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PubMed1996

Three-dimensional reconstruction of color Doppler flow convergence regions and regurgitant jets: an in vitro quantitative study.

OBJECTIVES: This study sought to investigate the applicability of a current implementation of a three-dimensional echocardiographic reconstruction method for color Doppler flow convergence and regurgitant jet imaging. BACKGROUND: Evaluation of regurgitant flow events, such as flow convergences or regurgitant jets, using two-dimensional imaging ultrasound color flow Doppler systems may not be robust enough to characterize these spatially complex events. METHODS: We studied two in vitro models using steady flow to optimize results. In the first constant-flow model, two different orifices were each mounted to produce flow convergences and free jets--a circular orifice and a rectangular orifice with orifice area of 0.24 cm(2). In another flow model, steady flows through a circular orifice were directed toward a curved surrounding wall to produce wall adherent jets. Video composite data of color Doppler flow images from both free jet and wall jet models were reconstructed and analyzed after computer-controlled 180 degrees rotational acquisition using a TomTec computer. RESULTS: For the free jet model there was an excellent relation between actual flow rates and three-dimensional regurgitant jet volumes for both circular and rectangular orifices (r = 0.99 and r = 0.98, respectively). However, the rectangular orifice produced larger jet volumes than the circular orifice, even at the same flow rates (p < 0.0001). Calculated flow rates by the hemispheric model using one axial measurement of the flow convergence isovelocity surface from two-dimensional color flow images underestimated actual flow rate by 35% for the circular orifice and by 44% for the rectangular orifice, whereas a hemielliptic method implemented using three axial measurements of the flow convergence zone derived using three-dimensional reconstruction correlated well with and underestimated actual flow rate to a lesser degree (22% for the circular orifice, 32% for the rectangular orifice). In the wall jet model, the jets were flattened against and spread along the wall and had reduced regurgitant jet volumes compared with free jets (p < 0.01). CONCLUSIONS: Three-dimensional reconstruction of flow imaged by color Doppler may add quantitative spatial information to aid computation methods that have been used for evaluating valvular regurgitation, especially where they related to complex geometric flow events.

Echocardiography, Doppler, Color↗
PubMed2001

Spatial distribution of Reissner's fiber glycoproteins in the filum terminale of the rat and rabbit.

The subcommissural organ secretes into the third ventricle glycoproteins that condense to form the Reissner's fiber (RF). At the distal end of the central canal of the spinal cord, the RF-glycoproteins accumulate in the form of an irregular mass known as massa caudalis. Antibodies against RF-glycoproteins and a set of lectins were used at the light and electron microscopic level to investigate the spatial distribution of the massa caudalis material in the rat and rabbit filum terminale. In the sacral region of the rat, the central canal presents gaps between the ependymal cells through which RF-glycoproteins spread out. The bulk of massa caudalis material, however, escapes through openings in the dorsal wall of the terminal ventricle. In the rabbit, the massa caudalis is formed within the ependymal canal, at the level of the second coccygeal vertebra, it accumulates within preterminal and terminal dilatations of the central canal, and it escapes out through gaps in the dorsal ependymal wall of the terminal ventricle. The existence of wide intercellular spaces and a large orifice (neuroporous) in the dorsal ependymal wall of the terminal ventricle, and the passage of RF-material through them, appear to be conserved evolutionary features. After leaving the terminal ventricle of the rat and rabbit, RF-glycoproteins establish a close spatial association with the numerous blood vessels irrigating the filum terminale, suggesting that in these species the blood vessels are the site of destination of the RF-glycoproteins escaping from the central canal, thus resembling the situation found in lower vertebrates. When passing from the RF stage to the massa caudalis stage, the rabbit RF-glycoproteins lose their sialic acid residues, exposing galactose as the terminal residue. Since this sialic acid-galactose modification of RF-glycoproteins had also been described in lamprey larvae, it may be regarded as a conserved evolutionary feature associated with the formation of the massa caudalis.

