Characterization of Polarimetric Properties in Various Brain Tumor Types Using Wide-Field Imaging Mueller Polarimetry.

Neuro-oncological surgery is the primary brain cancer treatment, yet it faces challenges with gliomas due to their invasiveness and the need to preserve neurological function. Hence, radical resection is often unfeasible, highlighting the importance of precise tumor margin delineation to prevent neurological deficits and improve prognosis. Imaging Mueller polarimetry, an effective modality in various organ tissues, seems a promising approach for tumor delineation in neurosurgery. To further assess its use, we characterized the polarimetric properties by analysing 45 polarimetric measurements of 27 fresh brain tumor samples, including different tumor types with a strong focus on gliomas. Our study integrates a wide-field imaging Mueller polarimetric system and a novel neuropathology protocol, correlating polarimetric and histological data for accurate tissue identification. An image processing pipeline facilitated the alignment and overlay of polarimetric images and histological masks. Variations in depolarization values were observed for grey and white matter of brain tumor tissue, while differences in linear retardance were seen only within white matter of brain tumor tissue. Notably, we identified pronounced optical axis azimuth randomization within tumor regions. This study lays the foundation for machine learning-based brain tumor segmentation algorithms using polarimetric data, facilitating intraoperative diagnosis and decision making.

Near-real-time Mueller polarimetric image processing for neurosurgical intervention.

PURPOSE: Wide-field imaging Mueller polarimetry is a revolutionary, label-free, and non-invasive modality for computer-aided intervention; in neurosurgery, it aims to provide visual feedback of white matter fibre bundle orientation from derived parameters. Conventionally, robust polarimetric parameters are estimated after averaging multiple measurements of intensity for each pair of probing and detected polarised light. Long multi-shot averaging, however, is not compatible with real-time in vivo imaging, and the current performance of polarimetric data processing hinders the translation to clinical practice. METHODS: A learning-based denoising framework is tailored for fast, single-shot, noisy acquisitions of polarimetric intensities. Also, performance-optimised image processing tools are devised for the derivation of clinically relevant parameters. The combination recovers accurate polarimetric parameters from fast acquisitions with near-real-time performance, under the assumption of pseudo-Gaussian polarimetric acquisition noise. RESULTS: The denoising framework is trained, validated, and tested on experimental data comprising tumour-free and diseased human brain samples in different conditions. Accuracy and image quality indices showed significant ( p < 0.05 ) improvements on testing data for a fast single-pass denoising versus the state-of-the-art and high polarimetric image quality standards. The computational time is reported for the end-to-end processing. CONCLUSION: The end-to-end image processing achieved real-time performance for a localised field of view ( ≈ 6.5 mm 2 ). The denoised polarimetric intensities produced visibly clear directional patterns of neuronal fibre tracts in line with reference polarimetric image quality standards; directional disruption was kept in case of neoplastic lesions. The presented advances pave the way towards feasible oncological neurosurgical translations of novel, label-free, interventional feedback.

Effects of formalin fixation on polarimetric properties of brain tissue: fresh or fixed?

Significance: Imaging Mueller polarimetry (IMP) appears as a promising technique for real-time delineation of healthy and neoplastic tissue during neurosurgery. The training of machine learning algorithms used for the image post-processing requires large data sets typically derived from the measurements of formalin-fixed brain sections. However, the success of the transfer of such algorithms from fixed to fresh brain tissue depends on the degree of alterations of polarimetric properties induced by formalin fixation (FF). Aim: Comprehensive studies were performed on the FF induced changes in fresh pig brain tissue polarimetric properties. Approach: Polarimetric properties of pig brain were assessed in 30 coronal thick sections before and after FF using a wide-field IMP system. The width of the uncertainty region between gray and white matter was also estimated. Results: The depolarization increased by 5% in gray matter and remained constant in white matter following FF, whereas the linear retardance decreased by 27% in gray matter and by 28% in white matter after FF. The visual contrast between gray and white matter and fiber tracking remained preserved after FF. Tissue shrinkage induced by FF did not have a significant effect on the uncertainty region width. Conclusions: Similar polarimetric properties were observed in both fresh and fixed brain tissues, indicating a high potential for transfer learning.

