Simulated increases in body fat and errors in bone mineral density measurements by DXA and QCT

Abstract

Major alterations in body composition, such as with obesity and weight loss, have complex effects on the measurement of bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA). The effects of altered body fat on quantitative computed tomography (QCT) measurements are unknown. We scanned a spine phantom by DXA and QCT before and after surrounding with sequential fat layers (up to 12 kg). In addition, we measured lumbar spine and proximal femur BMD by DXA and trabecular spine BMD by QCT in 13 adult volunteers before and after a simulated 7.5 kg increase in body fat. With the spine phantom, DXA BMD increased linearly with sequential fat layering at the normal (p < 0.01) and osteopenic (p < 0.01) levels, but QCT BMD did not change significantly. In humans, fat layering significantly reduced DXA spine BMD values (mean ± SD: -2.2 ± 3.7%, p = 0.05) and increased the variability of measurements. In contrast, fat layering increased QCT spine BMD in humans (mean ± SD: 1.5 ± 2.5%, p = 0.05). Fat layering did not change mean DXA BMD of the femoral neck or total hip in humans significantly, but measurements became less precise. Associations between baseline and fat-simulation scans were stronger for QCT of the spine (r(2)= 0.97) than for DXA of the spine (r(2)= 0.87), total hip (r(2) = 0.80), or femoral neck (r(2)= 0.75). Bland-Altman plots revealed that fat-associated errors were greater for DXA spine and hip BMD than for QCT trabecular spine BMD. Fat layering introduces error and decreases the reproducibility of DXA spine and hip BMD measurements in human volunteers. Although overlying fat also affects QCT BMD measurements, the error is smaller and more uniform than with DXA BMD. Caution must be used when interpreting BMD changes in humans whose body composition is changing.

Figure 1. Fat layering of spine phantom and adult volunteers

(a) Cross-sectional image of European Spine Phantom with 6 kg of fat layering. Posterior-anterior (b) and cross-sectional (c) images of an adult volunteer with 7.5 kg of fat layering.

Figure 2. Percent Change in Bone Mineral Density of the European Spine Phantom (EUS) by DXA and QCT After Fat Layering

Fat layering in amounts of 6 kg and above increased the measured DXA BMD values of the EUS (left panel, grey bars). Fat layering had no significant effect on BMD values of the EUS measured by QCT (right panel, white bars). Data shown are for the 200 mg/cm³ insert. * Indicates that percent change from baseline BMD exceeds LSC.

Figure 3. Percent Change in Bone Mineral Density of Human Volunteers by DXA and QCT After Fat Layering

Fat layering decreased DXA measurements of lumbar spine BMD and increased QCT measurements of trabecular BMD. Variability of bone density change in both DXA and QCT measurements was high, but the error was more pronounced for DXA measurements, especially at the total hip and femoral neck. * Indicates p<0.05 for percent change from baseline.

Figure 4. Correlation Graphs and Bland-Altman Plots After Fat Layering in Human Volunteers

The correlation after fat layering for DXA measurements of the lumbar spine, femoral neck, and total hip was weaker than for QCT measurement of the lumbar spine. In addition, the 95% confidence intervals were wider for all DXA measurements than for QCT measurements.