Measuring bone mineral density with fat-water MRI: comparison with computed tomography.

PURPOSE

To develop a method for measuring bone mineral density (BMD) with MRI, and to validate this method against quantitative computed tomography (QCT).

MATERIALS AND METHODS

A mathematical relationship between signal intensities from proton-density-weighted in-phase images generated by multi-fat-peak T2*-IDEAL MRI and BMD was derived using a set of calibration standards constructed from various concentrations of hydroxyapatite in water. Using these standards, the relationship between hydroxyapatite concentration and MRI signal intensity was examined. A T2*-IDEAL protocol was performed on the patella of 5 volunteers and the signal model was used to compute BMD of all voxels of the patella. The BMD data were validated by obtaining QCT scans of the same patella, computing QCT BMD of all voxels, and comparing the MRI and QCT BMD data by performing linear regression analysis on a voxel-by-voxel basis.

RESULTS

A strong linear correlation between hydroxyapatite concentration of the calibration standards and MRI signal intensities was observed (r = 0.98; P < 0.01). In the patella, BMD measurements (N = 28796 voxels) from the MRI signal model were significantly correlated with those from QCT (r = 0.82; P < 0.001; slope = 1.02; and intercept = -0.26).

CONCLUSION

A standardized phantom consisting of hydroxyapatite and water can be used to accurately quantify BMD in vivo using MRI.