VOLUMETRIC MEASUREMENTS OF BONE-MINERAL DENSITY OF THE LUMBAR SPINE -COMPARISON OF 3 GEOMETRICAL APPROXIMATIONS USING DUAL-ENERGY X-RAY ABSORPTIOMETRY (DXA)

Measurement of bone mineral density using dual-energy X-ray absorptiom etry (DXA) gives area values (g cm(-2)) rather than true volumetric va lues (g cm(-3)). To calculate the vertebral volume using planar poster o-anterior and lateral DXA values, several different geometrical appro ximations were used: cubic, cylindrical with a circular cross-section and cylindrical with an elliptical cross-section. The aim of this stud y was to compare these geometrical approximations with each other and with a reference standard, defined as the volume found on a computed t omographic (CT) scan. L2 and L3 were evaluated in a phantom study. Vol ume approximations by the cube or cylinder with circular cross-section geometry showed more than a 50% overestimation (range 54-74%). Howeve r, the elliptical cylinder approach showed very good agreement: 2.1% a nd 1.2% for L2 and L3, respectively, when compared to the CT volumes. In addition, we performed four patient studies with both CT and DXA to evaluate the elliptical cylinder estimate in a clinical setting. For L2 and L3, the mean relative difference was less than 2%. We conclude that the elliptical cylinder approach results in the most accurate bon e volume estimates in both the phantom and patients. ((C) 1998 Lippinc ott Williams & Wilkins).