3T chemical shift-encoded MRI: Detection of altered proximal femur marrow adipose tissue composition in glucocorticoid users and validation with magnetic resonance spectroscopy

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 $T_2^{*}$ (-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 $T_2^{*}$ (-5.4 msec, P < 0.05) in fTRIANGLE; and decreased $T_2^{*}$ (-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.