Animals↗
PubMed2000

Monte Carlo simulation of single-cell irradiation by an electron microbeam.

A model is presented for irradiation of a cellular monolayer by an electron microbeam. Results are presented for two possible window designs, cells plated on the vacuum-isolation window and cells plated on Mylar above the vacuum-isolation window. Even for the thicker dual-membrane window that facilitates tissue culture and allows the target cell to be centered relative to the electron beam, the majority of the calculated beam spreading was contained in a volume typical of the mammalian HeLa cell line. None of the 10(4) electrons simulated at 25 keV were scattered into the spatial region occupied by neighbors of the target cell. Dose leakage was largest at 50 keV where the mean energy deposited in all neighbors was 21% of that deposited in the target cell. This ratio was reduced to 5% at 90 keV, the highest beam energy simulated. Lineal energy spectra of energy deposition events scored in the nucleus of the target cell became progressively more like the gamma-ray spectrum as the electron beam energy increased. Hence, our simulations provide strong support for the feasibility of a low-LET, single-cell irradiator.

Cell Nucleus↗
PubMed2002

Fluid flow in an animal model of post-traumatic syringomyelia.

More than a quarter of patients with spinal cord injury develop syringomyelia, often with progressive neurological deficit. Treatment options remain limited and long-term failure rates are high. The current poor understanding is impeding development of improved therapies. The source and route of fluid flow into syringes has been investigated using cerebrospinal fluid (CSF) tracers. Previous work using a model of canalicular syringomyelia has shown that fluid enters the dilated central canal from perivascular spaces. The aim of this study was to determine the source and route of fluid flow in an animal model of extracanalicular (post-traumatic) syringomyelia. A model of post-traumatic syringomyelia was established in 25 Sprague-Dawley rats with intraparenchymal injections of quisqualic acid and kaolin-induced arachnoiditis. Rats survived for 6 weeks before injection of the CSF tracer horseradish peroxidase into the cisterna magna. Examination of the spatial distribution of horseradish peroxidase at 0, 3, 5, 10, or 20 min after injection was used to determine the route of fluid flow. Horseradish peroxidase rapidly spread to the ventromedian fissure, perivascular spaces, central canal, and extracanalicular syrinx. Flow occurred into the syrinx prior to significant perivascular flow in the rostral spinal cord. Preferential flow into the syrinx occurred from the perivascular spaces of the central penetrating branches of the anterior spinal artery in the grey matter. Transparenchymal flow into the syrinx was less prominent than perivascular flow. This is the first report of fluid flow within the spinal cord in a model of post-traumatic syringomyelia. Fluid from perivascular spaces moves preferentially into extracanalicular syringes and the surrounding parenchyma. Obstruction to CSF flow and loss of compliance from traumatic arachnoiditis might potentiate fluid flow in the perivascular space.

Animals↗
PubMed1980

Acoustic trauma in the guinea pig cochlea: early changes in ultrastructure and neural threshold.

Scanning microscopy was used to examine guinea pig cochleas for structural damage immediately after exposure to a pure tone ranging from 96 to 129 dB SPL. Functional changes to the cochlear neural sensitivity were assessed using the N1 audiogram. principal findings were: (1) The order of damage to receptor cells with increasing sound intensity was OHC1, then IHC, then OHC2 and OHC3. (2) The spatial distribution of damage to OHC1 and IHC differed with IHC tending to show damage mainly in the vicinity of the exposure frequently location and OHC1 damage spreading basal-ward of this point. (3) N1 threshold losses spread progressively to lower frequencies as exposure intensity increased. This was accompanied by an apical spread of damage to the receptor cells.

Animals↗
PubMedDate not supplied

Approaches to fast MR imaging.

