Mapping 3D fiber orientation in tissue using dual-angle optical polarization tractography

Three-dimensional fiber orientation mapping of the human brain at micrometer resolution

The accurate measurement of three-dimensional (3D) fiber orientation in the brain is crucial for reconstructing fiber pathways and studying their involvement in neurological diseases. Comprehensive reconstruction of axonal tracts and small fascicles requires high-resolution technology beyond the ability of current in vivo imaging (e.g. diffusion magnetic resonance imaging). Optical imaging methods such as polarization-sensitive optical coherence tomography (PS-OCT) and polarization microscopy can quantify fiber orientation at micrometer resolution but have been limited to two-dimensional in-plane orientation or thin slices, preventing the comprehensive study of connectivity in 3D. In this work we present a novel method to quantify volumetric 3D orientation in full angular space with PS-OCT. We measure the polarization contrasts of the brain sample from two illumination angles of 0 and 15 degrees and apply a computational method that yields the 3D optic axis orientation and true birefringence. We further present 3D fiber orientation maps of entire coronal cerebrum sections and brainstem with 10 μm in-plane resolution, revealing unprecedented details of fiber configurations. We envision that our method will open a promising avenue towards large-scale 3D fiber axis mapping in the human brain as well as other complex fibrous tissues at microscopic level.

Imaging of developing human brains with ex vivo PSOCT and dMRI

The human brain undergoes substantial developmental changes in the first five years of life. Particularly in the white matter, myelination of axons occurs near birth and continues at a rapid pace during the first 2 to 3 years. Diffusion MRI (dMRI) has revolutionized our understanding of developmental trajectories in white matter. However, the mm-resolution of in vivo techniques bears significant limitation in revealing the microstructure of the developing brain. Polarization sensitive optical coherence tomography (PSOCT) is a three-dimensional (3D) optical imaging technique that uses polarized light interferometry to target myelinated fiber tracts with micrometer resolution. Previous studies have shown that PSOCT contributes significantly to the elucidation of myelin content and quantification of fiber orientation in adult human brains. In this study, we utilized the PSOCT technique to study developing brains during the first 5 years of life in combination with ex vivo dMRI. The results showed that the optical properties of PSOCT quantitatively reveal the myelination process in young children. The imaging contrast of the optic axis orientation is a sensitive measure of fiber orientations in largely unmyelinated brains as young as 3-months-old. The micrometer resolution of PSOCT provides substantially enriched information about complex fiber networks and complements submillimeter dMRI. This new optical tool offers great potential to reveal the white matter structures in normal neurodevelopment and developmental disorders in unprecedented detail.

Multiscale label-free imaging of myelin in human brain tissue with polarization-sensitive optical coherence tomography and birefringence microscopy

The combination of polarization-sensitive optical coherence tomography (PS-OCT) and birefringence microscopy (BRM) enables multiscale assessment of myelinated axons in postmortem brain tissue, and these tools are promising for the study of brain connectivity and organization. We demonstrate label-free imaging of myelin structure across the mesoscopic and microscopic spatial scales by performing serial-sectioning PS-OCT of a block of human brain tissue and periodically sampling thin sections for high-resolution imaging with BRM. In co-registered birefringence parameter maps, we observe good correspondence and demonstrate that BRM enables detailed validation of myelin (hence, axonal) organization, thus complementing the volumetric information content of PS-OCT.

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

The accurate measurement of three-dimensional (3D) fiber orientation in the brain is crucial for reconstructing fiber pathways and studying their involvement in neurological diseases. Optical imaging methods such as polarization-sensitive optical coherence tomography (PS-OCT) provide important tools to directly quantify fiber orientation at micrometer resolution. However, brain imaging based on the optic axis by PS-OCT so far has been limited to two-dimensional in-plane orientation, preventing the comprehensive study of connectivity in 3D. In this work, we present a novel method to obtain the 3D fiber orientation in full angular space with only two illumination angles. We measure the optic axis orientation and the apparent birefringence by PS-OCT from a normal and a 15 deg tilted illumination, and then apply a computational method yielding the 3D optic axis orientation and true birefringence. We verify that our method accurately recovers a large range of through-plane orientations from -85 deg to 85 deg with a high angular precision. We further present 3D fiber orientation maps of entire coronal sections of human cerebrum and brainstem with 10 μm in-plane resolution, revealing unprecedented details of fiber configurations. We envision that further development of our method will open a promising avenue towards large-scale 3D fiber axis mapping in the human brain and other complex fibrous tissues at microscopic level.

