Synopsis

Brachial plexus is anatomically complex and may be involved in a variety of pathologies that leads to significant morbidity. MR neurography, based on diffusion-prepared MR neurography (SHINKEI), plays a major role in the diagnostic work-up of plexus pathologies. To solve the problems that caused by the current fat suppression techniques (SPAIR and STIR) with SHINKEI, we developed motion-sensitized driven inversion (MSDI). MSDI could simultaneously suppress fat, flow and muscle signals using only one pre-pulse module. SHINKEI with MSDI provides uniform fat suppression in the brachial plexus, as in STIR, with no significant decrease in SNR compared to SPAIR.

Purpose

The brachial plexus is anatomically complex and may be involved in a variety of pathologies that leads to significant morbidity. MR neurography plays a major role in the diagnostic work-up of plexus pathologies, substantially improving early diagnosis and patient management^1-3.

Recently, we proposed a diffusion-prepared MR neurography sequence (nerve-SHeath signal increased with INKed rest-tissue RARE Imaging: SHINKEI)⁴. By using this sequence, high-quality MR neurography has been obtained in the brachial plexus and the lumbosacral plexus^2-5. SHINKEI can be combined with different types of fat saturation techniques. Spectral attenuated inversion recovery (SPAIR) or short tau inversion recovery (STIR) are frequently used in the brachial plexus. Although SPAIR-SHINKEI facilitates excellent visibility of the nerves with high signal-to-noise ratio (SNR), image quality is often compromised by poor fat suppression. The origin of failed fat suppression can be found in poor homogeneity of the magnetic field (B0), which is mainly caused by susceptibility differences between air and tissue around the neck⁶. Although B0 image-based shimming can be applied, this still results in suboptimal homogeneity using standard shimming routines only³. On the other hand, STIR-SHINKEI provides uniform fat suppression; however, it suffers from significant decrease of SNR of the nerves compared with SPAIR-SHINKEI⁶.

To solve these problems caused by the fat suppression technique, we developed a new pre-pulse, motion-sensitized driven inversion (MSDI), to improve the image-quality of SHINKEI MR neurography in the brachial plexus.

Methods

Theory and pulse sequence:

Fig.1 shows the differences between MSDE and MSDI. MSDE is based on the improved MSDE (iMSDE)⁷, where additional eddy-current compensation is adopted in front of 90-degree pulse⁸. In MSDI the phase of the final 90-degree pulse is inverted.

Conventional SHINKEI consists of both fat suppression IR (SPAIR or STIR) and MSDE preparation, with a 3D T2-weighted TSE sequence to acquire the data. The MSDE module has a long duration to suppress flow and muscle signal simultaneously⁴ [Fig.2a]. MSDI-prepared SHINKEI uses only one pre-pulse module, but this can suppress the fat tissues by inverting the fat signals due to the final 90-degree pulse. Consequently, MSDI enables the suppression of fat, flow and muscle signals simultaneously [Fig.2b].

Experiments:

A total of five volunteers were examined on 3.0T systems (Ingenia, Philips Healthcare). The study was approved by the local IRB, and written informed consent was obtained from all subjects.

(1) Parameter optimization of MSDI-SHINKEI

To optimize the scan protocol of MSDI-SHINKEI for the brachial plexus, firstly we investigated the optimal MSDI delay time for suppressing fat signals. Subsequently we compared the two types of adiabatic refocusing pulses (the conventional hyperbolic secant and a broadband offset independent trapezoid) to stabilize the depiction of the nerves over the full field-of-view (FOV).

(2) Comparison of MSDI-SHINKEI with conventional SPAIR/STIR methods

To validate the quality of neurography images obtained by MSDI-SHINKEI, coronal brachial plexus images were acquired and were quantitatively compared with conventional methods. Neurography images were compared with conventional SHINKEI with SPAIR/STIR for image quality. We measured/calculated the SNR of the nerves (average value of both the left and right dorsal root ganglion from C5 to C8) by using noise measurement method⁹. The SNR were assessed by using one-way repeated measures analysis of variance and the post-hoc Tukey test. The imaging parameters common to all methods were: TR=2500ms, TE=63ms, ETL=100, echo spacing=4.0ms, voxel size=1x1x1mm³, 3D-volume size=220mm(FH) 310mm(RL) 100mm(AP), MSDE Venc=1cm/s³, MSDE prep-time=50ms and scan duration=5m24s.

Results and Discussion

Fig.3(a) shows the compassion of MSDI delay time. A delay time of 200ms is found to be optimal for fat suppression over the full FOV. Fig.3(b) shows the comparison of the two types of MSDI refocusing pulses. A broadband adiabatic pulse improves the depiction of the nerves in the right side of brachial plexus. Fig.4 shows the representative images of MSDI-SHINKEI images by using optimal parameters. MSDI-SHINKEI clearly shows the brachial plexus structure with high SNR and uniform fat suppression.

Fig.5 shows the comparison of representative images and SNR among SPAIR-SHINKEI, STIR-SHINKEI and MSDI-SHINKEI. The fat suppression using MSDI is better than using SPAIR. The SNR of MSDI is significantly better than that of STIR. Furthermore, SNR of MSDI showed similar values compared to that of SPAIR. There was no significant difference. MSDI-SHINKEI thus retains the fat suppression homogeneity of STIR-SHINKEI, while maintaining the high SNR of SPAIR-SHINKEI.

Conclusion

Acknowledgements

References

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