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PubMed1999

Anisotropy of water diffusion in corona radiata and cerebral peduncle in patients with hemiparesis.

Diffusion tensor imaging is a magnetic resonance method which provides quantitative measurements of the directionality (anisotropy) of diffusion. Anisotropy measurements can be used to obtain quantitative information about the microstructural integrity of white matter tracts. In intact tracts diffusion is restricted and directional because water molecules move predominantly longitudinally to tracts. The aim of this study was to measure the anisotropy of diffusion in patients with chronic hemiparesis. We measured in the corona radiata and the cerebral peduncle in 10 patients with a chronic hemiparesis and supratentorial lesions and 10 control subjects in regions of interest. In all patients anisotropy was reduced in the coronal radiata contralateral to the hemiparesis by more than 3 SD compared to control subjects. In three patients, each of which had a severe hemiparesis, anisotropy in the cerebral peduncle was reduced by more than 3 SD compared to normal control subjects. Our findings suggest that reduced anisotropy is associated with chronic hemiparesis.

Adult↗
PubMed2002

Colour-coded fractional anisotropy images: differential visualisation of white-matter tracts--preliminary experience.

Diffusion-tensor analysis allows quantitative assessment of diffusion anisotropy. Fractional anisotropy (FA) is commonly used to quantify anisotropy. One of the limitations of FA imaging is, however, that it does not contain information about the directionality of anisotropy and it is therefore difficult to identify white-matter tracts on FA images. Our purpose was to describe a simple method of making composite images containing information about both magnitude and direction of diffusion anisotropy. The composite colour-coded FA images enabled us to identify different adjacent fibre bundles of similar degrees of diffusion anisotropy, and might be helpful in assessment of these fasciculi.

Adult↗
PubMed2000

Effects of anisotropy on the development of cardiac arrhythmias associated with focal activity.

The anisotropy that normally exists in the myocardium may be either enhanced in peri-infarction zones by loss of lateral cell connections or reduced by redistribution of gap junctions. To test how the degree of anisotropy affects the development of ectopic focal activity, we carried out computer simulations in which a model of an ectopic focus is incorporated as the central element of a two-dimensional sheet of ventricular cells. At low values of intercellular coupling conductance (Gc), the focus region is spontaneously active, but the limited intercellular current flow inhibits propagation. At high Gc, automaticity is suppressed by the loading effects of the surrounding cells. At intermediate Gc, the ectopic activity may propagate into the sheet. In the case of isotropic coupling, the minimum size of the focus region for propagation to occur (in terms of number of collaborating cells within the focus) is as small as approximately ten cells, and this number decreases with increasing anisotropy. Thus, the presence of anisotropy facilitates the development of ectopic focal activity. We conclude that the remodeling that occurs in peri-infarction zones may create a substrate that either facilitates (enhanced anisotropy) or inhibits (reduced anisotropy) the development of cardiac arrhythmias associated with ectopic focal activity.

Action Potentials↗
PubMed1993

Quantification of ultrasonic anisotropy in normal myocardium with lateral gain compensation of two-dimensional integrated backscatter images.

Anisotropy of ultrasonic scattering and attenuation in heart tissue depends on the specific orientation of myofibers with respect to angle of insonification. We used lateral gain compensation (LGC) to correct two-dimensional cardiac images for physiologic anisotropy. Normal hearts excised from three dogs and five pigs were insonified in a water tank with both 2.5 and 5.0 MHz phased-array transducers. Integrated backscatter was measured from a short-axis approach in the anterior wall perpendicular to the principal fiber axis, and in the septum parallel to the fiber axis. The gain in a vertical sector encompassing the septum was adjusted to compensate the image for anisotropy by matching the intensity of scattering from septal and anterior regions. The average gain required to compensate the septum for anisotropy was 16 dB at 2.5 MHz, and 20 dB at 5.0 MHz. Five healthy volunteers underwent imaging with a 2.5 MHz transducer from a parasternal short-axis view. The LGC required in vivo was approximately 16 dB at 2.5 MHz and was equivalent to that required for correction of septal anisotropy in excised hearts. Thus, normal myocardium exhibits substantial ultrasonic anisotropy that can be quantified and compensated for with clinically applicable tissue characterization techniques.

