1. A patient monitor comprising: a 3D position determination module operable to determine measurements indicative of shape and location of at least part of a patient; a clock module operable to associate measurements indicative of the shape and location of at least part of a patient determined by the 3D position determination module with timing data, a model generation module operable to partition the shape and location and timing data into a plurality of groups of data each associated with different portions of the breathing cycle and utilize the portioned data to generate a plurality of component analysis models of the variation in shape and location of the at least part of a patient during the respective different portions of the breathing cycle; and a comparison module operable to utilize shape and location and timing data and a plurality of component analysis models generated by the model generation module to identify breathing which differs from that occurring in the images used to generate the component analysis models.