During a magnetic resonance imaging (MRI) study, local variations of the magnetic field in the brain exist around air/tissue interfaces. These local variations in the magnetic field behave similarly to apply imaging gradients and can affect both the quality of the acquisition and the functional weighting of the signal in a functional MRI (fMRI) acquisition. The main hypothesis of this proposal is that the sensitivity of fMRI in regions near air/tissue interfaces in the brain will have a strong dependence on: acquisition trajectory, acquisition timing, magnetic field shim, subject positioning, and magnetic field strength. These protocol choices and anatomical changes can be significant both individually and together, resulting in a wide range of sensitivity to functional signals throughout the brain. Because investigators are currently ignoring the effect of magnetic field distribution on echo time and fMRI sensitivity, the literature is creating results that are incomplete and, at times, misleading. This proposal will examine changes in magnetic field induced fMRI sensitivity as a function of: 1) acquisition sequence and timing, 2) subject positioning, and 3) magnetic field strength. Three-dimensional magnetic field maps will be measured on volunteer participants in multiple, specified orientations to examine the impact of variations in the pitch of a subjects head in an fMRI study. fMRI sensitivity changes due to position will be examined along with an analysis of the impact of anatomical variability among individuals. Additionally, changes in fMRI sensitivity will be validated by using a robust breath hold task that produces a signal similar to functional brain activations. The estimated fMRI sensitivity changes from susceptibility gradients will be examined to see if they explain significant variance in the breath hold data. Regions of the brain will be identified that have high variability in predicted fMRI sensitivity between subjects. Additionally, an fMRI sensitivity assessment tool will be developed and widely distributed that will allow neuroimaging researchers and clinicians to assess the quality of the functional data in various brain regions given their acquisition trajectory and a field map of a subject. This tool will enable the evaluation of protocol choices as they relate to magnetic field gradients. Further, the tool will provide motivation for future imaging protocols designed to study regions of the brain with high magnetic susceptibility.
Magnetic field variations due to air spaces next to the brain can have a dramatic effect on the sensitivity of functional magnetic resonance imaging in certain regions of the brain. Although this information is critical in interpreting functional imaging results, the effect is ignored in current neuroimaging studies. We propose a framework for evaluating the effect of magnetic field gradients around these airspaces and determine how protocol settings can influence their impact on functional imaging studies.
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