Development of a phantom for morphometric X-ray absorptiometry

Morphometric X-ray absorptiometry (MXA) has recently been developed to asse ss vertebral deformity status using dual energy X-ray absorptiometry (DXA) machines. In contrast to bone densitometry, a vertebral morphometry phantom is not supplied by any machine manufacturer. The aim of this study was to develop a suitable phantom to quantify the accuracy and precision or the ve rtebral measurement software on three DXA scanners in vitro and to perform a weekly quality control (QC) scan over a 30-month period to evaluate any d rift or changes in measurement accuracy over time. The phantom was construc ted from Perspex and aluminium to simulate sort tissue and bone, respective ly. 13 aluminium rectangles teach 30 mm wide, 25 mm high and 3 mm thick, wi th edges ("endplates") 6 mm thick) were set into one side of a solid Perspe x block to represent the vertebral bodies from the fourth thoracic (T4) to the fourth lumbar (L4). The phantom was scanned on both the Hologic QDR2000 plus and the QDR-4500A as well as the Lunar Expert-XL. Three consecutive la teral MXA scans were acquired on the Hologic machines using each of the sca n modes available. On the QDR-2000plus, the lateral scan modes available ar e fast, array and high definition, which are all dual energy modes. These t hree scan modes are also available on the QDR-4500A, with the addition of a single energy scan mode. Four lateral scans were acquired on the Expert-XL machine using the single scan mode available. Each MXA scan was analysed t wice by a trained operator using the standard software supplied by each man ufacturer. A QC scan was performed approximately weekly over a 30-month per iod on only the QDR-4500A machine, and total phantom height was measured fr om the inferior edge of L4 to the superior edge of T4. Accuracy of "vertebr al" height measurement varied between the three DXA machines and between th e scan modes available. All underestimated "true" Vertebral height by betwe en 0.4% and 8.6%, with the scan modes using finer collimation producing the most accurate results. Repeat analysis precision of vertebral height measu rement was best on the QDR-4500A, followed by the Expert-XL, and was poores t on the QDR-2001plus. The QC scans acquired on the QDR-4500A suggested tha t it was a highly stable machine, little affected by even major repairs. It must be remembered that these ir? vitro phantom results may not be represe ntative of the true in vivo situation. The MXA phantom appears to be a usef ul tool for documenting the stability of the mechanical instruments and for checking the long-term consistency of operator precision.