Fast MR scanning may be accomplished through a number of strategies as discussed below. Each technique accomplishes improvements in scan time at the expense of signal and noise, spatial resolution, contrast resolution or other imaging artifacts. The scanning strategies discussed are in a variety of stages of clinical implementation, although several are already in wide spread use. All the existing techniques offer improvements in scan time, but they are still far from the theoretical limits imposed by the physics of MR scanning and further reduction in MR scanning time maybe expected in the future.

Image Enhancement↗
PubMed2005

Computer simulations of successful defibrillation in decoupled and non-uniform cardiac tissue.

AIM: The aim of the present study is to investigate the origin and effect of virtual electrode polarization in uniform, decoupled and non-uniform cardiac tissue during field stimulation. METHODS: A discrete bidomain model with active membrane behaviour was used to simulate normal cardiac tissue as well as cardiac tissue that is decoupled due to fibrosis and gap junction remodelling. Various uniform and non-uniform electric fields were applied to the external domain of uniform, decoupled and non-uniform resting cardiac tissue as well as cardiac tissue in which spiral waves were induced. RESULTS: Field stimulation applied on non-uniform tissue results in more virtual electrodes compared with uniform tissue. The spiral waves were terminated in decoupled tissue, but not in uniform, homogeneous tissue. By gradually increasing local differences in intracellular conductivities, the amount and spread of virtual electrodes increased and the spiral waves were terminated. CONCLUSION: Fast depolarization of the tissue after field stimulation may be explained by intracellular decoupling and spatial heterogeneity present in normal and pathological cardiac tissue. We demonstrated that termination of spiral waves by means of field stimulation can be achieved when the tissue is modelled as a non-uniform, anisotropic bidomain with active membrane behaviour.

Anisotropy↗
PubMed2006

How accurate is Poisson-Boltzmann theory for monovalent ions near highly charged interfaces?

Surface sensitive synchrotron X-ray scattering studies were performed to obtain the distribution of monovalent ions next to a highly charged interface. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface to control surface charge density. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion (Cs+) distributions next to the negatively charged interfaces. Five decades in bulk concentrations are investigated, demonstrating that the interfacial distribution is strongly dependent on bulk concentration. We show that this is due to the strong binding constant of hydronium H3O+ to the phosphate group, leading to proton-transfer back to the phosphate group and to a reduced surface charge. The increase of Cs+ concentration modifies the contact value potential, thereby causing proton release. This process effectively modifies surface charge density and enables exploration of ion distributions as a function of effective surface charge-density. The experimentally obtained ion distributions are compared to distributions calculated by Poisson-Boltzmann theory accounting for the variation of surface charge density due to proton release and binding. We also discuss the accuracy of our experimental results in discriminating possible deviations from Poisson-Boltzmann theory.

Journal Article↗
PubMed2006

Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon).

Adaptive radiations have served as model systems for quantifying the build-up of species richness. Few studies have quantified the tempo of diversification in species-rich clades that contain negligible adaptive disparity, making the macroevolutionary consequences of different modes of evolutionary radiation difficult to assess. We use mitochondrial-DNA sequence data and recently developed phylogenetic methodologies to explore the tempo of diversification of eastern North American Plethodon, a species-rich clade of woodland salamanders exhibiting only limited phenotypic disparity. Lineage-through-time analysis reveals a high rate of lineage accumulation, 0.8 species per million years, occurring 11-8 million years ago in the P. glutinosus species group, followed by decreasing rates. This high rate of lineage accumulation is exceptional, comparable to the most rapid of adaptive radiations. In contrast to classic models of adaptive radiation where ecological niche divergence is linked to the origin of species, we propose that phylogenetic niche conservatism contributes to the rapid accumulation of P. glutinosus-group lineages by promoting vicariant isolation and multiplication of species across a spatially and temporally fluctuating environment. These closely related and ecologically similar lineages persist through long-periods of evolutionary time and form strong barriers to the geographic spread of their neighbours, producing a subsequent decline in lineage accumulation. Rapid diversification among lineages exhibiting long-term maintenance of their bioclimatic niche requirements is an under-appreciated phenomenon driving the build-up of species richness.

