MR-guided high-intensity focused ultrasound (MR-HIFU) is a noninvasive technique for depositing thermal energy in a controlled manner deep within the body. However, the MR-HIFU treatment of mobile abdominal organs is problematic since motion-related thermometry artifacts need to be corrected and the focal point position must be updated to follow the moving organ in order to avoid damaging healthy tissue. In this paper, a fat-selective pencil-beam navigator is proposed for real-time monitoring and compensation of through-plane motion. As opposed to the conventional spectrally non-selective navigator, the fat-selective navigator does not perturb the water-proton magnetization used for proton resonance frequency shift thermometry. This allows the proposed navigator to be placed directly on the target organ for improved motion estimation accuracy. The spectral and spatial selectivity of the proposed navigator pulse is evaluated through simulations and experiments, and the improved slice tracking performance is demonstrated in vivo by tracking experiments on a human kidney and on a human liver. The direct motion estimation provided by the fat-selective navigator is also shown to enable accurate motion compensated MR-HIFU therapy of in vivo porcine kidney, including motion compensation of thermometry and beam steering based on the observed three-dimensional kidney motion.Read less <