Animals↗
PubMed2004

Red-edge anisotropy microscopy enables dynamic imaging of homo-FRET between green fluorescent proteins in cells.

Steady-state fluorescence anisotropy measurements can be used to detect fluorescence resonance energy transfer (FRET) between identical fluorophores (homo-FRET). However, the contribution of homo-FRET to the steady-state anisotropy must be discerned from those due to the orientational distribution and rotational diffusion, which so far has required photobleaching controls, largely precluding dynamic measurements in live cells. We describe a variation of steady-state anisotropy microscopy in which the contribution of homo-FRET is dynamically isolated from the total anisotropy by exploiting the loss of energy transfer that occurs at red-edge excitation. Excitation of enhanced green fluorescent protein (EGFP) at the red-edge of its absorption band shows the shift in the emission spectrum compared to main-band excitation that is characteristic for photo-selection of static low energy S(0)-S(1) transitions that fail to exhibit FRET. An experimental setup for steady-state fluorescent anisotropy microscopy is described that can be used to acquire anisotropy images in live cells at main-band and red-edge excitation of EGFP. We demonstrate in live cells homo-FRET suppression of protein fusion constructs that consist of two and three EGFP molecules connected by short linkers. This methodology represents a novel approach for the dynamic measurement of homo-FRET in live cells that will be of utility in the biological sciences to detect oligomerization and concentration dependent interactions between identically labeled molecules.

Anisotropy↗
PubMed2006

The anisotropy of biological composites studied with ultrasonic technique.

One of the most common biological composites is wood material. This natural orthotropic like material is characterized by a high anisotropy determined by the special disposition of the microstructure elements. The anisotropy of wood can be described in various ways using the values of ultrasonic velocities of bulk waves (longitudinal and shear) observed on the velocity surface deduced from the theoretical relationships given by the Christofel's equation. The simultaneous view into the three symmetry planes of the anisotropic behavior of wood is presented on the velocity surface. The spatial filtering action of wood structure is easily connected with longitudinal and shear velocities. The first step in examining the anisotropy of wood is to relate the velocities to the symmetry axes. The simplest way to describe the anisotropy of wood is to express the ratios of velocities. These ratios can be calculated separately for longitudinal or shear waves or for a combination of both. The birefringence of shear waves have a particular interest for the fine definition of anisotropy. A more global appreciation of wood anisotropy than the values of individual velocities is given with acoustic invariants. The stability of calculation of acoustic invariants versus different propagation angles confirms the validity of the chosen model for the tested material. Wood species having high density and any important organized structure in the millimeter scale exhibit a high ratio of invariants. The acoustic behavior of tropical wood species is less anisotropic than that of species from a temperate zone having low density.

Algorithms↗
PubMed2003

Directional anisotropies reveal a functional segregation of visual motion processing for perception and action.

Human exhibits an anisotropy in direction perception: discrimination is superior when motion is around horizontal or vertical rather than diagonal axes. In contrast to the consistent directional anisotropy in perception, we found only small idiosyncratic anisotropies in smooth pursuit eye movements, a motor action requiring accurate discrimination of visual motion direction. Both pursuit and perceptual direction discrimination rely on signals from the middle temporal visual area (MT), yet analysis of multiple measures of MT neuronal responses in the macaque failed to provide evidence of a directional anisotropy. We conclude that MT represents different motion directions uniformly, and subsequent processing creates a directional anisotropy in pathways unique to perception. Our data support the hypothesis that, at least for visual motion, perception and action are guided by inputs from separate sensory streams. The directional anisotropy of perception appears to originate after the two streams have segregated and downstream from area MT.

Animals↗
PubMed2003

Complexation-induced chemical shifts--ab initio parameterization of transferable bond anisotropies.

