Synopsis

Inhomogeneous magnetization transfer (ihMT) is a promising MRI modality that provides high sensitivity and specificity to myelinated tissue. Demyelinating pathologies in the central nervous system could then be addressed by this technique. The goal of this work is to demonstrate that the combination of ihMT preparation with Rapid Acquisition Gradient Echo (ihMT-RAGE), as recently proposed for the brain, can be adapted to an ECG-triggered 3D exploration of the cervical spinal cord within a clinically-compatible scan time. The in-vivo ihMT results from healthy cervical spinal cord demonstrate the great potential of the ihMT-RAGE technique for future investigations of degenerative spinal cord pathologies.

Introduction

Inhomogeneous magnetization transfer (ihMT) is a promising MRI modality that provides high sensitivity and specificity to myelinated tissue^1-3. This technique is attractive to address demyelinating pathologies in the central nervous system.

State-of-the-art 3D sequences proposed for brain imaging have consisted of either ihMT prepared Rapid Acquisition Gradient Echo (ihMT-RAGE⁴) or interleaved ihMT gradient echo (ihMT-GRE⁵) repeated to reach a steady state.

As a small moving organ surrounded by pulsatile cerebrospinal fluid (CSF), the spinal cord is a difficult region to image using MRI. The first methodological developments of ihMT in cervical spine (c-spine) consisted of ECG-triggered 2D single-shot readouts, such as Haste or EPI, and were further applied to both healthy and patient populations^6-9. More recently, c-spine ihMT ratio (ihMTR) was measured at 3T using segmented 3D GRE acquisition¹⁰. Limitations of previous work consist in i. the use of 2D single-shot techniques which limits spatial coverage and induces blurring or susceptibility artefacts or ii. the use of 3D ihMT-GRE whose steady-state is not compatible with physiological triggering.

The goal of this work was to demonstrate that the ihMT-RAGE sequence can be adapted to an ECG-triggered 3D exploration of the cervical spinal cord in healthy volunteers.

Methods

A sensitivity-enhanced ihMT preparation^5,11 was implemented to mitigate sensitivity to B1+ spatial variations as recently proposed in the brain at 3T¹². Briefly, the ihMT RF irradiation was concentrated with 4 bursts of 16 Tukey-shaped RF pulses interleaved with a long mixing period, leading to a total ihMT preparation time of 1.06s. The ihMT preparation was directly followed by a centric-out RAGE readout to keep the ihMT signal maximum in the central part of k-space. The sequence chronogram and ihMT parameters are described in Fig1.

Experiments were performed on three healthy volunteers (age:40±4yo.) on a 3T Verio (Siemens Healthcare GmbH, Erlangen, Germany) using standard coil setup for c-spine imaging. Localized shim was performed prior to ihMT scanning at C1-C7 levels. A reference volume (M₀) and ihMT volumes (MT⁺/MT^+-/MT^-+/MT^-) repeated 3 times were acquired transversally with 0.9mm in-plane resolution and 10mm slice thickness. The ihMT-RAGE sequence was first repeated with and without ECG synchronization and then compared with single-slice ECG-triggered ihMT-Haste acquisition obtained at C3 level only⁷. Each scan lasted ~8-10min depending on the subject’s heartrate. TR_ihMT-RAGE was set to 2.2s for the non-triggered acquisition. An additional ihMT-RAGE was performed in one subject with factory shim settings to evaluate potential effects of B0 inhomogeneity.

All MT-prepared volumes were first co-registered to the M₀ reference using rigid motion correction. ihMTR was then calculated as Mean[MT⁺+MT^--MT^+--MT^-+]/M0. MT ratio was also computed for comparison using 1-MT⁺/M0 only. ihMTR values in grey and white matter (GM/WM) were extracted from manual segmentation within the cervical levels C1-C6.

Results & Discussion

Fig2 shows ihMTRs and MTRs from triggered and non-triggered ihMT-RAGE acquisitions as well as triggered ihMT-Haste obtained in Subject#1. The spinal cord specific WM and butterfly-shape GM can easily be seen in the triggered scan while the structure disappears in some cervical levels for the non-triggered one. The triggered ihMT-Haste scan exhibits a slightly lower ihMTR in whole cord at C3 level (6.1±0.3%vs.7.3±1.6% for ihMT-RAGE), as expected by the enhanced preparation¹². Additionally, because of in-plane blurring, the butterfly-shape GM is harder to distinguish and presented a similar value than WM as compared to ihMT-RAGE (GM:6.0±0.3%vs.5.2±0.6%; WM:6.1±0.3%vs.8.4±1.4%).

The importance of cardiac synchronization is further highlighted in Fig3, presenting two cervical levels with and without ECG-triggering. Repeating multiple 3D readouts without cardiac synchronization, i.e. with acquisitions performed during different phase of the CSF-pulsatility cycle, led to ihMTR contamination by small physiological motion. This behavior can also been seen in the sagittal reformat presented in Fig4. This figure also emphasizes the high importance of localized shimming in the c-spine.

Mean±SD ihMTR measured in the 3 subjects from C1-C6 was 9.2±2.5% in GM and 11.3±2.0% in WM. ihMTR values were higher than previously reported¹⁰.

On one hand, ihMT-RAGE appears more robust and reliable to provide contrasted GM/WM ihMTR on multiple cervical levels than single-slice ihMT-Haste or even multi-slice multi-angle (MSMA) 2D EPI acquisitions which suffer from blurring, susceptibility artefacts or slice-timing relaxation correction. On the other hand, partial volume effect (PVE) is expected to be more dominant using single 3D volume with thick slices as compared to MSMA 2D techniques, which could be disadvantageous when addressing patient population.

Conclusion

Acknowledgements

References

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