Extratemporal white matter abnormalities in mesial temporal lobe epilepsy demonstrated with diffusion tensor imaging

PURPOSE: Recent studies have demonstrated bilateral white matter abnormalities in temporal lobe epilepsy (TLE) patients with unilateral mesial temporal sclerosis (MTS). The purpose of this project was to determine whether abnormalities of water diffusion are seen in extratemporal white matter of patients with TLE and pathologically confirmed MTS and to determine whether these findings are associated with worse surgical outcome. METHODS: Eleven patients with TLE and unilateral MTS confirmed in surgical specimens and 14 controls were studied by using cerebrospinal fluid-suppressed diffusion tensor imaging (DTI) and T2 relaxometry. RESULTS: Hippocampal T2 signal for patients was significantly elevated both ipsilateral (p<0.001) and contralateral (p=0.006) to MTS. DTI demonstrated reduced fractional anisotropy of the genu of the corpus callosum (p=0.003) and external capsule (p=0.02) and elevated mean diffusivity of the genu (p=0.005), splenium (p=0.03), and external capsule (p<0.001). For both the genu and external capsule, parallel diffusion of patients was not different from that of controls (genu, p=0.81; external capsule, p=0.45), whereas perpendicular diffusion was elevated (genu, p=0.001; external capsule, p<0.001). With mean postsurgical follow-up of 18.5 months, eight of 11 patients were entirely seizure free and the remaining three had all experienced a worthwhile reduction in seizure frequency. CONCLUSIONS: Our findings suggest that although patients with TLE and MTS have extensive bilateral and extratemporal pathology, these findings may not be associated with a worse postsurgical outcome.     

Anisotropy and Spatial Heterogeneity in Quantitative Ultrasound Parameters: Relevance to the Study of the Human Cervix

Imaging biomarkers based on quantitative ultrasound can offer valuable information about properties that inform tissue function and behavior such as microstructural organization (e.g., collagen alignment) and viscoelasticity (i.e., compliance). For example, the cervix feels softer as its microstructure remodels during pregnancy, an increase in compliance that can be objectively quantified with shear wave speed and therefore shear wave speed estimation is a potential biomarker of cervical remodeling. Other proposed biomarkers include parameters derived from the backscattered echo signal, such as attenuation and backscattered power loss, because such parameters can provide insight into tissue microstructural alignment and organization. Of these, attenuation values for the pregnant cervix have been reported, but large estimate variance reduces their clinical value. That said, parameter estimates based on the backscattered echo signal may be incorrect if assumptions they rely on, such as tissue isotropy and homogeneity, are violated. For that reason, we explored backscatter and attenuation parameters as potential biomarkers of cervical remodeling via careful investigation of the assumptions of isotropy and homogeneity in cervical tissue. Specifically, we estimated the angle- and spatial-dependence of parameters of backscattered power and acoustic attenuation in the ex vivo human cervix, using the reference phantom method and electronic steering of the ultrasound beam. We found that estimates are anisotropic and spatially heterogeneous, presumably because the tissue itself is anisotropic and heterogeneous. We conclude that appropriate interpretation of imaging biomarkers of cervical remodeling must account for tissue anisotropy and heterogeneity.     

Glia and volume transmission during physiological and pathological states

Summary. Extrasynaptic communication between neurons or neurons and glia is mediated by the diffusion of neuroactive substances through the extracellular space (ECS). Structural changes and amino acid release occurring under physiological and pathological conditions result in cellular (particularly glial) swelling, leading to dynamic changes in the ECS volume and geometry that in turn affect ECS diffusion. Significant changes in ECS volume and in diffusion barriers occur during development and aging. They are often the result of cell death, astrogliosis, the rearrangement of astrocytic processes and changes in extracellular matrix molecules. Plastic changes in ECS volume, geometry and anisotropy significantly affect the spatial relation of glial processes towards synapses, glutamate or GABA spillover, synaptic cross-talk and neuron-glia communication/interaction. In addition, changes occurring during pathological states can be important for diagnosis, drug delivery and treatment.     

