Potential use of Oxygen as

We describe a novel MRI technique, detecting metabolism indirectly through changes in oxyhemoglobin: deoxyhemoglobin ratios and T2* signal change during “oxygen challenge” (OC, 5 mins 100% O2). During OC, T2* increase reflects O2 binding to deoxyhemoglobin, formed when metabolizing tissues take up oxygen. Here OC has been applied to identify tissue metabolism within ischemic brain. Permanent MCA occlusion was induced in rats. Series 1 scanning (n=5): Diffusion-weighted imaging (DWI), followed by echo-planar T2* acquired during OC, and perfusion-weighted imaging (PWI, arterial spin labeling, ASL). OC induced a T2* signal increased of 1.8%, 3.7% and 0.24% in contralateral cortex, ipsilateral cortex within the PWI/DWI mismatch zone, and ischemic core, respectively. T2* and ADC map co-registration revealed the T2* signal increase extended into the ADC lesion (3.4%). Series 2 (n=5): FLASH T2* and ADC maps co-registered with histology revealed a 4.9% T2* signal increase in histologically-defined border zone (55% normal neuronal morphology, located within ADC lesion boundary) compared with a 0.7% increase in cortical ischemic core (92% neuronal ischemic cell change, core ADC lesion). OC has potential clinical utility and, by distinguishing metabolically active and inactive tissue within hypoperfused regions, could provide a more precise assessment of penumbra.