Complexation-induced changes in proton chemical shifts provide a potent tool for conformational analysis, being highly dependent on intermolecular orientation. An important contribution to these shifts arises from the molecular magnetisability anisotropy, or more specifically from the anisotropy of certain groups, such as aromatic rings and unsaturated bonds. While the influence of aromatic rings has been well characterised via the ring current effect, unsaturated bonds have received much less attention and prediction of complexation shifts is hampered by the lack of accurate anisotropy parameters for these bonds. We have therefore used ab initio calculations at the HF/aug-cc-pVDZ level to obtain bond anisotropies for C-H, N-H, C=O, C=C, C triple bond N, N=N, C triple bond C, and C triple bond N. Fitting the anisotropies to bond magnetic dipoles (the McConnell equation) gives non-transferable values for C-H and N-H bonds. We have therefore expanded in terms of bond magnetic dipoles, quadrupoles, and octopoles for double and triple bonds only, obtaining highly accurate shielding surfaces in all cases. The transferable nature of the anisotropies is confirmed by comparing with shifts obtained in larger molecules containing unsaturated bonds.

Anisotropy↗
PubMed2001

Breakthrough waves during ventricular fibrillation depend on the degree of rotational anisotropy and the boundary conditions: a simulation study.

INTRODUCTION: The left ventricle (LV) and right ventricle (RV) are characterized by specific fiber orientation known as "rotational anisotropy." However, it remains unclear whether the LV and RV are different with regard to the effect of rotational anisotropy on the dynamics of scroll waves during ventricular fibrillation (VF). To resolve this issue, we used a computation-based model to study scroll wave behavior. METHODS AND RESULTS: We composed an environment of simulated three-dimensional ventricular wall slabs, with optional ratios of fiber rotation to wall thickness (0 degrees, 6 degrees, and 12 degrees/mm thickness; LV 10 mm, RV 5 mm), using Luo-Rudy phase I equations. When rotational anisotropy was not incorporated into the LV wall slab (theta endo to approximately theta epi = 0 degrees), most scroll waves rotated around the filaments perpendicular to the tissue surface, with only a few accompanying breakthrough waves. In a twisted LV model (theta endo to approximately theta epi = 60 degrees and 120 degrees), the scroll waves were demonstrated as multiple wavelets scattered spatiotemporally, frequently accompanied by breakthrough waves that were promoted by rotational anisotropy. In a twisted RV model (theta endo to approximately theta epi = 30 degrees and 60 degrees), single scroll waves and/or figure-of-eight reentrant waves appeared, with comparatively few breakthrough waves, regardless of the degree of fiber twist. CONCLUSION: The proportion of electrical effects of rotational anisotropy and tissue boundaries plays an important role in the genesis of breakthrough waves during VF, and the difference in wave propagating patterns and frequency spectrum of the ventricles may arise, in part, from the number of breakthrough waves promoted by rotational anisotropy.

Anisotropy↗
PubMed2005

A finite difference method with reciprocity used to incorporate anisotropy in electroencephalogram dipole source localization.

Many implementations of electroencephalogram (EEG) dipole source localization neglect the anisotropical conductivities inherent to brain tissues, such as the skull and white matter anisotropy. An examination of dipole localization errors is made in EEG source analysis, due to not incorporating the anisotropic properties of the conductivity of the skull and white matter. First, simulations were performed in a 5 shell spherical head model using the analytical formula. Test dipoles were placed in three orthogonal planes in the spherical head model. Neglecting the skull anisotropy results in a dipole localization error of, on average, 13.73 mm with a maximum of 24.51 mm. For white matter anisotropy these values are 11.21 mm and 26.3 mm, respectively. Next, a finite difference method (FDM), presented by Saleheen and Kwong (1997 IEEE Trans. Biomed. Eng. 44 800-9), is used to incorporate the anisotropy of the skull and white matter. The FDM method has been validated for EEG dipole source localization in head models with all compartments isotropic as well as in a head model with white matter anisotropy. In a head model with skull anisotropy the numerical method could only be validated if the 3D lattice was chosen very fine (grid size < or = 2 mm).

Algorithms↗
PubMed2001

Evaluation of white matter anisotropy in Krabbe disease with diffusion tensor MR imaging: initial experience.

