A direct fiber approach to model sclera collagen architecture and biomechanics.

Sclera collagen fiber microstructure and mechanical behavior are central to eye physiology and pathology. They are also complex, and are therefore often studied using modeling. Most models of sclera, however, have been built within a conventional continuum framework. In this framework, collagen fibers are incorporated as statistical distributions of fiber characteristics such as the orientation of a family of fibers. The conventional continuum approach, while proven successful for describing the macroscale behavior of the sclera, does not account for the sclera fibers are long, interwoven and interact with one another. Hence, by not considering these potentially crucial characteristics, the conventional approach has only a limited ability to capture and describe sclera structure and mechanics at smaller, fiber-level, scales. Recent advances in the tools for characterizing sclera microarchitecture and mechanics bring to the forefront the need to develop more advanced modeling techniques that can incorporate and take advantage of the newly available highly detailed information. Our goal was to create a new computational modeling approach that can represent the sclera fibrous microstructure more accurately than with the conventional continuum approach, while still capturing its macroscale behavior. In this manuscript we introduce the new modeling approach, that we call direct fiber modeling, in which the collagen architecture is built explicitly by long, continuous, interwoven fibers. The fibers are embedded in a continuum matrix representing the non-fibrous tissue components. We demonstrate the approach by doing direct fiber modeling of a rectangular patch of posterior sclera. The model integrated fiber orientations obtained by polarized light microscopy from coronal and sagittal cryosections of pig and sheep. The fibers were modeled using a Mooney-Rivlin model, and the matrix using a Neo-Hookean model. The fiber parameters were determined by inversely matching experimental equi-biaxial tensile data from the literature. After reconstruction, the direct fiber model orientations agreed well with the microscopy data both in the coronal plane (adjusted R2 = 0.8234) and in the sagittal plane (adjusted R2 = 0.8495) of the sclera. With the estimated fiber properties (C10 = 5746.9 MPa; C01 = -5002.6 MPa, matrix shear modulus 200 kPa), the model's stress-strain curves simultaneously fit the experimental data in radial and circumferential directions (adjusted R2's 0.9971 and 0.9508, respectively). The estimated fiber elastic modulus at 2.16% strain was 5.45 GPa, in reasonable agreement with the literature. During stretch, the model exhibited stresses and strains at sub-fiber level, with interactions among individual fibers which are not accounted for by the conventional continuum methods. Our results demonstrate that direct fiber models can simultaneously describe the macroscale mechanics and microarchitecture of the sclera, and therefore that the approach can provide unique insight into tissue behavior questions inaccessible with continuum approaches.

Autofluorescence enhancement for label-free imaging of myelinated fibers in mammalian brains.

Analyzing the structure of neuronal fibers with single axon resolution in large volumes is a challenge in connectomics. Different technologies try to address this goal; however, they are limited either by the ineffective labeling of the fibers or in the achievable resolution. The possibility of discriminating between different adjacent myelinated axons gives the opportunity of providing more information about the fiber composition and architecture within a specific area. Here, we propose MAGIC (Myelin Autofluorescence imaging by Glycerol Induced Contrast enhancement), a tissue preparation method to perform label-free fluorescence imaging of myelinated fibers that is user friendly and easy to handle. We exploit the high axial and radial resolution of two-photon fluorescence microscopy (TPFM) optical sectioning to decipher the mixture of various fiber orientations within the sample of interest. We demonstrate its broad applicability by performing mesoscopic reconstruction at a sub-micron resolution of mouse, rat, monkey, and human brain samples and by quantifying the different fiber organization in control and Reeler mouse's hippocampal sections. Our study provides a novel method for 3D label-free imaging of nerve fibers in fixed samples at high resolution, below micrometer level, that overcomes the limitation related to the myelinated axons exogenous labeling, improving the possibility of analyzing brain connectivity.

Analytical and fast Fiber Orientation Distribution reconstruction in 3D-Polarized Light Imaging.

