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.Comment: 32 pages, 15 figure

Microstructural Analysis of Human White Matter Architecture Using Polarized Light Imaging: Views from Neuroanatomy

To date, there are several methods for mapping connectivity, ranging from the macroscopic to molecular scales. However, it is difficult to integrate this multiply-scaled data into one concept. Polarized light imaging (PLI) is a method to quantify fiber orientation in gross histological brain sections based on the birefringent properties of the myelin sheaths. The method is capable of imaging fiber orientation of larger-scale architectural patterns with higher detail than diffusion MRI of the human brain. PLI analyses light transmission through a gross histological section of a human brain under rotation of a polarization filter combination. Estimates of the angle of fiber direction and the angle of fiber inclination are automatically calculated at every point of the imaged section. Multiple sections can be assembled into a 3D volume. We describe the principles of PLI and present several studies of fiber anatomy as a synopsis of PLI: six brainstems were serially sectioned, imaged with PLI, and 3D reconstructed. Pyramidal tract and lemniscus medialis were segmented in the PLI datasets. PLI data from the internal capsule was related to results from confocal laser scanning microscopy, which is a method of smaller scale fiber anatomy. PLI fiber architecture of the extreme capsule was compared to macroscopical dissection, which represents a method of larger-scale anatomy. The microstructure of the anterior human cingulum bundle was analyzed in serial sections of six human brains. PLI can generate highly resolved 3D datasets of fiber orientation of the human brain and has high comparability to diffusion MR. To get additional information regarding axon structure and density, PLI can also be combined with classical histological stains. It brings the directional aspects of diffusion MRI into the range of histology and may represent a promising tool to close the gap between larger-scale diffusion orientation and microstructural histological analysis of connectivity

A Jones matrix formalism for simulating three-dimensional polarized light imaging of brain tissue

The neuroimaging technique three-dimensional polarized light imaging (3D-PLI) provides a high-resolution reconstruction of nerve fibres in human post-mortem brains. The orientations of the fibres are derived from birefringence measurements of histological brain sections assuming that the nerve fibres - consisting of an axon and a surrounding myelin sheath - are uniaxial birefringent and that the measured optic axis is oriented in direction of the nerve fibres (macroscopic model). Although experimental studies support this assumption, the molecular structure of the myelin sheath suggests that the birefringence of a nerve fibre can be described more precisely by multiple optic axes oriented radially around the fibre axis (microscopic model). In this paper, we compare the use of the macroscopic and the microscopic model for simulating 3D-PLI by means of the Jones matrix formalism. The simulations show that the macroscopic model ensures a reliable estimation of the fibre orientations as long as the polarimeter does not resolve structures smaller than the diameter of single fibres. In the case of fibre bundles, polarimeters with even higher resolutions can be used without losing reliability. When taking the myelin density into account, the derived fibre orientations are considerably improved.Comment: 20 pages, 8 figure

Maximizing the Bandwidth Efficiency of the CMS Tracker Analog Optical Links

The feasibility of achieving faster data transmission using advanced digital modulation techniques over the current CMS Tracker analog optical link is explored. The spectral efficiency of Quadrature Amplitude Modulation -Orthogonal Frequency Division Multiplexing (QAM-OFDM) makes it an attractive option for a future implementation of the readout link. An analytical method for estimating the data-rate that can be achieved using OFDM over the current optical links is described and the first theoretical results are presented

3D Polarized Light Imaging Portrayed: Visualization of Fiber Architecture Derived from 3D-PLI

3D polarized light imaging (3D-PLI) is a neuroimaging technique that has recently opened up new avenues to study the complex architecture of nerve fibers in postmortem brains at microscopic scales. In a specific voxel-based analysis, each voxel is assigned a single 3D fiber orientation vector. This leads to comprehensive 3D vector fields. In order to inspect and analyze such high-resolution fiber orientation vector field, also in combination with complementary microscopy measurements, appropriate visualization techniques are essential to overcome several challenges, such as the massive data sizes, the large amount of both unique and redundant information at different scales, or the occlusion issues of inner structures by outer layers. Here, we introduce a comprehensive software tool that is able to visualize all information of a typical 3D-PLI dataset in an adequate and sophisticated manner. This includes the visualization of (i) anatomic structural and fiber architectonic data in one representation, (ii) a large-scale fiber orientation vector field, and (iii) a clustered version of the field. Alignment of a 3D-PLI dataset to an appropriate brain atlas provides expert-based delineation, segmentation, and, ultimately, visualization of selected anatomical structures. By means of these techniques, a detailed analysis of the complex fiber architecture in 3D is feasible

