Rapid six-degree-of-freedom motion detection using prerotated baseline spherical navigator echoes

A new spherical navigator echo (SNAV) registration technique is presented. This technique starts by collecting a set of SNAV templates at a reference position. These templates are acquired by rotating the gradient system to result in rotation angles that uniformly cover a predefined range of rotation. The rotation angles between an unknown physically transformed position and the reference position are subsequently determined by finding the template with the lowest sum of squared differences with SNAV at the transformed position. Translations are calculated from the phase differences between the best-match SNAV template and the SNAV acquired at the transformed position. In comparison with the conventional SNAV registration technique, the proposed technique is noniterative, robust, and can detect 3-dimensional rigid body motion in less than 50 msec. The technique was verified with phantom and in vivo experiments, which demonstrated subdegree rotational and submillimeter translational accuracy over a range of simultaneous ±20° and ±10° mm of motion.