Robust high-resolution reduced field-of-view MRI with sheared 2D RF excitation

Abstract

Reduced field-of-view (FOV) single-shot echo-planar imaging (ssEPI) is a widely applied imaging technique for diffusion-weighted magnetic resonance imaging (MRI), due to its robustness against in-plane off-resonance artifacts. Two-dimensional echo-planar RF (2D RF) excitation is a popular approach for reduced-FOV imaging due to its fat suppression capability and sharp slab profiles. However, long pulse durations render 2D RF pulses sensitive to through-plane off-resonance effects, causing local signal losses in reduced-FOV images. The standard 2D RF pulses also generate excitation replicas along the slice stack, limiting the slice coverage during multislice imaging. This thesis proposes a sheared 2D RF design for reduced-FOV imaging for significant reduction in pulse duration, leading to significant improvement in through-plane off-resonance ro-bustness. The proposed design also provides unlimited slice coverage and high fidelity fat suppression. Sheared k-space trajectories are designed such that the excitation replicas are positioned outside the slice stack to guarantee unlimited slice coverage, while ensuring identical k-space coverage as that of a standard 2D RF pulse. The efficacy of the sheared design is demonstrated by extensive simulations in terms of pulse duration, fat suppression capability, and signal com-parisons under off-resonance effects for a range of design parameters and hardware limits. The sheared and standard 2D RF pulses are then compared via imaging experiments on a custom head and neck phantom, and in vivo imaging experi-ments in the spinal cord at 3 T. The results show that in regions with high off-resonance effects, the sheared 2D RF pulse improves the signal by more than 50%when compared to the standard 2D RF pulse while preserving profile sharpness. Lastly, the benefits of the sheared design are demonstrated for low-cost low-field MRI systems via simulations and phantom experiments, making reduced-FOV imaging applicable on these systems. The proposed sheared 2D RF design will be especially beneficial in regions suffering from a variety of off-resonance effects, such as spinal cord and breast.