Brain Oximetry and the Quest for Quantified Metabolic Rate: Applications Using MRI and Near-Infrared Spectroscopy

Cerebral metabolic rate of oxygen (CMRO 2 ) is a robust marker of brain health. It represents the amount of oxygen consumed by the brain, and it has been proved to be more sensitive indicator than oxygenation level and cerebral blood flow alone. Quantitative assessment of CMRO 2 provides a useful insight into the viability of the brain tissue, the progression of a brain disease or action of a treatment. Therefore, there is a growing interest in developing methods that can quantify CMRO 2 , despite its complexity. Over the past years, many magnetic resonance imaging (MRI)-based methods and near-infrared spectroscopy (NIRS)-based methods have been developed for CMRO 2 quantification. Here, we review the available approaches based on these two techniques, their advantages, and disadvantages. Examples of application of these approaches in animal models, neonates and adults under normal and different physiological conditions are provided. Physiological correlates such as cerebral blood flow, venous oxygen saturation and oxygen extraction fraction in addition to CMRO 2 values found in the literature, are presented as well. We also show how the benefits of these two techniques can be combined to create a multimodal NIRS-MRI technique that can provide novel data, allowing better understanding of CMRO 2 and oxidative metabolism in the brain.