Bone Densitometry
Bone densitometry is a technique used to determine the average bone mineral density (BMD) within a particular region. Dual energy X-ray absorptiometry or 'DEXA" is the most accurate method available to perform this function. DEXA machines have an X-ray source consisting of a stationary anode X-ray tube which after filtration produces a pencil beam of X-rays of a specific energy. After passing through the region of interest, the X-rays which have not been absorbed in the body strike a detector consisting of sodium iodide. The X-rays cause a 'scintillation' of light in the detector. This light is measured and the reading digitized for analysis by a computer. By obtaining two sets of data using two different energies of X-ray it is possible to estimate the amount of calcium in the sample. In the clinical setting the regions most commonly scanned are the hip, lumbar spine and less often the forearm. The results obtained from a particular patient are compared with results from age matched and from young adult control groups. From the results, an estimate of fracture risk is obtained. Serial studies are used to monitor bone density especially in patients receiving treatment for known osteoporosis.
Osteoporosis is defined as low bone mass leading to increased bone fragility and greater susceptibility to fractures. It is a condition which is more common with increasing age and is a major contributor to the incidence of fractures in people over 60 years of age. Common sites of osteoporotic fractures are the thoracic and lumbar spines, the hip, the upper arm (humerus) and the wrist (radius).
