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UTE MRI can detect myelin loss in mice using an open field low intensity blast injury model of mild traumatic brain injury (mTBI)
Ya-Jun Ma1, Catherine E Johnson2, Jonathan Wong1,3, Hyungseok Jang1, Roland Lee1, Eric Y Chang1,3, Zezong Gu2, and Jiang Du1
1UC San Diego, San Diego, CA, United States, 2Missouri University of Science and Technology, Rolla, MO, United States, 3VA Health System, San Diego, CA, United States

Synopsis

Mild traumatic brain injury (mTBI) may cause significant myelin damage, leading to significant degradation of elaborate cognitive functions. However, conventional neuroimaging techniques are unable to accurately assess myelin, and fail to show abnormalities in the majority of mTBI cases. UTE MRI sequences with echo times (TEs) <0.1 ms allow direct imaging and quantitative assessment of myelin density. Here we aim to investigate whether the 3D IR-UTE sequence can detect myelin loss in mice using an open field low intensity blast injury model of mTBI. This technique provides a new approach for potentially more accurate diagnosis and treatment monitoring of mTBI.

Introduction

Mild traumatic brain injury (mTBI) is a major cause of long-term disability, with an annual 1.7 million Americans sustaining non-fatal TBI. Shear strains due to linear and rotational acceleration of the brain can severely damage axons and their myelin sheaths (1-3). Myelin is particularly vulnerable to secondary damage as a result of chemical cascades and neuroinflammation (1-3). Myelin impairment can disrupt axonal transport, integrity, and plasticity, leading to a massive reduction in signal transduction (4-6). Given its indispensable role in the development and maintenance of elaborate cognitive functions, loss of myelin could play a key role in the pathogenesis of mTBI. However, conventional neuroimaging techniques are unable to accurately assess myelin, and fail to show abnormalities in the majority of mTBI cases (7-9). Ultrashort echo time (UTE) MRI sequences with echo times (TEs) <0.1 ms allow direct detection of signals from myelin (10-14). In this study, we aim to investigate whether 3D UTE MRI can detect myelin loss in mice using an open field low intensity blast injury model of mTBI.

Methods and Materials

A 3D adiabatic inversion recovery prepared UTE (3D IR-UTE) sequence (Figure 1) was implemented on a 7T Bruker horizontal MRI system with 1,000 mT/m strength and 11,250 T/m/s slew rate. An adiabatic IR pulse is used to invert the longitudinal magnetizations of long T2 white matter (WML) (11-13). During this pulse, the longitudinal magnetization of myelin is not inverted, but is largely saturated due to its ultrashort T2. UTE data acquisition starts around the TI necessary for the inverted longitudinal magnetization of WML to reach its null point, leaving signals from myelin and some residual long T2 tissues to be detected by the FID. The second echo acquires signals from any residual long T2 tissues with zero signal from myelin. Subtraction of the second echo image from the first one provides selective imaging of myelin (11-13). The 3D IR-UTE sequence employed the following parameters: TR = 1000 ms, inversion time (TI) = 350 ms, TE = 0.02/2.0 ms, field of view (FOV) = 2.2×2.2×2.2 cm3, matrix = 128×128×128 cm3, flip angle (FA) = 20º. A total number of 21 spokes was acquired per IR preparation, leading to a total scan time of 100 min.

A total of 6 male C57BL/6 mice (Jackson Laboratories, Bar Harbor, ME) at ~8 weeks of age were studied according to the institutional guidelines. Mice were divided into mTBI (n=3) and sham (n=3) groups. The mTBI group were subject to a recently established highly reproducible open-field LIB injury murine model (15), where anesthetized mice were placed in the prone position three meters away from a detonation of 350g high-energy explosive C4 (both 1 m aboveground) (Figure 2). Sham mice underwent identical procedures, but without blast exposure. Four days later the animals were scanned with the 3D IR-UTE sequence followed by histological myelin assessment with Luxol Fast blue (LFB) staining. UTE measured myelin density was correlated with LFB myelin staining.