Animals↗
PubMed2005

Monovalent counterion distributions at highly charged water interfaces: proton-transfer and Poisson-Boltzmann theory.

Surface sensitive synchrotron-x-ray scattering studies reveal the distributions of monovalent ions next to highly charged interfaces. A lipid phosphate (dihexadecyl hydrogen phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the L3 Cs+ resonance, we provide spatial counterion distributions (Cs+) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H3O+ at the interface leads to proton transfer back to the phosphate group by a high contact potential, whereas high salt concentrations lower the contact potential resulting in proton release and increased surface charge density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions.

Journal Article↗
PubMed2005

Using adaline neural network for performance improvement of smart antennas in TDD wireless communications.

In time-division-duplex (TDD) mode wireless communications, downlink beamforming performance of a smart antenna system at the base station can be degraded due to variation of spatial signature vectors corresponding to mobile users especially in fast fading scenarios. To mitigate this, downlink beams must be controlled by properly adjusting their weight vectors in response to changing propagation dynamics. This can be achieved by modeling the spatial signature vectors in the uplink period and then predicting them to be used as beamforming weight vectors for the new mobile position in the downlink transmission period. We show that ADAptive LInear NEuron (ADALINE) network modeling based prediction of spatial signatures provides certain level of performance improvement compared to conventional beamforming method that employs spatial signature obtained in previous uplink interval. We compare the performance of ADALINE with autoregressive (AR) modeling based predictions under varying channel propagation (mobile speed, multipath angle spread, and number of multipaths), and filter order/delay conditions. ADALINE modeling outperforms AR modeling in terms of downlink SNR improvement and relative error improvement especially under high mobile speeds, i.e., V = 100 km/h.

Algorithms↗
PubMed1995

Spatiotemporal changes of Ca2+ during electrically evoked contractions in atrial and ventricular cells.

Spatial and temporal changes of intracellular calcium ion concentration ([Ca2+]i) during stimulated contractions were observed by confocal microscopy in rat ventricular and guinea pig atrial myocytes. Fluorescence intensity profiles in fluo 3-acetoxymethyl ester (fluo 3-AM)-loaded cells were collected from the entire cell, selected regions of the cell, or along a single scanned line across the cell. In rat ventricular myocytes, the increase of [Ca2+]i after a single stimulus from field electrodes occurred synchronously across the cell whether fluo 3 fluorescence was monitored in a narrow region aligned with the long axis of the cell or in line-scan images of a single z-line across the cell. However, during the onset of Ca2+ channel blockade by nifedipine (5 microM), electrical stimulation produced spatially nonuniform, focal increases of [Ca2+]i. In guinea pig atrial myocytes, stimulated increases of [Ca2+]i first appeared in focal regions at the cell periphery before spreading to the cell interior. Line-scan images showed the peripheral rise of [Ca2+]i led that at the center of the cell by 34 +/- 4 ms (mean +/- SE, n = 3). These data demonstrate that the t-tubular network ensures synchronous increases of [Ca2+]i throughout the cell during an action potential. In the absence of t tubules or when the number of sarcolemmal Ca2+ channels opened by membrane depolarization is greatly reduced, stimulated increases of [Ca2+]i can be observed to arise in focal regions of the cell.

Aniline Compounds↗
PubMed1998

Spatiotemporal patterns at the retinal output.

