[Contribution of medical imaging. What examinations for what indications?]

Osteoporosis is an obvious diagnosis in patients with fractures due to decreased bone mass. However, in many cases, major bone loss is infraclinical and requires quantification. Two methods are available to achieve this, i.e., dual energy X ray absorptiometry (DXA) and quantitative computerized tomography (QCT). QCT, which measures the central trabeculae in the lumbar vertebra, is a sensitive technique and has demonstrated a mean bone loss of 1.1 p. cent per year in cancellous bone. However, use of QCT is limited by the high radiation exposure and costs involved. Dual energy X ray absorptiometry is a highly reproducible method which ensures satisfactory correction of soft tissue effects and entails only low exposures to radiation. However, errors occur in patients with aortic calcifications, posteriorly located osteophytes or scoliosis. The optimal frequency of quantitative evaluations of bone varies with the type of disease; in patients with osteoporosis, it is unnecessary to perform evaluations more than once every three years to monitor the course of the disease.