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. Devia tions resulting from scoliotic curvature or poor patient positioning p roduce distortions of visible vertebral dimensions and may lead to ana lysis error. This study utilized a phantom developed at this centre to assess the effect of vertebral malalignment on the accuracy of the MX A technique on the Lunar Expert-XL. Measured vertebral heights were fo und 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 ef fect of rotation around the anteroposterior, lateral and superoinferio r axes. Rotation around the lateral axis produced little discernible e ffect. 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 a xial rotation produced an increase in observed height at small angles of rotation, and a rapid decrease in vertebral height as rotation incr eased. The results suggest that whilst kyphosis or lordosis of up to a t least 5.8 degrees has a minimal effect on MXA, scoliosis of 4.6 degr ees or above produces a distinctive effect on the defining crush heigh t ratios.