Three dimensional Polarized Light Imaging (3D-PLI) is an optical technique which allows mapping the spatial fiber architecture of fibrous postmortem tissues, at sub-millimeter resolutions. Here, we propose an analytical and fast approach to compute the fiber orientation distribution (FOD) from high-resolution vector data provided by 3D-PLI. The FOD is modeled as a sum of K orientations/Diracs on the unit sphere, described on a spherical harmonics basis and analytically computed using the spherical Fourier transform. Experiments are performed on rich synthetic data which simulate the geometry of the neuronal fibers and on human brain data. Results indicate the analytical FOD is computationally efficient and very fast, and has high angular precision and angular resolution. Furthermore, investigations on the right occipital lobe illustrate that our strategy of FOD computation enables the bridging of spatial scales from microscopic 3D-PLI information to macro- or mesoscopic dimensions of diffusion Magnetic Resonance Imaging (MRI), while being a means to evaluate prospective resolution limits for diffusion MRI to reconstruct region-specific white matter tracts. These results demonstrate the interest and great potential of our analytical approach.

FAConstructor: an interactive tool for geometric modeling of nerve fiber architectures in the brain.

PURPOSE: The technique 3D polarized light imaging (3D-PLI) allows to reconstruct nerve fiber orientations of postmortem brains with ultra-high resolution. To better understand the physical principles behind 3D-PLI and improve the accuracy and reliability of the reconstructed fiber orientations, numerical simulations are employed which use synthetic nerve fiber models as input. As the generation of fiber models can be challenging and very time-consuming, we have developed the open source FAConstructor tool which enables a fast and efficient generation of synthetic fiber models for 3D-PLI simulations. METHODS: The program was developed as an interactive tool, allowing the user to define fiber pathways with interpolation methods or parametric functions and providing visual feedback. RESULTS: Performance tests showed that most processes scale almost linearly with the amount of fiber points in FAConstructor. Fiber models consisting of < 1.6 million data points retain a frame rate of more than 30 frames per second, which guarantees a stable and fluent workflow. The applicability of FAConstructor was demonstrated on a well-defined fiber model (Fiber Cup phantom) for two different simulation approaches, reproducing effects known from 3D-PLI measurements. CONCLUSION: We have implemented a user-friendly and efficient tool that enables an interactive and fast generation of synthetic nerve fiber configurations for 3D-PLI simulations. Already existing fiber models can easily be modified, allowing to simulate many different fiber models in a reasonable amount of time.

MEDUSA: A GPU-based tool to create realistic phantoms of the brain microstructure using tiny spheres.

A GPU-based tool to generate realistic phantoms of the brain microstructure is presented. Using a spherical meshing technique which decomposes each microstructural item into a set of overlapping spheres, the phantom construction is made very fast while reliably avoiding the collisions between items in the scene. This novel method is applied to the construction of human brain white matter microstructural components, namely axonal fibers, oligodendrocytes and astrocytes. The algorithm reaches high values of packing density and angular dispersion for the axonal fibers, even in the case of multiple white matter fiber populations and enables the construction of complex biomimicking geometries including myelinated axons, beaded axons, and glial cells. The method can be readily adapted to model gray matter microstructure.

Improvement of water saturation shift referencing by sequence and analysis optimization to enhance chemical exchange saturation transfer imaging.

PURPOSE: To optimize B0-field inhomogeneity correction for chemical exchange saturation transfer (CEST) imaging by investigating different water saturation shift referencing (WASSR) Z-spectrum shapes and different frequency correction techniques. METHODS: WASSR Z-spectra were simulated for different B1-fields and pulse durations (PD). Two parameter settings were used for further simulations and experiments (WASSR1: B1=0.1 µT, PD=50ms; WASSR2: B1=0.3 µT, PD=40ms). Four frequency correction techniques were investigated: 1) MinW: Minimum of the spline-interpolated WASSR-spectrum; 2) MSCF: maximum symmetry center frequency algorithm; 3) PMSCF: further development of MSCF algorithm; 4) BFit: fit with Bloch equations. Performance of frequency correction was assessed with Monte-Carlo simulations and in-vivo MR examinations in the brain and intervertebral disks. RESULTS: Different shapes of WASSR-Z-spectra were obtained by changing B1 and PD including spectra with one (1-Peak) or two (2-Peak) minima. WASSR1 resulted in 1-Peak WASSR-spectrum, whereas WASSR2 resulted in 2-Peak WASSR-spectrum. Both Monte-Carlo simulations and in-vivo MR examinations revealed highest accuracy of field-inhomogeneity correction with WASSR1 combined with PMSCF or BFit. CONCLUSION: Using a WASSR sequence, which results in a Z-spectrum with a single absorption peak, in combination with advanced postprocessing algorithms enables improved B0-field inhomogeneity correction for CEST imaging.

