Artifacts caused by patient motion during scanning remain a serious problem in most MRI applications. The prospective motion correction technique attempts to address this problem at its source by keeping the measurement coordinate system fixed with respect to the patient throughout the entire scan process. In this study, a new image-based approach for prospective motion correction is described, which utilizes three orthogonal two-dimensional spiral navigator acquisitions, along with a flexible image-based tracking method based on the extended Kalman filter algorithm for online motion measurement. The spiral navigator/extended Kalman filter framework offers the advantages of image-domain tracking within patient-specific regions-of-interest and reduced sensitivity to off-resonance-induced corruption of rigid-body motion estimates. The performance of the method was tested using offline computer simulations and online in vivo head motion experiments. In vivo validation results covering a broad range of staged head motions indicate a steady-state error of less than 10% of the motion magnitude, even for large compound motions that included rotations over 15 deg. A preliminary in vivo application in three-dimensional inversion recovery spoiled gradient echo (IR-SPGR) and three-dimensional fast spin echo (FSE) sequences demonstrates the effectiveness of the spiral navigator/extended Kalman filter framework for correcting three-dimensional rigid-body head motion artifacts prospectively in high-resolution three-dimensional MRI scans. Artifacts caused by patient motion during scanning remain a serious problem in most clinical and research MRI applications. In fast single-shot sequences, such as dynamic two-dimensional (2D) echo-planar imaging (EPI), between-scan motion can introduce significant motionrelated variance to the voxel-time courses and disrupt the spin excitation history of the acquisition (1,2). In multishot 2D and three-dimensional (3D) sequences, withinscan patient motion results in k-space data inconsistencies, causing artifacts such as ghosting, blurring, and ringing in the images themselves. Offline image registration can mitigate most between-scan motion artifacts in time-series data (3-5) but cannot correct for changes in the spin excitation history caused by through-plane motion. In addition, while some within-scan motion artifacts can be corrected retrospectively using knowledge of the motion history derived from either navigator scans (6,7) or overlapping k-space data (8,9), most of these methods are limited by the inability to (1) fully correct for through-plane motion in 2D sequences and (2) avoid k-space data inconsistencies caused by interpolation errors.An alternative approach to motion correction, which shares none of these drawbacks, is modify the pulsesequence online, in real-time, during the acquisition itself. Some of the first real-time prospective motion correction methods used straight-line navigators to correct for linear translations of organs in the chest (10-12). Since then, navigator...