Synopsis

A total of 39 volunteers underwent an imaging and spectroscopy protocol on a 3T Ingenia Philips system with an axial 3D spoiled-gradient multiple echo sequence on abdominal region and a set of three STEAM sequences acquired on subcutaneous adipose tissue, visceral adipose tissue and liver. The quantification of Proton density fat fraction (PDFF), proportion of saturated (SFA), monounsaturated (MUFA) or polyunsaturated (PUFA) fatty acids from both MRI and MRS methods were compared. Good correlation with a little bias was found for the liver PDFF. Values of PUFA, MUFA and SFA from both techniques were poorly correlated.

Introduction

Subjects and methods

Imaging and spectroscopy protocol

A total of 39 healthy male volunteers underwent an imaging and spectroscopy protocol on a 3T Ingenia Philips system constituted by an axial 3D spoiled-gradient multiple echo sequence (MGE) on abdominal region (encompassing lumbar L1 to L5 and the liver) and a set of three STEAM sequences acquired on SAT, VAT and liver (parameters in Table 1 and Figure 1).

Data processing

The segmentation of adipose tissue and the quantification of FA (PUFA, MUFA and SFA) from a MGE has been previously described^2,5. On MR images, the liver volume was manually defined on a unique slice, SAT and VAT volumes were automatically segmented on multiple slices (21 ± 2) from lumber L2 to L4. We use the same spectral model function to process the imaging and spectroscopic signal to quantify FA composition. Peak amplitude were directly expressed by the parameters characterizing a triglyceride molecular structure (chain length, number of double bonds and number of methylene interrupted by double bond)⁶. This fitting model has been described in previous work⁷ and has given robust results in vitro and in vivo.

Results comparison

PDFF, PUFA, MUFA and SFA quantified in each single MRS volume (for the liver, only PDFF) were compared to the mean value from MRI calculated on the total segmented SAT and VAT volumes. The correlation was evaluated by the coefficient of determination r² and bias was measured by Bland-Altman plot.

Results/Discussion

PDFF in liver

On 39 subjects, 15 MRS measurements were null and were removed from the analyses. Hepatic fat is weak in healthy subjects and difficult to detect. Two subjects presented an abnormal level of PDFF in the liver (PDFF >10%) with MRI measurements. Thus analyses were made with and without these subjects (Figure 2). As demonstrated in the literature, Figures 2-A and C show good correlation between MRI and MRS measurements (figure C, r²=0.76 vs figure A, r²=0.87). However, MRI-based measurements slightly overestimated PDFF compared to MRS measurements (bias=1.9 for the Figure2-B and bias=1.7 D).

Fatty acid composition in SAT and VAT

On 39 subjects, 9 MRS VAT spectra were not analyzable because of poor quality and were removed from the analyses. Regarding MRI results from the whole segmented volume versus the MRS results, the comparison of FA composition of SAT and VAT appeared to be substantially equivalent in average (Figure3). In both cases, paired t-tests showed significant difference between SAT and VAT FA composition (p<0.001). However, MRS and MRI FA composition estimations were poorly correlated (r²=0.43 for PUFA, r²=0.18 for MUFA, r²=0.36 for SFA, n=69). PUFA MRI values were significantly higher than MRS values (bias=+4.1, paired t-test p<0.001). SFA MRI values were significantly higher than MRS values (bias=-4.2, paired t-test p=0.005). No significant difference in MUFA measurements was found. The inter-subject variability was higher in localized MRS measurements than in the MRI measurements which were averaged over the whole adipose tissue volumes.

Conclusion

Acknowledgements

References

1. OMS Infobase globale: www.who.int
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