Inhomogeneity in body fat distribution may result in inaccuracy in the measurement of vertebral bone mass

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Abstract
When bone mineral content (BMC) is measured by dual X-ray absorptiometry (DXA), the X-ray beam is attenuated by bone and soft tissue. Since the component of the attenuation caused by the soft tissue overlying bone cannot be measured, the attenuation caused by soft tissue adjacent to bone is measured and is used in the calculation of BMC. The assumption underlying this approach is that the amount and composition of this adjacent soft tissue is the same as overlying bone. The aim of this study was to examine the validity of this assumption by determining whether fat distribution over and adjacent to bone differ and whether this introduces accuracy errors in the measurement of BMC by postero-anterior (PA) and lateral scanning. BMC (posterior processes plus vertebral body, g) of the third lumbar vertebra was 17.3 ± 0.7 by PA and 17.0 ± 0.7 by lateral scanning in 27 premenopausal women (p = NS), but 2.7 g or 20% higher by PA than lateral scanning in 27 postmenopausal women (14.4 ± 0.7, 11.7 ± 0.5, p < 0.01). Thus, the respective diminutions across age by PA scanning was about half that by lateral scanning (16.8 ± 3.9%, 31.2 ± 3.0%, p < 0.01). Percent fat in the soft tissue baseline (lateral to bone, ST-lat) used to derive BMC by PA scanning, was higher than in the soft tissue baseline (anterior to bone, ST-ant) used to derive BMC by lateral scanning by 2.6 ± 0.7% in premenopausal women and 7.5 ± 1.0% in postmenopausal women (both p < 0.01). After adjusting for these differences in percent fat, BMC by PA and lateral scanning no longer differed. Fat content (cm2) measured by quantitative computed tomography (QCT) in an additional 46 women, was higher in the soft tissue baseline (ST-lat) than anterior to bone in the 18 premenopausal (30.7 ± 4.2, 24.3 ± 3.5, p < 0.01) and the 28 postmenopausal women (41.6 ± 3.7, 34.1 ± 3.5, p < 0.01). Fat content in the soft tissue baseline region (ST-ant) was higher than on either side of bone in premenopausal women (31.3 ± 5.7, 26.5 ± 4.2, p < 0.05) but less than on either side of bone in postmenopausal women (44.7 ± 4.3, 50.6 ± 5.1, p < 0.05). In summary, the differing composition of soft tissue anterior 6and lateral to bone calls to question the validity of the assumption of tissue homogeneity needed for accurate measurement of BMC by DXA. In the elderly, BMC may be too high by PA scanning, resulting in a reduced cross-sectional diminution with age. Thus accuracy errors due to fat inhomogeneity compound those caused by osteophytes and suggests caution is needed in making inferences regarding the magnitude of age-related bone loss determined in vivo using bone densitometry.
Funding Information
  • Austin Hospital Medical Research Foundation