Motion compensation is one of the more eminet problems in magnetic resonance imaging (MRI). Motion artifacts in MRI vary with the temporal and spatial extent of the motion in relation to the specifications of the acquisition sequence. The problem of motion compensation in MRI technology deals with 1) identification of the source of motion, 2)optimizing the image acquisition sequence with respect to the requirements of the imaging modality in order to minimize the effect of motion, 3)obtaining a mathematical model of motion that can be used to extract and then invert the motion effects. In light of increased interest in functional MRI (fMRI), to obtain a quantative measure of the extent of the movement of the subject between different frames in an fMRI examination, we have proposed a novel 3D orbital navigator technique that addresses the speed requirements of a BOLD (Blood oxygen level dependant) echo planar imaging (EPI) acqiosition. The term navigator in MRI refers to a repeated and invariant acquisition of a small portion of the field of view that serves to mark any variations between different acquisitions with respect to a reference.

The outline of the presentation includes:

• Types of motion
• mathematical description of rotational and translational motion in the Fourier domain
• brief review of MR-based motion compensation techniques
• navigator echoe technique
• design and implementation of 3D orbital navigators
• digital simulation, phantom and in-vivo experimental results
• future of navigator-based motion compensation in fMRI