Analytic and quantitative exposition of patient-specific systematic inaccuracies inherent in planar DXA-derived in vivo BMD measurements.

An extensive analytic exposition is developed of systematic inaccuracies inherent in planar dual-energy x-ray absorptiometry (DXA) in vivo bone mineral density (BMD) measurements arising directly from the dual-energy facet of this methodology. Generalized and specific criteria governing these BMD inaccuracies are derived for the presence of relevant absorptiometrically distinguishable extra- and intra-osseous soft tissues (lean muscle tissue, interposed adipose, marrow, and cerebrospinal fluid). These analytic findings are utilized in comprehensive quantitative simulation studies which evaluate the magnitude and stipulate the direction (under- or over-estimate) of such systematic errors for typical, realistic DXA in vivo lumbar vertebral BMD scans over the ranges of soft tissue parameters and trabecular bone volumes (TBV) encountered clinically. It is shown that inherent systematic inaccuracies as high as +/- approximately 20% may be anticipated in typical patient-specific planar DXA vertebral BMD scans, particularly so for older, osteoporotic-prone, and osteoporotic individuals. These inaccuracies exceed considerably the DXA precision error and may mask or exaggerate clinically significant true changes in BMD. Both the scale and trends of patient-specific inaccuracies found to pertain in the present work are in keeping with those observed by recent investigators in actual specimen-specific in situ and in vitro DXA scans of given cadaveric bones. Insofar as the range of BMD inaccuracies found here is comparable to that of variations in the DXA-measured interpatient, age-moderated BMD of normal individuals, the intrinsic specificity of planar DXA for screening/diagnostic purposes is examined critically. It is shown that these inherent inaccuracies may prompt erroneous diagnoses, assessments, and interpretations of DXA-derived BMD measurements undertaken to screen, monitor, and help evaluate patient-specific predictive bone fragility, and to assess the efficacy of drug and other therapeutic regimens.