Comparison of different models for interpreting bone mineral density measurements using DXA and MRI technology

Bone mineral density measurements using dual X-ray absorptiometry (DXA) are commonly expressed as areal density (g/cm2). However, areal BMD (BMDareal) is dependent on bone size and this can lead to erroneous interpretations of BMD values. We have previously presented a simple method for calculating apparent volumetric bone mineral density (BMDvol) using ancillary DXA-derived data. In the present study we tested the validity of our model using in vivo volumetric data obtained from magnetic resonance imaging (MRI) of lumbar vertebrae. BMDareal and BMDvol of L3 were measured from sixteen pairs of identical twins (24 men, 8 women), aged 25-69 years. The dimensions of the lumbar vertebra L3 were measured from MR images and BMD values were corrected for these dimensions. The DXA-derived apparent volumetric bone mineral density (BMDvol) correlated moderately with MRI-derived BMDs (r values from 0.665 to 0.822). In contrast to BMDareal, BMDvol and MRI-derived BMDs were not related to body size variables. All these volume-corrected BMDs diminished the erroneous effect of vertebral size on areal BMD. We conclude that the simple DXA-derived BMDvol can be used for normalization of bone mineral density values in subjects of different body sizes, and especially in growing children.