T 2 mapping with magnetization-prepared 3D TSE based on a modified BIR-4 T 2 preparation

The purpose of this work was to investigate the performance of the modified BIR-4 T preparation for T mapping and propose a method to remove T quantification errors in the presence of large B and B offsets. The theoretical investigation of the magnetization evolution during the T preparation in the presence of B and B offsets showed deviations from a mono-exponential T decay (two parameter fit). A three parameter fit was used to improve T accuracy. Furthermore, a two parameter fit with an additional saturation preparation scan was proposed to improve T accuracy and precision. These three fitting methods were compared based on simulations, phantom measurements and an in vivo healthy volunteer study of the neck musculature using a 3D TSE readout. The results based upon the pure two parameter fit overestimated T in regions with high B offsets (up to 40% in phantoms). The three parameter fit T values were robust to B offsets but with higher standard deviation (up to 40% in simulations). The two parameter fit with the saturation preparation yielded high robustness towards B offsets with a noise performance comparable to that of the two parameter fit. In the volunteer study the T values obtained by the pure two parameter fit showed a dependence on the field inhomogeneities, whereas the T values from the proposed fitting approach were shown to be insensitive to B offsets. The proposed method enabled accurate and precise T mapping in the presence of large B and B offsets.