Gender, BMI and T2 dependencies of glycosaminoglycan chemical exchange saturation transfer in intervertebral discs.

PURPOSE: The purpose was to investigate the dependence of glycosaminoglycan chemical exchange saturation transfer (gagCEST) effect of lumbar intervertebral discs (IVD) on gender, body mass index and T2 value. METHODS: T2 imaging and gagCEST imaging was performed in 34 healthy volunteers (17 males, 17 females) without low back pain at a 3T MRI system (Magnetom Trio, A Tim System, Siemens Healthcare, Erlangen, Germany). The body mass index was determined for each volunteer. The mean and standard deviation of MTRasym and T2 values were calculated for nucleus pulposus (NP) and annulus fibrosus (AF) as descriptive statistics for females and males. An unpaired student's t-test was applied in order to validate obtained differences. Pearson correlation was determined in order to reveal, if gagCEST effect and T2 values decrease with increasing body mass index (BMI). Pearson correlation analysis was additionally performed between gagCEST and T2 values. RESULTS: GagCEST effect and T2 values were significantly higher in females compared to males [gagCEST effect (nucleus pulposus, females)=3.58±1.49%; gagCEST effect (nucleus pulosus, males)=3.01±1.63%, p-value (gagCEST effect, nucleus pulposus)=0.02); T2 (nucleus pulposus, females)=134.56±30.27 ms, T2 (nucleus pulposus, males)=122.35±27.64 ms, p-value (T2, nucleus pulposus)=0.01)]. Pearson correlation analysis showed a significant negative relation between BMI and gagCEST effect (nucleus pulposus: ρ=-0.16, p=0.03) and between BMI and T2 values (nucleus pulposus: ρ=-0.30, p<0.01). The correlation between gagCEST effect and T2-values was highly significant (nucleus pulposus: ρ=0.59, p<0.01). CONCLUSIONS: Significantly lower gagCEST effects were found in males compared to females and with increased body mass index. The gagCEST effect was highly correlated with quantitative T2 imaging.

Glycosaminoglycan Chemical Exchange Saturation Transfer of Lumbar Intervertebral Discs in Healthy Volunteers.

STUDY DESIGN: Evaluation of a new quantitative imaging technique in a prospective study design. OBJECTIVE: To assess glycosaminoglycan (GAG) content of lumbar intervertebral discs (IVDs) in healthy volunteers with chemical exchange saturation transfer (CEST). SUMMARY OF BACKGROUND DATA: Biochemical alterations of lumbar discs are present before the appearance of morphological changes. GAG loss plays a central role in these degenerative processes. METHODS: Lumbar intervertebral discs of healthy controls (26 women, 22 men; mean age 31 ±â€Š8 years; range: 21-49 years) without lumbar back pain were examined at a 3 Tesla magnetic resonance imaging (MRI) scanner in this prospective study. None of the participants were overweight or had previous surgery of the lumbar spine. The MRI protocol included standard morphological, sagittal and transversal T2-weighted (T2w) images to assess Pfirrmann score and to detect disc disorders according to the Combined Task Force classification of five lumbar IVDs (L1 to S1). A prototype glycosaminoglycan chemical exchange saturation transfer (gagCEST) sequence was applied to measure GAG content of the nucleus pulposus (NP) and annulus fibrosus (AF) by identifying the magnetization transfer asymmetry ratio (MTRasym) in a region-of-interest analysis. Morphological and biochemical imaging analysis were statistically tested for quantitative differences between different grades of IVD degeneration and disc disorders. RESULTS: gagCEST values of NP demonstrated a significant negative correlation with morphological Pfirrmann score (r = -0.562; P < 0.0001). The MTRasym values were higher in non-degenerative lumbar IVDs (Pfirrmann 1-2) compared with degenerative lumbar discs (Pfirrmann 3-5; 2.92% ±â€Š1.42% vs. 0.78% ±â€Š1.38%; P < 0.0001). The MTRasym values of NP were significantly higher in normal appearing discs compared with herniated IVDs (2.83% ±â€Š1.52% vs. 1.55% ±â€Š1.61%; P < 0.0001). We found a significant negative correlation between gagCEST values and the graduation of disc herniation (r = -0.372; P < 0.0001). CONCLUSION: Biochemical imaging with gagCEST distinguished morphologically degenerative from non-degenerative lumbar IVDs (in NP and AF) of healthy volunteers at a clinical 3T-MRI system. The depletion of GAG content in degenerative lumbar discs correlated significantly with the morphological disc classification. We could demonstrate that disc disorders, such as protrusion and extrusion, were accompanied by lower GAG content. LEVEL OF EVIDENCE: 2.

