Single-point macromolecular proton fraction mapping at 7T in healthy and demyelinated mouse brain
Lucas Soustelle1,2, Maria-Cristina Antal2, Paulo Loureiro de Sousa2, and Laura Harsan2

1Aix-Marseille Univ. CRMBM UMR 7339, Marseille, France, 2Université de Strasbourg, CNRS, ICube, FMTS, Strasbourg, France


Assessment of myelin content in the brain is essential for monitoring pathologies such as multiple sclerosis. Quantitative MRI methods including quantitative magnetization transfer imaging (qMTI) have been employed in animal and human studies to assess demyelination processes. Animal studies have reported high correlations between myelin content and the macromolecular proton fraction (MPF), a metric derived from qMTI. The single-point MPF mapping method requires the acquisition of a single MT-weighted image, hence reducing protocol scan duration. In this work, we propose the adaptation of this method at 7T in a study involving healthy and demyelinated mice.


The macromolecular proton fraction (MPF) derived from quantitative Magnetization Transfer (qMT) imaging is a potential biomarker of myelin in brain tissues1,2. Methods to reduce the necessary amount of MT-weighted images for 3D-MPF mapping were previously proposed and adapted at 1.5T (humans only)3, 3T (humans and animals)3-8 and 11.7T (animals only)9. Yarnykh's single-point MPF (SP-MPF) method consists in estimating the MPF by constraining the values of other two-pool model parameters; $$$R$$$ (cross-relaxation rate constant), $$$T_{2,r}$$$ ($$$T_2$$$ of the restricted pool), and $$$R_{1,f}T_{2,f}$$$ (product of the relaxation quantities $$$R_{1,f}$$$ and $$$T_{2,f}$$$ of the free pool).

In this study we provide an optimized protocol for robust SP-MPF mapping at 7T, based on a comprehensive qMTI method of healthy and demyelinated mice.


Experiments were conducted on a 7T preclinical scanner (Bruker BioSpec, Ettlingen, Germany), using a 86-mm volume Tx coil and a mouse head surface Rx coil. Male C57BL/6 mice (4 healthy vs. 3 cuprizone-fed with a respective mean weight of $$$28.1\pm2.1$$$ g and $$$23.2\pm0.9$$$ g) were scanned at 16 weeks of age. Demyelination was induced by cuprizone administration in standard chow (0.2%)10 for 8 consecutive weeks prior to scanning. During scans, animal respiratory rate and temperature were monitored and maintained at $$$100\pm10$$$ breaths-per-minute and $$$37.6\pm0.1~^\circ$$$C, respectively.

Table 1 shows the acquisition protocol. In T1 (Variable Flip Angle GRE), B1 (Actual flip angle imaging; AFI) and qMT (MT-GRE) protocols, a strong gradient spoiling was systematically included to crush the remaining transverse magnetization through diffusion effects, along with an appropriate RF spoiling increment11,12. B0 maps were acquired throughout the protocol to provide an additional correction in the qMT post-processing step. Axial FOV were tilted so that their respective edge was parallel to the corpus callosum along the genu-splenium line determined on a contrasted T2-weighted sagittal scan.

All images were denoised using BM4D13 and subject-wise rigidly registered onto the 10-degrees volume of the $$$T_1$$$ mapping protocol using ANTs14. Partial brain extraction (from genu to splenium) were manually performed using ITK-Snap. Finally, $$$T_1$$$ maps were estimated using an in-house implementation, and qMT maps ($$$T_{2,f}$$$, $$$T_{2,r}$$$, MPF and $$$R$$$) were estimated using a two-pool model15 implemented in the qMTLab toolbox16, with $$$R_{1,r}=R_{1,f}$$$.

MPF maps for each animal were reconstructed by the single-point method for each combination of saturation pulse frequency offset $$$\Delta$$$ and flip angle $$$\alpha_\text{sat}$$$ as follow:
- #1: using the medians of the estimated histograms parameters from the present study (i.e. respectively for Control and Cuprizone groups);
- #2: using the medians of the estimated joint histogram parameters from the present study;
- #3: using published constrained parameters estimated at 3T for the human brain6.