不同弥散强度对脑组织ADC值及FA值测定的影响

探讨不同弥散强度对弥散张量成像中表观弥散系数值及部分各向异性指数值测定的影响.材料和方法:使用七个不同的弥散梯度(b值分别为b0=0、b1=500s/mm2、b2=1000s/mm2、b3=1500s/mm2、b4=2000s/mm2、b5=2500s/mm2和b6=3000s/mm2)对20名正常成人脑组织进行弥散张量成像,对7个不同的b值分别进行组合(组合方式为b0b1、b0b2、b0b3、b0b5、b0b6和b0b6)及后处理,分别获得其ADC图和FA图,然后在ADC图和FA图上分别测定尾状核头、内囊后肢、丘脑、半卵圆中心、侧脑室体部及胼胝体膝部和压部的数值并进行统计学分析.结果:脑内各个解剖部位的ADC值均随着弥散强度的增加而显著降低(P＜0.05);FA值不随弥散强度的改变而改变(P＞0.05).结论:FA值不随弥散强度的改变而发生改变;随着弥散强度的改变,ADC值也发生改变,在进行临床研究时要充分考虑弥散强度对ADC值测定的影响.     

Visual field anisotropy revealed by perceptual filling-in

Four experiments were performed to investigate how the time required for perceptual filling-in varies with the position of the target in the visual field. Conventional studies have revealed that filling-in is facilitated by a target with greater eccentricity, while no systematic studies have examined the effect of polar angle. Experiment 1 examined the effect of polar angle when the target and surround differed in luminance. Filling-in was facilitated as the target position changed from the horizontal to the vertical meridian. This dependency was more prominent in the upper field than in the lower, although no asymmetry was found between the left and right visual fields. These features were observed in both monocular and binocular viewing. These results were replicated in a modified stimulus configuration, in which the surround was a circular region concentric with the target (Experiment 2). Moreover, it was confirmed that the asymmetry was not due to fluctuation in the retinal image (i.e., eye movement) (Experiment 3). Finally, Experiment 4 examined whether this anisotropy was observed when two differently oriented gratings were presented in the target and surround regions. Again, filling-in was facilitated for a target close to the vertical meridian, irrespective of the relationship between the target and surround orientations. The underlying mechanism of this anisotropy is discussed from the viewpoints of cortical magnification and neural connections in the visual cortex.     

Extracellular Measurement of Anisotropic Bidomain Myocardial Conductivities. I. Theoretical Analysis

The passive electrical properties of cardiac tissue, such as the intracellular and interstitial conductivities along the longitudinal and transverse axes, have not been often measured because intracellular electrodes are usually needed for these measurements. In this paper, we present a theoretical analysis of two myocardial models developed to estimate these properties by analyzing potentials recorded with a pair of extracellular electrodes while injecting alternating current between another pair of electrodes. First, the cardiac tissue is represented by a standard bidomain model which includes a membrane capacitance; second, this model is modified by adding an intracellular capacitance representing the intercalated disks. Numerical solutions are computed with a fast Fourier transform algorithm without constraining the anisotropy ratios of the interstitial and intracellular domains. We systematically investigate the effects of changes in the bidomain parameters on the voltage-to-current ratio curves. We also demonstrate how the bidomain parameters can be theoretically estimated by fitting, with a modified Shor's r algorithm, the simulated potentials along the longitudinal and transverse axes for different frequencies between 10 and 10000 Hz. An important finding is that the interelectrode distance must be similar to the myocardial space constant so as to obtain frequency dependent measurements. © 2001 Biomedical Engineering Society. PAC01: 8719Nn, 8719Hh, 8716Uv, 0230Uu, 8716Ac     

Increase in bone volume fraction precedes architectural adaptation in growing bone

