Quantification of Fatty Acid Composition in Short T2' Tissue Using MRI – Comparison of 3 T and 7 T

Sammanfattning: Purpose: To optimize a method and develop an algorithm to quantify Fatty Acid Composition (FAC) in bone marrow adipose tissue. By using different reconstruction models, compare different MRI systems (3 T and 7 T) to find the optimal field strength and reconstruction model for FAC quantification. Also, to develop a reference phantom with bone marrow-like characteristics. Method: Multi-echo GRE (Gradient Echo) data was acquired from a 3 T Siemens and a 7 T Philips system using phantoms made from different commercial butters. Proton density fat fraction (PDFF), Saturated fatty acids (SFA) and poly-unsaturated fatty acids (PUFA) were estimated using three different reconstruction models: constrained, free and original. Quantification agreement and method robustness was compared between systems and models using linear regression, Bland-Altman analysis, and relative errors. Results: When comparing the MRI-measurements with the theoretical values from the commercial butters, overall, PDFF is the most robust variable with high quantification agreement. In general, SFA was underestimated across all models for both field strengths, while PUFA in general was slightly overestimated. For both SFA and PUFA, comparing 3 T and 7 T, the 3 T system shows more robustness to errors and changes in the method than the 7 T system. Model-wise, the constrained model showed both more robust results and higher agreement than the free and original models. The algorithm that has been developed works for reconstruction of FAC images for 3 T Philips and 7 T Siemens MRI systems. The constructed reference phantoms may be used to estimate FAC at low T2' and different FAC, however, cannot be used to estimate FAC at different T2'. Conclusion: Commercial butters has proven to be a reasonable reference phantom to simulate tissue with short T2'. For future studies of FAC in bone marrow adipose tissue, 3 T with a constrained approach is preferred over 7 T. Keywords: MRI, fatty acid composition, bone marrow adipose tissue, fat quantification, field strength comparison