Summary and target audience

The volume of a mouse brain is approximately 3000 times smaller than that of a human brain. Revealing anatomical details of the small animal brain requires higher resolution (e.g. 0.1-0.2mm isotropic). Ultra-high field (UHF) (e.g. 7.0 Tesla, 9.4 Tesla, 11.7 Tesla, or even 17 Tesla) is needed for sufficient signal-to-noise ratio (SNR) of animal diffusion MR images. The audience will understand current technical challenges and possible solutions of animal diffusion imaging at UHF. Several applications of animal diffusion imaging at UHF will be introduced. These applications include delineation of mouse embryo anatomical structures with high contrasts of diffusion tensor microimaging, microstructural measurement of animal brain cerebral cortex and white matter tracts, tractography of mouse brain white matter tracts, establishment of animal brain atlases and biomarker detection with diffusion MRI of animal model.

Technical challenges

High magnetic field has been used to enhance spatial resolution in human brain diffusion tensor imaging (DTI) [e.g. 1-3]. The primary technical challenge in animal diffusion MRI is to achieve sufficient SNR at high spatial resolution. The spatial resolution is theoretically limited by SNR that can be achieved with MRI. Diffusion MRI is inherently a noisy technique because the signal magnitude in diffusion weighted images is attenuated by diffusion sensitizing gradients, posing further challenges on the front of SNR. With the volume of a mouse brain is approximately 3000 times smaller than that of a human brain, 0.1-0.2mm isotropic resolution in the mouse brain could only reveal the anatomical details similar to those in human brain diffusion MRI at typical 2mm isotropic resolution. 0.1-0.2 mm isotropic resolution, however, results in 3 orders of magnitude loss in MR signal strength, as the signal is proportional to the volume of the voxel. Therefore, ultra-high field magnets (e.g. 7.0 Tesla, 9.4 Tesla, 11.7 Tesla, or even 17 Tesla) are necessary for animal diffusion MRI. Higher magnetic field induces problems of smaller T2 and higher field inhomogeneity.

Possible imaging solutions

Possible solutions include two approaches based on spin echo: multiple spin echo (MSE) and fast spin echo (FSE) [4]. In both MSE and FSE acquisitions, there are multiple refocusing pulses and echoes after the initial excitation pulse. In the FSE acquisition, the multiple spin echoes sample different lines in the k-space, whereas in MSE acquisition, the multiple spin echoes sample the same line in the k-space. In both cases, the number of echoes that can be acquired is limited by the T2 decay. From the MSE experiments, multiple images will be obtained and added together to increase the SNR. Selection of FSE or MSE acquisition depends on imaging time, resolution, and SNR requirements. Usually FSE is more time efficient and well-suited for in vivo experiments.

Applications

High contrasts of developing mouse embryos: With the high contrasts offered by DTI-derived maps, the critical structures such as neuroepithelium, cortical plate and axonal structures of developing mouse embryos can be delineated and three-dimensionally reconstructed [5]. The high contrasts of DTI-derived maps at 0.1x0.1x0.1mm3 resolution are shown in Figure 1. The microstructural measurement of animal brain: The microstructure of both cerebral cortex [6-9] and white matter tracts [10] can be measured to characterize the animal brain developmental pattern. The cortical microstructural maturational pattern of the developing rat brains can be delineated by cortical fractional anisotropy maps, shown in Figure 2. Tractography of rodent brain white matter tracts: Based on tractography with the high resolution mouse brain DTI, The traced and reconstructed corpus callosum of a mouse brain is shown in Figure 3. Establishment of animal brain atlases: The animal brain atlases can be established with high contrasts and high resolution of diffusion MRI obtained with UHF. As an example, the macaque brain atlas [11,12] with the DTI resolution of 0.3x0.44x0.45mm3 is demonstrated in Figure 4. Biomarker detection of diffusion MRI of animal model: A suite of genotypes and phenotypes with animal diffusion MRI for biomarker of neurological and psychiatric disorders will be covered.

Acknowledgements

No acknowledgement found.

References

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