Measurement of the hyperelastic properties of ex vivo brain tissue slices

The elastic and hyperelastic properties of brain tissue are of interest to the medical research community as there are several applications where accurate characterization of these properties is crucial for an accurate outcome. The linear response is applicable to brain elastography, while the non-linear response is of interest for surgical simulation programs. Because of the biological differences between gray and white matter, it is reasonable to expect a difference in their mechanical properties. The goal of this work is to characterize the elastic and hyperelastic properties of the brain gray and white matter. In this method, force-displacement data of these tissues were acquired from 25 different brain samples using an indentation apparatus. These data were processed with an inverse problem algorithm using finite element method as the forward problem solver. Young's modulus and the hyperelastic parameters corresponding to the commonly used Polynomial, Yeoh, Arruda-Boyce, and Ogden models were obtained. The parameters characterizing the linear and non-linear mechanical behavior of gray and white matters were found to be significantly different. Young's modulus was 1787±186 and 1195±157Pa for white matter and gray matter, respectively. Among hyperelastic models, due to its accuracy, fewer parameters and shorter computational time requirements, Yeoh model was found to be the most suitable. Due to the significant differences between the linear and non-linear tissue response, we conclude that incorporating these differences into brain biomechanical models is necessary to increase accuracy.     

Residual stress in the adult mouse brain

This work provides direct evidence that sustained tensile stress exists in white matter of the mature mouse brain. This finding has important implications for the mechanisms of brain development, as tension in neural axons has been hypothesized to drive cortical folding in the human brain. In addition, knowledge of residual stress is required to fully understand the mechanisms behind traumatic brain injury and changes in mechanical properties due to aging and disease. To estimate residual stress in the brain, we performed serial dissection experiments on 500-mum thick coronal slices from fresh adult mouse brains and developed finite element models for these experiments. Radial cuts were made either into cortical gray matter, or through the cortex and the underlying white matter tract composed of parallel neural axons. Cuts into cortical gray matter did not open, but cuts through both layers consistently opened at the point where the cut crossed the white matter. We infer that the cerebral white matter is under considerable tension in the circumferential direction in the coronal cerebral plane, parallel to most of the neural fibers, while the cerebral cortical gray matter is in compression. The models show that the observed deformation after cutting can be caused by more growth in the gray matter than in the white matter, with the estimated tensile stress in the white matter being on the order of 100–1,000 Pa.     

Dielectric properties of porcine brain tissue in the transition from life to death at frequencies from 800 to 1900 MHz

Ten experiments on pigs were performed to investigate possible postmortem changes of the dielectric properties of brain gray matter in the frequency range of 800-1900 MHz. After keeping the animals in stable anaesthesia for at least 45 min, they were euthanatised by an intravenous injection of hypertonic potassium chloride (KCl), causing cardiac arrest within 3 min. Measurements of the dielectric properties were performed repeatedly from at least 45 min prior to death to 18 h after euthanasia. The anaesthesia regimen was chosen to minimize influence on brain tissue characteristics such as brain water content, intracranial blood volume, and cerebral blood flow. The data showed a decline of mean gray matter equivalent conductivity of about 15% at 900 MHz and about 11% at 1800 MHz within the first hour after death. The decline in permittivity was less pronounced (about 3-4%) and almost frequency independent. The results indicate that in vitro measurements of dielectric properties of brain tissue underestimate equivalent conductivity as well as permittivity of living tissue. These changes may affect the generally accepted data of dielectric properties of brain tissue widely used in RF dosimetry.     

Regional,directional, and age-dependent properties of the brain undergoing large deformation

The large strain mechanical properties of adult porcine gray and white matter brain tissues were measured in shear and confirmed in compression. Consistent with local neuroarchitecture, gray matter showed the least amount of anisotropy, and corpus callosum exhibited the greatest degree of anisotropy. Mean regional properties were significantly distinct, demonstrating that brain tissue is inhomogeneous. Fresh adult human brain tissue properties were slightly stiffer than adult porcine properties but considerably less stiff than the human autopsy data in the literature. Mixed porcine gray/white matter samples were obtained from animals at "infant" and "toddler" stages of neurological development, and shear properties compared to those in the adult. Only the infant properties were significantly different (stiffer) from the adult.     

Differential distribution of the 5-alpha-reductase in the central nervous system of the rat and the mouse: are the white matter structures of the brain target tissue for testosterone action?

