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。

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.