Effects of oxygen saturation on BOLD and arterial spin labelling perfusion fMRI signals studied in a motor activation task

Effects of oxygen availability on blood oxygenation level dependent (BOLD) and arterial spin labelling (ASL) perfusion functional magnetic resonance imaging (fMRI) signal changes upon motor activation were studied. Mild hypoxic hypoxia was induced by reducing the inspired oxygen content (FIO2) to 12%, decreasing blood oxygen saturation (Y) from 0.99 ± 0.01 to 0.85 ± 0.03. The fMRI signal characteristics were determined during finger tapping. BOLD activation volume decreased as a function of declining Y in the brain structures involved in execution of the motor task, however, the BOLD signal increase in activated parenchyma was not influenced by Y. ASL fMRI showed that the baseline CBF of 61.8 ± 3.6 ml/100 g/min was not affected by hypoxic hypoxia. Similar to the BOLD fMRI, the volume of motor cortex areas displaying increase in perfusion by ASL fMRI decreased, but the signal change due to perfusion increase was not influenced in hypoxia. The present fMRI results show distinct patterns of haemodynamic and metabolic responses in the brain to motor task between normoxia and hypoxia. On one hand, neither BOLD nor ASL fMRI signal changes are influenced by hypoxia during motor activation. On the other hand, hypoxia attenuates increase in both BOLD and perfusion fMRI signals upon finger tapping from the levels determined in normoxia. These observations indicate that haemodynamic and metabolic responses may be heterogeneous in brain during execution of motor functions in mild hypoxia.