The authors have no conflict of interest
Patient-Specific DXA Bone Mineral Density Inaccuracies: Quantitative Effects of Nonuniform Extraosseous Fat Distributions†
Version of Record online: 1 JUN 2003
Copyright © 2003 ASBMR
Journal of Bone and Mineral Research
Volume 18, Issue 6, pages 1020–1027, June 2003
How to Cite
Bolotin, H., Sievänen, H. and Grashuis, J. (2003), Patient-Specific DXA Bone Mineral Density Inaccuracies: Quantitative Effects of Nonuniform Extraosseous Fat Distributions. J Bone Miner Res, 18: 1020–1027. doi: 10.1359/jbmr.2003.18.6.1020
- Issue online: 2 DEC 2009
- Version of Record online: 1 JUN 2003
- Manuscript Accepted: 16 DEC 2002
- Manuscript Revised: 12 DEC 2002
- Manuscript Received: 31 JUL 2002
- bone mineral density inaccuracies;
- 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.
THE INCREASING INCIDENCE of osteoporosis in the vulnerable population groups (1–3) has intensified the need for effective and reliable means of diagnosis and assessment of the condition and for the evaluation of the patient-specific therapeutic efficacy of its treatments. For more than a decade, the appraisal of skeletal bone mineral status in clinical practice and research has been based mainly on noninvasive planar DXA measurements of in vivo bone mineral areal density (BMD, g/cm2).(4) However, notwithstanding the widespread clinical use of, and extensive reliance on, DXA for clinical research, diagnostics of osteoporosis, and prognostics of the risk of osteoporotic fractures,(4–13) it is particularly disquieting that there is mounting evidence and serious concern that these measurements are subject to sizable inherent patient-specific inaccuracies.
These inaccuracies derive from the known inapplicability of planar DXA methodology to bone sites comprised of more than two absortiometrically distinguishable components within the scan region of interest (ROI)—the two-component DXA limitation.(14–26) The extent of these inaccuracies is sufficient to call into question a number of aspects of osteopenic and osteoporotic diagnoses/prognoses, bone fragility, remedial bone therapy effectiveness, and other related systematic features gleaned primarily from DXA in vivo BMD measurements. Faulkner(13) has reviewed the relevant evidence and explored the prospect that measured in vivo BMD, in and of itself, may not be homologous with bone strength. Marshall et al.(10) concluded that in vivo DXA measurement of BMD is not sufficiently definitive to be relied on to identify those specific individuals who will develop a future bone fracture. The likelihood that these inherent DXA in vivo BMD inaccuracies and unsettling clinical observations(10,13, 18, 19, 24–29) are linked(20–24, 30)—the former effectively manifest in the latter—looms as a fundamental issue relevant to the intrinsic viability and reliability of patient-specific planar DXA in vivo bone densitometry and bone fragility studies in clinical practice and research.
The principal objective of the present work was to extend the scope of these previous quantitative studies of inherent DXA in vivo BMD measurement inaccuracies by systematic investigation of the extent to which nonuniform distributions of extraosseous fat within the scan ROI further contribute to and affect these BMD measurement inaccuracies. It is noted that such a systematic and comprehensive study would not be feasible or practical using actual patients or in situ cadavers, because in neither could the true BMD and/or the requisite details of soft tissue composition and distribution within the scan ROI be ascertained(20) [the “true” value of BMD, (BMD)true, being that BMD value that would be obtained from a DXA in vivo measurement free of attendant inaccuracies]. Furthermore, in neither case could the investigator determine or preselect for interrogation any of these pertinent anthropometric and X-ray absorptiometric particulars to an extent sufficient to elucidate the underlying causative relationships fully. Thus, the use of realistic phantom arrays that closely replicate these soft tissue anthropometrics becomes essential in any thorough quantitative endeavor to establish and understand the fundamental causes of these inaccuracies and the extent to which they can affect patient-specific DXA in vivo BMD measurements. The present work differs from earlier, ostensibly similar, efforts (6, 14–19, 25, 26, 31–33) in that it develops directly from the underlying source of these inaccuracies and does so across the entire practical clinical range of BMD and soft tissue anthropometrics.
