Figure 5.

Simulation of the effect of baseline physiological variability on two calibration methods. (a) Using the standard approach to hyperoxia calibration results in a high degree of uncertainty in the resultant measurement of the cerebral metabolic rate of oxygen consumption (CMRO₂) because of the use of an assumed haematocrit (Hct) and resting oxygen extraction fraction (OEF). The blood oxygenation level-dependent (BOLD) signal change normalised by the BOLD scaling parameter (δ S/M) is plotted against the change in cerebral blood flow (CBF) normalised to baseline (CBF₀). (b) If these values are known on an individual subject basis, this uncertainty is markedly reduced. (c) For comparison, the standard approach to hypercapnia does not suffer from the same uncertainty as it does not require assumptions about baseline physiology.