COMPARISON OF 2 HOLOGIC-DXA-SYSTEMS (QDR-1000 AND QDR-4500 A)/

Bone mineral content is reliably measured by dual energy X-ray absorpt iometry (DXA), if manufacturers' recommendations and quality control ( QC) procedures are followed. Several phantoms (Hologic anthropomorphic spine phantom, the Groupe de Recherche et d'Informations sur les Oste oporoses (GRIO) test objects and the European semi-anthropomorphic pha ntoms) were used to evaluate reproducibility, linearity, accuracy and spatial resolution of two DXA devices in vitro. These parameters were also evaluated in vivo from measurements performed on 120 volunteer pa tients. It was found that when one device (a single beam monodetector QDR 1000) is replaced by another (a fan beam multidetector QDR 4500/A) , the novel combination of procedures described here, ensures that the accuracy of DXA study results is maintained when both devices are use d in succession for the same patient. To study the possible responses in clinical situations, the influence of bone environment (soft and ad ipose tissues) was also evaluated. In both systems, similar performanc es (in vitro coefficients of variation of 0.5%) were established. At e xtreme bone density values, slight differences in linearity were found , as well as differences in accuracy and spatial resolution. Lumbar sp ine and femoral neck measurements were performed with both systems in 120 volunteers, both measurements being made on the same day. The corr esponding bone mineral density (BMD) values were highly correlated (r( 2) = 0.985 for lumbar spine and 0.948 for the femoral neck), and the m ean BMD differences were 0.68% and 0.37% for each anatomical site, res pectively. Although small, these differences add to the precision erro r of the method, which is near 1%. A calibration curve has to be obtai ned in order that both devices can be equally used in regular clinical study. We concluded that when a DXA system is replaced by a new one, appropriate QC procedures must be strictly observed.