Quantitative Measurements of Cerebral Metabolic Rate of Oxygen (CMRO2) Using MRI
Recently, we have demonstrated that a quantitative estimate of cerebral blood oxygen saturation can be obtained in vivo via blood oxygen level dependent contrast. With a ROI analysis, a mean cerebral blood oxygen saturation (CBOS) of 58.4% ± 1.8% was obtained from 8 normal healthy volunteers. When converting the MR measured CBOS to the oxygen extraction fraction (OEF) via the Hill equation as well as the oxygen dissociation curve, an MR-OEF of 42.6% was obtained, in excellent agreement with the known OEF under normal physiological conditions via PET. While it is known that the level of OEF is a strong indicator of the functional status of brain tissue, OEF by itself does not uniquely indicate brain ischemia or brain viability. Therefore, CMRO2, which includes both CBF and OEF, has been utilized to better characterize the balance between oxygen supply and demand in brain tissue. In order to obtain a quantitative estimate of MR-CMRO2, a measure of both OEF and CBF is required. In total, 5 normal healthy volunteers were studied. A 2D multiecho gradient/spin echo sequence was employed to acquire images, which were subsequently post-processed to obtain an estimate of MR-OEF. In addition, a dynamic imaging approach was utilized to acquire images before, during and after the injection of a paramagnetic contrast agent and post-processed via the singular value decomposition so that a quantitative estimate of CBF can be obtained. Finally, pixe-by-pixel MR-CMRO2 maps were calculated as the product of MR-OEF and MR-CBF. Four ROIs: one located in the cortical and one in the subcortical regions of each hemisphere, were defined to obtain measurements of MR-CMRO2 in all volunteers. A mean MR-CMRO2 of 27.7±6.2 ml/100gm/min and 10.5±3.2 ml/100gm/min was obtained for the cortical and subcortical regions, respectively. Notice the MR-CMRO2 differs from the conventionally measured CMRO2 via PET. No normalization with respect to the arterial oxygen content of each volunteer was made for the MR-CMRO2. With the non-invasive nature of MRI and the ability to provide quantitative estimates of cerebral oxygen metabolism in vivo, MR-CMRO2 should open a new avenue for the investigation of cerebrovascular disease.