Development and evaluation of a phantom for morphometric X-ray absorptiometry.

Morphometric X-ray absorptiometry (MXA) provides the potential to assess vertebral deformity using a technique with much lower radiation dose to the patient than standard radiographic procedures. MXA overcomes many other limitations such as cone beam distortion observed in conventional plane radiographs. A phantom has been designed to assess the accuracy of the MXA technique, to monitor long-term precision and to assess inter- and intra-operator variability. The phantom consists of two columns of 12 cylinders representing the vertebral bodies, one of regular components and one representing vertebral deformities. Each column may be inserted, as required, into a Perspex torso-mimicking block. Initial assessment on the Lunar Expert-XL demonstrates that the phantom provides image parameters reflecting those found clinically. Measurement of vertebral height was found to be consistently underestimated by 4.9%. Operator precision ranged from 0.6% for posterior height measurement to 1.0% for middle height measurement of the regular component column. The corresponding precision range for the column representing vertebral deformation was 0.6% (posterior) and 1.1% (middle). Analysis of 10 scans of each column by two independent operators demonstrated a few significant differences in height assessment confined to the 'thoracic' region of the regular column. However, inter-operator variability was found to increase with increasing complexity of vertebral shape producing several differences, particularly in posterior height assessment of the deformed column.