1. A method of processing a time-sequence of separate image data sets which record induced changes in pixel values of successive images of a subject, each set comprising a plurality of image data items which each represent the location of an image pixel of the image subject according to a common reference frame within which the subject is located, the method including the steps of: (a) selecting from each of a plurality of said separate image data sets an image data item which represents an image pixel located at the same fixed image pixel location, thereby to generate a time-domain image data set containing only image data items which represent an image pixel at the same said image pixel location; (b) determining according to a measure of said induced changes as between all of the pixel values of the image data items of the time-domain image data set whether the image data items thereof are associated with the presence of a specified tissue within the image subject, which includes: forming a time-domain image vector wherein each image data item of the time-domain image data set represents a separate vector component of the time-domain image vector, and determining the measure according to a property of the time-domain image vector; wherein, said measure is determined according to the degree to which a time-domain image vector differs from an identity vector.