1. A method for compensating for respiration-induced motion of a structure imaged by a magnetic resonance scanner, comprising the steps of: in a first time interval: obtaining from the magnetic resonance scanner a sequence of navigator samples representative of a progressive displacement of the structure; and adjusting a waveform model of predefined initial parameters to follow the sequence of navigator samples; in a second time interval which follows the first time interval and during which the magnetic resonance scanner takes an imaging segment composed of a number of image projection views or lines: estimating an expected displacement of the structure at the times at which at least some of the image projection lines are taken by extrapolating the waveform model and estimating displacements predicted by the extrapolated waveform model; and adjusting a position at which at least some of the image projection lines are taken based on the estimated displacement of the structure at the time that those image projection lines are taken; wherein the predefined initial parameters of the waveform model include that the waveform model is sinusoidal and has an initial frequency approximately equal to a breathing rate of a subject to be imaged; providing a control system into which the sequence of navigator samples is input and which is configured to have the waveform model of predefined initial parameters as its output, the control system being operable to adjust the waveform model by: computing a feedback error equal to the difference between the navigator samples and the output, and providing the feedback error as an input to the control system, the output of the control system thereby converging upon the sequence of navigator samples so that the waveform model mimics the actual displacement of the structure caused by the respiration-induced motion.