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functional magnetic resonance imaging
We have been developing novel functional MRI protocols based on either multiple echo time (TE) or variable TE echo-planar imaging (EPI). For example, in our variableTE fMRI protocol, the chosen TE value varies with functional time point and the TE sampling strategy is integrated with the desired block-design functional paradigm. Mathematical simulation and experiment data show that the blood oxygenation level-dependent (BOLD) sensitivity achieved with this approach is comparable to that achieved using a conventional constant-TE protocol. The developed variable-TE fMRI protocol provides valuable information that cannot be obtained with the constant-TE protocol. First, a field inhomogeneity map can be derived from the multi-TE data and used to correct EPI geometric distortions. Second, changes of T2* values due to the BOLD effect can be quantified. Third, for brain regions with pronounced susceptibility field gradients, the reduced BOLD sensitivity may be compensated for when the acquired multi-TE data are processed appropriately (e.g., with weighted summation). Fourth, large venules and veins may possibly be identified (depending on the vessel orientation and volume fraction) by evaluating the phase values of the multi-TE data. Finally, magnetic field drift over time can be measured from dynamic field maps available with this protocol.