1. A method of estimating the spatial variation of magnetic resonance imaging radio frequency (RF) signal intensities within an object from measured RF signal intensities of a uniform spin density medium surrounding the object, said method comprising: acquiring a magnetic resonance image of an object bounded by a medium which is of essentially uniform spin density, on the length scale of resolution of the image; formulating a semi-empirical mathematical model of the spatial variation in RF signal intensity within the said object; and fitting said model to selected measured RF signal intensities of said surrounding medium to obtain an estimate of the spatial variation of magnetic resonance imaging radiofrequency (RF) signal intensities within said object.