MATERIALS AND METHODS
Phantom and DXA scans
Phantom materials formulated(23) as X-ray absorptiometric equivalents of bone material (B), red and yellow marrows (RM and YM, respectively), lean muscle tissue (L), and fat (F) were assembled into 225 different “anthropometric” arrays, as schematically represented in Fig. 1. These arrays spanned the full range of soft tissue anthropometrics encountered clinically, and as well, a broad range of nonuniform distributions of extraosseous fat and lean muscle tissue. The fabrication of these phantom tissues was based on the essentials of the loaded-epoxy resin method of White et al.(34) All phantom materials were identical in composition and overall dimensions to those used in our earlier publication.(23)
In each such phantom assembly, the extraosseous “soft tissue” material lateral to the “bone” was the absorptiometric equivalent of a specific preselected homogeneous F-to-L areal density ratio q = (ρFyF/ρLyL); y is the linear anterior-posterior (A/P) dimension (cm) of the given extraosseous “tissue” along X-ray paths lateral to bone, and q is the lateral extraosseous soft tissue descriptor relevant to the extraction of BMD from DXA scans.(20–23) Along X-ray paths traversing “bone material” in the phantom array, the linear extent of the extraosseous soft tissue and its F-to-L areal density ratio are denoted by x and Q, respectively, so that Q = (ρFxF/ρLxL). However, in this study, q and Q were generally not the same, and y was not equal to x.(23) The various separate values of q and Q used (0.0, 0.1, 0.3, 0.6, and 1.0) spanned the full range encountered clinically,(35) ranging from 0% to 50% F by weight in the scan ROI.
The thicknesses of phantom bone material inserted into the “bone site” of each phantom array were selected to be (BMD)true values of 0.6, 0.8, and 1.4 g/cm2, ranging(9, 36) from that typical of the osteoporotic, through osteopenic, to high BMD values, respectively. The remaining bone-site space was completely filled with various combinations of RM and YM phantom slabs. The fraction of the marrow volume that was yellow in each case, g = (YM)/(YM + RM) = 0.6, 0.8, and 1.0 (40% RM, 20% RM, and wholly YM, respectively), spanned the marrow constitutions typical of mature and elderly patients.(37, 38) In this way, phantom arrays replicating 225 different combinations of BMD and intra-/extraosseous soft tissue anthropometrics and absorptiometrics were assembled and DXA scanned.
DXA simulation studies of phantom arrays
As dual poly-energetic DXA simulated BMD values (after beam-hardening corrections were made) were in excellent quantitative agreement (< ∼0.2%) with actual DXA poly-energetic BMD measurement results and with dual mono-energetic DXA BMD simulation findings (intrinsically free of beam-hardening) for similar 20-cm-thick phantom arrays,(23) dual mono-energetic DXA BMD simulations of each of the phantom arrays were carried out.(20–22)
The BMD values of all simulated scans, (BMD)sim, were evaluated (as are the BMD values obtained in standard clinical DXA scans) using the analytic DXA equation,(20)
where ln(J1/J01) and ln(I1/I01) represent the natural logarithms of the fraction of incident low-energy X-ray photon spectral region transmitted through the specimen lateral to and through the “bone,” respectively, and ln(J2/J02) and ln(I2/I02) are the corresponding values for the high-energy X-ray region, with λ1B and λ2B being, respectively, the low- and high-energy X-ray mass attenuation coefficients (cm2/g) of bone material at the two energies.
However, the DXA equation (Eq. 1) will yield the true value of BMD if, and only if, proper account can be taken of the presence of marrow (and its composition) and any differences that pertain between the makeup of the extraosseous fat/lean tissue in the X-ray “shadow” of the bone and lateral to it (q,Q differences).(20) As DXA cannot, (BMD)meas may differ markedly from (BMD)true, with DXA methodology unable to estimate the extent of these BMD inaccuracies. However, these inaccuracies can be quantitatively assessed in the present study, because in both the actual DXA scans and the corresponding simulation studies of each phantom array, all anthropometric and X-ray absorptiometric particulars, no matter how disparate, are known a priori.
