Dictionary-based T2-mapping with Multi-Echo Turbo-Spin Echo

Multi-echo turbo-spin echo (ME-TSE) is a pulse sequence commonly used for T2 mapping in MRI. As compared to other pulse sequences such as MESE, ME-TSE is largely faster. It has been previously shown that dictionary-based T2-mapping can be used to provide accurate T2 values from MESE datasets but the corresponding accuracy on ME-TSE datasets has never been assessed. In the present study, we aimed to investigate the impact of combining effective echo signals in a ME-TSE pulse sequence on the accuracy of T2 mapping using a dictionary-based reconstruction method. We initially compared three different combinations of echo signals on phantom scans and identified the corresponding differences. We then determined the best combination among them. In the pulse sequence with an echo train length (ETL) of 3 and 3 contrasts, we found that the most accurate and homogeneous combination was achieved when the first echo was assigned to the center in the first contrast, the second echo in the second contrast, and the third echo in the third contrast. Next, we compared the use of dictionaries generated for a single slice versus multiple slices (N = 5) in the reconstruction process for this specific combination. The dictionaries were generated using a Bloch simulator, taking into account the saturation effect between slices during dictionary generation for the multi-slice case. Our results show that using single-slice dictionaries for reconstructing T2 phantom maps with no gap between slices can lead to variations in T2 values between slices. However, this variation can be reduced when using a multi-slice dictionary so that more accurate T2 values can be obtained.