0187

Distinguishing microgliosis and tau deposition in the mouse brain using paramagnetic and diamagnetic susceptibility source separation

Jayvik Joshi¹, Minmin Yao^2,3, Wenzhen Duan^2,3, and Manisha Aggarwal⁴
¹Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD, United States, ²Department of Psychiatry and Behavioral Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, United States, ³Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD, United States, ⁴Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, United States

Synopsis

Keywords: Susceptibility/QSM, Microstructure, Brain

Motivation: Susceptibility source separation methods to disentangle sub-voxel paramagnetic and diamagnetic susceptibility sources may provide higher specificity to distinguish tissue microstructural alterations.

Goal(s): Our goal was to investigate sub-cellular histopathological alterations in an established tauopathy mouse model using quantitative susceptibility source separation.

Approach: Brains of PS19 mice and wild-type controls (n = 5 each) were imaged at 11.7 T. We used the DECOMPOSE-QSM model to calculate paramagnetic and diamagnetic component susceptibility maps.

Results: Susceptibility maps revealed significant localized alterations in specific regions of the hippocampus and entorhinal cortex, which were found to correspond to regional microgliosis and tau deposition seen with immunohistology.

Impact: Our findings demonstrate unique sensitivity of paramagnetic and diamagnetic susceptibility changes to distinguish regional microgliosis and tau deposition in the brain. Quantitative magnetic susceptibility source separation may therefore provide a sensitive method to assess sub-cellular histopathological alterations in tauopathies.

Introduction

Quantitative susceptibility mapping is sensitive to the molecular and cellular composition of brain tissue^1,2. Recently, susceptibility source separation methods^3-7 to disentangle para- and diamagnetic source distributions at the sub-voxel level have been proposed, which may provide a more specific probe of tissue microstructural alterations. Tauopathies, like Alzheimer’s disease, involve complex changes in the brain including phosphorylated-tau deposits and microgliosis. Here, we investigate the potential of quantitative susceptibility source separation to distinguish sub-cellular alterations in an established tauopathy mouse model⁸. The results reveal unique sensitivity of paramagnetic and diamagnetic susceptibility source separation maps to distinct histopathological alterations.

Methods

Adult PS19 mice, which exhibit tau pathology in the brain, were used in this study. Brains of 9-month old PS19 mice (referred to as Tau mice) (n = 5) and age-matched wild-type (WT) controls (n = 5) were perfused with 4% PFA. MRI was performed on an 11.7 T Bruker scanner using a 20-mm birdcage transceiver coil. A 3D monopolar multi-gradient-echo sequence was used for acquisition, with flip-angle 30°, 8 echoes, TE1/ΔTE/TR = 4/3.8/100 ms, receiver bandwidth = 70 kHz, and isotropic spatial resolution of 70 µm.

Phase maps were unwrapped using 3D Laplacian-based unwrapping⁹, followed by V-SHARP¹⁰ background removal. Dipole inversion to calculate quantitative susceptibility maps was performed using the STAR-QSM method in StiSuite¹¹. To estimate sub-voxel susceptibility source contributions, the synthesized local complex signal was fit to the DECOMPOSE-QSM three-pool model⁴, implemented in Matlab;
$$S(t;C_0,C_+,C_-,χ_+,χ_-,R_{2,0}^*)=C_0e^{-R_{2,0}^*t}+C_+e^{-(R_{2,0}^*+aχ_+-i\frac{2}{3}χ_+γB_0)t}+C_-e^{-(R_{2,0}^*-aχ_--i\frac{2}{3}χ_-γB_0)t}$$
where $$$C_0,C_+,C_-$$$ represent concentrations of reference, paramagnetic, and diamagnetic susceptibility components, respectively. Paramagnetic (PCS) and diamagnetic (DCS) component susceptibility maps were calculated according to Ref. 4. Group-wise averaged templates of the Tau and WT mouse brains were generated using diffeomorphic registrations based on the sum of squared magnitude images. For structural annotation, we nonlinearly registered the Allen reference atlas¹² to the average group templates. Following MRI, the brains were processed for histology, and immunostained with antibodies for microglia (iba1) and p-tau (AT8).

Results

Fig. 1 shows representative slices from the group-averaged QSM template of WT mouse brains, revealing distinctly highlighted neuronal layers and gray matter regions. The laminar structure of the hippocampus delineated based on susceptibility variations is clearly seen in Fig. 1. Fig. 1e further shows markedly high paramagnetic susceptibility in the pyramidal cell layer of the dorsal hippocampus, reflecting its high iron content. Fig. 2 shows the signal fraction ($$$C_0,C_+,C_-$$$) and source separation maps from fitting the three-pool signal model for the WT mouse brains. The resulting PCS and DCS maps revealed distinct tissue contrasts consistent with variations in iron and myelin content in the mouse brain. Zoomed-in views of select regions further show cortical layers and neuronal cell layers in the hippocampus distinguished in the PCS maps (Fig. 2).