Edge enhancement in the retina is thought to be mediated by classical center-surround antagonism, first encountered as the interactions between horizontal cells and cones. But in the salamander retina these interactions do little to enhance edges. Instead, a robust dynamic interaction between amacrine and bipolar cells appears to be responsible for a sharp edge enhancement. To demonstrate this we recorded extracellularly from a single ganglion cell and moved a flashed square, 300 micro(m) on a side, over a 1.5 x 1.0 mm2 grid at 25-micro(m) increments. Playing back all of these recordings simultaneously simulated the pattern of responses that would have been measured from an array of ganglion cells. The emerging pattern of ganglion cell activity first faithfully represented the flashed square, but after approximately 60 ms the center of the representation collapsed, leaving a representation of only the edges. We inferred that the feedback synapse from amacrine to bipolar cells at gamma-aminobutyric acid-C (GABAC) receptors mediated this effect: bicuculline and strychnine were ineffective in altering the response pattern, but in picrotoxin the center of the representation did not collapse. The GABAergic amacrine cells thought to mediate this effect have quite narrow spread of processes, so the existence of this edge-enhancing effect suggests a mechanism quite different from classical lateral inhibition, namely the delayed inhibition of a spatially expanding input pattern.

Ambystoma↗
PubMed1986

Pioneer growth cone morphologies reveal proximal increases in substrate affinity within leg segments of grasshopper embryos.

We have compared the morphologies of approximately 5000 antibody-labeled afferent pioneer growth cones fixed at various stages of growth along their characteristic path over the epithelium in the legs of grasshopper embryos, and have used growth cone morphology as an indicator of differences in the affinity of the epithelial substrate for pioneer growth cones in vivo. Growth cone morphologies differ markedly between different locations in limb buds, and also in the same location in limbs at different stages of differentiation. Growth cones characteristically extend branches and lamellae circumferentially along segment boundaries, and filopodia and lamellae are retained (or extended) longer there. Where they contact a relatively well-differentiated segment boundary, the growth cones also abruptly reorient circumferentially. In the proximal regions of limb segments, growth cones consistently have a high degree of branching and lamellae; previously formed axons also extend secondary branches and spread there as development progresses. Low incidence of these morphologies is observed at all stages in the distal regions of limb segments. Thus, neuronal morphologies correlate both spatially and temporally with the differentiation of limb segmentation. These results suggest the following: Detailed growth cone morphology is a reliable indicator of differences in extrinsic guidance cues. The affinity of the epithelial substrate for afferent pioneer growth cones increases proximally within segments, with a peak at the segment boundary. (This affinity could be based on surface density of adhesion molecules or on nonadhesive molecules that actively regulate growth cone extension.) Increasing epithelial affinity within segments appears to act as a proximal guidance cue for afferent pioneer growth cones. Pioneer growth cones are observed to navigate proximally in circumstances where proximally located guidepost cells differentiate too late to guide them.

Animals↗
PubMed2000

Propagation of spontaneous synchronized activity in cortical slice cultures recorded by planar electrode arrays.

The spatial propagation of synchronized activity in cortical slice cultures was characterized by multi-site extracellular recording. Spontaneous activity was studied in normal culture medium, and in bicuculline- or kainic acid-containing media. A common feature in all these conditions was that activity was generated first in superficial layers (i.e., layer I/II) before spreading over the whole area of the slice. In culture medium or bicuculline-containing medium, the initiation site of the activity was not constant and showed a large variety of patterns of horizontal propagation. Kainic acid induced epileptiform activity, consisting of intense initial bursts followed by repetitive after-discharges. Though the patterns of spatial propagation of the bursts were variable as in the other conditions, the after-discharges followed a constant path. Cross-correlation analysis indicated that the network moved in a graded fashion to a steady state during the sequence of after-discharges.

Animals↗
PubMed1996

The role of postlesion seizures and spreading depression in the upregulation of glial fibrillary acidic protein mRNA after entorhinal cortex lesions.

