The intrinsic magnetic field symmetries of the spiral birdcage coil

INTRODUCTION Transmit array spatial encoding (TRASE) is a novel MRI technique that utilizes B1-field phase gradients as the means for traversing k-space [1-3]. This method allows MRI to be performed without the usual application of B0-field amplitude gradients, thereby possibly eliminating the equipment, complexity, power consumption and acoustic noise associated with traditional static-field gradient coils. As a result, TRASE might provide a comfortable (i.e. silent) imaging experience in a lower-cost, simplified scanner. The primary requirement of TRASE is an RF phase-gradient coil, which ideally generates a transverse RF field of constant magnitude and linearly varying phase over the sample volume. Previous work [1-3] did not focus on the design and optimization of RF phase-gradient coils. Rather, basic coil structures with roughly the correct field and phase symmetries were employed. Most notably, a spiral birdcage coil (Fig. 1) was used to generate a transverse RF field with phase varying along B0. Here we explore the intrinsic properties of such a coil in order to guide and motivate the future design of phase-gradient coils.