Simultaneous integrated functional magnetic resonance imaging and optical tomography in the near infrared spectrum (SINFONIS) for studying hemodynamic and neuronal responses in the human brain

Blood oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) study has become an important method for neuroimaging. However, the underlying mechanism of the BOLD signal has not yet been understood completely. In contrast, near-infrared optical spectroscopy (NIBS) can be used for direct measurements of oxy- and deoxyhemoglobin concentrations. Its response time is on the order of milliseconds, which makes possible studying the "fast" neuronal in addition to the "slow" hemodynamic responses. Previous studies have demonstrated that the integration of the two techniques can decouple the contributions from blood flow and deoxyhemoglobin concentration to the BOLD signal, although with very limited spatial resolution.; In this study, a complete methodology of human brain mapping with simultaneous integrated fMRI and optical tomography in the near-infrared spectrum (SINFONIS) has been established. It features an MRI-compatible optical probe that can be placed over any region of the human brain and fits into a standard MRI birdcage head-coil producing minimal MR image distortion. True three-dimensional optical imaging has been achieved by diffuse optical tomography (DOT) using a perturbation approach of the diffusion equation. A Monte Carlo method has been used to simulate light transport in a subject-specific realistic head model and to subsequently determine the sensitivity function of the discretized Born solution to the diffusion equation. As a result, the linearized inverse problem could be solved using a variety of existing algorithms. The further integration of additional MRI or optical measurement techniques into the current method could be achieved without significant modification to existing hardware and protocols of experiment.; The hemodynamic responses reconstructed from DOT have shown reasonably good consistency with that of the BOLD fMRI result. It has been discovered from the correspondence between the two results that the change in oxyhemoglobin concentration may also play a secondary role in the BOLD signal. However, this hypothesis needs further systematic studies, such as determining the effect of oxyhemoglobin on the gradient of the extravascular magnetic susceptibility and its subsequent effect on T2*.