Controlling sensitivity bias in functional MRI studies due to field inhomogeneity

描述(申请人提供)：在磁共振成像(MRI)研究期间，大脑中的磁场在空气/组织界面周围存在局部变化。磁场中的这些局部变化的行为类似于应用成像梯度，并且可能影响采集的质量和功能磁共振成像(FMRI)采集中信号的功能加权。这一建议的主要假设是，大脑中空气/组织界面附近区域的fMRI的敏感性将强烈依赖于：采集轨迹、采集时间、磁场填补、受试者位置和磁场强度。这些方案的选择和解剖变化可能是显著的，无论是单独的还是共同的，导致对整个大脑的功能信号的广泛敏感性。由于研究人员目前忽略了磁场分布对回波时间和功能磁共振灵敏度的影响，这些文献正在创造不完整的结果，有时甚至具有误导性。这项建议将检查磁场诱导的fMRI灵敏度的变化，作为以下因素的函数：1)采集顺序和时间，2)对象定位，以及3)磁场强度。在一项功能磁共振研究中，志愿者将在多个特定方向上测量三维磁场图，以检查受试者头部间距变化的影响。将检查由于位置引起的fMRI敏感性变化，同时还将分析个体之间的解剖变异的影响。此外，fMRI敏感性的变化将通过使用健壮的屏气任务来验证，该任务产生类似于大脑功能激活的信号。根据敏感度梯度估计的fMRI灵敏度变化将被检查，以确定它们是否可以解释屏气数据中的显著差异。将确定在不同受试者之间预测的fMRI敏感度具有高度变异性的大脑区域。此外，还将开发并广泛分发功能磁共振敏感性评估工具，使神经成像研究人员和临床医生能够根据不同大脑区域的获取轨迹和受试者的野外地图来评估其功能数据的质量。该工具将能够评估协议选择，因为它们与磁场梯度有关。此外，该工具将为未来的成像方案提供动力，该方案旨在研究大脑中具有高磁化率的区域。 与公共健康相关：大脑旁边的空间导致的磁场变化可能会对大脑某些区域的功能性磁共振成像的灵敏度产生巨大影响。虽然这一信息在解释功能成像结果时至关重要，但在目前的神经成像研究中，这种影响被忽略了。我们提出了一个评估这些空间周围磁场梯度效果的框架，并确定协议设置如何影响它们对功能成像研究的影响。

DESCRIPTION (provided by applicant): 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. PUBLIC HEALTH RELEVANCE: 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.