1. A method of analysing an image of a subject to identify spatial areas which contain periodic intensity variations representing valid temporal information about the subject, comprising the steps of: dividing said image into a plurality of spatial areas and obtaining respective signals representative of an image intensity for each spatial area; spectrally analysing the intensity variations of each representative signal by fitting respective autoregressive models to the representative signals, each of the fitted autoregressive models comprising a plurality of poles representing spectral components of the intensity variations of each representative signal, each pole having a magnitude dependent upon the strength of the spectral component and a phase angle dependent upon the frequency of the spectral component; and selecting those spatial areas whose fitted autoregressive model has a dominant pole whose magnitude is greater than a predetermined threshold and identifying those spatial areas as containing periodic intensity variations representing said valid temporal information.