Patient-specific DXA bone mineral density inaccuracies: quantitative effects of nonuniform extraosseous fat distributions.

Nonuniform extraosseous fat is shown to raise the magnitude of inaccuracies in DXA in vivo BMD measurements into the range of 20-50% in clinically relevant cases. Hence, DXA-based bone fragility diagnoses/ prognoses and evaluations of bone responsiveness to treatment can be unreliable. Patient-specific DXA in vivo bone mineral areal density (BMD) measurements have been demonstrated to be inherently inaccurate even when extraosseous fat (F) and lean muscle tissue (L) are uniformly distributed throughout the scan region of interest (ROI). The present work extends these investigations to quantitative evaluation of the extent to which clinically realistic soft tissue inhomogeneities external to the bone within the DXA scan ROI affect patient-specific in vivo BMD measurement inaccuracies. The results are particularly relevant to patient-specific lumbar vertebral and proximal femoral sites. Norland, Hologic, and Lunar DXA scans and corresponding DXA simulation studies of the same set of 225 different phantom arrays were carried out. The phantoms were specially fabricated absorptiometric replications of bone mineral material (B), red marrow (RM), and yellow marrow (YM) mixtures, and extraosseous F and L combinations spanning the anthropometric ranges encountered clinically. The three different DXA scanners yielded BMD results that effectively coincided, were in excellent agreement with the findings of the present corresponding DXA-simulation studies in each case, and confirmed the validity of the DXA BMD inaccuracy analysis formalism. It was found that only relatively small extraosseous soft tissue inhomogeneities within the ROI of DXA BMD scans can increase substantially the already sizable BMD inaccuracies shown earlier to pertain for uniformly distributed extraosseous soft tissues. The extent of these in vivo BMD inaccuracies (%) are shown to depend on the mean extraosseous F-to-L areal density ratio and its degree of nonuniformity within the local bone scan ROI, the marrow thickness and specific composition, and the actual BMD in any given case. It was found that patient-specific DXA-measured in vivo BMD inaccuracies can, in many clinically encountered cases, be as large as 20-50%, particularly so for osteopenic, osteoporotic, and elderly patients. It is concluded that, because these DXA in vivo BMD inaccuracies are unavoidable and clinically unpredictable, diagnoses/ prognoses of bone fragility and evaluations of bone responsiveness to treatment of individual patients based mainly on DXA in vivo BMD measurements can be unreliable.