PURPOSE: To compare diffusion tensor magnetic resonance imaging with conventional T2-weighted imaging for evaluation of white matter changes in patients with Krabbe disease. MATERIALS AND METHODS: In eight patients with Krabbe disease and eight age-matched control subjects, anisotropy maps were generated with diffusion tensor data by using echo-planar imaging with diffusion gradient encoding in six directions. Anisotropy maps and T2-weighted images were visually inspected. Relative anisotropy (RA) and normalized T2-weighted signal intensity in white matter tracts and gray matter nuclei were quantitatively compared between patients and controls (paired Student t test). RESULTS: Loss of diffusion anisotropy appeared on anisotropy maps as areas of decreased hyperintensity in patients with Krabbe disease. Differences in RA between Krabbe disease patients and control subjects were significant in eight of nine white matter structures studied (P =.001-.01) and in basal ganglia (P =.04). T2-weighted signal intensity was also significantly different in the same white matter structures (P =.006-.049) but not in basal ganglia. In the three patients imaged after stem cell transplantation, mean RA was between the RAs of untreated patients and control subjects. CONCLUSION: Diffusion tensor-derived anisotropy maps (a) provide a quantitative measure of abnormal white matter in patients with Krabbe disease, (b) are more sensitive than T2-weighted images for detecting white matter abnormality, and (c) may be a marker of treatment response.

Anisotropy↗
PubMed2002

Cardiomyocyte cultures with controlled macroscopic anisotropy: a model for functional electrophysiological studies of cardiac muscle.

Structural and functional cardiac anisotropy varies with the development, location, and pathophysiology in the heart. The goal of this study was to design a cell culture model system in which the degree, change in fiber direction, and discontinuity of anisotropy can be controlled over centimeter-size length scales. Neonatal rat ventricular myocytes were cultured on fibronectin on 20-mm diameter circular cover slips. Structure-function relationships were assessed using immunostaining and optical mapping. Cell culture on microabraded cover slips yielded cell elongation and coalignment in the direction of abrasion, and uniform, macroscopically continuous, elliptical propagation with point stimulation. Coarser microabrasion (wider and deeper abrasion grooves) increased longitudinal (23.5 to 37.2 cm/s; r=0.66) and decreased transverse conduction velocity (18.1 to 9.2 cm/s; r=-0.84), which resulted in increased longitudinal-to-transverse velocity anisotropy ratios (1.3 to 3.7, n=61). A thin transition zone between adjacent uniformly anisotropic areas with 45 degrees or 90 degrees difference in fiber orientation acted as a secondary source during 2x threshold field stimulus. Cell culture on cover slips micropatterned with 12- or 25- micro m wide fibronectin lines and previously coated with decreasing concentrations of background fibronectin yielded transition from continuous to discontinuous anisotropic architecture with longitudinally oriented intercellular clefts, decreased transverse velocity (16.9 to 2.6 cm/s; r=-0.95), increased velocity anisotropy ratios (1.6 to 5.6, n=70), and decreased longitudinal velocity (36.4 to 14.6 cm/s; r=-0.85) for anisotropy ratios >3.5. Cultures of cardiac myocytes with controlled degree, uniformity and continuity of structural, and functional anisotropy may enable systematic 2-dimensional in vitro studies of macroscopic structure-related mechanisms of reentrant arrhythmias. The full text of this article is available at http://www.circresaha.org.

Action Potentials↗
PubMed1998

Dependence of broadband ultrasound attenuation on the elastic anisotropy of trabecular bone.

The effect of trabecular elastic anisotropy on broadband ultrasound attenuation (BUA) and bone mineral density (BMD) was investigated with human and bovine cubic cancellous bones. Ultrasonic parameters describing trabecular anisotropy were found from the three orthogonal ultrasound velocities. BMD was measured using quantitative computed tomography. Three elastic anisotropy ratios were compared to BUA in all three directions and to BMD. The combined effect of anisotropic characteristics and BMD was also correlated with BUA. The results showed that the anisotropy ratios were significantly related to BUA (p < 0.05). There was, however, no correlation between BMD and the elastic anisotropy ratios. The combination of BMD and the anisotropy produced a significantly enhanced relationship with BUA.

Aged↗
PubMed2002

Diffusion anisotropy in the corpus callosum.