In the brain of several animal species testosterone is converted into a series of 5-alpha-reduced metabolites, and especially into 17-beta-hydroxy-5-alpha-androstan-3-one (DHT), by the action of the enzyme 5-alpha-reductase. The formation of DHT has never been evaluated in the white matter structures of the brain, which are composed mainly of myelinated axons. The experiments here described were performed in order to study, in the rat and the mouse, the DHT forming activity of several white matter structures, in comparison with that of the cerebral cortex and of the hypothalamus. Two sampling techniques were used in the rat: microdissection under a stereo-microscope from frozen brain sections of fragments of corpus callosum, optic chiasm and cerebral cortex; fresh tissue macrodissection of subcortical white matter, cerebral cortex and hypothalamus. Only macrodissection was used in the mice. The data show that, independently from the sampling technique used, there are considerable quantitative differences in the distribution pattern of the 5-alpha-reductase activity within different brain structures. Both in the rat and in the mouse, the enzyme appears to be present in higher concentrations in the white matter structures, than in the cerebral cortex and in the hypothalamus. The present results clearly show that the subcortical white matter and the corpus callosum are at least three times as potent as the cerebral cortex in converting testosterone into DHT. An even higher 5-alpha-reductase activity has been found in the optic chiasm. Further work is needed in order to understand the possible physiological role of DHT formation in the white matter structures.     

Shear linear behavior of brain tissue over a large frequency range

The literature review about the shear linear properties of brain tissue reveals both a large discrepancy in the existing data and a crucial lack of information at high frequencies associated with traffic road and non-penetrating ballistic impacts. The purpose of this study is to clarify and to complement the linear material characterisation of brain tissue. New data at small strains and high frequencies were obtained from oscillatory experiments. The tests were performed on thin porcine white matter samples (corona radiata) using an original custom-designed oscillatory shear testing device. At 37 degrees C, the results showed that the mean storage modulus (G') and the mean loss modulus (G') increased with the frequency (0.1 to 6310 Hz) from 2.1+/-0.9 kPa to 16.8+/-2.0 kPa and from 0.4+/-0.2 kPa to 18.7+/-2.3 kPa respectively. The reliability of these new dynamic data was checked over a partially common frequency range by conducting similar experiments using a standard rheometer (Bohlin C-VOR 150). Data were also compared in the time field. From these experiments, the relaxation modulus (G(t)) was found to decrease from 24.4+/-2.1 kPa to 1.0+/-0.3 kPa between 10(-5) s and 270 s.     

Lipid and Fatty Acid Composition of Brain Tissue from Adrenoleukodystrophy Patients

Abstract: White matter and active plaque tissue from adrenoleukodystrophy (ALD) patients were analysed for lipid class and fatty acid compositions and the results compared with white matter from normal brain. ALD white matter was characterized by increased levels of cholesteryl esters and decreased levels of phosphatidylethanola- mine, including phosphatidylethanolamine plasmalogen, in comparison with normal brain white matter. In addition to even higher levels of cholesteryl esters, ALD plaque tissue had reduced levels of cerebrosides as well as phosphati-dylethanolamines. The loss of phosphatidylethanolamine plasmalogen is indicative of early demyelination. Total lipid from ALD white matter and ALD plaque tissue contained nearly five times and seven times, respectively, more 26:0 than total lipid from normal brain white matter. The 26:0 in ALD white matter was elevated in all lipid classes except phosphatidylinositol, but was located mainly in cerebrosides, phosphatidylcholine, sphingomyelin, and sulfatides. Most of the 26:0 in ALD plaque tissue was present in cholesteryl esters, followed by phosphatidylcholine and sphingomyelin, with reduced amounts in cerebrosides as compared with ALD white matter. The results are consistent with an initial accumulation of very-long-chain fatty acids in ALD white matter, primarily in sphingolipids and phosphatidylcholine, and subsequent accumulation of very-long- chain fatty acids in cholesteryl esters during demyelination. In addition, it was notable that the sphingolipids, especially sphingomyelin in ALD brain, had decreased levels of 24:1 and increased levels of 18:0, as well as increased levels of very-long-chain fatty acids. The extent to which the data shed light on mechanisms of demyelination in ALD is discussed.     

