What Causes the Discrepancy between Quantitative Computed Tomography and Dual Energy X-Ray Absorptiometry?

Objective: Dual energy X-ray absorptiometry (DXA) is a standard tool for measuring the bone mineral density (BMD), but it sometimes provides inaccurate results. In contrast, quantitative computed tomography (QCT) measures the BMD more accurately by directly measuring the trabecular bone. This study examined the factors that cause the discrepancy between DXA and QCT with advanced spinal imaging.Methods: The medical records of 59 patients who underwent DXA, QCT, and a lumbar spine magnetic resonance imaging (MRI) were reviewed retrospectively. The following values were measured on plain radiographs, computed tomography, and MRI: lumbar lordosis, segmental scoliosis, compression fracture, spondylosis, abdominal aortic calcification, facet joint degeneration, and Modic change. The significant parameters contributing to the discordance results between DXA and QCT were evaluated by logistic regression analysis.Results: QCT identified osteoporosis in 43 patients (72.88%). DXA identified 15 osteoporotic patients (25.42%). Of the 44 patients not diagnosed with osteoporosis by DXA, 30 patients (68.18%) were diagnosed with osteoporosis by QCT. Multivariate logistic regression analysis showed that the result of DXA might be overestimated by a degenerated facet joint (odds ratio [OR] 4.58; 95% confidence interval [CI], 1.1-19.07) and measurements at a fracture site (OR, 1.63; 95% CI, 1.23-2.15).Conclusion: DXA might miss a diagnosis of osteoporosis. Facet joint hypertrophy due to degeneration revealed a stronger association with the overestimation of the BMD by DXA than the formation of osteophytes at the vertebral body. Therefore, the results of DXA should be interpreted carefully, considering the possibility of interruption.