Lingzhi Shi

Ultra High Field MRI and Polarized Light Imaging of postmortem Human Brain Tissue

Grey matter is the major component of the central nervous system, and the white matter consists of long-ranged myelinated fibers. The white matter fiber pathways are responsible for the functional interactions between different grey matter regions of the brain. Thus, understanding of the white matter fiber architecture is critical for investigating the function of neural networks. In order to investigate white matter fiber tracts in this project, the diffusion weighted magnetic resonance imaging (DW-MRI) and polarized light imaging (PLI) were performed on a postmortem sample from the human occipital lobe. The DW-MRI was carried out with voxel sizes of 300 µm using two techniques: diffusion weighted spin echo with echo planar imaging (DW-SE-EPI) and diffusion weighted steady-state fast precession (DW-SSFP). In addition the latter sequence was also applied with a voxel-size of 150 µm. Polarized light imaging with the voxel sizes of 2.5 µm was applied on the same sample after cryo-sectioning into 100 µm thick slices. We found that DW-SSFP presents apparent advantages in efficiency and data quality over DW-SE-EPI. Furthermore, the images from DW-MRI and PLI were compared, and it was shown that while DW-SSFP yields 3D information, PLI is suitable for identifying small fibers, owing to its high spatial resolution. The findings of this project demonstrated that diffusion weighted magnetic resonance imaging and polarized light imaging is complementary to each other in the area of whiter matter fiber tracking.

Ultra High Field MRI and Polarized Light Imaging of Human Occipital Lobe

The goal of this project is to investigate the white matter fiber tracts in the human occipital lobe using two different methods: diffusion weighted magnetic resonance imaging and polarized light imaging.

Nowadays, the diffusion weighted magnetic resonance imaging is the most promising tool for the white matter fiber tracking, and because it is a non-invasive approach the in-vivo experiment can be performed. Diffusion weighted magnetic resonance imaging can provide 3D information. However, it is limited by its resolution, which makes it difficult to track small fibers in the white matter. Polarized Light microscope provides a new possibility for white matter fiber tracking in high resolution.

In Fig.1 an example of the polarized light image of the human occipital lobe is presented, and fiber bundles with different directions and the calcarine sulcus can be observed from the image. The polarized light microscopic images are presented with different colors, even though no staining of the sample has been performed. The different colors show different angles, meaning different directions of the fiber in plane. The satuation codes represent the inclination of the fiber, from highest intensity for in-plane fibers to lowest intensity for fibers with high inclination (orthogonal to the plane). From Fig.46, the grey matter can be observed with the green color, while the white matter is shown in multiple colors because of the myelinated fibers.

Fig.1 Polarized light microscopic image of the human occipital lobe with 25x magnification and a resolution of 5 µm/pixel. The marked area of the image of the whole slice is magnified, where fine fibers are presented.

In Fig.2 the microscopic images from nine consecutive slices are presented, and because slices are consecutive, the pathway of the different fiber bundles can be estimated and followed.

Fig.2 Polarized light microscopic images of the human occipital lobe with 25x magnification and a resolution of 5 µm/pixel. Images are obtained from consecutive slices.

In order to appreciate the significance of the polarized light imaging, the image acquired with the diffusion weighted MRI is presented in Fig.48. For comparison between the two techniques for visualizing white matter fiber tracts, the selected area of the images is magnified. From the magnified images, the microscopic image has far better resolution than the MR image, which allows the visualization of the fine white matter fibers.

Fig.3 Visualization of the white matter fiber tracts in the human occipital lobe using different techniques: the fiber direction map from diffusion weighted MRI with an isotropic resolution of 300 µm (left), and the polarized light microscopic image with a resolution of 5 µm/pixel. In order to highlight the improved resolution of the microscopic image compared to the diffusion weighted MR image, the selected area of each image is magnified.

From the image shown from our experiment (Fig.4), some fine fibers are just becoming observable, and for the clear visualization of those fibers, higher resolution is necessary.

Fig.4 Polarized light microscopic image of the human occipital lobe with 25x magnification and with a resolution of 5 µm/pixel, in the marked area small fibers are starting to show their existance.