Towards noninvasive MRI measurement of cerebral metabolic rate of oxygen consumption: Techniques for measuring and validating deoxyhemoglobin concentration

Functional magnetic resonance imaging has become an important tool in investigating brain function. However, it suffers due to the qualitative nature of the changes that are measured. In order to obtain absolute quantitative values for metabolic changes using only endogenous contrast, a quantitative measurement of deoxyhemoglobin is required. Magnetic resonance imaging (MRI) and near-infrared spectroscopy (NIRS) techniques for measuring deoxyhemoglobin concentration were investigated.;Estimates of deoxyhemoglobin concentration obtained with MRI are based on field inhomogeneities created by deoxygenated blood. Background macroscopic magnetic fields create considerable problems in isolating these effects. In order to minimize background magnetic fields, a pulse sequence that enables rapid and accurate measurement of the in vivo magnetic field was implemented. Used in conjunction with maps of the fields created by shim coils, this allows the magnetic field homogeneity in any arbitrary region to be optimized. These optimizations are required in order to reduce signal loss and make quantification of deoxyhemoglobin concentration feasible.;With optimized magnetic field homogeneity, the effect of blood vessel geometry on MRI signal can be investigated. Due to the statistical nature of analytical models relating deoxyhemoglobin concentration to MRI signal evolution, it is necessary to validate the regime in which the assumptions of the model apply. A three dimensional simulation was developed to allow various different vascular geometries to be investigated. It was found that the minimum resolution was limited by the statistical averaging of the vascular structure rather than MRI hardware. Voxel dimensions must be large enough to ensure a pseudo random vessel distribution. It was also shown that diffusion weights results to larger venous vessels, as the reversible effects of smaller vessels were obscured and arterial vessels contain minimal deoxyhemoglobin.;Preliminary measurements of deoxyhemoglobin concentration during changes to inspired fraction of oxygen using MRS and MRI techniques showed good agreement. Regions traditionally difficult to shim present the greatest challenge and will require regional shimming. Combining the MRI technique with measures of blood flow, such as dual coil arterial spin labeling, will allow the calculation of oxygen metabolism under the assumption of complete utilization.;Transcranial continuous wave NIRS requires accurate knowledge of the optical pathlength. Although second derivative methods of quantifying the mean optical pathlength are available, the wavelength variations are typically assumed from literature values even though they can change with tissue type and geometry. In situ measurements of wavelength dependence of optical pathlength were made from measurements of cardiac correlated changes in attenuation in the adult human head. A Fourier analysis technique eliminated the positive error bias inherent with using the magnitude of the Fourier coefficients. This in situ correction allows accurate calculation of deoxyhemoglobin concentration for comparison with MRI based techniques.;Keywords: magnetic resonance imaging, nuclear magnetic resonance, susceptibility, near-infrared spectroscopy, oxygenation, hemoglobin, automatic shimming, B0 inhomogeneity, phase mapping, simulation, cerebral metabolic rate of oxygen consumption...