A reference-based PET/MRI method for quantifying activation-induced changes in cerebral oxygen metabolism

Hybrid PET/MRI can overcome the complexity of PET imaging of the cerebral metabolic rate of oxygen (CMRO₂), while retaining the ability to directly measure oxygen uptake in the brain. One technique, PMROx, incorporates complementary MRI methods acquired simultaneously with [¹⁵O]O₂-PET. Specifically, the MRI-based method arterial spin labelling (ASL) to image cerebral blood flow (CBF) and MR-susceptometry to measure whole-brain CMRO₂. PMROx is non-invasive with imaging times around 5 min, making it feasible to image CMRO₂ under different conditions in one session. This study presents the first application of PMROx to humans with the aims of evaluating its reliability and sensitivity to increased CMRO₂ during functional activation (right-handed sequential finger tapping). In addition, blood-oxygen level dependent (BOLD) images were acquired to compare BOLD contrast to underlying changes in CBF and CMRO₂. Across 14 participants, mean CMRO₂ was 3.2 ± 0.5 mLO₂/100 g/min with excellent within-session repeatability. Significant increases in CBF, CMRO₂ and BOLD contrast were detected in the primary sensorimotor cortex, supplementary motor area and secondary somatosensory cortex. This study demonstrated the ability of PMROx to image CMRO₂ non-invasively, its sensitivity to increased regional CMRO₂ and how PET/MRI provides the opportunity to compare BOLD contrast to underlying changes in perfusion and oxygen metabolism.