A phantom based study on the effect of subject positioning on morphometric X-ray absorptiometry using the Lunar Expert-XL.

Morphometric X-ray absorptiometry (MXA) relies on accurate measurement of vertical dimensions of vertebrae from a lateral perspective. Deviations resulting from scoliotic curvature or poor patient positioning produce distortions of visible vertebral dimensions and may lead to analysis error. This study utilized a phantom developed at this centre to assess the effect of vertebral malalignment on the accuracy of the MXA technique on the Lunar Expert-XL. Measured vertebral heights were found to be consistently underestimated by an average of 3.7%. Precision ranged from 0.79% for anterior height measurement to 1.03% for middle height measurement. Vertebral malalignment was investigated as the effect of rotation around the anteroposterior, lateral and superoinferior axes. Rotation around the lateral axis produced little discernible effect. However, superoinferior axial rotation showed a change of more than two standard deviations in the mid/posterior ratios of biconcave vertebrae at comparatively small angles of rotation. Anteroposterior axial rotation produced an increase in observed height at small angles of rotation, and a rapid decrease in vertebral height as rotation increased. The results suggest that whilst kyphosis or lordosis of up to at least 5.8 degrees has a minimal effect on MXA, scoliosis of 4.6 degrees or above produces a distinctive effect on the defining crush height ratios.