1. A magnetic resonance imaging (MRI) method comprising: performing a pulse sequence including excitation pulses and magnetic gradients designed to excite first and second spin populations within a slice of an object, to have different phase histories; acquiring MRI signals from the first and second spin populations at different times; using the MRI signals to determine a phase difference between the MRI signals from the first and second spin populations; using the determined phase difference to determine a magnetic field strength difference between different portions of the slice; and using the determined magnetic field strength difference between different portions of the slice, preparing a frequency map indicative of differences in a precessional frequency of the spin populations in the slice.