Measurement of Bone Density: Current Techniques

Abstract

Since the discovery of X-rays by Röntgen in November 1895 there has been interest in utilising them for the examination of bone. In fact as early as January 1896 the first paper appeared which contained a radiograph. There can be little doubt as to the utility of radiographs in the examination of the skeleton, in particular for the location of fractures and dislocations. Plain radiographs also have a role in the assessment of bone mineral density; advanced osteopenic change or established osteoporosis can easily be detected using, for example, lateral views of the thoracolumbar spine. However, many problems have been found in using this approach. Many attempts have been made to quantify bone mineral from images on radiographic film starting in the 1930s with the work of the American dentist Hodge. Together with his co-workers he examined many variables likely to affect the measurement of bone mineral content (BMC) using direct radiographic methods. In the late 1930s a system which shone a collimated beam of light through a radiograph was used by Pauline Mack to obtain quantitative values from radiographs. This system, albeit modified, was in use into the early 1970s when it was used for measuring the bones of astronauts who had undergone weightlessness during space flight. In 1951 it was pointed out by Ardran that bone destruction could not be shown on radiographs as the images appeared normal until a loss of BMC of the order of 20-30% was present. He also demonstrated the poor reproducibility of the plain film methodology. A need was established for an imaging modality, which was capable of producing not just useful images of the bone anatomy, but also quantitative data that was not subject to the limitations of work based around the use of radiographs. In 1963 a direct method of measuring BMC was developed by Cameron and Sorenson and was used to quantify the loss of bone due to osteoporosis in the forearm bones. This technique known as single photon absorptiometry made the reproducible measurement of bone mineral content a reality. This technique and others that were developed from it is examined in more detail in the section that follows.