Confounding factors in multi-parametric q-MRI protocol: A study of bone marrow biomarkers at 1.5 T

Object: The MRI tissue characterization of vertebral bone marrow includes the measurement of proton density fat fraction (PDFF), T1 and T-2* relaxation times of the water and fat components (T-1W, T-1F, T-2*W, T-2*F), IVIM diffusion D, perfusion fraction f and pseudo-diffusion coefficient D*.However, the measurement of these vertebral bone marrow biomarkers (VBMBs) is affected with several confounding factors.In the current study, we investigated these confounding factors including the regional variation taking the example of variation between the anterior and posterior area in lumbar vertebrae, B-1 inhomogeneity and the effect of fat suppression on f.Materials and methods: A fat suppressed diffusion-weighted sequence and two 3D gradient multi-echo sequences were used for the measurements of the seven VBMBs. A turbo flash B-1 map sequence was used to estimate B1 inhomogeneities and thus, to correct flip angle for T1 quantification. We introduced a correction to perfusion fraction f measured with fat suppression, namely f(PDFF).Results: A significant difference in the values of PDFF, f and f(PDFF), T1F, T-2* W and D was observed between the anterior and posterior region. Although, little variations of flip angle were observed in this anterior-posterior direction in one vertebra but larger variations were observed in head-feet direction from L1 to L5 vertebrae.Discussion: The regional difference in PDFF, f(PDFF) and T-2* W can be ascribed to differences in the trabecular bone density and vascular network within vertebrae.The regional variation of VBMBs shows that care should be taken in reproducing the same region-of-interest location along a longitudinal study. The same attention should be taken while measuring f in fatty environment, and measuring T-1. Furthermore, the MRI-protocol presented here allows for measurements of seven VBMBs in less than 6 min and is of interest for longitudinal studies of bone marrow diseases.