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Changes in Quantitative Magnetization Transfer MR in the Mouse Brain After Transient Cerebral Ischemia
Grzegorz Kwiatkowski1, Georgios Louloudis1, Jan Klohs1, and Sebastian Kozerke2
1Institute for Biomedical Engineering, ETH Zurich, Zurich, Switzerland, 2ETH Zurich, Zurich, Switzerland

Synopsis

Quantitative analysis of magnetization transfer (qMT) based on a two-pool model was employed to characterized changes in the mouse brain following a transient middle cerebral artery occlusion (tMCAO) model of cerebral ischemia.The changes in qMT were compared to the standard MR metrics of an ischemic lesion (T1, T2, ADC, FLAIR) to examine the possible overlap of mechanisms affecting these magnetic resonance imaging contrasts. Notable changes of all MR metrics were found in the brain of the tMCAO group while only the qMT analysis revealed significant alterations in the sham-operated animals.

Introduction

Quantitative MT (qMT) is a sensitive measure of a macromolecular content of tissue1,2. It enables measuring the physicochemical characteristics of the macromolecular proton pools including semi-solid proton fraction, relaxation times of the semi-solid/free water pool and the corresponding exchange rate between them. These characteristics were previously employed to assess changes of brain tissue in Alzheimer's disease3,4, multiple sclerosis5 or cancer6,7, pointing to a potentially increased qMT sensitivity towards macromolecular changes in the tissue, not accessible with standard MR contrasts. In this study, qMT analysis along with a standard MR protocol for characterization of ischemic lesions were employed to reveal similarities and differences in changes of MR contrasts after 24 h and 48 h post tMCAO.

Methods

Animal handling: All animal experiments were performed with adherence to the Swiss Federal Act on Animal Protection and were approved by the Cantonal Veterinary Office Zurich. A total of 17 male C57/BL6 mice were studied, weight range 20–25 g, age range 8–10 weeks. The surgical procedure was carried out as previously described8–10. The middle cerebral artery was occluded for 60 min. tMCAO/sham-operated animals were assessed with MRI at 24h and 48h after reperfusion.

Data acquisition: All MR data were recorded with a 9.4 T small animal MRI scanner (Bruker BioSpec, Ettlingen, Germany), equipped with a cryogenically cooled 2x2 phased array. The standard MR protocol to characterize ischemic lesion was employed as described earlier10 and encompassed acquisition of T1, T2 and ADC maps. For the acquisition of qMT maps, the following acquisition parameters were used: FOV: 20x20 mm2, matrix: 128x128, TE/TR: 3.3/5000 ms, RARE factor: 32, MT module: 32 irradiation offsets between 5-250 ppm spaced logarithmically, saturation power: 2,4,6,8μT, saturation duration: 500,1000ms.

Data analysis: All data were reconstructed in ParaVision 6.0.1. (Bruker, Ettlingen, Germany) and analyzed in MatLab 20018b (Mathworks, Natick, MA). The relaxation/ADC maps were fitted using a mono-exponential function. MT data were analyzed using a two-pool tissue model consisting of the free water pool (A) and the semi-solid macromolecular pools (B). The qMT fitting was performed in transient state using the Bloch-McConnel equations11,12:
$$\left\{ \begin{array}{ll}\frac{dM_{X,A}}{dt} = -\frac{M_{X,A}}{T_{2,A}}-2\pi\Delta M_{Y,A}\\\frac{dM_{Y,A}}{dt} = -\frac{M_{Y,A}}{T_{2,A}}-2\pi\Delta M_{X,A}-\omega_{1}M_{Z,A}\\\frac{dM_{Z,A}}{dt} = R_{A}(M_{0,A}-M_{Z,A})-RM_{0,B}M_{Z,A}+RM_{0,A}M_{Z,B}+\omega_{1}M_{Y,A}\\\frac{dM_{Z,B}}{dt} = R_{B}(M_{0,B}-M_{Z,B})-RM_{0,A}M_{Z,B}+RM_{0,B}M_{Z,A}+R_{RF,B}M_{Z,B}\end{array} \right.$$
with four free parameters: R – exchange constant between the two pools, T2,B – traverse relaxation of the semi-solid pool, RM0,B/RA – amount of magnetization transfer and 1/RAT2,A – saturation due to the direct effect. A super Lorentzian line shape was used for the absorption rate constant RRF,B of the semi-solid pool11,12:
$$
R_{RF,B} = \pi\omega_{1}\int_{0}^{\frac{\pi}{2}}\sin\theta\sqrt{\frac{2}{\pi}}\frac{T_{2,B}}{|3\cos^{2}\theta-1|}\exp[-2(\frac{2\pi T_{2,B}}{|3\cos^{2}\theta-1|})^{2}]d\theta
$$
The qMT model was solved with matrix exponentials and fitted with “fminsearch” function in MatLab using all eight MT curves simultaneously.

