1. A method of comparing a dimension of a feature using sequential X-ray images, comprising steps of: providing an X-ray source; locating a first target adjacent the feature between the source and the feature; providing an X-ray detector on an opposite side of the feature from the source and the first target; locating a second target between the feature and the detector, the second target having a known size relationship to the first target; forming a first X-ray image of the feature, the first image including an image of both the first target and the second target; at a later time, locating the first target adjacent the feature and the second target between the feature and the detector; at the later time, forming a second X-ray image of the feature, the second image including an image of both the first target and the second target; calculating a first scale factor for the first X-ray image based on the relative sizes of the images of the first and second targets in the first X-ray image; calculating a second scale factor for the second X-ray image based on the relative sizes of the images of the first and second targets in the second X -ray image; measuring a dimension of the feature in the first and second X-ray images; adjusting the measured dimension of the feature in the first and second X-ray images by the respective first and second scale factors; and comparing the adjusted measured dimensions.