence to assess quantitatively the relationship between the fractional venous blood oxygenation (1-Y_v) and transverse relaxation rate of grey matter (ΔR_2⁢GM^*), without inducing a change in vCBV. The results demonstrate that the relationship between ΔR₂^* and fractional venous oxygenation at all magnet field strengths studied was adequately described by a linear relationship. The gradient of this relationship did not increase monotonically with field strength, which may be attributed to the relative contributions of intravascular and extravascular signals which will vary with both field strength and blood oxygenation.