Changes in Rat Brain Tissue Microstructure and Stiffness during the Development of Experimental Obstructive Hydrocephalus

Understanding neural injury in hydrocephalus and how the brain changes during the course of the disease in-vivo remain unclear. This study describes brain deformation, microstructural and mechanical properties changes during obstructive hydrocephalus development in a rat model using multimodal magnetic resonance (MR) imaging. Hydrocephalus was induced in eight Sprague-Dawley rats (4 weeks old) by injecting a kaolin suspension into the cisterna magna. Six sham-injected rats were used as controls. MR imaging (9.4T, Bruker) was performed 1 day before, and at 3, 7 and 16 days post injection. T2-weighted MR images were collected to quantify brain deformation. MR elastography was used to measure brain stiffness, and diffusion tensor imaging (DTI) was conducted to observe brain tissue microstructure. Results showed that the enlargement of the ventricular system was associated with a decrease in the cortical gray matter thickness and caudate-putamen cross-sectional area (P < 0.001, for both), an alteration of the corpus callosum and periventricular white matter microstructure (CC+PVWM) and rearrangement of the cortical gray matter microstructure (P < 0.001, for both), while compression without gross microstructural alteration was evident in the caudate-putamen and ventral internal capsule (P < 0.001, for both). During hydrocephalus development, increased space between the white matter tracts was observed in the CC+PVWM (P < 0.001), while a decrease in space was observed for the ventral internal capsule (P < 0.001). For the cortical gray matter, an increase in extracellular tissue water was significantly associated with a decrease in tissue stiffness (P = 0.001). To conclude, this study characterizes the temporal changes in tissue microstructure, water content and stiffness in different brain regions and their association with ventricular enlargement. In summary, whilst diffusion changes were larger and statistically significant for majority of the brain regions studied, the changes in mechanical properties were modest. Moreover, the effect of ventricular enlargement is not limited to the CC+PVWM and ventral internal capsule, the extent of microstructural changes vary between brain regions, and there is regional and temporal variation in brain tissue stiffness during hydrocephalus development.     

Multidimensional analysis and detection of informative features in human brain white matter

The white matter contains long-range connections between different brain regions and the organization of these connections holds important implications for brain function in health and disease. Tractometry uses diffusion-weighted magnetic resonance imaging (dMRI) to quantify tissue properties along the trajectories of these connections. Statistical inference from tractometry usually either averages these quantities along the length of each fiber bundle or computes regression models separately for each point along every one of the bundles. These approaches are limited in their sensitivity, in the former case, or in their statistical power, in the latter. We developed a method based on the sparse group lasso (SGL) that takes into account tissue properties along all of the bundles and selects informative features by enforcing both global and bundle-level sparsity. We demonstrate the performance of the method in two settings: i) in a classification setting, patients with amyotrophic lateral sclerosis (ALS) are accurately distinguished from matched controls. Furthermore, SGL identifies the corticospinal tract as important for this classification, correctly finding the parts of the white matter known to be affected by the disease. ii) In a regression setting, SGL accurately predicts “brain age.” In this case, the weights are distributed throughout the white matter indicating that many different regions of the white matter change over the lifespan. Thus, SGL leverages the multivariate relationships between diffusion properties in multiple bundles to make accurate phenotypic predictions while simultaneously discovering the most relevant features of the white matter.     

Cortical tissue from syngenous brain as a stimulator of immunogenesis in thymectomized mice]

Experiments on CBA mice showed that the gray matter homogenate of the syngeneic brain contained components reconstituting the T-cell population in the spleen of thymectomized animals and possessing a stimulating effect on the immune response to sheep red blood cells. The white matter homogenate was much less active, it is not excluded that the trace activity is due to the gray matter admixture in the preparation. Syngeneic muscle tissue homogenate possessed no biological activity.     

Dynamic mechanical response of bovine gray matter and white matter brain tissues under compression

Dynamic responses of brain tissues are needed for predicting traumatic brain injury (TBI). We modified a dynamic experimental technique for characterizing high strain-rate mechanical behavior of brain tissues. Using the setup, the gray and white matters from bovine brains were characterized under compression to large strains at five different strain rates ranging from 0.01 to 3000/s. The white matter was examined both along and perpendicular to the coronal section for anisotropy characterization. The results show that both brain tissue matters are highly strain-rate sensitive. Differences between the white matter and gray matter in their mechanical responses are recorded. The white matter shows insignificant anisotropy over all strain rates. These results will lead to rate-dependent material modeling for dynamic event simulations.     

A comparative study of brain and kidney glycolipids in metachromatic leucodystrophy

Sulphatides and cerebrosides from white matter of brains of patients with metachromatic leucodystrophy (MLD) have been isolated and compared in fatty acid composition to those glycolipids found in MLD kidney tissue. A marked difference in glycolipid composition was found between the brain and kidney tissues. The sulphatides accumulated in MLD kidney have the same fatty acid profile as those found in normal kidney tissue and are typical‘kidney sulphatides.’The neutral glycolipids of MLD kidney retain larger amounts of the longer chain acids than do the cerebrosides of MLD brain white matter and thus resemble more closely in fatty acid composition, glycolipids of normal tissue. Structurally, the sulfate group is located at the C-3 position of the galactose molecule in sulphatides from normal and MLD tissue. As in the brain white matter, the sulphatides which accumulate in the kidney tissue of patients with MLD are normal in structure and composition.     

