Osteoporosis is a disease of weak bone and increased fracture risk caused by low bone mass and microarchitectural deterioration of bone tissue. The standard-of-care test used to diagnose osteoporosis, dual-energy x-ray absorptiometry (DXA) estimation of areal bone mineral density (BMD), has limitations as a tool to identify patients at risk for fracture and as a tool to monitor therapy response. Magnetic resonance imaging (MRI) assessment of bone structure and microarchitecture has been proposed as another method to assess bone quality and fracture risk in vivo. MRI is advantageous because it is noninvasive, does not require ionizing radiation and can evaluate both cortical and trabecular bone. In this review article, we summarize and discuss research progress on MRI of bone structure and microarchitecture over the last decade, focusing on in vivo translational studies. Single center, in vivo studies have provided some evidence for the added value of MRI as a biomarker of fracture risk or treatment response. Larger, prospective, multicenter studies are needed in the future to validate the results of these initial translational studies.

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Background Osteoporosis (OP) results in weak bone and can ultimately lead to fracture. Drugs such as glucocorticoids can also induce OP (glucocorticoid‐induced osteoporosis [GIO]). Bone marrow adipose tissue composition and quantity may play a role in OP pathophysiology, but has not been thoroughly studied in GIO compared to primary OP. Purpose/Hypothesis Chemical shift‐encoded (CSE) MRI allows detection of subregional differences in bone marrow adipose tissue composition and quantity in the proximal femur of GIO compared to OP subjects and has high agreement with the reference standard of magnetic resonance spectroscopy (MRS). Study Type Prospective. Subjects In all, 18 OP and 13 GIO subjects. Fields Strength 3T. Sequence Multiple gradient‐echo, stimulated echo acquisition mode (STEAM). Assessment Subjects underwent CSE‐MRI in the proximal femurs, and for each parametric map regions of interest (ROIs) were assessed in the femoral head (fHEAD), femoral neck (fNECK), Ward's triangle (fTRIANGLE), and the greater trochanter (GTROCH). In addition, we compared CSE‐MRI against the reference standard of MRS performed in the femoral neck and Ward's triangle. Statistical Tests Differences between OP/GIO were investigated using the Mann–Whitney nonparametric test. Bland–Altman methodology was used to assess measurement agreement between CSE‐MRI and MRS. Results GIO compared with OP subjects demonstrated: decreased monounsaturated fat fraction (MUFA) (–2.1%, P < 0.05) in fHEAD; decreased MUFA (–3.8%, P < 0.05), increased saturated fat fraction (SFA) (5.5%, P < 0.05), and decreased normalT2* (–3.8 msec, P < 0.05) in fNECK; decreased proton density fat fraction (PDFF) (–15.1%, P < 0.05), MUFA (–9.8%, P < 0.05), polyunsaturated fat fraction (PUFA) (–1.8%, P < 0.01), increased SFA (11.6%, P < 0.05), and decreased normalT2* (–5.4 msec, P < 0.05) in fTRIANGLE; and decreased normalT2* (–1.5 msec, P < 0.05) in GTROCH. There was high measurement agreement between MRI and MRS using the Bland–Altman test. Data Conclusion 3T CSE‐MRI may allow reliable assessment of subregional bone marrow adipose tissue (bMAT) quantity and composition in the proximal femur in a clinically reasonable scan time. Glucocorticoids may alter the lipid profile of bMAT and potentially result in reduced bone quality. Level of Evidence: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:490–496.

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