INHOMOGENEITY IN BODY-FAT DISTRIBUTION MAY RESULT IN INACCURACY IN THE MEASUREMENT OF VERTEBRAL BONE MASS

When bone mineral content (BMC) is measured by dual X-ray absorptiomet ry (DXA), the X-ray beam is attenuated by bone and soft tissue. Since the component of the attenuation caused by the soft tissue overlying b one cannot be measured, the attenuation caused by soft tissue adjacent to bone is measured and is used in the calculation of BMC. The assump tion underlying this approach is that the amount and composition of th is adjacent soft tissue is the same as overlying bone, The aim of this study was to examine the validity of this assumption by determining w hether fat distribution over and adjacent to bone differ and whether t his introduces accuracy errors in the measurement of BMC by postero-an terior (PA) and lateral scanning. BMC (posterior processes plus verteb ral body, g) of the third lumbar vertebra was 17.3 +/- 0.7 by PA and 1 7.0 +/- 0.7 by lateral scanning in 27 premenopausal women (p = NS), bu t 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 di minutions across age by PA scanning was about half that by lateral sca nning (16.8 +/- 3.9%, 31.2 +/- 3.0%, p < 0.01). Percent fat in the sof t tissue baseline (lateral to bone, ST-lat) used to derive BMC by PA s canning, 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 pr emenopausal 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 (cm(2)) measured by quantitative computed tomography (QCT) in an additional 46 women, w as higher in the soft tissue baseline (ST-lat) than anterior to bone i n 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 co ntent in the soft tissue baseline region (ST-ant) was higher than on e ither 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 wome n (44.7 +/- 4.3, 50.6 +/- 5.1, p < 0.05). In summary, the differing co mposition of soft tissue anterior and lateral to bone calls to questio n the validity of the assumption of tissue homogeneity needed for accu rate measurement of BMC by DXA. In the elderly, BMC may be too high by PA scanning, resulting in a reduced cross-sectional diminution with a ge. Thus accuracy errors due to fat inhomogeneity compound those cause d by osteophytes and suggests caution is needed in making inferences r egarding the magnitude of age-related bone loss determined in vivo usi ng bone densitometry.