Automatic Segmentation of Eight Tissue Classes in Neonatal Brain MRI

Purpose

Volumetric measurements of neonatal brain tissues may be used as a biomarker for later neurodevelopmental outcome. We propose an automatic method for probabilistic brain segmentation in neonatal MRIs.

Materials and Methods

In an IRB-approved study axial T1- and T2-weighted MR images were acquired at term-equivalent age for a preterm cohort of 108 neonates. A method for automatic probabilistic segmentation of the images into eight cerebral tissue classes was developed: cortical and central grey matter, unmyelinated and myelinated white matter, cerebrospinal fluid in the ventricles and in the extra cerebral space, brainstem and cerebellum. Segmentation is based on supervised pixel classification using intensity values and spatial positions of the image voxels. The method was trained and evaluated using leave-one-out experiments on seven images, for which an expert had set a reference standard manually. Subsequently, the method was applied to the remaining 101 scans, and the resulting segmentations were evaluated visually by three experts. Finally, volumes of the eight segmented tissue classes were determined for each patient.

Results

The Dice similarity coefficients of the segmented tissue classes, except myelinated white matter, ranged from 0.75 to 0.92. Myelinated white matter was difficult to segment and the achieved Dice coefficient was 0.47. Visual analysis of the results demonstrated accurate segmentations of the eight tissue classes. The probabilistic segmentation method produced volumes that compared favorably with the reference standard.

Conclusion

The proposed method provides accurate segmentation of neonatal brain MR images into all given tissue classes, except myelinated white matter. This is the one of the first methods that distinguishes cerebrospinal fluid in the ventricles from cerebrospinal fluid in the extracerebral space. This method might be helpful in predicting neurodevelopmental outcome and useful for evaluating neuroprotective clinical trials in neonates.     

Studies of the immunohistochemical and biochemical localization of the cytochrome P-450scc-linked monooxygenase system in the adult rat brain

Immunohistochemical and biochemical studies were performed on the brains of adult female and male rats using a specific antibody against bovine adrenocortical cytochrome P-450scc. The results showed that in both male and female rats, the myelinated regions of the white matter are selectively immunostained throughout the brain and that even in rats pretreated with colchicine, there is never positive staining of neuronal cell bodies and their dendrites in any brain region. Western immunoblotting with the P-450scc antibody and enzymatic assays revealed that P-450scc and cholesterol side-chain cleavage activity were present in a homogenate derived from the cortical white matter, but not detectable in that from the cerebral cortex. Furthermore, quantitation of the P-450scc protein in the immunoblots indicated that the concentration of P-450scc in the cortical white matter of both female and male rat brains is approx. 3-4 pmol per mg tissue protein. Thus it could be concluded that in the adult rat brain, P-450scc and cholesterol side-chain cleavage activity are selectively localized only in the myelinated region of the white matter.     

Effect of substances of a polypeptide nature isolated from the cerebral cortex on the immune response of mice

The influence of low molecular weight polypeptides (Mol wt below 10,000) isolated by acetic acid extraction from the cortex and white matter of the brain and from the bone marrow of cows on the primary immune response to SRBC was studied in experiments on 248 male CBA mice. The results showed that after the subcutaneous injections of the preparations from the cortex (where the theta-antigen cross-reacting with the thymocytes was localized) for 5 days prior to and 3 days post immunization, hemagglutinin titres and the quantity of direct (IgM) and indirect (IgG) antibody-forming cells increased 2.5 times in comparison with the control. The preparations from the white matter of the brain and from the bone marrow failed to demonstrate such an effect.     

Bovine brain gray and white matter exhibit differential protein prenyl transferase activity

Protein farnesyl transferase and geranylgeranyl transferase-I activities were determined in gray and white matter from various regions of bovine brain. Farnesyl transferase activity was 3–8 times greater than geranylgeranyl transferase-I activity. However, farnesyl transferase activity was about 2 times greater in the white matter than in the gray matter in all regions of the brain. Mixing experiments indicated lack of farnesyl transferase activators in white matter. This difference in farnesyl transferase activity may be due to enzyme content and may have implications in brain cell function.     

