1. A method of operating an NMR imaging system in which there is applied a pulsed field having a gradient in a direction lying in a cross-sectional slice of a body being examined, the nuclei in said slice having been selectively excited to resonance, the resonance signal is sensed during the pulsed field, the sensed signal is sampled and the samples analysed by a method including Fourier transformation, the sampling interval being varied with time in such a way that the higher frequencies after transformation orginate from a sampling which is more widely spaced than the sampling producing the lower spatial frequencies.