Fifteen additional similar phantom arrays were also assembled with the same standard intra- and extraosseous dimensions, each constituted with q = Q, and all marrow slabs replaced by material identical in composition to that of the extraosseous phantom tissue, q, of the same array. Because these assemblies were comprised of only two absortiometrically distinct materials (bone material plus a single intra-/extraosseous soft tissue equivalent), the two-component DXA limitation was satisfied fully, allowing DXA-measured BMD to be determined correctly (i.e., without inaccuracy). These 15 phantom assemblies were DXA scanned using all five q = Q values for each of the three selected phantom BMD values. Scans of these q = Q = g arrays established (calibrated) the three true (actual) BMD values, (BMD)true, of phantom bone material used in the other 225 phantom arrays. In this way, the corresponding overall percentage inaccuracies in both DXA-measured and normalized DXA-simulated BMD [(BMD)meas and (BMD)sim, respectively] were obtained for each of the 225 phantom array cases:
Estimates of q, Q differences in DXA in vivo vertebral and proximal femoral scans
To obtain reasonable estimates of the extent to which the extraosseous soft tissue areal density ratios along X-ray paths passing lateral to (q) and those intersecting bone (Q) in planar DXA in vivo BMD scans actually differ within the scan ROI of lumbar vertebral and proximal femoral bone sites for most patients encountered clinically, a number of anatomical atlases were consulted. These depicted actual cross-sectional anatomical cadaveric slices,(39) computerized tomographic,(40–41) and magnetic resonance imaging(41) sections in which extraosseous adipose tissues (both subcutaneous and intraperitoneal) are distinguishable from lean muscle tissue and internal organs.
Careful measurements were made of the separate overall extraosseous fat and lean tissue dimensions along a large number of individual notional A/P X-ray paths passing lateral to and through the bone in each depicted contiguous transverse anatomical section image through the lumbar spinal and proximal femoral bone sites, as exemplified in Norland, Hologic, and Lunar DXA manuals and various texts.(42) In addition, the selected soft tissue ROI boundaries laterally adjacent to the bone were varied in breadth, and the new soft tissue anthropometrics encompassed were again evaluated. In each case, the separate average fat and lean tissue A/P dimensions were weighted by their respective volumetric densities,(23) ρF and ρL, to obtain the mean F-to-L areal density ratios, q and Q, and the (q − Q) difference (positive and/or negative). In doing so, all nonfat tissues (internal organs, muscle tissue, etc.) present in the scan ROI were assigned the same volumetric density and linear X-ray attenuation coefficients of lean muscle tissue.(23) In this way, the difference between the average extraosseous F-to-L areal density ratio lateral to the bone and that along each notional X-ray path intersecting bone mineral material in the ROI (q − Q) was obtained.
Because no transverse sections of particularly obese or exceedingly slender persons were depicted in the above noted anatomical atlases, the overall A/P torso and hip thicknesses of the full range of patients presenting for DXA BMD scans were not represented. To obtain the (q − Q) value for any given q value that might reasonably be anticipated clinically, various A/P uniform fat layers were notionally added or removed from the depicted “normal” cross-sectional anatomical atlas slices, and new overall resultant q, Q, and (q − Q) values were simply derived from them in each case. Because adult weight gain (loss) largely results from the accretion (loss) of subcutaneous (and/or visceral) fat, the thickness of the fat added (lost) seems to be spread relatively uniformly across the local scan ROI when viewed in A/P projection DXA BMD measurements of supine patients. On this basis, extrapolations from the “normal” 20-cm-thick cross-sectional atlas slices were transformed into reasonably realistic representations of A/P torso thicknesses ranging from 16 to 28 cm, effectively covering the full range of F-to-L areal density ratios and A/P torso thicknesses encountered clinically. These estimates of inhomogeneous fat distributions in the lateral and bone-shadow segments of DXA scans of these bone sites were found to be contained within the limits −0.10 ≤ (q − Q) ≤ +0.03 in most cases, for all q values pertaining to A/P lumbar spinal and proximal femoral DXA scan regions of most patients presenting for BMD measurement.
The percentage inaccuracies found in the present DXA-measured BMD scans using Norland XR-26, Hologic QDR-1000, and Lunar DPX-α densitometers were effectively indistinguishable for each of the “anthropometrically” different 225 phantom arrays, as was also the case in our earlier phantom studies.(23) The results of the present investigation need therefore be displayed for any single densitometer (arbitrarily selected here for our Norland data only). Because the full span of the (q − Q) differences DXA scanned and simulated in the present study may not be encountered clinically, the percentage BMD inaccuracies (Eq. 2) displayed in Figs. 2, 3, and 4 are largely restricted to those data and results corresponding to anthropometric factors [q, Q, (q − Q), g, and (BMD)true] anticipated to be clinically relevant for adult patients (some results outside these ranges are also included to elucidate the trends and extent of inherent DXA BMD percentage inaccuracies that could result from whatever inhomogeneities in extraosseous soft tissue values, (q − Q), might pertain, or be introduced, in some research or clinical studies). These data also make clear the sensitivity of the percentage BMD inaccuracies to inhomogeneous anthropometric and X-ray absorptiometric aspects of the various intra- and extraosseous soft tissues.
