Diattenuation of brain tissue and its impact on 3D polarized light imaging.

3D-polarized light imaging (3D-PLI) reconstructs nerve fibers in histological brain sections by measuring their birefringence. This study investigates another effect caused by the optical anisotropy of brain tissue - diattenuation. Based on numerical and experimental studies and a complete analytical description of the optical system, the diattenuation was determined to be below 4 % in rat brain tissue. It was demonstrated that the diattenuation effect has negligible impact on the fiber orientations derived by 3D-PLI. The diattenuation signal, however, was found to highlight different anatomical structures that cannot be distinguished with current imaging techniques, which makes Diattenuation Imaging a promising extension to 3D-PLI.

Estimating Fiber Orientation Distribution Functions in 3D-Polarized Light Imaging.

Research of the human brain connectome requires multiscale approaches derived from independent imaging methods ideally applied to the same object. Hence, comprehensible strategies for data integration across modalities and across scales are essential. We have successfully established a concept to bridge the spatial scales from microscopic fiber orientation measurements based on 3D-Polarized Light Imaging (3D-PLI) to meso- or macroscopic dimensions. By creating orientation distribution functions (pliODFs) from high-resolution vector data via series expansion with spherical harmonics utilizing high performance computing and supercomputing technologies, data fusion with Diffusion Magnetic Resonance Imaging has become feasible, even for a large-scale dataset such as the human brain. Validation of our approach was done effectively by means of two types of datasets that were transferred from fiber orientation maps into pliODFs: simulated 3D-PLI data showing artificial, but clearly defined fiber patterns and real 3D-PLI data derived from sections through the human brain and the brain of a hooded seal.

A multiscale approach for the reconstruction of the fiber architecture of the human brain based on 3D-PLI.

Structural connectivity of the brain can be conceptionalized as a multiscale organization. The present study is built on 3D-Polarized Light Imaging (3D-PLI), a neuroimaging technique targeting the reconstruction of nerve fiber orientations and therefore contributing to the analysis of brain connectivity. Spatial orientations of the fibers are derived from birefringence measurements of unstained histological sections that are interpreted by means of a voxel-based analysis. This implies that a single fiber orientation vector is obtained for each voxel, which reflects the net effect of all comprised fibers. We have utilized two polarimetric setups providing an object space resolution of 1.3 µm/px (microscopic setup) and 64 µm/px (macroscopic setup) to carry out 3D-PLI and retrieve fiber orientations of the same tissue samples, but at complementary voxel sizes (i.e., scales). The present study identifies the main sources which cause a discrepancy of the measured fiber orientations observed when measuring the same sample with the two polarimetric systems. As such sources the differing optical resolutions and diverging retardances of the implemented waveplates were identified. A methodology was implemented that enables the compensation of measured different systems' responses to the same birefringent sample. This opens up new ways to conduct multiscale analysis in brains by means of 3D-PLI and to provide a reliable basis for the transition between different scales of the nerve fiber architecture.

Target sites for transcallosal fibers in human visual cortex - A combined diffusion and polarized light imaging study.

Transcallosal fibers of the visual system have preferential target sites within the occipital cortex of monkeys. These target sites coincide with vertical meridian representations of the visual field at borders of retinotopically defined visual areas. The existence of preferential target sites of transcallosal fibers in the human brain at the borders of early visual areas was claimed, but controversially discussed. Hence, we studied the distribution of transcallosal fibers in human visual cortex, searching for an organizational principle across early and higher visual areas. In-vivo high angular resolution diffusion imaging data of 28 subjects were used for probabilistic fiber tracking using a constrained spherical deconvolution approach. The fiber architecture within the target sites was analyzed at microscopic resolution using 3D polarized light imaging in a post-mortem human hemisphere. Fibers through a seed in the splenium of the corpus callosum reached the occipital cortex via the forceps major and the tapetum. We found target sites of these transcallosal fibers at borders of cytoarchitectonically defined occipital areas not only between early visual areas V1 and V2, V3d and V3A, and V3v and V4, but also between higher extrastriate areas, namely V4 (ventral) and posterior fusiform area FG1 as well as posterior fusiform area FG2 and lateral occipital cortex. In early visual areas, the target sites coincided with the vertical meridian representations of retinotopic maps. The spatial arrangement of the fibers in the 'border tuft' region at the V1/V2 border was found to be more complex than previously observed in myeloarchitectonic studies. In higher visual areas, our results provided additional evidence for a hemi-field representation in human area V4. The fiber topography in posterior fusiform gyrus indicated that additional retinotopic areas might exist, located between the recently identified retinotopic representations phPITv/phPITd and PHC-1/PHC-2 in lateral occipital cortex and parahippocampal gyrus.

Understanding fiber mixture by simulation in 3D Polarized Light Imaging.

