1. A method of comparing the size of a feature in sequential X-ray images, comprising the steps of: a) forming first and second sequential X-ray images including an image of the feature, an image of a first target located directly adjacent the feature and an image of a second target located between the object and an X-ray detector; b) calculating scale factors for the first and second X-ray images based on the relative sizes of the images of the first and second targets in the X-ray images; c) measuring the sizes of the feature in the first and second X-ray images; d) adjusting the measured sizes of the feature in the first and second X-ray images by the respective scale factors; and e) comparing the adjusted measured sizes.