Also displayed in Figs. 2, 3, and 4 are those demarcations of the limits [−0.10 ≤ (q − Q) ≤ +0.03] found in the present investigation to reasonably represent the range of extraosseous soft tissue areal density ratio differences (q − Q) encountered in actual A/P DXA lumbar spine and proximal femoral BMD scans of typical patients. For example, in the particular case of an osteopenic individual [(BMD)true = 0.8 g/cm2] with a marrow composition 80% yellow (g = 0.8) and a lateral F-to-L areal density ratio q = 0.3, a DXA-measured BMD value may well underestimate the true BMD by between ∼14% [(BMD)meas ≈ 0.69 g/cm2] and ∼23% [(BMD)meas ≈ 0.62 g/cm2], leading to possible misdiagnosis of this patient as osteoporotic. For an osteoporotic individual, (BMD)true = 0.6 g/cm2, with the same soft tissue compositions (Fig. 2), the underestimates of the true BMD value range between ∼19% [(BMD)meas ≈ 0.49 g/cm2] and ∼31% [(BMD)meau ≈ 0.41 g/cm2].
It is also seen (Figs. 2, 3, and 4) that the red/yellow marrow compositional effect (g-dependence) on DXA-measured BMD inaccuracies still pertains, no matter the value of q, Q, (q − Q), or (BMD)true.
The extent of these inaccuracies have been quantitatively delineated on a foundation of anatomically realistic phantom representation of in vivo soft tissue and bone anthropometrics and X-ray absorptiometrics relevant to the broad range of patients presenting for clinical DXA BMD scan procedures. Of particular importance, the present study has extended the earlier quantitative expositions(20–24) of DXA in vivo BMD inaccuracies to those more anatomically realistic cases in which the extraosseous fat and lean soft tissue mix is not uniform throughout the scan ROI. In particular, it has been shown here that the values of the extraosseous fat-to-lean soft tissue areal density ratios lateral to and in the bone-shadow regions, and their differences, (q − Q), in the local bone site DXA scan ROI are anthropometric parameters directly relevant to and affecting the extent of BMD inaccuracies manifest in DXA-derived in vivo measurements. This is of some clinical importance, because it is clear that fat and lean tissues are not distributed uniformly throughout the body, and that the general distribution of soft tissues and accretion/loss of extraosseous fat is significantly different for children and adults, men and women, and pre and postmenopausal women.(43) Furthermore, the specific local bone-site F-to-L areal density ratio in the scan ROI may vary, sometimes considerably, as a function of age, physical condition, bone site, and other external factors such as the specific boundaries selected for the scan ROI. If the lateral segments of the ROI defining the extraosseous soft tissue areal density, q, can be varied by the DXA operator (or are DXA instrument dependent), somewhat different values for q and (q − Q) could pertain for scans of a given bone site, thereby altering the effective BMD inaccuracy and the measured value of BMD. As such, measures of patient whole body fat-mass/total body-mass/body mass index, although indicative, may not necessarily be reliable or dependable descriptors of particular local bone site F-to-L areal density ratios within the scan ROI. Just as q is bone-site dependent, so too can be the quantity (q − Q). In addition to other scan particulars, the relevant q and (q − Q) values can also differ in A/P, lateral, and other DXA scan projections of the same bone site, leading to different BMD inaccuracies and, therefore, different measured BMD values. (It is noted here that the present study, by design, was unaffected by bone-edge detection concerns or uncertainties related to bone alignment relative to incident X-ray paths.)