Robustness of the wide-field imaging Mueller polarimetry for brain tissue differentiation and white matter fiber tract identification in a surgery-like environment: an ex vivo study.

During neurooncological surgery, the visual differentiation of healthy and diseased tissue is often challenging. Wide-field imaging Muller polarimetry (IMP) is a promising technique for tissue discrimination and in-plane brain fiber tracking in an interventional setup. However, the intraoperative implementation of IMP requires realizing imaging in the presence of remanent blood, and complex surface topography resulting from the use of an ultrasonic cavitation device. We report on the impact of both factors on the quality of polarimetric images of the surgical resection cavities reproduced in fresh animal cadaveric brains. The robustness of IMP is observed under adverse experimental conditions, suggesting a feasible translation of IMP for in vivo neurosurgical applications.

Polarimetric visualization of healthy brain fiber tracts under adverse conditions: ex vivo studies.

We suggest using the wide-field imaging Mueller polarimetry to contrast optically anisotropic fiber tracts of healthy brain white matter for the detection of brain tumor borders during neurosurgery. Our prior studies demonstrate that this polarimetric imaging modality detects correctly the in-plane orientation of brain white matter fiber tracts of a flat formalin-fixed thick brain specimen in reflection geometry [IEEE Trans. Med. Imaging39, 4376 (2020)10.1109/TMI.2020.3018439]. Here we present the results of ex vivo polarimetric studies of large cross-sections of fresh calf brain in reflection geometry with a special focus on the impact of the adverse measurement conditions (e.g. complex surface topography, presence of blood, etc.) on the quality of polarimetric images and the detection performance of white matter fiber tracts and their in-plane orientation.

Method to calibrate a full-Stokes polarimeter based on variable retarders.

We present a calibration method for a full-Stokes polarimeter. The polarimeter uses two liquid-crystal variable retarders (LCVR) and a linear polarizer to measure the four Stokes parameters. The calibration method proposed in this paper calculates the errors in the experimental setup by fitting the experimental intensity measurements for a set of calibration samples to a theoretical polarimeter with errors. The errors calculated in the method include the axes alignment errors and the errors in the retardance values of both LCVRs. The resulting calibration parameters are verified by measuring the polarization state of a light beam passing through a rotating linear polarizer, a half-wave plate, and a quarter-wave plate and comparing with the predictions for an ideal, error-free polarimeter. It is found that an average reduction in rms error of 55.8% can be obtained with the proposed method.

Experimental limits for eigenvalue calibration in liquid-crystal Mueller-matrix polarimeters.

A numerical study is carried out to find the experimental conditions necessary for the eigenvalue calibration procedure to work correctly in a liquid-crystal variable-retarder-based Mueller-matrix polarimeter. Using the error between the simulated experimental Mueller matrix in a polarimeter with errors and the expected ideal Mueller matrices for four calibration samples, the maximum experimental errors are estimated for a successful eigenvalue calibration. It is found that the retarder axes' orientations have smaller permitted errors than the retardation values.

Permitted experimental errors for optimized variable-retarder Mueller-matrix polarimeters.

An optimized Mueller-matrix polarimeter is simulated. The polarimeter is optimized by finding the configurations of the polarization state generator and polarization state analyzer that give the minimum condition number. Noise is included in the measurement of the polarimeter intensities, and the eigenvalue calibration procedure is used to reduce the errors in the final Mueller matrix. Controlled errors are introduced to the polarimeter configuration, and the error in the final measured Mueller matrix is calculated as a function of these configuration errors. It is found that the alignment of the retarder axes in the polarimeter is much more important than the use of the ideal, optimized retardance values. In particular, the misalignment of the retarders farthest from the sample is the error source with the highest impact in the precision of the polarimeter.

Calibration and data extraction in nonoptimized Mueller matrix polarimeters.

We present a method for calibration and data extraction for a nonoptimized Mueller matrix polarimeter. The advantage of this type of method is a reduction in measurement time for multiwavelength systems or in systems with slow response times. The calibration process requires the measurement of four known polarization devices. Here we use free-space transmission, a horizontal and a vertical linear polarizer, and a quarter-wave retarder with its fast axis at 30° to the horizontal. Experimental measurements of rotating quarter-wave and half-wave retarders show that accurate results can be obtained with the proposed method.