Date Published: February 14, 2017
Publisher: Public Library of Science
Author(s): Elisabeth Klupp, Dominik Weidlich, Sarah Schlaeger, Thomas Baum, Barbara Cervantes, Marcus Deschauer, Hendrik Kooijman, Ernst J. Rummeny, Claus Zimmer, Jan S. Kirschke, Dimitrios C. Karampinos, Miklos S. Kellermayer.
To propose a T2-prepared 3D turbo spin echo (T2prep 3D TSE) sequence for B1-insensitive skeletal muscle T2 mapping and compare its performance with 2D and 3D multi-echo spin echo (MESE) for T2 mapping in thigh muscles of healthy subjects.
The performance of 2D MESE, 3D MESE and the proposed T2prep 3D TSE in the presence of transmit B1 and B0 inhomogeneities was first simulated. The thigh muscles of ten young and healthy subjects were then scanned on a 3 T system and T2 mapping was performed using the three sequences. Transmit B1-maps and proton density fat fraction (PDFF) maps were also acquired. The subjects were scanned three times to assess reproducibility. T2 values were compared among sequences and their sensitivity to B1 inhomogeneities was compared to simulation results. Correlations were also determined between T2 values, PDFF and B1.
The left rectus femoris muscle showed the largest B1 deviations from the nominal value (from 54.2% to 92.9%). Significant negative correlations between T2 values and B1 values were found in the left rectus femoris muscle for 3D MESE (r = -0.72, p<0.001) and 2D MESE (r = -0.71, p<0.001), but not for T2prep 3D TSE (r = -0.32, p = 0.09). Reproducibility of T2 expressed by root mean square coefficients of variation (RMSCVs) were equal to 3.5% in T2prep 3D TSE, 2.6% in 3D MESE and 2.4% in 2D MESE. Significant differences between T2 values of 3D sequences (T2prep 3D TSE and 3D MESE) and 2D MESE were found in all muscles with the highest values for 2D MESE (p<0.05). No significant correlations were found between PDFF and T2 values. A strong influence of an inhomogeneous B1 field on the T2 values of 3D MESE and 2D MESE was shown, whereas the proposed T2prep 3D TSE gives B1-insensitive and reproducible thigh muscle T2 mapping.
Acute inflammatory edematous alterations of skeletal muscles, reflecting disease activity, as well as fatty infiltration of chronically affected muscles are two main characteristics of neuromuscular diseases [1–3]. Conventional diagnostic magnetic resonance imaging (MRI), based on T1-weighted, T2-weighted and Short Tau Inversion Recovery (STIR) sequences, is only able to detect qualitative pathological changes in the muscle tissue. Semi-quantitative scales exist for judging the extent of fatty infiltration or edematous muscular alterations based on conventional MR images [4–6]. However, the evaluation of conventional images can become strongly dependent on the reader’s judgment. There is therefore an emerging need for objective evaluation of fatty infiltration and inflammatory skeletal muscle alterations based on quantitative imaging.
Fig 2a shows the simulated large dependency of the 2D MESE sequence on the transmit B1 field for a range of B1 values between 60% and 120%. The deviation from the real T2 value is increased with decreasing B1 field amplitude. In Fig 2b the performance of the 3D MESE sequence is illustrated. The estimated T2 value is also dependent on the B1 field amplitude but the simulated T2 value is stable over a much wider range of possible B1 errors. Fig 2c shows a stable T2 quantification for the employed T2-preparation. The range of B0 errors over which the employed T2-preparation provides a stable T2 quantification decreases as the B1 value decreases (Fig 2c). For B0 offsets between -60 Hz and 60 Hz (typical range for thigh muscle imaging at 3 T), the proposed T2-preparation results in stable T2 quantification for B1 varying in the entire range between 60% and 120%.
The present work proposed a newly developed T2 mapping sequence (T2prep 3D TSE) for skeletal muscle T2 mapping and compared the developed T2 mapping sequence with standard 2D and 3D MESE in healthy thigh muscles. The results showed a dependency of the T2 values from 2D MESE and 3D MESE on transmit B1 field, whereas the developed T2prep 3D TSE was less affected by the transmit B1 field and showed a good reproducibility in the thigh musculature of young and healthy subjects.
In conclusion, the present work proposed the use of a T2-prepared 3D TSE for robust muscle T2 mapping and was able to show that this method gave B1-insensitive and reproducible T2 values in the thigh muscles of young and healthy volunteers. T2-prepared 3D TSE might therefore be useful as an alternative to standard T2 mapping sequences in settings where strong B1 errors are expected (i.e. in extremities and at high field strengths).