Unilateral lesions of the entorhinal cortex have been shown to lead to dramatic increases in GFAP mRNA levels in denervated zones in the hippocampus and dentate gyrus and sometimes (but not always) in nondenervated zones in the contralateral hippocampus and dentate gyrus. The variable distribution of the increases in GFAP mRNA expression suggests that the events which trigger changes in GFAP mRNA levels occur to a variable extent in individual animals. The companion paper characterizes two candidate triggering events: spreading depression (SD) that occurs to a variable extent at the time of the lesion and recurrent seizures that occur during the early postlesion interval. The goal of the present study was to evaluate whether individual differences in the extent or spatial distribution of lesion-induced increases in GFAP mRNA are related to the occurrence of either SD or seizures. We quantified the increases in GFAP mRNA levels in individual animals that had been monitored physiologically to define the incidence of SD and postlesion seizures. The results revealed that the quantitative extent of the increases in GFAP mRNA in denervated zones and was not related to either SD or postlesion seizures. The increases in GFAP mRNA in nondenervated zones also were not related to episodes of spreading depression that occurred at the time of lesion production but were related to the spontaneous seizures that developed during the first 24 h postlesion after the animals had recovered from the surgical anesthesia. Taken together, these data indicate that physiological events that occur during the early postlesion interval can play an important role in determining the pattern and extent of altered cellular gene expression in response to an injury.

Animals↗
PubMed1986

A model for the polarization of neurons by extrinsically applied electric fields.

A model is presented for the subthreshold polarization of a neuron by an applied electric field. It gives insight into how morphological features of a neuron affect its polarizability. The neuronal model consists of one or more extensively branched dendritic trees, a lumped somatic impedance, and a myelinated axon with nodes of Ranvier. The dendritic trees branch according to the 3/2-power rule of Rall, so that each tree has an equivalent cylinder representation. Equations for the membrane potential at the soma and at the nodes of Ranvier, given an arbitrary specified external potential, are derived. The solutions determine the contributions made by the dendritic tree and the axon to the net polarization at the soma. In the case of a spatially constant electric field, both the magnitude and sign of the polarization depend on simple combinations of parameters describing the neuron. One important combination is given by the ratio of internal resistances for longitudinal current spread along the dendritic tree trunk and along the axon. A second is given by the ratio between the DC space constant for the dendritic tree trunk and the distance between nodes of Ranvier in the axon. A third is given by the product of the electric field and the space constant for the trunk of the dendritic tree. When a neuron with a straight axon is subjected to a constant field, the membrane potential decays exponentially with distance from the soma. Thus, the soma seems to be a likely site for action potential initiation when the field is strong enough to elicit suprathreshold polarization. In a simple example, the way in which orientation of the various parts of the neuron affects its polarization is examined. When an axon with a bend is subjected to a spatially constant field, polarization is focused at the bend, and this is another likely site for action potential initiation.

Action Potentials↗
PubMed2006

Supramolecular assemblies of adsorbed collagen affect the adhesion of endothelial cells.

The behavior of endothelial cells (HUVECs) in contact with thin collagen films presenting different supramolecular organizations was investigated. Collagen was adsorbed on polystyrene (PS) and plasma-oxidized PS (PSox) in conditions ensuring the formation of continuous layers presenting an increasing density of fibrillar structures. Discontinuous collagen layers were also prepared on PS by adsorption followed by dewetting. The morphology of the obtained collagen films was checked by using atomic force microscopy. HUVECs adhesion was evaluated in terms of cell number, cell area, cell shape, and actin structure after 4 h of contact with the prepared collagen layers. In the presence of serum, no adhesion was observed on PS, whereas a substantial adhesion was found on PSox. This is explained by the competition for adsorption, which turns in favor of adhesive proteins secreted by the cells on the hydrophilic PSox, but turns in favor of serum albumin on the hydrophobic PS. The progressive coating of PS by smooth collagen films increased cell adhesion and spreading. However, cell spreading and cytoskeleton organization were adversely affected by the appearance of a high density of collagen fibrillar structures. This latter trend was similarly observed on PSox. On the other hand, HUVECs spreading and cytoskeleton organization were clearly enhanced on discontinuous collagen layers compared with continuous ones. A possible explanation for these observations lies in the modification of exposure and/or spatial distribution of recognition sequences due to spontaneous collagen self-assembly on fibril formation or to collagen aggregation on dewetting.

Adsorption↗