Errors in dual-energy X-ray absorptiometry of the lumbar spine owing to fat distribution and soft tissue thickness during weight change.

It has previously been shown that the non-uniform distribution of fat leads to errors in the determination of spine bone mineral. If the fat error changes during alterations of weight, conclusions about bone mineral measurements might be compromised. To test this possibility, the distribution of adipose tissue was determined from X-ray CT scans taken for diagnostic purposes from patients whose weight had increased or decreased due to disease or treatment. For AP scanning the change in mean fat thickness over the vertebrae was similar to that in the background area, so that there was little change in fat error and therefore no misinterpretation of bone mineral measurements. Results were more variable for lateral scanning, sufficiently so as to question its validity in this context. The dependence of bone mineral measurements on the soft tissue thickness was investigated with three different makes of dual-energy X-ray absorptiometer and four spine models. A complex dependence was found, which varied with the bone mineral density, the thickness, the machine and the model. However, the magnitude of the dependence was not sufficient to falsify the conclusions about changes in bone mineral in two clinical studies in which patients had changed weight.