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，其变化不超过百万分之几。不受控制的偏离均匀性被称为“非共振”，并导致有害的图像伪影。非共振效应可以是时不变的，例如由空气-脑组织界面的磁化率不匹配产生的效应，或者是动态的，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像的方法，但尚未实现对非共振场分布的准确表征和对高度局域非均匀磁场（特别是动态磁场）的鲁棒性减少。在这个发现基金中，我建议减轻人类大脑MRI的非共振效应的挑战。我将特别关注颞叶和眶额皮质：这两个区域在神经学、精神疾病和神经科学研究中都很重要，但却普遍受到非共振伪影的影响。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见的静态主场（B0）不均匀性测量外，还将开发专用磁场探针来表征由大块运动和呼吸引起的动态B0不均匀性分布的形状和强度-长时间MRI扫描中动态B0不均匀性的两个主要来源。这些场扰动测量将用于减少图像失真，并通过后处理提高信号稳定性。研究人员将开发两种多线圈（MC）衬垫阵列，以在患者的颞叶和眶额皮质中提供强大的非共振抑制。每个MC垫片阵列将有几组线圈，以解释种群中非共振分布的可变性。磁场探头也将集成到调节垫片电流，以抑制动态非共振效应，从而提供一个空间上更加均匀和时间上稳定的磁场。作为一个示例性验证，MC片阵列和场探针将用于在执行语言和决策任务时减轻快速fMRI（每秒10脑容量，5毫米各向同性分辨率）的非共振效应。我们的方法有望揭示更大的激活脑区，更强的fMRI信号，以及更显著的fMRI时间差异，以区分颞叶和眶额皮质的实验条件。在这项工作完成后，新的MRI技术将被开发出来，为临床应用提供鲁棒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.