In vivo quantitative assessment of proximal femoral cortical bone microstructure using double-echo ultrashort echo time magnetic resonance imaging

Background: Quantitative assessment of cortical bone microstructure is crucial for the evaluation of osteoporosis, yet current clinical methods such as dual-energy X-ray absorptiometry (DXA) have many limitations. The in vivo quantitative assessment without radiation can be achieved by ultrashort echo time (UTE) magnetic resonance imaging (MRI), where double-echo UTE has high clinical feasibility. However, related studies have mainly focused on distal extremities, and there is a lack of in vivo studies on the proximal femur. This cross-sectional study, as a preliminary study for in vivo quantitative assessment of proximal femoral cortical bone in healthy adults by double-echo UTE MRI, aimed to evaluate the repeatability and explore the impact of potential influencing factors on UTE measurements, thereby providing a reference for the further clinical application of this technique.

Methods: Healthy volunteers without osteoporosis risk factors were enrolled and underwent double-echo UTE MRI of the proximal femur. Porosity index (PI) and cortical bone thickness (CbTh) were obtained manually by two radiologists independently on double-echo UTE images using image processing software. Repeatability of PI and CbTh measurements were evaluated using intra-class correlation coefficient (ICC) analysis. PI and CbTh of different limbs and sexes were compared, and P<0.05 indicated statistical significance for these analyses. Correlations of PI and CbTh with age and body mass index (BMI) were assessed by Pearson or Spearman correlation coefficient as well as the partial Pearson correlation coefficient. Curve estimation was used to assess non-monotonic variable correlations in scatterplots. For these analyses, P<0.025 indicated statistical significance according to the Bonferroni correction.

Results: A total of 52 healthy cases (33 males, 19 females) aged 22-55 years were included for analysis, where no statistical difference in age was found between sexes (P=0.586) and BMI of males was greater than that of females (P=0.007). The repeatability of PI and CbTh measurements was excellent (ICC 0.985 and 0.943, respectively). The proximal femoral cortical PI was greater on the non-dominant side (P<0.001). PI was greater in males than in females (non-dominant: P<0.001, dominant: P=0.032) and CbTh was greater on the non-dominant side in males than in females (P=0.036) after excluding the effect of confounding factor (BMI). PI on the dominant side was positively correlated with BMI in males (r=0.535, P=0.001), and CbTh on the non-dominant side was positively correlated with BMI in males (r=0.482, P=0.005). There was a U-shaped curve trend between dominant side cortical PI and age in females (y = 65.32 - 1.88x + 0.02x², R²=0.348, P=0.033), although there was no statistical significance after Bonferroni correction.

Conclusions: Double-echo UTE MRI enables in vivo quantitative assessment of proximal femoral cortical microarchitecture with excellent repeatability. We identified the effects of limb dominance, sex, age, and BMI on UTE measurements in healthy adults, which can serve as a reference for future in vivo studies on proximal femoral cortical bone and is essential before clinical application.