Cytomembrane visualization using Stokes parameter confocal microscopy

A new, to the best of our knowledge, method for Stokes vector imaging is proposed to achieve imaging and dynamic monitoring of a non-labeled cytomembrane. In this work, a polarization state vector is described by a Stokes vector and expressed in chrominance space. A physical quantity called polarization chromaticity value (PCV) corresponding to a Stokes vector is used as the imaging parameter to perform Stokes vector imaging. By using the PCV imaging technique, the Stokes vector can be expressed in three-dimensional real space rather than in a Poincare sphere. Furthermore, a four-way Stokes parameter confocal microscopy system is designed to measure four Stokes parameters simultaneously and obtain micro-imaging. Label-free living onion cell membranes and their plasmolysis process are selected as the representative micro-anisotropy experimental analysis. It is proved that PCV imaging can perform visualization of cytomembranes, and further, microscopic orientation is demonstrated. The prospect of universal measurement of anisotropy details for analysis and diagnosis is provided.

Impact-induced cartilage damage assessed using polarisation-sensitive optical coherence tomography

Non-invasive determination of structural changes in articular cartilage immediately after impact and rehydration provides insight into the response and recovery of the soft tissue, as well as provides a potential methodology for clinicians to quantify early degenerative changes. In this study, we use polarisation-sensitive optical coherence tomography (PS-OCT) to examine subtle alterations of the optical properties in healthy and early-stage degenerate articular cartilage immediately after impact loading to identify structurally relevant metrics required for understanding the mechanical factors of osteoarthritic initiation and progression. A custom-designed impact testing rig was used to deliver 0.9 J and 1.4 J impact energies to bovine articular cartilage. A total of 52 (n=26 healthy, n=26 mildly degenerate) cartilage-on-bone samples were imaged before, immediately after, and 3 h after impact. PS-OCT images were analyzed to assess changes relating to surface irregularity, optical attenuation, and birefringence. Mildly degenerate cartilage exhibits a significant change in birefringence following 1.4 J impact energies compared to healthy samples which is believed to be attributable to degenerate cartilage being unable to fully utilise the fluid phase to distribute and dampen the energy. After rehydration, the polarisation-sensitive images appear to 'optically-recover' reducing the reliability of birefringence as an absolute metric. Surface irregularity and optical attenuation encode diagnostically relevant information and may serve as markers to predict the mechanical response of articular cartilage. PS-OCT with its ability to non-invasively image the sub-surface microstructural abnormalities of cartilage presents as an ideal modality for cartilage degeneration assessment and identification of mechanically vulnerable tissue.

High-resolution 3D tractography of fibrous tissue based on polarization-sensitive optical coherence tomography

Fibrous tissues play important roles in many parts of the body. Their highly organized directional structure is essential in achieving their normal biomechanical and physiological functions. Disruption of the typical fiber organization in these tissues is often linked to pathological changes and disease progression. Tractography is a specialized imaging method that can reveal the detailed fiber architecture. Here, we review recent developments in high-resolution optical tractography using Jones matrix polarization-sensitive optical coherence tomography. We also illustrate the use of this new tractography technology for visualizing depth-resolved, three-dimensional fibrous structures and quantifying tissue damages in several major fibrous tissues.

Simultaneous tractography and elastography imaging of the zone-specific structural and mechanical responses in articular cartilage under compressive loading

We quantified the precise zonal cartilage structural and mechanical responses to unconfined compressive loading by using simultaneous PSOCT based optical tractography and elastography imaging. Twelve bovine knee articular cartilage samples from six animals were imaged under bulk compression from 4% to 20%. The results revealed strong evidence that the conventional radial zone could be divided into two sub-zones with distinct mechanical properties. The "upper" part of the radial zone played a critical role in "absorbing" the mechanical compression. The study also showed that the zonal fiber organization greatly affected the cartilage structural and mechanical responses. A strong correlation was observed between the optical birefringence and logarithm of the Young's modulus. These new results provide useful information for improving mechanical modeling of articular cartilage and developing better cartilage-mimetic biomaterials.

