Improved MRI by correcting static and dynamic magnetic field inhomogeneity

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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.