THE ACCURACY OF VOLUMETRIC BONE-DENSITY MEASUREMENTS IN DUAL X-RAY ABSORPTIOMETRY

New developments in dual x-ray absorptiometry (DXA) allow the performa nce of high precision anteroposterior (AP) and lateral scans of spinal bone mineral density (BMD, units: g/cm(2)) without the patient moving from the supine position. Data from both projections may be combined to give an estimate of the true volumetric bone mineral density (VBMD, units: g/cm(3)) of the lumbar vertebral bodies. This report presents a cadaver study designed to validate DXA measurements of volumetric bo ne density. Sections of whole lumbar spine were scanned in AP and late ral projections in a water tank to simulate soft tissue. Individual ve rtebrae were then divided to separate the vertebral body from the neur al arch, and vertebral body volume was measured using the displacement of sand. The bone mineral content (BMC) of vertebral bodies and neura l arches was measured by ashing at 250 degrees C for 60 hours followed by 500 degrees C for a further 24 hours. The results showed that DXA scanning systematically underestimated ashing data by 14% for AP BMC, 33% for vertebral body BMC, 23% for vertebral body volume, and 12% for VBMD. Despite these significant systematic errors, the DXA measuremen ts and ashing values were highly correlated (r = 0.979-0.992). The res ults suggested that after allowing for the systematic errors, lateral DXA parameters related closely to true BMC, volume, and VBMD.