Backscattering Mueller Matrix polarimetry on whole brain specimens shows promise for minimally invasive mapping of microstructural orientation features

Abstract

Understanding microscale physiology and microstructural cellular features of the brain is key to understanding mechanisms of neurodegenerative diseases and injury, as well as prominent changes undergone in development and aging. Non-invasive imaging modalities sensitive to the microscale, especially diffusion magnetic resonance imaging (dMRI), are promising for mapping of cellular microstructure of brain tissues; however, there is a need for robust validation techniques to verify and improve the biological accuracy of information derived. Recent advances in dMRI have moved toward probing of the more complex grey matter architecture, challenging current validation techniques, which are largely based on ex vivo staining and microscopy focusing on white matter. Polarized light imaging (PLI) has been shown to be successful for high resolution, direct, microstructural imaging and has been applied to dMRI validation with clear advantages over staining and microscopy techniques. Conventionally, PLI is applied to thin, sectioned samples in transmission mode, but PLI has also been extended to operate in reflectance mode to bridge the gap toward in vivo measurements of the brain. In this report we investigate the use of backscattering Mueller Matrix polarimetry to characterize the microstructural content of intact ferret brain specimens. The results show that backscattering polarimetry can probe white matter fiber coherence and fiber orientation, and show promise for probing grey matter microstructure. Ultimately, this motivates further study to fully understand how best to implement backscattering polarimetry for in vivo microstructural imaging of the brain.

Reflectance full Mueller matrix polarimetry for microstructural validation of diffusion magnetic resonance imaging

“…The artificial fiber data seems to be in agreement with previous work simulating and measuring the effects of crossing fibers in transmission PLI systems, 32 which found that crossing fibers would create a patchwork effect. Alternatively, a recent study with this same microscope system found evidence to suggest that the retardance angle data would be averaged over in regions of crossing fibers, 17 which is a more accurate description of what is happening here with the optic chiasm retardance angle data at higher wavelengths. These disagreements could come down to resolution, the artificial fibers used here are much larger than the resolution limit of the microscope, where the neural fibers will be in actuality much finer and thinner potentially resulting in averaging effects in regions where the fibers cannot be resolved.…”

“…In a recently published study, 17 preserved, whole ferret brain samples were imaged using a back scattering PLI microscope and several of these challenges were observed and discussed. However, in working with tissue samples it can be challenging to isolate individual tissue properties as there are multiple confounding factors.…”

“…While the diameters of the fibers were considerably larger than that of a neural fiber, the Fiber optic cable had a diameter of 0.31mm, the Polyester strand-0.06mm, and the monofilament thread-0.07mm, ideally they will still be indicative of the sensitivity of the polarimeter. In addition to the fibers, ferret brains used in several previous studies 17,30 were imaged. The brains were removed from the ferrets postmortem after they underwent a perfusion fixation with 4% paraformaldehyde (PFA).…”

Mueller matrix polarization imaging of gastrinoma shows promise for tumor localization

“…The resulting pixelwise Mueller matrix data is then further processed using a Python program which uses the pySCATMECH package as described here 24 to perform a Lu-Chipman decomposition on the Mueller matrixes (Figure 2), resulting in separated polarization parameters for each pixel, including depolarization, diattenuation, and retardance (Figure 3). Using the pixel resolved depolarization data as a reference and comparing that to the hematoxylin and eosin (H&E)-stained images of the same slide (Figure 1B), tissue regions of interest (ROIs) were selected, and binary masks were drawn to isolate those areas using ImageJ (Figure 1C) 20 . The ROIs selected consist of Brunner's glands (BGs) and the tumor.…”

“…Of various subsets of polarized light imaging, Mueller Matrix polarimetry is powerful due to its sensitivity to micro and nanoscale structures as predicted by scattering theory, which are often influenced with the onset of cancer. Recently, many works have implemented polarized light imaging for identifying diseased tissues 20,21,22 , suggesting that it could be a promising approach for surgical localization of DGASTs, but this has yet to be tested systematically. In our work, we evaluate the suitability of Mueller Matrix polarimetry for localizing DGASTs We conducted a preliminary imaging study of FFPE DGASTs samples using a five-wavelength Mueller Matrix polarimeter, finding that regions of interest corresponding to disease showed significant differences in depolarization and diattenuation characteristics at certain wavelengths when compared to normal adjacent tissues.…”