Fig. 5.

Simulations show that potential ΔCBV_a quantification errors from our MT-varied BOLD data are small using simplified Eq. [5] vs. Eq. [3] with Δν_ν = 0%. Eq. [3] accounts for non-negligible venous blood contributions in the absence of free exchange between capillary and tissue water; simulations are for baseline venous blood oxygenation levels (Y) of 0.6 (a) and 0.7 (b), as a function of stimulus-induced venous oxygenation changes (ΔY) and ratio of venous MTR to tissue MTR (κ). See text for parameters. Horizontal lines in (a) and (b) represent the ΔCBV_a value of 0.46 ml/100 g obtained with Eq. [5], which ignores the venous (third) term of Eq. [3]. A reasonable lower limit for κ is 0.4, based on measurements of a stationary blood phantom (9,10), and the fact of that significant exchange between blood and tissue during the MT spin-preparation period should increase κ; regions that are not gray therefore indicate likely in vivo κ values. It should be noted that Eq. [5] is valid under the conditions of free exchange (κ = 1), even with a non-negligible venous blood term in Eq. [3].