Figs. 3 and 4 compare group-averaged susceptibility and source separation maps of the control and Tau mouse brains, revealing highly localized PCS and DCS alterations in the Tau brains. Bilaterally increased paramagnetic susceptibility in specific hippocampal regions of the Tau brains is seen clearly seen in Fig. 3. Coronal slices in Fig. 4a further show drastically elevated PCS in the Tau group, in bilateral regions that correspond closely to the anatomic borders of hippocampal CA1, subiculum, and dentate gyrus (c.f. structural delineations in Fig. 5a). The PCS increase is consistent with widespread microgliosis and presence of iron-laden microglia in the hippocampus of Tau brains⁸. In comparison, the entorhinal cortex (EC) and caudal hippocampus exhibited significantly increased DCS in the Tau brains relative to controls (Fig. 4b).

Fig. 5 shows structural delineations from the reference atlas overlaid on QSM maps of the WT and Tau brains, with plots of DCS and PCS values in select regions. Significant (p < 0.01) increases in the absolute values of PCS and DCS are evident in the hippocampal CA1 and EC, respectively, with no significant differences in other gray matter areas (Fig. 5a). Immunostained sections in Fig. 5b reveal extensive microgliosis in the hippocampal CA1 of the Tau mouse brains and dominant p-tau deposits in the EC, which correspond closely to the localized differences in susceptibility source separation maps.

Discussion and Conclusion

The results of this study demonstrate unique sensitivity of quantitative susceptibility source separation to distinguish sub-cellular alterations in the mouse brain. Localized increases in absolute PCS and DCS were observed in distinct brain regions, and found to correspond to localized microgliosis and tau deposition seen with immunohistology. Intracellular tau accumulation and proliferation of microglia with iron loading is a hallmark of tauopathies. The ability to probe these sub-cellular alterations with MRI therefore has important implications to gain insights into the disease pathogenesis.

Acknowledgements

This work was supported in part by the National Institutes of Health (NIH) grant R01AG057991.

References

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Figures

Fig. 1: a-d) Representative axial slices from the group-averaged QSM template of WT mouse brains. The anatomical location of the slices is indicated by dashed lines in the scout magnitude image on the bottom left. e,f) Zoomed-in views of select regions within the white boxes show distinctly highlighted neuronal layers and gray matter regions, including the hippocampal pyramidal cell layer (Py) and globus pallidus (GP), reflecting their high iron content. The laminar structure of the hippocampus delineated based on susceptibility variations is clearly seen in (e).

Fig. 2: Parametric maps from fitting the 3-pool complex signal model for source separation in WT mouse brains. Signal fraction (C₀, C₊, C_-) maps and resulting paramagnetic component susceptibility (PCS) and diamagnetic component susceptibility (DCS) maps are shown for a representative axial slice. The averaged PCS and DCS maps reveal distinct tissue contrasts reflecting predominantly variations in iron and myelin content. Magnified views of select regions (right) show cortical layers (arrows) and neuronal layers in the hippocampus (arrowheads) distinguished in the PCS map.

Fig. 3: Comparison of quantitative susceptibility and source separation maps of the WT control and Tau mouse brains. a) A representative axial slice from the group-averaged susceptibility maps is shown, revealing bilateral hyperintense areas localized to specific hippocampal regions of the Tau mouse brains. b) Zoomed-in views of PCS and DCS maps corresponding to the white boxes in (a) show drastically increased PCS with disruption of the laminar structure in the hippocampus of the Tau brains.

Fig. 4: Comparison of coronal susceptibility source separation maps of the WT control and Tau mouse brains. Two slices from group averaged magnitude and susceptibility source separation maps are shown in (a, b). PCS maps in (a) reveal drastically high paramagnetic susceptibility in the hippocampus of Tau brains compared to controls. In contrast, bilateral regions with high diamagnetic susceptibility are seen in the entorhinal cortex and caudal hippocampus of Tau brains in (b). The PCS and DCS differences are found to be localized to specific areas, as seen in the group difference maps.

Fig. 5: a) Structural delineations from the Allen mouse atlas overlaid on the susceptibility maps of the WT control and Tau groups, showing hippocampal and limbic areas. Mean DCS and PCS values for five mice in each group are plotted in the graphs. Data points represent mean values for each brain, with group mean and standard deviation indicated by the bars; **p < 0.01. b) Immunohistology from mouse brains in each group, stained for iba1 (microglia) and AT8 (p-tau), reveal extensive microgliosis in the hippocampus and dominant deposits of p-tau in the entorhinal cortex (EC) of Tau brains.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)

0187

DOI: https://doi.org/10.58530/2024/0187