Age-dependency of glycosaminoglycan content in lumbar discs: A 3t gagcEST study.

PURPOSE: To analyze age-dependency of glycosaminoglycan content using gagCEST (glycosaminoglycan chemical exchange saturation transfer) imaging in lumbar intervertebral discs of healthy volunteers. MATERIALS AND METHODS: In all, 70 volunteers without low back pain (mean age 44 ± 14 years, range: 21-69 years) were examined with T2 -weighted and gagCEST imaging with a 3T MR scanner, with approval of the local Ethics Committee after written informed consent was obtained. Pfirrmann grading and classification into discs without bulging and herniation, discs with bulging, and discs with herniation were performed. Only intervertebral discs without bulging and herniation were analyzed. A region-of-interest-based gagCEST analysis of nucleus pulposus (NP) and annulus fibrosus (AF) was performed. Correlation between age and gagCEST was tested within groups of equal Pfirrmann score. RESULTS: GagCEST effects decreased significantly from 3.09 ± 1.12% in 20-29 years old volunteers to -0.24 ± 1.36% in 50-59 years old volunteers (P < 0.001). In the case of Pfirrmann scores 2 and 3, a significant correlation was observed between gagCEST effect and age (Pfirrmann score 2, NP: ρ = -0.558, P < 0.001; Pfirrmann score 3, NP: ρ = -0.337, P = 0.048). For Pfirrmann scores 1 and 4, no significant correlation was obtained (Pfirrmann score 1, NP: ρ = -0.046, P = 0.824; Pfirrmann score 4, NP: ρ = -0.316, P = 0.188). CONCLUSION: We show a decreased gagCEST effect likely corresponding to decreasing glycosaminoglycans with aging. Hence, age-matched analysis of gagCEST imaging may be necessary in future studies.

Glycosaminoglycan chemical exchange saturation transfer of lumbar intervertebral discs in patients with spondyloarthritis.

PURPOSE: To assess glycosaminoglycan (GAG) content of lumbar intervertebral discs (IVD) in patients with spondyloarthritis (SpA) using glycosaminoglycan chemical exchange saturation transfer (gagCEST). MATERIALS AND METHODS: Ninety lumbar intervertebral discs of nine patients with SpA and nine age-matched healthy controls (eight patients with ankylosing spondylitis; one patient with spondylitis related to inflammatory bowel disease; mean age: 44.1 ± 14.0 years; range: 27-72 years) were examined with a 3T magnetic resonance imaging (MRI) scanner in this prospective study. The MRI protocol included standard morphological, sagittal T2 -weighted (T2 w) images to assess Pfirrmann score of the five lumbar IVDs (L1 to S1) and biochemical imaging with gagCEST to calculate a region of interest analysis of nucleus pulposus (NP) and annulus fibrosus (AF). Prior to statistical testing of gagCEST effects (MTRasym values in percent) in patients and controls, IVDs were classified according to the Pfirrmann score. RESULTS: Significantly lower gagCEST values of NP and AF were found in SpA patients compared with healthy volunteers (NP: 1.41% ± 0.41%, P = 0.001; 95% confidence interval, CI [0.600%-2.226%]; AF: 1.19% ± 0.32%, P < 0.001; CI [0.560%-1.822%]) by comparing the differences of the means. Pooled nondegenerative IVDs (Pfirrmann 1 and 2) had significantly lower gagCEST effects in patients suffering from SpA compared with healthy controls in NP (P < 0.001; CI [1.176%-2.337%]) and AF (P < 0.001; CI [0.858%-1.779%]). No significant difference of MTRasym values was found in degenerative IVDs between patients and controls in NP (P = 0.204; CI [-0.504%-2.170%]). CONCLUSION: GagCEST analysis of morphologically nondegenerative IVDs (Pfirrmann score 1 and 2) in T2 w images demonstrated significantly lower GAG values in patients with spondyloarthritis in NP and AF, possibly representing a depletion of GAG in spondyloarthritis in the absence of morphologic degeneration.