BACKGROUND AND PURPOSE: The corpus callosum is a heterogeneous white-matter tract that connects the cerebral hemispheres. The purpose of this investigation was to study its microstructural architecture in normal human adult brains by using diffusion tensor imaging (DTI). METHODS: Two hundred consecutive patients referred for brain MR imaging underwent additional DTI with a high gradient field strength applied in six directions. Forty-two patients met the following inclusion criteria: 1) normal brain and 2) age greater than 18 years. Anisotropy maps were generated, and regions of interest were drawn around specified regions within the corpus callosum. Results were stratified by sex and age. In addition, available histologic specimens of the corpus callosum from cadaver brains were analyzed with conventional and specialized vascular staining. RESULTS: Anisotropy values in the various regions of the corpus callosum differed significantly. Average values of the anisotropy index for the genu, body, and splenium of the corpus callosum were 0.400, 0.456, and 0.539, respectively. The differences between these values are statistically significant (P <.01). Increased anisotropy was present in posterior areas in both sexes and in all three age groups examined. CONCLUSION: The results of this investigation show a statistically significant increase in anisotropy of the corpus callosum in its more posterior portions compared with its more anterior portions across sex and age groups. Although the microstructural etiology for this apparent increase in anisotropy is unclear, a number of possible mechanisms are presented.

Adult↗
PubMed1996

Plasma membrane fluidity of keratinocytes of normal and psoriatic skin: a study using fluorescence anisotropy of trimethylammoniumdiphenylhexatriene (TMA-DPH).

The aim of this study was to investigate plasma membrane fluidity in human keratinocytes using fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and its cationic derivative 1-[4-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH). Keratinocytes from normal or psoriatic skin were isolated using trypsin-EDTA or dispase. In keratinocytes isolated from normal skin, TMA-DPH anisotropy values were higher than those observed using DPH; the difference must be related to the different localization of the two probes. In fact, DPH in whole cells localizes in plasma as well as intracellular membranes, yielding an average value of fluidity, while the cationic derivative TMA-DPH resides in the plasma membrane of the whole cells for a sufficient time for anisotropy measurements. Moreover, it has to be considered that plasma membrane is more ordered than intracellular membranes. The kinetics of incorporation of TMA-DPH was similar in keratinocytes isolated using trypsin-EDTA and those isolated using; dispase, however, the fluorescence anisotropy values were lower in keratinocytes isolated with dispase (0.260 +/- 0.01 vs 0.270 +/- 0.01, p = 0.029). This difference is probably related to modifications of lipid-protein interactions after trypsin treatment. Since no damage to plasma membrane after incubation with dispase seems to have been reported, we decided to use this separation procedure to study plasma membrane fluidity in psoriasis, a human pathological condition characterized by excessive cell proliferation and incomplete differentiation. Lower anisotropy values (0.260 +/- 0.01 vs 0.270 +/- 0.01, p = 0.001), indicating an increase in fluidity, were observed in keratinocytes isolated from skin of psoriatic patients than in epidermal cells isolated from normal human skin. We suggest that the measurement of fluorescence anisotropy in living cells is a convenient and useful tool to study membrane fluidity in human keratinocytes isolated from normal and diseased skin. Its application represents a technical advance because plasma membrane fluidity can be measured using very limited amounts of tissue, as obtained from biopsies.

Diphenylhexatriene↗
PubMed1993

Inhomogeneity and anisotropies for motion detection in the monocular visual field of human observers.

