Introduction

MR thermometry plays an important role for the real-time monitoring of MRI-guided ablation therapies. Fast MR thermometry with short TR (<100 ms)¹ allows the acquisition of motion-free images in moving organs using single-shot EPI. Precision of MR-thermometry is directly related to TE which should ideally be matched to the T₂* of the targeted tissue. Multi-echo sequences can improve the precision of thermometry as previously demonstrated using GRE or segmented EPI acquisitions². In this study, we sought to investigate the potential of dual-echo (dTE) single-shot EPI to improve the precision of MR-thermometry and combine it with simultaneous multi-slice (SMS) imaging³ to provide increased spatial coverage.

Methods

A diagram of the proposed prototype SMS dual-echo single-shot EPI sequence is shown in Figure 1. SMS EPI was achieved using a blipped CAIPI approach⁴. The imaging parameters were optimized so the second TE matches the targeted T₂* time. Temperature maps were obtained from the phase images using the PRFS method⁵. Temperature maps were reconstructed for each TE and were combined using weights as described by Odéen and Parker²:

$$$w_j (\bar{r})=a (\textrm{TE}_j |m_j (\bar{r})|)^2$$$,

where $$$\textrm{TE}_j$$$ was the echo time of the j-th image, $$$m_j (\bar{r})$$$ the complex MRI image value at position $$$\bar{r}$$$, and $$$a$$$ the normalization factor such that $$$∑_j w_j (\bar{r})=1$$$.

The sequence was evaluated in a standardized phantom (T1mes)⁶ using a 1.5T scanner (MAGNETOM Aera, Siemens Healthcare, Erlangen, Germany). Of this phantom, 4 vials were considered that had T₂^* values between 41 and 47 ms (vials D, F, G, and I, with T₁ values between 300 and 1000 ms).

In the first experiment, the prototype dTE EPI sequence was acquired in single band mode in a single slice (TE1 = 20 ms, TE2 = 57 ms, TR = 117 ms, FOV 180x180 mm², voxel size 1.6x1.6 mm², slice thickness 6 mm, BW 1540 Hz/pixel, flip angle 35˚, partial Fourier 75%, GRAPPA factor 2, number of dynamics: 150) and compared to a conventional single-echo EPI acquisition with TE = 20 ms, TR = 80 ms and otherwise matching imaging parameters. Scans in the second experiment were acquired with the same parameters, but now acquiring 2 slices using SMS factor 2 for both the prototype dTE and single TE acquisitions (slice distance 6 mm).

Stability maps were calculated as the standard deviation over time of the temperature in each voxel. Stability maps were produced using temperature maps derived from single echo only or using the proposed combined dTE.

Results

In the single slice experiment, combining the dual-echo data always led to the best temperature stability (Figure 2), with an average increase in stability of 19% compared to the temperature stability obtained from using only the 2^nd echo in the dTE sequence. The faster conventional sequence always showed the lowest stability.

In the experiment with SMS, the combined dual-echo data also outperformed the other acquisitions in all vials (Figure 3). Although the overall stability was lower than in the single band experiment, the improvement due to adding TE2 to TE1 was similar at 21% and 20% in these slices.

Discussion

Temperature stability using dTE was consistently highest in both experiments. TE2 was longer than the T₂* of the vials, it remains to be seen if larger improvements are possible with closer matching TE2. These experiments were conducted in a static phantom, the benefit of the dTE single-shot EPI technique in-vivo remains to be demonstrated.

Conclusion

MR thermometry using dual-echo single-shot EPI resulted in better temperature stabilities than the single echo acquisitions. This sequence was successfully combined with SMS which also resulted in higher temperature stabilities than the single echo acquisitions.

Acknowledgements

This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) grant (EP/R010935/1), the British Heart foundation (BHF) grant (PG/19/11/34243), The Innovate UK grant (68539), the Wellcome EPSRC Centre for Medical Engineering at Kings College London (WT 203148/Z/16/Z), the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy’s and St Thomas’ NHS Foundation Trust and King’s College London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.

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