An Insertable Nonlinear Gradient Coil for Phase Compensation in SEA Imaging

Abstract

In magnetic resonance imaging with array coils with many elements, as the radiofrequency (RF) coil dimensions approach the voxel dimensions, the phase gradient due to the magnetic field pattern of the coil causes signal cancellation within each voxel. In single echo acquisition (SEA) imaging with coil arrays, a gradient pulse can be applied to compensate for this effect. However, because RF coil phase varies with distance from the array and reverses on opposite sides of a dual-sided array, this method of phase compensation can be optimized for only a single slice at a time. In this study, a nonlinear gradient coil was implemented to provide spatially varying phase compensation to offset the coil phase with slice position for dual-sided arrays of narrow coils. This nonlinear gradient coil allows the use of one phase compensation pulse for imaging multiple slices through a slab, and, importantly, is shown to enable simultaneous SEA imaging from opposite sides of a sample using a dual-sided receive array.

Exploration of highly accelerated magnetic resonance elastography using high-density array coils

“…However, the B 1 phase gradient varies with distance from the array and reverses sign depending on whether the array is placed above or below the sample, preventing SEA and highly accelerated imaging using both arrays simultaneously. To overcome this, an insertable biplanar nonlinear (x-, y-) gradient coil, shown in Figure 4, was employed to provide opposite phase compensation for both arrays (18). This coil was designed using a target field approach to produce a linear x-gradient, which also varies linearly in y.…”

“…Applications such as turbulent flow (17) and microscopy of excised brain slices have been investigated. The system was further extended to facilitate imaging of thicker volumes using a biplanar receive array and an insertable x-, y-gradient coil to compensate for the phase gradient due to the RF coil elements (18). Combined with strong and fast gradients, imaging at 1,000 frames per second has been shown (19).…”