In mature trabecular bone, both density and trabecular orientation are adapted to external mechanical loads. Few quantitative data are available on the development of architecture and mechanical adaptation in juvenile trabecular bone. We studied the hypothesis that a time lag occurs between the adaptation of trabecular density and the adaptation of trabecular architecture during development. To investigate this hypothesis we used ten female pigs at 6, 23, 56, 104, and 230 weeks of age. Three-dimensional morphological and mechanical parameters of trabecular bone samples from the vertebra and proximal tibia were studied using microcomputed tomography and micro-finite element analysis. Both bone volume fraction and stiffness increased rapidly in the initial growth phase (from 6 weeks on), whereas the morphological anisotropy started increasing only after 23 weeks of age. In addition, the anisotropy reached its highest value much later in the development than did bone volume fraction. Hence, the alignment of trabeculae was still progressing at the time of peak bone mass. Therefore, our hypothesis was supported by the time lag between the increase in trabecular density and the adaptation of the trabecular architecture. The rapid increase of bone volume fraction in the initial growth phase can be explained by the enormous weight increase of the pigs. The trabeculae aligned at later stages when the increase in weight, and thus the loading, was slowed considerably compared with the early growth stage. Hence, the trabecular architecture was more efficient in later years. We conclude that density is adapted to external load from the early phase of growth, whereas the trabecular architecture is adapted later in the development.     

Finite element modeling of soft tissues: Material models,tissue interaction and challenges

Background

Musculoskeletal soft tissues, such as articular cartilage, ligaments, knee meniscus and intervertebral disk, have a complex structure, which provides elasticity and capability to support and distribute the body loads. Soft tissues describe an inhomogeneous and multiphasic structure, and exhibit a nonlinear, time-dependent behavior. Their mechanical response is governed by a substance composed of protein fiber-rich and proteoglycan-rich extracellular matrix and interstitial fluid. Protein fibers (e.g. collagen) give the tissue direction dependent stiffness and strength. To investigate these complex biological systems, the use of mathematical tools is well established, alone or in combination with experimental in vitro and in vivo tests. However, the development of these models poses many challenges due to the complex structure and mechanical response of soft tissues.

Methods

Non-systematic literature review.

Findings

This paper provides a summary of different modeling strategies with associated material properties, contact interactions between articulating tissues, validation and sensitivity of soft tissues with special focus on knee joint soft tissues and intervertebral disk. Furthermore, it reviews and discusses some salient clinical findings of reported finite element simulations.

Interpretation

Model studies extensively contributed to the understanding of functional biomechanics of soft tissues. Models can be effectively used to elucidate clinically relevant questions. However, users should be aware of the complexity of such tissues and of the capabilities and limitations of these approaches to adequately simulate a specific in vivo or in vitro phenomenon.     

DTI在足月新生儿窒息后脑损伤中的应用

目的：探讨磁共振弥散张量成像(diffusion tensor imaging DTI)在评价足月新生儿窒息后脑损伤中的价值。方法搜集临床确诊为新生儿缺氧缺血性脑病(hypoxic-ischemic encephalopathy HIE)的患儿15例,有窒息史但临床未诊断为HIE的患儿15例及正常组新生儿15例,应用1.5T磁共振仪对所有患儿及正常新生儿出生后行颅脑常规序列和DTI序列扫描。测量HIE组、窒息非HIE组及对照组新生儿颅脑各感兴趣区的各向异性分数(fractional anisotropy FA)值并比较各组间FA值是否具有统计学意义。结果 HIE组患儿额叶白质、内囊后肢、胼胝体压部、大脑脚及半卵圆中心FA值较对照组患儿降低,差异具有统计学意义(P＜0.05);窒息非HIE组患儿额叶白质、内囊后肢、胼胝体压部及半卵圆中心FA值较对照组新生儿降低,差异具有统计学意义(P＜0.05);HIE组患儿较窒息非HIE组患儿内囊后肢FA值降低,差异具有统计学意义(P＜0.05)。结论磁共振弥散张量成像可以早期发现窒息患儿脑损伤并指导临床进行早期干预以改善预后。