Gene Expression Profile of Glioblastoma Peritumoral Tissue: An Ex Vivo Study

The gene expression pattern of glioblastoma (GBM) is well documented but the expression profile of brain adjacent to tumor is not yet analysed. This may help to understand the oncogenic pathway of GBM development. We have established the genome-wide expression profiles of samples isolated from GBM tumor mass, white matter adjacent to tumor (apparently free of tumor cells), and white matter controls by using the Affymetrix HG-U133 arrays. Array-CGH (aCGH) was also performed to detect genomic alterations. Among genes dysregulated in peritumoral white matter, 15 were over-expressed, while 42 were down-regulated when compared to white matter controls. A similar expression profile was detected in GBM cells. Growth, proliferation and cell motility/adhesion-associated genes were up-regulated while genes involved in neurogenesis were down-regulated. Furthermore, several tumor suppressor genes along with the KLRC1 (a member of natural killer receptor) were also down-regulated in the peritumoral brain tissue. Several mosaic genomic lesions were detected by aCGH, mostly in tumor samples and several GBM-associated mosaic genomic lesions were also present in the peritumoral brain tissue, with a similar mosaicism pattern. Our data could be explained by a dilution of genes expressed from tumor cells infiltrating the peritumour tissue. Alternatively, these findings could be substained by a relevant amount of “apparently normal” cells presenting a gene profile compatible with a precancerous state or even “quiescent” cancer cells. Otherwise, the recurrent tumor may arise from both infiltrating tumor cells and from an interaction and recruitment of apparently normal cells in the peritumor tissue by infiltrating tumor cells.     

White Matter Microstructure Alterations: A Study of Alcoholics with and without Post-Traumatic Stress Disorder

Many brain imaging studies have demonstrated reductions in gray and white matter volumes in alcoholism, with fewer investigators using diffusion tensor imaging (DTI) to examine the integrity of white matter pathways. Among various medical conditions, alcoholism and post-traumatic stress disorder (PTSD) are two comorbid diseases that have similar degenerative effects on the white matter integrity. Therefore, understanding and differentiating these effects would be very important in characterizing alcoholism and PTSD. Alcoholics are known to have neurocognitive deficits in decision-making, particularly in decisions related to emotionally-motivated behavior, while individuals with PTSD have deficits in emotional regulation and enhanced fear response. It is widely believed that these types of abnormalities in both alcoholism and PTSD are related to fronto-limbic dysfunction. In addition, previous studies have shown cortico-limbic fiber degradation through fiber tracking in alcoholism. DTI was used to measure white matter fractional anisotropy (FA), which provides information about tissue microstructure, possibly indicating white matter integrity. We quantitatively investigated the microstructure of white matter through whole brain DTI analysis in healthy volunteers (HV) and alcohol dependent subjects without PTSD (ALC) and with PTSD (ALC+PTSD). These data show significant differences in FA between alcoholics and non-alcoholic HVs, with no significant differences in FA between ALC and ALC+PTSD in any white matter structure. We performed a post-hoc region of interest analysis that allowed us to incorporate multiple covariates into the analysis and found similar results. HV had higher FA in several areas implicated in the reward circuit, emotion, and executive functioning, suggesting that there may be microstructural abnormalities in white matter pathways that contribute to neurocognitive and executive functioning deficits observed in alcoholics. Furthermore, our data do not reveal any differences between ALC and ALC+PTSD, suggesting that the effect of alcohol on white matter microstructure may be more significant than any effect caused by PTSD.     

On the characterization of the heterogeneous mechanical response of human brain tissue

The mechanical characterization of brain tissue is a complex task that scientists have tried to accomplish for over 50 years. The results in the literature often differ by orders of magnitude because of the lack of a standard testing protocol. Different testing conditions (including humidity, temperature, strain rate), the methodology adopted, and the variety of the species analysed are all potential sources of discrepancies in the measurements. In this work, we present a rigorous experimental investigation on the mechanical properties of human brain, covering both grey and white matter. The influence of testing conditions is also shown and thoroughly discussed. The material characterization performed is finally adopted to provide inputs to a mathematical formulation suitable for numerical simulations of brain deformation during surgical procedures.     

Specific binding properties of 125I-apamin in various structures of the rat central nervous system

The properties of 125I-apamin binding with rat central nervous system slices were analysed in vitro using computerized densitometric autoradiography. Scatchard analysis performed for the data of binding experiments in rat brain and spinal cord demonstrates that apamin binds to a single class of non-interacting binding sites in all investigated structures. The dissociation constant values (KD) were similar in all investigated structures (31-38 pM). The maximal binding capacity (Bmax) was observed in the structures of limbic olfactory system (30 fmol/mg protein), the lowest in brain white matter (0.5 fmol/mg protein). It is concluded that the observed pattern of 125I-apamin binding might represent the topography of a class of Ca2+ dependent K+ channels in the rat central nervous system.