Enabling axial diffusion tensor imaging of the human cervical spinal cord at 7T
Aurélien Massire1,2, Pierre Besson1,2, Maxime Guye1,2, Jean-Philippe Ranjeva1,2, and Virginie Callot1,2

1Centre de Résonance Magnétique Biologique et Médicale (CRMBM), UMR 7339, CNRS, Aix-Marseille Université, Marseille, France, 2Centre d'Exploration Métabolique par Résonance Magnétique (CEMEREM), Hôpital de la Timone, Pôle d’imagerie médicale, AP-HM, Marseille, France

Synopsis

MRI at 7T has recently demonstrated its ability to provide high-quality anatomical images of the spinal cord (SC), yet no diffusion tensor imaging (DTI) study was reported so far. Single-shot echo-planar imaging (ss-EPI) is the method of choice for DTI but the sequence is seriously limited by strong susceptibility artifacts. This work demonstrates that a thoughtful implementation of ss-EPI at 7T combined with distortion correction post-processing from two acquisitions with opposed phase-encoding directions can generate high-resolution axial DTI images of the cervical SC with added value compared to lower field standard protocols making SC DTI ready for UHF clinical investigations.

Introduction

MRI of the spinal cord (SC) at 7T has recently demonstrated its ability to provide high-quality anatomical images [1], with consecutive better characterization of pathologies [2]. Although routinely used to diagnose pathologies and probe SC microstructure at lower field [3], diffusion tensor imaging (DTI) has however, to the authors’ knowledge, not been reported so far on the SC at 7T. Two main reasons may explain this absence of investigation. Firstly, single-shot echo-planar imaging (ss-EPI), which is well established as the method of choice for DTI, is seriously limited at high field by the increased level of artifacts associated with magnetic susceptibility variations at tissue interfaces. Secondly, SC imaging also presents its own challenges such as physiological pulsatility of the cerebrospinal fluid, partial volume effects due to the small physical dimensions of the cord and last, severe magnetic susceptibility-induced image distortions coming from adjacent bone structures. Several original approaches, largely applied in the brain, were proposed to reduce image distortions [4,5]. One of them, which includes post-processing of two ss-EPI acquisitions with opposed phase-encoding directions [6,7], has been used in this study. The work demonstrates that a quite simple yet thoughtful implementation of ss-EPI at 7T combined with distortion correction post-processing can generate high-resolution axial DTI images of the cervical SC with added value compared to lower field standard protocols for an identical acquisition time.

Methods

- Whole-body actively-shielded 7T system with an eight-channel transceiver cervical SC coil array.

- Seven healthy volunteers (22±2 years) scanned with approval of the local Ethic Committee.

- Anatomical imaging: sagittal 2D T2-w TSE sequence (0.6x0.6x2 mm3) for positioning (Figure 1.a), and axial 2D T2*-w GRE sequence with multiple TEs (4 TE, 0.18x0.18x3 mm3, 12 slices). The sum of squares of all echoes (Figure 1.e) was used for anatomical reference after resampling to DTI resolution (c3D, ITK-SNAP) (Figure 1.f).

- DTI with two opposite phase-encoding acquisitions: R>L (Figure 1.b) and L>R (Figure 1.c) using ss-EPI sequence, at the C3 level (0.8x0.8x3 mm3, b-values: (0,800) s/mm2, 12 slices, 12 directions, TE 53ms, PAT 3, 5 averages, pulse trigger, 3-4 concatenations depending on heart rate, and ~7 min acquisition).

- Total acquisition time, including system adjustments (2nd order B0 shimming: FWHM~150 Hz, B1+ calibration): 30 min/subject.

- DTI post-processing using FSL Topup (FMRIB) [7] and FSL DTIFIT.

- Quantification within specific WM and GM regions of interest (Figure 1.j) after manual segmentation using FSL.

- Last subject also enrolled for 3T acquisitions: optimized “3T DTI” protocol (~7 min acquisition, 0.9x0.9x10 mm3, same b-values, 6 slices, 30 directions, pulse trigger, 4 averages, 3 concatenations; Figure 1.k), and “7T-like” protocol (Figure 1.l).

Results

Figures 1.b to 1.d illustrate how well Topup corrects image distortions (subject #7). Outlines of the cord parenchyma were manually drawn in R>L (Figure 1.b), L>R (Figure 1.c) and in corrected (Figure 1.d) images, and subsequently compared to the reference cord outline (Figure 1.f). Related zoom (Figure 1.g) depicts all outlines, with significant overlap between the reference (red) and the corrected (green) outlines, showing accurate distortion corrections. The sum of squares of all GRE echoes provided clear delineation of the GM butterfly (Figure 1.e). DTI metrics, here MD (Figure 1.h, mean diffusivity) and FA (Figure 1.i, fractional anisotropy) maps enable cord and GM butterfly visualization respectively, with exquisite delineation of the posterior horns (Figure 1.I). Figure k. and l. exhibit FA maps of subject #7 acquired at 3T for “3T DTI” and “7T-like” protocols, respectively. The “3T DTI” protocol was acquired within the same acquisition time but with a lower spatial resolution. It shows decent SNR and GM butterfly delineation yet slice thickness was 3.33 times larger, precluding visualization of small tissue alteration. The “7T like” protocol was too noisy to interpret data. Quantitative results for all subjects at 7T (Figure 2) were found comparable to values measured at 3T in this work and coherent with [8]. Furthermore, higher spatial resolution and slice thickness helped minimizing partial volume effect while obtaining DTI quantification within the posterior horns.

Conclusion

A quite simple implementation of ss-EPI at 7T combined with distortion correction post-processing can generate useful high-resolution axial DTI images of the cervical SC. Obtained results confirmed the added value of the 7T in terms of image resolution, metrics evaluation in precise ROIs and total sequence coverage, compared to lower field standard protocols for an equivalent acquisition time. This study lays the groundwork for high-resolution DTI of the SC at 7T. The proposed methodology can direcly be used to explore degenerative SC pathologies and quantify tissue alterations. Future studies will focus on higher spatial resolution and refined sequences.

Acknowledgements

Fundings: 7T-AMI ANR-11-EQPX-0001, A*MIDEX-EI-13-07-130115-08.38-7T-AMISTART & A*MIDEX ANR-11-IDEX-0001-02.

References

[1] Sigmund et al., NMR Biomed 25, 2012. [2] Dula et al., Mult Scler, 2015. [3] Wheeler-Kingshott et al., Neuroimage 84, 2014. [4] Heidemann et al., MRM 64, 2010 [5] Dowell et al., JMRI 29, 2009. [6] Andersson et al., Neuroimage 20, 2003. [7] Smith et al., Neuroimage 23, 2004. [8] Xu et al., Neuroimage 67, 2013.

Figures

Figure 1.a: TSE image. Opposite EPI acquisitions with outlines (b: R>L and c: L>R). d: Corrected image with outline. e: Anatomical image, f: resampled with reference outline, g: zoomed with outlines. Corresponding h: MD map, i: FA map, j: with ROIs. FA maps at 3T (k: standard, l: “7T-like” protocols).

Figure 2: Quantitative DTI results (FA and MD) obtained at 7T and 3T, at the C3 cervical level, in all ROIs after manual segmentation (purple: anterior GM horns, cyan: “sensory” and light green: “motor” WM tracts, cf. Figure 1.j).



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