3D Polarized Light Imaging (3D-PLI) is a neuroimaging technique that has opened up new avenues to study the complex architecture of nerve fibers in postmortem brains. The spatial orientations of the fibers are derived from birefringence measurements of unstained histological brain sections that are interpreted by a voxel-based analysis. This, however, implies that a single fiber orientation vector is obtained for each voxel and reflects the net effect of all comprised fibers. The mixture of various fiber orientations within an individual voxel is a priori not accessible by a standard 3D-PLI measurement. In order to better understand the effects of fiber mixture on the measured 3D-PLI signal and to improve the interpretation of real data, we have developed a simulation method referred to as SimPLI. By means of SimPLI, it is possible to reproduce the entire 3D-PLI analysis starting from synthetic fiber models in user-defined arrangements and ending with measurement-like tissue images. For the simulation, each synthetic fiber is considered as an optical retarder, i.e., multiple fibers within one voxel are described by multiple retarder elements. The investigation of different synthetic crossing fiber arrangements generated with SimPLI demonstrated that the derived fiber orientations are strongly influenced by the relative mixture of crossing fibers. In case of perpendicularly crossing fibers, for example, the derived fiber direction corresponds to the predominant fiber direction. The derived fiber inclination turned out to be not only influenced by myelin density but also systematically overestimated due to signal attenuation. Similar observations were made for synthetic models of optic chiasms of a human and a hooded seal which were opposed to experimental 3D-PLI data sets obtained from the chiasms of both species. Our study showed that SimPLI is a powerful method able to test hypotheses on the underlying fiber structure of brain tissue and, therefore, to improve the reliability of the extraction of nerve fiber orientations with 3D-PLI.

Classification of ambiguous nerve fiber orientations in 3D polarized light imaging.

3D polarized light imaging (3D-PLI) has been shown to measure the orientation of nerve fibers in post mortem human brains at ultra high resolution. The 3D orientation in each voxel is obtained as a pair of angles, the direction angle and the inclination angle with unknown sign. The sign ambiguity is a major problem for the correct interpretation of fiber orientation. Measurements from a tiltable specimen stage, that are highly sensitive to noise, extract information, which allows drawing conclusions about the true inclination sign. In order to reduce noise, we propose a global classification of the inclination sign, which combines measurements with spatial coherence constraints. The problem is formulated as a second order Markov random field and solved efficiently with graph cuts. We evaluate our approach on synthetic and human brain data. The results of global optimization are compared to independent pixel classification with subsequent edge-preserving smoothing.

High-resolution fiber tract reconstruction in the human brain by means of three-dimensional polarized light imaging.

Functional interactions between different brain regions require connecting fiber tracts, the structural basis of the human connectome. To assemble a comprehensive structural understanding of neural network elements from the microscopic to the macroscopic dimensions, a multimodal and multiscale approach has to be envisaged. However, the integration of results from complementary neuroimaging techniques poses a particular challenge. In this paper, we describe a steadily evolving neuroimaging technique referred to as three-dimensional polarized light imaging (3D-PLI). It is based on the birefringence of the myelin sheaths surrounding axons, and enables the high-resolution analysis of myelinated axons constituting the fiber tracts. 3D-PLI provides the mapping of spatial fiber architecture in the postmortem human brain at a sub-millimeter resolution, i.e., at the mesoscale. The fundamental data structure gained by 3D-PLI is a comprehensive 3D vector field description of fibers and fiber tract orientations - the basis for subsequent tractography. To demonstrate how 3D-PLI can contribute to unravel and assemble the human connectome, a multiscale approach with the same technology was pursued. Two complementary state-of-the-art polarimeters providing different sampling grids (pixel sizes of 100 and 1.6 µm) were used. To exemplarily highlight the potential of this approach, fiber orientation maps and 3D fiber models were reconstructed in selected regions of the brain (e.g., Corpus callosum, Internal capsule, Pons). The results demonstrate that 3D-PLI is an ideal tool to serve as an interface between the microscopic and macroscopic levels of organization of the human connectome.

Clinical optical coherence tomography combined with multiphoton tomography of patients with skin diseases.

We report on the first clinical study based on optical coherence tomography (OCT) in combination with multiphoton tomography (MPT) and dermoscopy. 47 patients with a variety of skin diseases and disorders such as skin cancer, psoriasis, hemangioma, connective tissue diseases, pigmented lesions, and autoimmune bullous skin diseases have been investigated with (i) state-of-the-art OCT systems for dermatology including multibeam swept source OCT, (ii) the femtosecond laser multiphoton tomograph, and (iii) dermoscopes. Dermoscopy provides two-dimensional color images of the skin surface. OCT images reflect modifications of the intratissue refractive index whereas MPT is based on nonlinear excitation of endogenous fluorophores and second harmonic generation. A stack of cross-sectional OCT "wide field" images with a typical field of view of 5 x 2 mm(2) gave fast information on the depth and the volume of the lesion. Multiphoton tomography provided 0.36 x 0.36 mm(2) horizontal/diagonal optical sections within seconds of a particular region of interest with superior submicron resolution down to a tissue depth of 200 mum. The combination of OCT and MPT provides a unique powerful optical imaging modality for early detection of skin cancer and other skin diseases as well as for the evaluation of the efficiency of treatments.