A 3-D spherical navigator (SNAV) echo technique for MRI that can measure rigid body motion in all six degrees of freedom simultaneously by sampling a spherical shell in k-space is under development. Rigid body 3-D rotations of an imaged object simply rotate the k-space data, while translations alter phase in a known way. A computer controlled motion phantom was used to execute known rotations and translations to evaluate the technique. Accurate detection was possible with a double SNAV echo acquisition following a 3-D helical spiral trajectory. Motion detection for retrospective or prospective correction in MRI with spherical navigator echoes is thus feasible and practical.
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