In vivo and in vitro comparison of densitometers in the NOREPOS study

The purpose of this study was to assess the agreement of in vivo hip scans on 3 densitometers (1 GE Lunar DPX-IQ and 2 GE Lunar Prodigy scanners) and to evaluate whether the European Spine Phantom (ESP) was able to reproduce the in vivo variability. Sixteen subjects had 3 repeated scans (with repositioning) on each densitometer, and the ESP was measured on each densitometer at least 40 times. Mean differences between hip scans on the Prodigy scanners were small and insignificant, and the in vivo results were not significantly different from the in vitro results. Bland and Altman plots showed no systematic differences between the Prodigy scanners over the range of bone mineral density (BMD). On the other hand, differences between Prodigy and DPX-IQ changed systematically over the range of BMD. The ESP did not fully reproduce the in vivo difference between Prodigy and DPX-IQ. In conclusion, the ESP is a valid substitute when assessing agreement between Prodigy scanners. However, when assessing agreement between different types of scanners, substitution of in vivo with in vitro measurements should be made with caution.