The invention provides a method of measuring characteristics of an organ or part thereof from multiple images of the organ or part thereof, the method comprising the steps of defining the spatial position of at least two of the images; defining a reference model of the organ or part thereof scaled according to the distance between reference markers on the images; defining one or more boundary guide points associated with one or more images for which the spatial positions have been defined; converting the guide points to three-dimensional coordinates; defining an estimate model by fitting the reference model to the guide points; and calculating the characteristics from the estimate model. The invention also provides a system and a computer program for measuring characteristics of an organ or part thereof of a subject from multiple images of the subject's organ or part thereof.