Results and Discussion

Figure 3 shows coronal T2-FSE and IR-UTE imaging of a normal mouse. Cortical bone in the skull and myelin in white matter are depicted with high signal and contrast on the IR-UTE images, but are invisible using FSE imaging.

Figure 4 shows 3D IR-UTE imaging of a control C57BL/6 mouse and an mTBI mouse four days post an open-field LIB injury. About 25% myelin loss was observed in the CC of mTBI mice, as confirmed by LFB results.

We have demonstrated that that 3D IR-UTE sequences can detect myelin loss in mice induced by the open-field LIB injury model. More recently, myelin sheath defects were identified in the corpus callosum (CC) of mice subject to low-intensity blast (LIB) exposure, a highly reproducible open-field LIB injury murine model of mTBI (15). Myelin defects appeared as extensive split layers, dense degeneration, myelin ballooning, myelin disruption or myelin detachment at 7 days post-blast injury (DPI), and returned to normal appearance 30 days DPI, consistent with evidence from other related research (15). Our results were largely consistent with the reported results, and further confirmed that the 3D IR-UTE sequence could be used to monitor myelin loss in mice subject to the open-field LIB injury, which might be more robust than the traditional controlled cortical impact model of mTBI. A systematic study of two larger groups of mice (normal vs. mTBI induced by the LIB injury) will be performed, and will likely confirm the technical capability in evaluation of demyelination and remyelination in mice subjected to mTBI, and their association with behavioral testing.

Conclusion

The 3D IR-UTE sequence allows quantitative imaging of myelin in mouse brain, and can reliably measure myelin loss in white matter of the brain induced by the open-field low intensity blast. This technique provides a new approach for potentially more accurate diagnosis and treatment monitoring of mTBI.

Acknowledgements

The authors acknowledge grant support from the NIH (R01NS092650, R01AR075825 and R21AR075851), Veterans Affairs (Merit Awards I01CX001388 and I01RX002604), and GE Healthcare.

References

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12. Du J, Ma G, Li S, Carl M, Szeverenyi N, VandenBerg S, Corey-Bloom J, Bydder GM. Ultrashort TE echo time (UTE) magnetic resonance imaging of the short T2 components in white matter of the brain using a clinical 3T scanner. NeuroImage 2013; 87C:32-41. PMID: 24188809.

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Figures

Figure 1. The basic 3D UTE sequence employs a short rectangular pulse for signal excitation, followed by 3D radial ramp sampling with a minimal nominal TE of 20 µs and a second echo of 2 ms. (B) The 3D IR-UTE sequence employs an adiabatic IR pulse followed by multiple- spoke (Nsp) 3D UTE data acquisition each with a duration t. The inversion time (TI) is adjusted for the inverted long T2 WM (WML) longitudinal magnetization to reach the nulling point. An appropriate TR and TI combination is required for robust suppression of signals from WML.

Figure 2. Experimental set ups for the open-field low intensity blast (LIB) injury murine model of mTBI. Mild TBI is induced by placing anesthetized mice in the prone position three meters away from a detonation of 350 g of high-energy explosive C4 (both 1 m above ground level).

Figure 3. Representative axial images of an adult control C57BL/6 mouse: (a) T2-FSE, (b) 3D IR-UTE imaging at TE = 0.020 ms (c), and TE = 2 ms (windowed 10X), where myelin (thin arrows) and bone (thick arrows) signals dropped to near zero, consistent with short T2* relaxation times.

Figure 4. 3D IR-UTE imaging of myelin in a control mouse (A) and a mouse four days after open-field LIB injury (B). LFB for the control and mTBI mice shows a significant reduction in staining intensity within the corpus callosum (CC) (red arrows) for the mTBI mouse (C), consistent with demyelination induced by LIB. UTE-measured myelin density for the CC was reduced by ~25% from 10.6±0.8% for the control mouse to 7.9±0.9% for the mTBI mouse (F), largely consistent with LFB staining (C).

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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