Comparison of T2-prepared 3D TSE with multi-echo spin-echo sequences for T2 mapping of thigh muscles in healthy volunteers
Elisabeth Klupp1, Dominik Weidlich2, Thomas Baum2, Barbara Cervantes2, Marcus Deschauer3, Hendrik Kooijman4, Ernst J. Rummeny2, Claus Zimmer1, Jan S. Kirschke1, and Dimitrios C. Karampinos2

1Neuroradiology, Technische Universität München, München, Germany, 2Radiology, Technische Universität München, München, Germany, 3Neurology, Technische Universität München, München, Germany, 4Philips Healthcare, Hamburg, Germany

Synopsis

There is a growing interest for applying T2 mapping for non-invasively tracking inflammatory changes in patients with neuromuscular diseases. T2 has been traditionally quantified using multi-echo spin-echo (MESE) sequences with known problems related to the refocusing pulses in presence of B1-inhomogeneity and slice profiles effects. The present work proposes the combination of an adiabatic T2-preparation with 3D TSE for B1-insenstive T2 mapping. The proposed method is compared with 2D-MESE and 3D-MESE, in terms of reproducibility on T2 quantification and sensitivity to B1 effects, in the thigh musculature of ten healthy subjects.

Purpose

Edematous alterations and fatty infiltration of skeletal muscles are main characteristics of neuromuscular diseases. Both pathologies increase the total muscle T2 relaxation-time1-3. T2 has been routinely quantified by multi-echo spin-echo (MESE) with known problems related to the refocusing pulses in presence of B1-inhomogeneity (3D-MESE) and additional slice profiles issues (2D-MESE)4. An adiabatic BIR-4 RF pulse previously used for generating T2-weighted contrast5 and recently adjusted with gaps (modified BIR-4)6 can be alternatively used for B1-insensitive T2 mapping. In addition, presence of fat within muscle can influence T2 quantification7. Purpose of this work was to compare a newly developed T2 mapping-sequence (T2Prep-3DTSE) with 2D- and 3D-MESE regarding T2 values, influence of B1-field and proton density fat fraction (PDFF) on it as well as reproducibility in thigh muscles of healthy subjects.

Methods

Ten young and healthy subjects (age: 27.2±1.7yrs, BMI: 22.9±4.3kg/m², n=5 male) with no history of diabetes, neuromuscular disorders and Quadriceps muscle injuries were recruited.

MR measurements: The bilateral thigh muscles were scanned on a 3.0T system (Ingenia, Philips Healthcare) using anterior and posterior coil arrays. The FOV was centered at the femur mid-length using greater trochanter and tibial plateau as anatomical landmarks7. T2 mapping was performed using 2D-MESE, 3D-MESE and T2prep-3DTSE. Three subjects were scanned three times to assess the reproducibility of the three methods. The T2prep-3DTSE used T2 preparation based on a modified BIR-4 RF pulse, where two gaps with equal duration were introduced to achieve a module with variable TE (Fig. 1). For the MESE-sequences standard implementations were used. SPAIR was used for all sequences. The common sequence parameters were: FOV = 42×26×12cm3, TR/TE = 1.5 s/20ms and acquisition voxel = 2×2×4mm3. The TSE sequence had a TSE factor = 50 and T2Prep durations of 20/40/60/80ms. In MESE sequences echoes have been acquired in 10ms steps. Additionally, B1-map was acquired with dual TR method, B0- and PDFF-maps were measured based on a 6-echo gradient echo sequence.

Data analysis: Data at the same echo times for all three T2 mapping sequences was fitted by a 2-parameter fit. The first echo was excluded for MESE data. B0 and PDFF maps were determined based on a signal model accounting for the multi-peak fat spectrum and single T2* decay effects. Two muscles (Rectus femoris, Vastus lateralis) of the Quadriceps muscle, mainly affected in neuromuscular disorders, were selected. ROIs were drawn manually in the interior of these muscles avoiding vessels and fasciae. To assess reproducibility errors of the different T2 mapping-sequences, root mean square coefficients of variation (RMSCV)8 were computed. T2 values of the different T2 mapping sequences were compared and correlations were calculated between T2 values and PDFF and B1.