Signal-to-noise-ratio (SNR) thresholds were measured for the detection of coherent motion in moving random pixel arrays of constant root-mean-square contrast (35%) and constant average luminance (48 cd/m2) for 8 or 16 directions of motion at 25 positions in the visual field of the right eye. Five observers took part in this perimetric study of motion detection. The 24 eccentric positions were chosen on 8 equally spaced radial lines at the eccentricities 6, 24, and 48 degrees, the 25th position was centred on the fovea. At these positions we analysed the threshold SNR-value as a function of motion direction alpha. A significant modulation of the threshold with alpha is called an anisotropy. Anisotropies were found for low to medium velocities at positions on and near the vertical meridian, where the thresholds proved to be highest for vertical motion directions (up or down). On the horizontal meridian no significant anisotropies were found. Also on the oblique radials anisotropies were found, especially at 225 degrees (lower nasal quadrant of the visual field, upper temporal quadrant of the retina), but these were milder than those on the vertical meridian. The diameter of the stimulus is an important parameter and its influence was explored, albeit incompletely. Also inhomogeneities were found. This is defined as a consistent modulation of the threshold SNR-value with position A, the position along an equi-eccentricity circle (A-inhomogeneity), or with eccentricity E (E-inhomogeneity) or both. A simple acuity-scaling optimized for the nasal retina takes care of most of the E-inhomogeneity, but an A-inhomogeneity stays rather prominent. It too is characterized by higher thresholds near the vertical meridian than near the horizontal meridian. The findings suggest that iso-threshold curves are elliptical or egg-shaped with their long axis on the horizontal meridian and shifted somewhat out of naso-temporal symmetry towards the nasal half of the retinal field. As with the anisotropies the inhomogeneity grows in amplitude for decreasing velocity below medium velocity values of 1-2 pixels/frame, but in contradistinction to the anisotropies it is present and even increases in amplitude for increasing velocities above these medium values of 1-2 pixels/frame as well. The results are discussed in the light of other perimetric studies of motion detection and acuity, in the light of a model postulating the cooperation of groups of velocity-tuned bilocal motion detectors, and in the light of recent ideas on structure and function of primate cortical areas and processing streams.

Adult↗
PubMed1993

Fluorescence depletion measurements in various experimental geometries provide true emission and absorption anisotropies for the study of protein rotation.

Use of fluorescence depletion methods for measuring slow protein rotational diffusion has been limited by failure to obtain, from depletion data, well-defined anisotropy functions dependent on the distribution of either fluorophore emission or absorption transition dipoles, but not both. Such anisotropies would be directly comparable to those obtained from phosphorescence emission or triplet absorption measurements. We now describe such procedures applicable to cuvet and microscope experimental geometries, together with supporting experimental results. In cuvet measurements, the pump and probe beams are colinear and fluorescence is collected at 90 degrees to this axis. The data analysis procedure for this geometry has been suggested by Wegener (Biophys. J., 46 (1984) 795) and permits calculation of the absorption and emission anisotropies and the interdipole angle. In microscope experiments, fluorescence emission is collected along the pump/probe beam axis. For microscope measurements, a new experimental procedure permits evaluation of absorption and emission anisotropies when the interdipole angle is independently known. In either case multiple depletion measurements are required, each with different relative orientations of the probe beam polarization, pump beam polarization and emission polarizer axis. We have used these methods to calculate the time-dependent anisotropies for eosin-derivatized BSA rotation in glycerol solutions in both experimental geometries. These data correspond well with those obtained from time-resolved phosphorescence anisotropy measurements.

Chemistry, Physical↗
PubMed1987

Enhanced resolution of fluorescence anisotropy decays by simultaneous analysis of progressively quenched samples. Applications to anisotropic rotations and to protein dynamics.

Enhanced resolution of rapid and complex anisotropy decays was obtained by measurement and analysis of data from progressively quenched samples. Collisional quenching by acrylamide was used to vary the mean decay time of indole or of the tryptophan fluorescence from melittin. Anisotropy decays were obtained from the frequency-response of the polarized emission at frequencies from 4 to 2,000 MHz. Quenching increases the fraction of the total emission, which occurs on the subnanosecond timescale, and thereby provides increased information on picosecond rotational motions or local motions in proteins. For monoexponential subnanosecond anisotropy decays, enhanced resolution is obtained by measurement of the most highly quenched samples. For complex anisotropy decays, such as those due to both local motions and overall protein rotational diffusion, superior resolution is obtained by simultaneous analysis of data from quenched and unquenched samples. We demonstrate that measurement of quenched samples greatly reduces the uncertainty of the 50-ps correlation time of indole in water at 20 degrees C, and allows resolution of the anisotropic rotation of indole with correlation times of 140 and 720 ps. The method was applied to melittin in the monomeric and tetrameric forms. With increased quenching, the anisotropy data showed decreasing contributions from overall protein rotation and increased contribution from picosecond tryptophan motions. The tryptophan residues in both the monomeric and the tetrameric forms of melittin displayed substantial local motions with correlation times near 0.16 and 0.06 ns, respectively. The amplitude of the local motion is twofold less in the tetramer. These highly resolved anisotropy decays should be valuable for comparison with molecular dynamics simulations of melittin.

Acrylamide↗