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 mi neral measurements might be compromised. To test this possibility, the distribution of adipose tissue was determined from X-ray CT scans tak en 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 backg round 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 vali dity in this context. The dependence of bone mineral measurements on t he soft tissue thickness was investigated with three different makes o f dual-energy X-ray absorptiometer and four spine models. A complex de pendence was found, which varied with the bone mineral density, the th ickness, the machine and the model. However, the magnitude of the depe ndence was not sufficient to falsify the conclusions about changes in bone mineral in two clinical studies in which patients had changed wei ght.