Overcoming Coil Phase Effects in Highly Accelerated Imaging with a Dedicated Fourth Gradient Channel

Introduction: Magnetic Resonance Microscopy (MRM) has become an important tool for in-vivo microscopy and in-vitro/exvivo histology [1,2]. One challenge is that very small RF coils are used to obtain high signal-to-noise ratio in small voxel sizes. These small coils limit the field-of-view, necessitating the use of arrays or multiple studies. In this abstract, the extension of our previous work in Single-Echo-Acquisition (SEA) imaging to enable wide-field-ofview MRM is discussed. SEA MRI uses arrays of small coils along with multiple receiver channels to obtain entire images in a single echo [3,4]. In Wide FOV Microscopy, multiple phase encode lines are obtained, but the narrow dimension of the coils remains similar to the voxel dimensions, leading to a phase dispersion across the voxel due to the coil sensitivity pattern. This phase dispersion in turn shifts the center of k-space (peak signal) away from the origin, impacting which k-space lines one would obtain in a highly accelerated parallel acquisition. This phase shift is dependent on the relative position of the voxel to the coil, complicating 3D imaging, commonly used in MR Microscopy. Additionally, as the phase can be shown to reverse from one side of the coil to the other, it effectively prevents imaging on both sides of the array or using two arrays in a “sandwich” configuration as might be desirable for histological applications. Here we describe the addition of a fourth gradient channel to a small animal MRI system in order to provide a spatially dependent, non-linear gradient field that helps offset the spatially dependent phase dispersion of the array elements. This gradient amplifier is controlled by the host MRI system, and one or two gradient coils, placed underneath or around the RF coil array, minimize the signal dropout due to the coil phase pattern, improving 3D MR Microscopy and enabling the use of “sandwich” coils.