Methodologies for the measurement of bone density and their precision and accuracy

Radiographic methods of determining bone density have been available for many years, but recently most of the efforts in this field have focused on the development of instruments which would accurately and automatically measure bone density by absorption, or by the use of x-ray computed tomography (CT). Single energy absorptiometers using I-125 have been available for some years, and have been used primarily for measurements on the radius, although recently equipment for measuring the os calcis has become available. Accuracy of single energy measurements is about 3% to 5%; precision, which has been poor because of the difficulty of exact repositioning, has recently been improved by automatic methods so that it now approaches 1% or better. Dual energy sources offer the advantages of greater accuracy and the ability to measure the spine and other large bones. A number of dual energy scanners are now on the market, mostly using gadolinium-153 as a source. Dual energy scanning is capable of an accuracy of a few percent, but the precision when scanning patients can vary widely, due to the difficulty of comparing exactly the same areas; 2 to 4% would appear to be typical. Quantitative computed tomography (QCT) can be used to directly measure the trabecular bone within the vertebral body. The accuracy of single-energy QCT is affected by the amount of marrow fat present, which can lead to underestimations of 10% or more. An increase in marrow fat would cause an apparent decrease in bone mineral. However, the precision can be quite good, 1% or 2% on phantoms, and nearly as good on patients when four vertebrae are averaged. Dual energy scanning can correct for the presence of fat, but is less precise, and not available on all CT units. QCT of the femoral neck has recently been attempted, but presents difficulties that make it unlikely to become widely accepted. Recently there has been much interest in using gamma cameras for dual energy bone density measurements. Although this can present some difficulties, several groups reportedly have overcome them, and this technique may have more widespread application in the future.