Proton density fat fraction MRI of vertebral bone marrow: Accuracy, repeatability, and reproducibility among readers, field strengths, and imaging platforms

Background Chemical shift‐encoding based water‐fat MRI is an emerging method to noninvasively assess proton density fat fraction (PDFF), a promising quantitative imaging biomarker for estimating tissue fat concentration. However, in vivo validation of PDFF is still lacking for bone marrow applications. Purpose To determine the accuracy and precision of MRI‐determined vertebral bone marrow PDFF among different readers and across different field strengths and imager manufacturers. Study Type Repeatability/reproducibility. Subjects Twenty‐four adult volunteers underwent lumbar spine MRI with one 1.5T and two different 3.0T MR scanners from two vendors on the same day. Field Strength/Sequence 1.5T and 3.0T/3D spoiled‐gradient echo multipoint Dixon sequences. Assessment Two independent readers measured intravertebral PDFF for the three most central slices of the L1–5 vertebral bodies. Single‐voxel MR spectroscopy (MRS)‐determined PDFF served as the reference standard for PDFF estimation. Statistical Tests Accuracy and bias were assessed by Pearson correlation, linear regression analysis, and Bland–Altman plots. Repeatability and reproducibility were evaluated by Wilcoxon signed rank test, Friedman test, and coefficients of variation. Intraclass correlation coefficients were used to validate intra‐ and interreader as well as intraimager agreements. Results MRI‐based PDFF estimates of lumbar bone marrow were highly correlated ( r 2 = 0.899) and accurate (mean bias, –0.6%) against the MRS‐determined PDFF reference standard. PDFF showed high linearity ( r 2 = 0.972–0.978) and small mean bias (0.6–1.5%) with 95% limits of agreement within ±3.4% across field strengths, imaging platforms, and readers. Repeatability and reproducibility of PDFF were high, with the mean overall coefficient of variation being 0.86% and 2.77%, respectively. The overall intraclass correlation coefficient was 0.986 as a measure for an excellent interreader agreement. Data Conclusion MRI‐based quantification of vertebral bone marrow PDFF is highly accurate, repeatable, and reproducible among readers, field strengths, and MRI platforms, indicating its robustness as a quantitative imaging biomarker for multicentric studies. Level of Evidence: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1762–1772.