Variable-Density Oblique Spherical Navigator Echoes to Improve Performance in Measuring 3 D Motion

A. F. Costa, D. Petrie, M. Drangova Robarts Research Institute, London, Ontario, Canada, Department of Medical Biophysics, The University of Western Ontario, London, Ontario, Canada Introduction: Correcting for subject motion remains a challenging problem in Magnetic Resonance Imaging. One-dimensional navigator echoes have been used successfully in a number of applications, but can only measure translations in one direction. Spherical navigator (SNAV) echoes have the potential to accurately measure and correct for rigid-body motion in 3D, however, several design considerations in the acquisition of helical-spiral SNAVs can greatly affect the overall performance of this technique. For example, the slew rate limitation prevents acquisition of the poles of the spherical shell for typical combinations of sampling density and k-space radius (1). In addition, higher accuracy is attained in SNAV registration when the rotations are predominantly along the threads of the helical-spiral trajectory, rather than “cross-thread” (2). In this study we tested modifications to the SNAV trajectories that overcome the slew rate limitation and acquire points on the entire spherical shell. The modified trajectories were found to improve upon the SNAV technique’s accuracy in detecting 3-dimensional compound rotations by up to one degree rms.