The mean absolute percentage error per voxel for each SP-MPF reconstructions was computed as6: $$\delta_f=\frac{1}{N}\sum_\nu^N\frac{|f_\text{ref}(\nu)-f_\text{SP}(\nu)|}{f_\text{ref}(\nu)},$$
where $$$\nu$$$ is a voxel from the brain parenchyma across all mice.

Optimal $$$\Delta$$$ and $$$\alpha_\text{sat}$$$ were determined for MPF reconstruction #1 and #2 as the values minimizing $$$\delta_f$$$. To evaluate the agreement between the reference qMTI method and the SP-MPF method within optimal setup conditions, mean values of MPF estimated in the somato-sensory cortex and the corpus callosum were plotted against each other.


Averaged histograms for $$$R_{1,f}T_{2,f}$$$, $$$T_{2,r}$$$, $$$R$$$ and MPF parameters from both groups are shown in Figure 1. Although $$$R_{1,f}T_{2,f}$$$ does not seem to vary much between the two groups, $$$R$$$, $$$T_{2,r}$$$ and MPF distributions are clearly affected by the global brain demyelination following the cuprizone diet10. Medians of the separate Control and Cuprizone distributions amounted to $$$R_{1,f}T_{2,f}=0.0191/0.0185$$$, $$$R=41.9/45.4~\text{s}^{-1}$$$ and $$$T_{2,r}=10.2/9.6~\mu\text{s}$$$, respectively. Alternatively, the joint distributions resulted in $$$R_{1,f}T_{2,f}=0.0190$$$, $$$R=43.6~\text{s}^{-1}$$$ and $$$T_{2,r}=9.9~\mu\text{s}$$$.

The SP-MPF reconstructions #1 and #2 for $$$\Delta=6$$$ kHz and $$$\alpha_\text{sat}=600^\circ$$$ were found to provide the smallest experimental mean percentage errors ($$$\delta_f=7.4\%$$$ and $$$\delta_f=7.6\%$$$, respectively) at 7T (Figure 2).

A good agreement was found between reference MPF values and those estimated using the SP-MPF method from reconstruction #1 and #2 (Figures 3 and 4). Using group-specific parameters does not seem to improve the accuracy of the results. In contrast, MPF values were overestimated using published constrained parameters, estimated at 3T in the human brain.


We demonstrated that MPF maps in mouse brain at 7T can be estimated accurately and efficiently using the SP-MPF methodology. We also showed that constrained parameters and optimal $$$\Delta$$$ and $$$\alpha_\text{sat}$$$ values must be experimentally determined for robust and accurate SP-MPF mapping.



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Table 1: Detailed sequences parameters of the complete MPF protocol. Fractional k-space acquisition factors were ordered in the slice direction.

Figure 1: Normalized histograms of Control (CTL; solid line) and Cuprizone (CPZ; dashed line) mice corresponding to the averaged distributions of $$$R$$$ (a; bin size = 0.5 s-1), $$$R_{1,f}T_{2,f}$$$ (b; bin size = 0.00022), $$$T_{2,r}$$$ (c; bin size = 0.12 $$$\mu$$$s) and MPF (d; bin size = 0.0006). Medians from the respective (Reconstruction #1) and joint (Reconstruction #2) groups parameters are indicated in the upper-right part of each histogram.

Figure 2: Absolute deviations $$$\delta_f$$$ (averaged across all mice) using Reconstruction #1 (a) Reconstruction #2 (b) and Reconstruction #3 (c) along $$$\alpha_\text{sat}$$$ and $$$\Delta$$$.

Figure 3: Comparison of averaged MPF values from ROIs in the somato-sensory cortex (gray bullets) and corpus callosum (black bullets) across all mice between Reference and Reconstructions #1 (a), #2 (b), and #3 (c). The solid line corresponds to the line of unity, indicating the deviation of the SP-MPF reconstruction from the Reference.

Figure 4: Representative axial views of MPF maps (from left to right column: Reference; Reconstruction #1; Reconstruction #2; Reconstruction #3) of typical Control (upper row) and Cuprizone (lower row) mice.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)