Introduction

Chemical shift-encoded magnetic resonance imaging (CSE-MRI) is a useful method for evaluating proton density fat fraction (PDFF) across various organs, such as the liver and vertebral bone. This method utilizes a low-flip angle to minimize T₁ bias (i.e., differences in T₁ values between water and fat), which affects the PDFF. Different arm positions during MRI scan can significantly affect the radio frequency magnetic field (B₁) distribution, which caused variability in the flip angle and PDFF.¹ Although simply changing the arm position during spine imaging can improve B₁ heterogeneity,² its effects on the PDFF of vertebral bone marrow are yet to be determined; therefore, this study sought to evaluate the influence of arm position on the PDFF in vertebral bone marrow using CSE-MRI.

Methods

This study was approved by the Institutional Review Board, and written informed consent was obtained from all study participants. All scans were performed using a 3-T MRI scanner (Skyra; Siemens Healthcare, Erlangen, Germany). This study enrolled 15 volunteers (12 men and 3 women; average age, 39.6 years [22–58 years]) who underwent lumbar spine scan using CSE-MRI with different arm positions (elevated and side arm positions). Scans were performed twice in each position (Fig. 1). The scan parameters were as follows: TR, 9.0 ms; first TE and delta TE, 1.18 ms; number of echoes, 6; flip angle, 4 degrees; field of view, 300 mm × 300 mm; matrix, 160 × 160; slice thickness 4 mm; parallel imaging, CAIPIRINHA; parallel imaging factor, 2 × 2; and scan time, 22 s. Participants were instructed to hold their breath with each inhalation for each scan. Regions of interest were established on each lumbar vertebra (L1–L5) in the central slice of the PDFF map. To determine whether the variability in the observed PDFF was due to arm position, the variability of the CSE-MRI sequence itself needed to be evaluated; thus, the PDFFs of two consecutive scans in each arm position were initially compared to evaluate the within-examination variability using the Bland–Altman plot and Wilcoxon test. Thereafter, the PDFFs in the side arm and elevated arm positions were also compared. A P value of ≤ 0.05 indicated statistical significance.

Results and Discussion

Variability in the PDFF between two sequential scans in different arm positions is shown in the Bland–Altman plot (Fig. 2). The mean difference was 0.07% (95% confidence interval: −0.09%–0.22%), whereas the limit of agreement (the mean difference ± 1.96 standard deviations) was −1.85%–1.99%. PDFFs at all lumbar vertebrae (L1–L5) did not significantly differ between scans (all P > 0.05) (Table 1). The Bland–Altman plots that compared the PDFF between different arm positions are shown in Fig. 3. The mean difference was 0.77% (95% confidence interval: 0.50%–1.04%), whereas the limit of agreement was −1.50%–3.05%. PDFFs at all lumbar vertebrae (L1–L5) were slightly but significantly larger in the elevated arm position than in the side arm position (all P < 0.05) (Table 2). The aforementioned results suggested that PDFF variability between different arm positions exceeds the variability of the PDFF sequence itself within two consecutive scans.

Conclusion

Although arm position significantly affects the PDFF in vertebral marrow during MRI, the change was exceptionally small. This suggests that PDFF measurements using CSE-MRI in the lumbar spine are not affected by clinically problematic PDFF variability caused by B₁ heterogeneity with different arm positions.

Acknowledgements

No acknowledgement found.