Dense Fiber Modeling for 3D-Polarized Light Imaging Simulations

3D-Polarized Light Imaging (3D-PLI) is a neuroimaging technique used to study the structural connectivity of the human brain at the meso- and microscale. In 3D-PLI, the complex nerve fiber architecture of the brain is characterized by 3D orientation vector fields that are derived from birefringence measurements of unstained histological brain sections by means of an effective physical model. To optimize the physical model and to better understand the underlying microstructure, numerical simulations are essential tools to optimize the used physical model and to understand the underlying microstructure in detail. The simulations rely on predefined configurations of nerve fiber models (e.g. crossing, kissing, or complex intermingling), their physical properties, as well as the physical properties of the employed optical system to model the entire 3D-PLI measurement. By comparing the simulation and experimental results, possible misinterpretations in the fiber reconstruction process of 3D-PLI can be identified. Here, we focus on fiber modeling with a specific emphasize on the generation of dense fiber distributions as found in the human brain's white matter. A new algorithm will be introduced that allows to control possible intersections of computationally grown fiber structures

Diattenuation Imaging reveals different brain tissue properties

When transmitting polarised light through histological brain sections, different types of diattenuation (polarisation-dependent attenuation of light) can be observed: In some brain regions, the light is minimally attenuated when it is polarised parallel to the nerve fibres (referred to as D+), in others, it is maximally attenuated (referred to as D-). The underlying mechanisms of these effects and their relationship to tissue properties were so far unknown. Here, we demonstrate in experimental studies that diattenuation of both types D+ and D- can be observed in brain tissue samples from different species (rodent, monkey, and human) and that the strength and type of diattenuation depend on the nerve fibre orientations. By combining finite-difference time-domain simulations and analytical modelling, we explain the observed diattenuation effects and show that they are caused both by anisotropic absorption (dichroism) and by anisotropic light scattering. Our studies demonstrate that the diattenuation signal depends not only on the nerve fibre orientations but also on other brain tissue properties like tissue homogeneity, fibre size, and myelin sheath thickness. This allows to use the diattenuation signal to distinguish between brain regions with different tissue properties and establishes Diattenuation Imaging as a valuable imaging technique.Comment: 18 pages, 9 figure

Scattered Light Imaging: Resolving the substructure of nerve fiber crossings in whole brain sections with micrometer resolution

For developing a detailed network model of the brain based on image reconstructions, it is necessary to spatially resolve crossing nerve fibers. The accuracy hereby depends on many factors, including the spatial resolution of the imaging technique. 3D Polarized Light Imaging (3D-PLI) allows the three-dimensional reconstruction of nerve fiber tracts in whole brain sections with micrometer in-plane resolution, but leaves uncertainties in pixels containing crossing fibers. Here we introduce Scattered Light Imaging (SLI) to resolve the substructure of nerve fiber crossings. The measurement is performed on the same unstained histological brain sections as in 3D-PLI. By illuminating the brain sections from different angles and measuring the transmitted (scattered) light under normal incidence, SLI provides information about the underlying nerve fiber structure. A fully automated evaluation of the resulting light intensity profiles has been developed, allowing the user to extract various characteristics, like the individual directions of in-plane crossing nerve fibers, for each image pixel at once. We validate the reconstructed nerve fiber directions against results from previous simulation studies, scatterometry measurements, and fiber directions obtained from 3D-PLI. We demonstrate in different brain samples (human optic tracts, vervet monkey brain, rat brain) that the 2D fiber directions can be reliably reconstructed for up to three crossing nerve fiber bundles in each image pixel with an in-plane resolution of up to 6.5 $\mu$m. We show that SLI also yields reliable fiber directions in brain regions with low 3D-PLI signals coming from regions with a low density of myelinated nerve fibers or out-of-plane fibers. In combination with 3D-PLI, the technique can be used for a full reconstruction of the three-dimensional nerve fiber architecture in the brain.Comment: 30 pages, 16 figure

The Qualification of Silicon Microstrip Detector Modules for the CMS Inner Tracking Detector

For the construction of the CMS inner tracking detector 15,232 silicon microstrip detectors had to be produced. This large number required a fast, easy to use and cost-efficient test setup for the quality assurance at different production steps in all laboratories participating in the production of the detector modules. This article describes typical faults occurring on modules as well as the test procedures used to identify and classify them provided by the APV Readout Controller (ARC) system. To establish the final test procedures the data of more than 500 tracker endcap modules had been analysed in detail. An optimal combination of measures was found that prove to be extremely efficient in detecting and properly identifying all relevant failure modes of a detector module. Finally the quality of all modules for the CMS silicon microstrip tracker is quoted