Refractive-index matching enhanced polarization sensitive optical coherence tomography quantification in human brain tissue

Abstract

The importance of polarization-sensitive optical coherence tomography (PS-OCT) has been increasingly recognized in human brain imaging. Despite the recent progress of PS-OCT in revealing white matter architecture and orientation, quantification of fine-scale fiber tracts in the human brain cortex has been a challenging problem, due to a low birefringence in the gray matter. In this study, we investigated the effect of refractive index matching by 2,2’-thiodiethanol (TDE) immersion on the improvement of PS-OCT measurements in ex vivo human brain tissue. We show that we can obtain fiber orientation maps of U-fibers that underlie sulci, as well as cortical fibers in the gray matter, including radial fibers in gyri and distinct layers of fibers exhibiting laminar organization. Further analysis shows that index matching reduces the noise in axis orientation measurements by 56% and 39%, in white and gray matter, respectively. Index matching also enables precise measurements of apparent birefringence, which was underestimated in the white matter by 82% but overestimated in the gray matter by 16% prior to TDE immersion. Mathematical simulations show that the improvements are primarily attributed to the reduction in the tissue scattering coefficient, leading to an enhanced signal-to-noise ratio in deeper tissue regions, which could not be achieved by conventional noise reduction methods.

Quantitative imaging of three-dimensional fiber orientation in the human brain via two illumination angles using polarization-sensitive optical coherence tomography

“…We applied refractive index matching to our samples to increase the SNR in the PS-OCT measurements. Previously, we showed that index matching significantly improved the SNR and subsequentially enhanced the accuracy of in-plane orientation and apparent birefringence measurements 21 . It is important to note that, without index matching, the limited SNR in the data would prevent the successful application of our orientation estimation from the two illumination measurements.…”

“…We cut two blocks in the corpus callosum region from one of the MGH samples and one coronal slab from the other MGH sample. We applied refractive index matching to all the samples by incubations in 2,2-thiodiethanol solutions as described in our previous work 21 . The ex vivo imaging procedures are approved by the Institutional Review Board of the MGH.…”

Improved in vivo optical coherence tomography imaging of animal peripheral nerves using a prism nerve holder

“…All of these issues do not appear when ex vivo tissue images are merged because excised tissue would normally be imaged either through a glass slide or in a bath with refractive index matching fluid. 39 Index matching is not feasible during surgery because any additional liquid in the surgical pocket is contaminated with red blood cells which in turn significantly degrade the OCT scan quality due to excessive scattering and water absorption. Despite our best effort to remove excessive liquid pooling in the nerve holder channel after the rehydration, some of our acquired volumes suffered signal degradation due to liquid trapped in the meniscus between the nerve and the prisms.…”

“…Methods have been previously demonstrated by other groups to remove OCTA tail artifacts, 37 , 38 which could be applied prior to merging OCTA volumes. All of these issues do not appear when ex vivo tissue images are merged because excised tissue would normally be imaged either through a glass slide or in a bath with refractive index matching fluid 39 . Index matching is not feasible during surgery because any additional liquid in the surgical pocket is contaminated with red blood cells which in turn significantly degrade the OCT scan quality due to excessive scattering and water absorption.…”

Multi‐Scale Label‐Free Human Brain Imaging with Integrated Serial Sectioning Polarization Sensitive Optical Coherence Tomography and Two‐Photon Microscopy

“…Image Acquisition and Postprocessing-Scattering and Birefringence Fitting: Extraction of scattering coefficient, [71,74] back-scattering coefficient, and birefringence [30] followed previously reported methods, by fitting the depth profiles of PSOCT intensity and retardance.…”