Abstract #2790

Zhao F, Wang P, Jin T, Hu X, Kim S

The phase change of MRI signal was previously observed in both BOLD fMRI and CBV-weighted fMRI at 4.7T and 9.4T. The observed phase changes are not caused by the intravascular (IV) signal since the IV signal is minimal at high magnetic field; they are not caused by the temperature change during the stimulation either since the phase changes of the BOLD and CBV-weighted fMRI are opposite. The fMRI signal phase changes are caused by the volume-averaged magnetization change due to the Lorentz Sphere effect during the stimulation and its demagnetization. Theory of using the Lorentz Sphere concept to describe the magnetic field inhomogeneity of the heterogeneous cortex was provided. A simulation based on the real fMRI data was conducted. The simulation results are consistent with the experimental results.