Results

Examples of MT curves recorded in the contralateral and ipsilateral side at 24h post tMCAO are shown in Fig.1. Significantly lower saturation levels are observed in the lesion side as compared to the contralateral side. These changes were further characterized by employing a two-pool exchange model. The examples of parametric maps of qMT and standard MR metrics are shown in Fig.2. The longitudinal changes in qMT and other MR metrics assessed at 24 and 48h post-reperfusion are compared using box plots, shown in Fig.3 for the cortex and in Fig.4 for the striatum. Significant changes (see Tab.1) of all MR contrast were found in tMCAO group, reflecting post-ischemia tissue remodeling.

Discussion

Characteristic feature of T2 hyperintensities and reduced ADC values confirmed development of an ischemic lesion. Those changes correlate well with the changes in qMT maps. In particular, reduced fraction of semi-solid pool and increased exchange rate indicates breakdown of macromolecules, possibly evoked by edema and tissue necrosis as shown previously13–15. Moreover, statistically significant changes between brain sides were found also in sham-operated animals, which could not be detected with standard MR metrics (T1/T2/ADC). Similar to tMCAO group decreased fraction of semi-solid pool and increased exchange rates were observed. This was accompanied by an increase in the relaxation rate of the free water pool 48h post-surgery and could be likely linked to hypoperfusion and activation of lymphatic system.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 820374.

References

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Figures

Figure 1: Example of magnetization transfer saturation curves measured in contralateral (red) and ipsilateral ischemic hemisphere (blue) of a mouse brain 24h post tMCAO. The curves were recorded following saturation module applied at different off-resonance frequency, with varying B1 power (2,4,6 and 8 μT) and saturation duration, tp (0.5 and 1 s). Significant differences were observed for curves recorded at different brain sides, reflecting changes in the magnetization transfer between free solute and bound water protons of the underlying tissue.

Figure 2: Examples of MR metric maps obtained for the sham-operated and tMCAO animals, 24h post-reperfusion. Each MR contrast was plotted with the same scale between different animal groups.

Figure 3: Box and whisker plots of MR metrics obtained for naive, sham-operated (sham) and tMCAO animals at 24 h (black) and 48 h (orange) post-reperfusion. The changes in MR metrics were measured for contralateral (Cl.) and ischemic ipsilateral (Ip.) hemisphere. ROIs were placed symmetrically in the cortex.

Figure 4: Box and whisker plots of MR metrics obtained for naive, sham-operated (sham) and tMCAO animals at 24 h (black) and 48 h (orange) post-reperfusion. The changes in MR metrics were measured for contralateral (Cl.) and ipsilateral (Ip.) hemisphere. ROIs were placed symmetrically in the striatum.

Table 1: A comparison of the statistical significance of different MR metrics obtained for healthy control, sham-operated and tMCAO animals. The analysis compared changes in a given MR metric between contralateral- and ipsilateral side of the brain in cortex and striatum, at 24 h and 48 h after reperfusion. The statistical significance was tested using a paired t-Student test.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)
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