Measurement of biaxial optical birefringence in articular cartilage

Optical birefringence is a valuable parameter in evaluating collagen organization and assessing collagen degeneration in articular cartilage. A uniaxial birefringent model is implied when using existing methods such as polarized light microscopy in characterizing birefringence in cartilage. However, some studies suggest the existence of a sheet-like collagen organization in articular cartilage, which requires a biaxial birefringence model to describe. In this study, we applied a multi-incident birefringence measurement procedure to investigate the biaxial birefringence in articular cartilage. The results supported the existence of a small yet significant biaxial birefringence effect, which was in agreement with the expectation from a sheet-like collagen organization.

Robust reconstruction of local optic axis orientation with fiber-based polarization-sensitive optical coherence tomography

It is challenging to recover local optic axis orientation from samples probed with fiber-based polarization-sensitive optical coherence tomography (PS-OCT). In addition to the effect of preceding tissue layers, the transmission through fiber and system elements, and imperfect system alignment, need to be compensated. Here, we present a method to retrieve the required correction factors from measurements with depth-multiplexed PS-OCT, which accurately measures the full Jones matrix. The correction considers both retardation and diattenuation and is applied in the wavenumber domain, preserving the axial resolution of the system. The robustness of the method is validated by measuring a birefringence phantom with a misaligned system. Imaging ex-vivo lamb trachea and human bronchus demonstrates the utility of reconstructing the local optic axis orientation to assess smooth muscle, which is expected to be useful in the assessment of airway smooth muscle thickness in asthma, amongst other fiber-based applications.

Optic axis mapping with catheter-based polarization-sensitive optical coherence tomography

Birefringence offers an intrinsic contrast mechanism related to the microstructure and arrangement of fibrillary tissue components. Here we present a reconstruction strategy to recover not only the scalar amount of birefringence but also its optic axis orientation as a function of depth in tissue from measurements with catheter-based polarization sensitive optical coherence tomography. A polarization symmetry constraint, intrinsic to imaging in the backscatter direction, facilitates the required compensation for wavelength-dependent transmission through system elements, the rotating catheter, and overlying tissue layers. Applied to intravascular imaging of coronary atherosclerosis in human patients, the optic axis affords refined interpretation of plaque architecture.

Parametric imaging of collagen structural changes in human osteoarthritic cartilage using optical polarization tractography

Collagen degeneration is an important pathological feature of osteoarthritis. The purpose of this study is to investigate whether the polarization-sensitive optical coherence tomography (PSOCT)-based optical polarization tractography (OPT) can be useful in imaging collagen structural changes in human osteoarthritic cartilage samples. OPT eliminated the banding artifacts in conventional PSOCT by calculating the depth-resolved local birefringence and fiber orientation. A close comparison between OPT and PSOCT showed that OPT provided improved visualization and characterization of the zonal structure in human cartilage. Experimental results obtained in this study also underlined the importance of knowing the collagen fiber orientation in conventional polarized light microscopy assessment. In addition, parametric OPT imaging was achieved by quantifying the surface roughness, birefringence, and fiber dispersion in the superficial zone of the cartilage. These quantitative parametric images provided complementary information on the structural changes in cartilage, which can be useful for a comprehensive evaluation of collagen damage in osteoarthritic cartilage.

Heart structural remodeling in a mouse model of Duchenne cardiomyopathy revealed using optical polarization tractography [Invited]

We investigated the heart structural remodeling in the mdx4cv mouse model of Duchenne cardiomyopathy using optical polarization tractography. Whole heart tractography was obtained in freshly dissected hearts from six mdx4cv mice. Six hearts from C57BL/6J mice were also imaged as the normal control. The mdx4cv hearts were significantly larger than the control hearts and had significantly higher between-subject variations in myofiber organization. While both strains showed classic cross-helical fiber organization in the left ventricle, the rate of the myocardial fiber orientation change across the heart wall was significantly altered in the right ventricle of the mdx4cv heart.