MRI Measures of Cortical Bone Water Concentration: Dependence on Age and Pore Volume Fraction
Objective: To quantify bulk bone water to test the hypothesis that bone water concentration (BWC) is negatively correlated with bone mineral density (BMD) and is positively correlated with age, and to propose the suppression ratio (SR) (the ratio of signal amplitude without to that with long-T2 suppression) as a potentially stronger surrogate measure of porosity, which is evaluated ex vivo and in vivo.
Results: BWC was positively correlated with age (r = 0.52; 95% confidence interval [CI]: 0.22, 0.73; P = .002) and negatively correlated with vBMD at the same location (r = 20.57; 95% CI: 20.76, 20.29; P , .001). Data were suggestive of stronger associations with SR (r = 0.64, 95% CI: 0.39, 0.81, P , .001 for age; r = 20.67, 95% CI: 20.82, 20.43, P , .001 for vBMD; P , .001 for both), indicating that SR may be a more direct measure of porosity. This interpretation was supported by ex vivo measurements showing SR to be strongly positively correlated with micro-CT porosity (r = 0.88; 95% CI: 0.64, 0.96; P , .001) and with age (r = 0.87; 95% CI: 0.62, 0.96; P , .001).
Conclusion: The MR imaging–derived SR may serve as a biomarker for cortical bone porosity that is potentially superior to BWC, but corroboration in larger cohorts is indicated.