Introduction

Currently, theT2*-weighted Multi-Echo Data Imaging Combination (MEDIC) sequence is the most commonly used sequence for cervical spine imaging because of the monopolar readout gradients and the multiple-echo combination used, which are beneficial to avoid off resonance effects and generate a high signal-to-noise ratio [1]. However, during scanning, respiration-induced field variations may produce phase errors among the echoes, which decrease the signal-to-noise ratio and potentially induce severe image artifacts. To address these issues, we developed a navigated MEDIC sequence to monitor the phase instability caused by the field variation, and a segmented linear phase correction model was used to reduce the phase errors.

Method

Sequence: In the developed prototype navigated MEDIC sequence, image echoes are followed by an additional navigator echo in each repetition period [Fig1]. By comparing the phase variation between the navigator echo and its predecessor navigator reference echo, the phase variation between the two navigators was calculated, then echoes acquired between these two moments were respectively calibrated with a compensation phase calculated with a segmented linear phase correction model [Fig2]. The idea of using predecessor navigator echo as correction reference is to ensure the accuracy of phase correction and reduce possible phase wrapping effect.
Experiments: The IRB-approved c-spine imaging was performed in healthy volunteers on a 3T scanner (MAGNETOM Spectra, Siemens Healthcare, Shenzhen, China) with a 16-channel head/neck coil. The optimized parameters for the developed navigated MEDIC sequence included: FOV = 180 x 180 mm², flip angle = 30^o; slice number = 30; distance factor = 10%; slice thickness = 3 mm; TE/TR = 15/545 ms; BW/pixel = 260 Hz; combined echoes = 4; and scan time = 4:58 minutes. A conventional MEDIC sequence was also conducted for comparison, with similar parameters: FOV = 180 x 180mm², flip angle = 30^o; slice number = 30; distance factor = 10%; slice thickness = 3 mm; TE/TR = 15/550 ms; BW/pixel = 260 Hz; combined echoes = 4; and scan time = 4:38 minutes.

Result

Compared to the conventional MEDIC sequence, the developed navigated MEDIC sequence can effectively improve the image quality. In contrast to conventional MEDIC, the images obtained by the newly developed sequence showed less artifact and clearer tissue interfaces [Fig3]. Furthermore, the images obtained from the developed sequence looked much sharper [Fig4]. Respiratory motion during MR scans results in strong blurring and ghosting, and these negative effects may not only be directly caused by motion, but also by motion-induced field variations, and the latter are particularly more common at 3T or higher magnetic field. The segmented linear phase correction model used in this work can compensate for the phase variations.

Conclusion

We developed and evaluated a new navigated MEDIC sequence for cervical spine imaging. The preliminary study has demonstrated that the technique may outperform the conventional MEDIC sequence, and might be a potential technique for the diagnosis of the cervical spine. Clinical studies are currently underway to validate the clinical value of the technique.

Acknowledgements

No acknowledgement found.