It is also seen that within reasonable limits of extraosseous soft tissue disparity and composition (Figs. 2, 3, and 4), the inherent (BMD)meas inaccuracies in patient-specific DXA in vivo measurements can readily be expected to exceed 20% in clinically realistic cases. Particularly for osteoporotic patients (low true BMD), for postmenopausal and elderly individuals (marrow tending to higher yellow content in the lumbar vertebrae and proximal femoral regions),(37, 38) and for nonobese and lean patients (moderate and low relative fat content within and surrounding the lean tissues within the DXA scan ROI), the inaccuracies in DXA-derived BMD measurements can be quite large (30–50%). As a result, patient-specific DXA-measured in vivo BMD values can be seriously in error, more often than not considerably underestimating the true BMD of the very patients whose BMD it is most important for DXA to assess accurately. Given the reported 2-fold increase in population-based fracture risk per each SD decrement in BMD,(44) these sizable inaccuracies can be seen to insinuate confounding aspects into the clinical assessments of patient-specific fracture risk and diagnoses of osteoporosis based on the simple T-score criterion (T-score < −2.5).
As a consequence of the inability of planar DXA methodology to assess the absorptiometrics of both the extraosseous F-to-L areal density ratio in the bone-shadow region, Q, and the bone marrow, g, (one of the most labile of body tissues),(36–38, 45–47) the inaccuracies in DXA-measured in vivo BMD values may be both large and unpredictable in any given patient case. This is as expected, because the presence of marrow, irrespective of its specific red/yellow mix, g, effectively modifies the overall soft tissue areal density ratio (combined intra- and extraosseous tissues) along all X-ray paths traversing marrow within any given scanned bone. The marrow composition (largely affected through hematopoietic activity) can vary with patient age, disease, drug therapy, immobilization, physical activity regimen, bed confinement, infection, etc. This is the case particularly in the vertebrae and proximal extremities of appendicular bones where hematopoietic activity can be quite variable.(18, 37, 38, 45, 46) Similarly, the extraosseous fat-to-lean muscle tissue areal density ratio in the ROI of DXA BMD scans of patients can separately alter as a result of dietary regimens, hormonal changes, drug therapy, exercise programs, illness, etc. Consequently, changes in the soft tissue parameters, with or without accompanying true changes in bone mineral material, can be reflected in changes in DXA-measured BMD that may mislead interpretations of prospective BMD measurements of individual patients. Indeed, all DXA-measured in vivo BMD values will reflect this variability to some unknown extent, and as a result, may lead to interpretations and evaluations of diagnostic, prognostic, longitudinal, or cross-sectional BMD measurements that are seriously flawed.
In this context, it is important to stress that it is not the overall thickness or the difference in the linear thicknesses of fat alone or of lean tissues alone (or the linear extent of the body section alone) in either or both segments of the scan ROI (lateral to the bone and in the bone-shadow) that are relevant to the magnitude of inherent inaccuracies in DXA in vivo BMD measurements. Rather, it is the overall F-to-L areal density ratio, q, and the effective difference between the F-to-L areal density ratios (q and Q) along X-ray paths through the scan ROI that is of consequence. Thus, the local bone-site ratio of the F-to-L masses along the X-ray paths, and not the overall patient fat mass or lean mass or body mass singly, is the relevant soft tissue anthropometric parameter affecting DXA-measured in vivo BMD values. It is on these grounds that previous conflicting reports(48–61) specifying patient fat mass rather than body lean mass (and vice versa) to be the soft tissue parameter correlated strongly with DXA-measured BMD may be resolved, because neither one of these anthropometric quantities alone underpins the inherent DXA in vivo BMD inaccuracies from which these ostensible BMD correlations arise.
It is clear that when true BMD decreases, the bone must be more fragile to some extent (effectively the definition of “osteoporosis”). However, the present results have shown equally clearly that because of the sizable inherent, patient-specific DXA in vivo BMD measurement inaccuracies demonstrated here and in earlier works,(14–24, 30) a decrease (increase) in measured BMD may not necessarily signal any change in the bone fragility for a given individual. As a consequence of demonstrated large inherent systematic patient-specific DXA in vivo BMD inaccuracies, it is of utmost importance that it be recognized that DXA-measured BMD and true BMD are not necessarily synonymous. It is also difficult not to conclude from this and past investigations that introspective review and critical assessment of those aspects of present consensual knowledge of in vivo bone fragility and bone responsiveness to treatment based primarily on DXA BMD measurements are very much warranted.
This research was supported in part by Grant 202011 of the National Health and Medical Research Council (Australia) and the Research Fund of Tampere University Hospital (Finland).
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