Improved MRI by correcting static and dynamic magnetic field inhomogeneity

高质量的磁共振成像(MRI)需要高度均匀的磁场分布，变化不超过百万分之几。不受控制的对均匀性的偏离被称为非共振，并且会导致有害的图像伪影。非共振效应可以是时间不变的，例如由空气-脑组织界面的磁化率不匹配所产生的效应，也可以是动态的，例如由大量患者运动和呼吸引起的磁场。虽然已经有了减少静态非共振伪影的方法，但还没有实现对非共振场分布的准确表征和高度局部化的非均匀磁场的稳健减少，特别是动态磁场。 在这项发现拨款中，我建议减轻人脑核磁共振成像中非共振效应的挑战。我将特别关注颞叶和眼眶额叶皮质：这两个区域在神经学、精神障碍和神经科学研究中都很重要，但却广泛地受到非共振伪影的影响。提出了三个目标： 目标1：测量和减少人脑中的静态和动态非共振效应 除了常用的静态主场(B0)不均匀性测量外，还将开发专用磁场探头来表征由整体运动和呼吸引起的动态B0不均匀性分布的形状和强度--这是长时间MRI扫描中动态B0不均匀性的两个主要来源。这些场扰动量测将用于减少图像失真，并通过后处理提高信号稳定性。 目标2：使用局部垫片线圈阵列缓解静态和动态脱谐效应 将开发两个多线圈(MC)垫片阵列，以在患者的颞叶和眶前皮质提供强大的非共振抑制。每个MC垫片阵列将具有几组线圈，以说明种群中非共振分布的可变性。现场探头也将被集成，以调整填补电流，以抑制动态非共振效应，以提供空间上更均匀和时间上更稳定的磁场。 目的3：语言处理和决策的功能磁共振成像(FMRI)技术的验证 作为一个示例性验证，当执行语言和决策任务时，MC垫片阵列和场探头将用于减轻快速fMRI(每秒10个脑体积，各向同性分辨率为5 mm)中的非共振效应。我们的方法预计将显示更大的激活脑区，更强的fMRI信号，以及更显著的fMRI计时差异，以区分颞叶和眶前皮质的实验条件。 意义 这项工作完成后，新的磁共振成像技术将被开发出来，为临床应用提供稳健和有效的非共振伪影抑制。

High-quality magnetic resonance imaging (MRI) requires a highly homogeneous magnetic field distribution that varies by no more than a few parts-per-million. The un-controlled deviation from the homogeneity is termed “off-resonance” and causes detrimental image artifacts. Off-resonance effects can be time-invariant, such as those produced by the magnetic susceptibility mismatch at air-brain tissue interfaces, or dynamic, such as the magnetic fields caused by bulk patient motion and breathing. While there have been methods of reducing static off-resonance artifacts, accurate characterization of the off-resonance field distribution and robust reduction of highly localized inhomogeneous magnetic fields, particularly dynamic ones, have yet to be achieved. In this Discovery grant, I propose to mitigate the challenge of off-resonance effects in MRI of the human brain. I will focus specifically on the temporal lobes and the orbitofrontal cortex: two regions that are important in neurological as well as psychiatric disorders and neuroscience studies, yet suffer extensively from off-resonance artifacts. Three aims are proposed: Aim 1: Measure and reduce the static and dynamic off-resonance effects in the human brain In addition to the common static main field (B0) inhomogeneity measurement, dedicate magnetic field probes will be developed to characterize the shapes and strengths of dynamic B0 inhomogeneity distributions caused by bulk motion and breathing - two major sources of dynamic B0 inhomogeneity in lengthy MRI scans. These field disturbance measurements will be used to reduce image distortion and improve signal stability by post-processing. Aim 2: Mitigate static and dynamic off-resonance effects using localized shim coil arrays Two multi-coil (MC) shim arrays will be developed to offer robust off-resonance suppression in the temporal lobes and orbitofrontal cortex across patients. Each MC shim array will have a few sets of coils to account for the variability of the off-resonance distributions in the population. Field probes will also be integrated to adjust shim currents to suppress dynamic off-resonance effects to offer a much more spatially homogeneous and temporally stable magnetic field. Aim 3: Validation of the technology by functional MRI (fMRI) of language processing and decision making As an exemplary validation, the MC shim arrays and field probes will be used to mitigate off-resonance effects in fast fMRI (10 brain volumes per second with 5 mm isotropic resolution) when performing language and decision-making tasks. Our approach is expected to reveal larger activated brain areas, stronger fMRI signals, and more significant differences in fMRI timing to distinguish among experimental conditions at temporal lobes and orbitofrontal cortices. Significance On completion of this work, novel MRI technology will have been developed to provide robust and effective suppression of off-resonance artifacts for clinical applications.