Results

Representative T2 maps and the respective B1-map are shown in Fig. 2. When the B1 error was large, T2 was significantly higher with the MESE-sequences compared to T2Prep-3DTSE (arrow in Fig. 2). When the B1 error was small, T2 values from 2D-MESE were higher than T2 values from 3D-MESE and T2Prep-3DTSE (circle in Fig. 2). Significant differences between T2 values of T2Prep-3DTSE and T2 values of 2D-MESE respectively 3D-MESE were found in all four muscles with the lowest values for T2Prep-3DTSE (p<0.05; Fig. 3). No significant correlations were found between PDFF and T2 values. The left Rectus femoris muscle showed the highest variance of T2 values (T2Prep-3DTSE: 30.94±1.28ms, 3D-MESE: 41.7±7.8ms; 2D-MESE: 36.6±2.4ms) and B1-field (54.1±15.6%) with RMSCVs: T2Prep-3DTSE: 1.7%; 3D-MESE: 19.9%; 2D-MESE: 1.8% (Fig. 4). Significant negative correlations between T2values and B1-field were found in the left Rectus femoris muscle for 3D-MESE (r=-0.86, p<0.001) and 2D-MESE (r=-0.54, p=0.033), but not for T2Prep-3DTSE (p=0.71; Fig. 5).

Discussion & Conclusion

The results show a low reproducibility of the T2 values measured by 3D-MESE. Both, T2 values of 3D-MESE and 2D-MESE are strongly affected by an inhomogeneous B1 field. MESEs suffer from the presence of stimulated echoes, induced by B1 effects (2D- or 3D-MESE) and slice profile effects (2D-MESE). Recent work has shown that the application of extended phase graphs can compensate for stimulated echoes due to B1 effects in MESE-based T2 quantification9. However, MESE can lead to T2 overestimation in fatty infiltrated muscles (see subcutaneous fat region; Fig. 2), whereas T2prep-3DTSE leads to T2 values in subcutaneous fat in the same range as healthy muscle (Fig. 2). The remaining fat signal after SPAIR in the three sequences requires further investigations. In conclusion, we proposed the use of a B1-insensitive T2-prepared 3D TSE for robust muscle T2 mapping and showed that the proposed method gave reliable and reproducible T2 values in the thigh muscles of ten healthy volunteers.

Acknowledgements

The present work was supported by Philips Healthcare and the European Union (ERC-StG-2014 iBack).

References

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Figures

Fig. 1: Pulse sequence diagram for T2prep-3DTSE, showing amplitude and frequency modulation of the modified BIR4 T2 preparation module, spoiler and readout. The dotted lines indicate the position of the gaps in the T2 preparation.

Fig. 2: B1-map and T2-maps in one representative subject. The red arrow indicates an area with low B1, the red circle a region with B1 close to 100%. With low B1, 2D- and especially the 3D-MESE are significantly affected. With no B1-error, 2D-MESE shows higher T2 values than 3D MESE and T2Prep-3DTSE.

Fig. 3: Means +/- standard deviations of the three different T2 mapping methods (in ms, y-axis) in ten young and healthy subjects presented for the four investigated muscles of the thigh region (x-axis).

Fig. 4: Reproducibility of the three different T2mapping methods in the left Rectus femoris muscle presented as mean +/- standard deviation (in ms, y-axis) of the three scanning times for the three participating subjects (x-axis). RMSCVs (in %) are given in the text.

Fig. 5: Relationship between the B1-field (in %, x-axis) and the three different T2 mapping methods (in ms, y-axis) in the left Rectus femoris muscle presented for 16 scans (n=10 young and healthy subjects plus 6 repeated measures).



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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