Magnetic Resonance Myelin G-ratio mapping for the Brain and Cervical Spinal Cord: 10 Minutes Protocol for Clinical Application
Masaaki Hori1,2, Nikola Stikov3, Ryuji Nojiri2, Yasuaki Tsurushima2, Katsutoshi Murata4, Keiichi Ishigame2, Kouhei Kamiya5, Yuichi Suzuki5, Koji kamagata1, and Shigeki Aoki1

1Radiology, Juntendo University School of Medicine, Tokyo, Japan, 2Tokyo Medical Clinic, Tokyo, Japan, 3Ecole Polytechnique, University of Montreal, Montreal, QC, Canada, 4Siemens Japan, Tokyo, Japan, 5Radiology, The University of Tokyo, Tokyo, Japan

### Synopsis

The purpose of this exhibit is to present the feasibility of MR g-ratio mapping for whole brain and cervical spinal cord using a 10 minute protocol. This protocol is appropriate for clinical use and provides reasonable image quality for clinical diagnosis. In vivo MR g-ratio mapping warrants daily clinical use as it has potential to provide additional information about WM microstructure and its changes in pathology.

### Target audience

Researchers and clinicians who investigate the brain and spinal cord by using diffusion-weighted imaging and quantitative MRI.

### Purpose

In addition to structural conventional MR imaging, magnetic resonance (MR) g-ratio mapping has been introduced as an important technique because it can provide microstructural information about the myelin thickness in vivo, which is not available from other imaging techniques1. G-ratio is defined as the ratio of the axon to the axon plus myelin diameter of the white matter fiber. MR g-ratio maps are usually generated from MR data consisting of two quantitative MR measurements, myelin volume fraction (MVF) and axon volume fraction (AVF). Several MR imaging methods were proposed to acquire MVF and AVF in vivo, however, the combination of these techniques often needs more than 30 minutes, which is too long for patients in a clinical setting. We optimized the imaging parameters for quantitative T1 mapping for MVF and neurite orientation dispersion and density imaging (NODDI)2 for AVF. As a result, total scan time for both imaging sequence is approximately 10 minutes and ready for daily clinical use. The purpose of this exhibit is to present the feasibility of MR g-ratio mapping for whole brain and cervical spinal cord using a 10 minute protocol for clinical use.

### Methods

We will demonstrate the basics of a new MR g-ratio mapping technique and discuss its benefits compared to other approaches, such as T2 relaxometry and quantitative magnetization transfer methods for MVF imaging, and NODDI or other diffusion MR imaging methods for AVF. We chose quantitative T1 mapping sequence for MVF for our protocol because of its relatively shorter scanning time and its image quality. The key for the MVF quantification using T1 value is the linear relationship between macromolecular tissue volume and T13, keeping in mind that approximately half of the macromolecules of white matter (WM) are comprised of myelin4. Moreover, we chose NODDI for AVF as described in the literature1, with use of multiband EPI technique for reducing scanning time5. As a result, total scanning time for whole brain and cervical spinal cord g-ratio mapping were 10 min. 39 sec. and 10min. 18 sec., respectively. Details for each protocol is shown in the table 1. MR g-ratio using MVF and AVF is defined as ($g = \sqrt{1 - \frac{MVF}{FVF}}$)1.

### Results

We present MR g-ratio maps of a normal volunteer (Figure 1) and a clinical case of multiple sclerosis in brain (Figure 2) and cervical spinal cord. We observe that g-ratio is constant in healthy and normal-appearing white matter, but changes in regions with demyelination. MR g-ratio mapping would provide additional information of microstructural changes in WM, in degenerative and demyelinating disease in particular.

### Discussion

The g-ratio values computed with our protocol agree with literature. The decreased g-ratio in a lesion shows our method is sensitive to demyelination. However, the trend is opposite of what we expect (higher g-ratio due to demyelination), which could be due to improper calibration of the MVF technique.

### Summary

This exhibit demonstrates the clinical feasibility of MR g-ratio mapping for whole brain and cervical spinal cord using a 10 minutes protocol. This protocol seems to be tolerable for clinical use and reasonable image quality for clinical diagnosis. In vivo MR g-ratio mapping warrants daily clinical use and has the potential to provide additional information of WM microstructure and its changes in pathology.

N/A

### References

1. Stikov N, Campbell JS, Stroh T, et al. In vivo histology of the myelin g-ratio with magnetic resonance imaging. Neuroimage. 2015;118:397-405.

2. Zhang H, Schneider T, Wheeler-Kingshott CA, Alexander DC.NODDI: practical in vivo neurite orientation dispersion and density imaging ofthe human brain. Neuroimage. 2012;61(4):1000-16.

3. Mezer A, Yeatman JD, Stikov N, et al. Quantifying the local tissue volume and composition in individual brains with magnetic resonance imaging. Nat Med. 2013 ;19(12):1667-72.

4. Norton, W.T., Autilio, L.A. The lipid composition of purified bovine brain myelin. J. Neurochem. 1966;13, 213–222.

5. Xu J, Moeller S, Auerbach EJ, et al. Evaluation of slice accelerations using multiband echo planar imaging at 3 T.Neuroimage. 2013;83:991-1001.

### Figures

Table 1.

Figure 1. A whole brain g-ratio mapping of a normal volunteer. The g-ratio is higher in the corpus callosum and brain stem white matter.

Figure 2. A small intensity spot is seen in the right deep white matter, indicating demyelinated lesion in a patient with multiple sclerosis. Corresponding myelin volume fraction is low, axon volume fraction is almost normal and g-ratio showed iso- or slightly lower than surrounding whit matter. These changes indicates the pathological changes of demyelination and gliosis.

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