Bone densitometry of the forearm: comparison of single-photon and dual-energy X-ray absorptiometry

Forearm bone mineral densitometry was performed initially by single-photon absorptiometry (SPA), but is now achievable by dual-energy X-ray absorptiometry (DXA) as well, with a good correlation between both measurements. However, it is still unknown whether: (1) short-term precision of DXA is superior to SPA and (2) identical regions of interest (ROI) are mandatory to correlate SPA with DXA. The aim of this study was to answer these questions using a commercial system for DXA (DXA-FAS) and to test an in-house system using spine DXA and a soft-tissue compensator (DXA-STC). In ten subjects, four measurements on the same day showed significantly lower (p < 0.05) coefficients of variation (CV) for bone mineral density (BMD) by DXA-FAS (proximal site: 0.74%; ultradistal site: 1.20%) than by SPA (1.26% and 2.25%). However, the CV for bone mineral content (BMC) were similar for DXA-FAS (0.73% and 1.58%) and SPA (0.79% and 1.34%). The significant difference (p < 0.05) for surface calculation by DXA-FAS (1.24% and 0.93%) compared with SPA (2.36% and 1.28%) explains all the advantages of DXA-FAS for short-term precision. The measurements taken on the same day on the ulna and the radius or on the radius alone by SPA, DXA-FAS, and DXA-STC on 108 subjects aged 18-80 years were highly correlated [r ranging from 0.925 to 0.995 (p < 0.0001) and standard error of the estimate from 3.15% to 8.89%]. The need for a manual adjustment of the ROI was found to be mandatory for BMC but not BMD assessment. The use of DXA-STC is a fast method for forearm bone densitometry and its correlation with SPA is very high. However, its short-term precision for BMC (3.00% and 1.54%), BMD (2.15% and 1.12%), and surfaces (1.99% and 1.12%) is significantly higher (p < 0.05) than that of DXA-FAS. We conclude that short-term precision of DXA is better than that of SPA only for BMD and surface measurement but not for BMC. ROI should be adjusted manually for the assessment of BMC but not for that of BMD.