Abstract
Non-invasive quantitative imaging of cerebral oxygen metabolism (CMRO(2)) in small animal models is crucial to understand the role of oxidative metabolism in healthy and diseased brains. In this study, we developed a multimodal method combining near-infrared spectroscopy (NIRS) and MRI to non-invasively study oxygen delivery and consumption in the cortex of mouse and rat models. The term CASNIRS is proposed to the technique that measures CMRO(2) with ASL and NIRS. To determine the reliability of this method, CMRO(2) values were compared with reported values measured with other techniques. Also, the sensitivity of the CASNIRS technique to detect changes in CMRO(2) in the cortex of the animals was assessed by applying a reduction in core temperature, which is known to reduce CMRO(2). Cerebral blood flow (CBF) and CMRO(2) were measured in five mice and five rats at a core temperature of 37 °C followed by another measurement at 33 °C. CMRO(2) was 7.8 ± 1.8 and 3.7 ± 0.9 (ml/100 g/min, mean ± SD) in mice and rats respectively. These values are in good agreement with reported values measured by (15)O PET, (17)O NMR, and BOLD fMRI. In hypothermia, we detected a significant decrease of 37% and 32% in CMRO(2) in the cortex of mice and rats, respectively. Q10 was calculated to be 3.2 in mice and 2.7 in rats. In this study we showed that it is possible to assess absolute values of metabolic correlates such as CMRO(2), CBF and oxygen extraction fraction (OEF) noninvasively in living brain of mice and rats by combining NIRS with MRI. This will open new possibilities for studying brain metabolism in patients as well as the many mouse/rat models of brain disorders.