Multiband Monte Carlo modeling of upconversion emission in sub 10 nm ß-NaGdF4:Yb3+, Er3+ nanocrystals-Effect of Yb3+ content.

In this work, we report the results of theoretical modeling supported and confirmed by experimentally measured emission, emission decay curves, and power dependent emission spectra for sub 10 nm ß-NaGdF4:Er3+,Yb3+ nanocrystals with different Yb3+ content (0.5%-15%). For the theoretical analysis, we develop a stochastic Monte Carlo model which is based on two components: (i) formation of clusters composed of Er3+ ion and Yb3+ neighbors, which gives insight into the role of local parameters and (ii) a simplified kinetic model of excitation and relaxation phenomena in pairs of Er3+and Yb3+ ions. The quantitative agreement between experimental data and modeling was obtained for the relative emission ratio of upconversion luminescence in green, red, and blue spectral ranges. Theoretical predictions of impact of excitation pulse duration and pumping light power on upconversion luminescence are presented.

Liquid crystal hyperbolic metamaterial for wide-angle negative-positive refraction and reflection.

We show that nanosphere dispersed liquid crystal (NDLC) metamaterial can be characterized in near IR spectral region as an indefinite medium whose real parts of effective ordinary and extraordinary permittivities are opposite in signs. Based on this fact we designed an electro-optic effect: an external electric-field-driven switch between normal refraction, negative refraction, and reflection of TM incident electromagnetic wave from the boundary vacuum/NDLC. A detailed analysis of its functionality is given based on effective medium theory combined with a study of negative refraction in anisotropic metamaterials and finite elements simulations.

Infrared cylindrical cloak in nanosphere dispersed liquid crystal metamaterial.

We present a design of an infrared cylindrical cloak using nanosphere dispersed nematic liquid crystal (NLC) metamaterial following the approach of Smith's group [Science 314, 977 (2006)]. Cloaking conditions require spatial distribution of liquid crystal birefringence with constant extraordinary index of refraction and radially dependent ordinary index of refraction. An approximate analytical formula for the latter is derived. Finite element (FE) simulations confirm the cloaking effect. Owing to the tunable birefringence of the liquid crystal component, such cloaking material offers the interesting possibilities of real-time control of invisibility. The possibility of experimental realization is briefly discussed.

[Ischemic stroke patients following intravenous thrombolysis. Outcome in a regional stroke center]. / Intravenöse Thrombolyse des ischämischen Schlaganfalls. Ergebnisse in einem regionalen Schlaganfallzentrum.

BACKGROUND: We wanted to compare the outcome following IV thrombolysis in our patients with ischemic stroke with the outcome reported from the phase 3 registration trials and other large trials. PATIENTS AND METHODS: From January 2008 through December 2009 we treated 225 patients with ischemic stroke with IV thrombolysis. Retrospectively, we analyzed their clinical data upon admission, during their stay, and upon discharge with special reference to eligibility criteria for IV thrombolysis, symptomatic hemorrhage, and clinical outcome. RESULTS: The average age of the patients was 74 years, with 34% being older than 80 years. The median time between stroke onset and initiation of treatment was 112 min. The initial median NIHSS was 11 points (Rankin score ≥ 4 in 79% of patients). The rate of symptomatic hemorrhage was 3%. The median follow-up was 14 days. At the end of the observation period, clinical symptoms had improved in 73% of patients (Rankin score ≤ 2 in 45% of patients). The mortality rate was 12%. In 55% of patients, IV thrombolysis was off label (age > 80 years in 34% of patients). CONCLUSION: The clinical results of IV thrombolysis in our stroke center are similar to the outcome reported from the registration trials and larger clinical series although we treated off label in more than half of the patients. Even larger studies have since shown that the age limit under 80 years for patients is not reasonable. The eligibility criteria should be adapted to reality at this point.

On effective electric field nano-octupoling in two dimensions.

Conditions towards effective electric field poling in two dimensions (2D) of octupolar molecules which can be achieved are being addressed, based on a lattice model which mimics the basic features of poling. The model is studied using the complementary approaches of analytical methods in statistical mechanics and Monte Carlo simulations. The poling field is imparted by a system of adequately shaped cylindrical electrodes. A topologically rich structure of local and global inhomogeneous octupolar order, including octupolar vortices, is present in the system. The poling criteria are shown to vary strongly throughout the cell: in close proximity to the contact points of neighboring electrodes, a high quality local octupolar order appears at temperature T ≃ 0.1 K while octupoling in the center of the cell requires temperatures as low as 10(-4) K. The highly demanding octupoling criteria are ascribed to symmetry-driven effects which decrease the quality of the octupolar phase even in the ground state, as well as to thermal fluctuations and numerical factors at above zero temperatures. Based on our results and using plausible conjectures related to the generalization of the model, it is argued that a weak global octupolar order can be reached at liquid Helium temperatures (a few Kelvins), based on current advances in optical techniques and nanotechnologies.

Second-harmonic generation in poled polymers: pre-poling history paradigm.

Experimental studies of second harmonic generation (SHG) from electric-field poled PMMA - DR1 system show occurrence of a maximum in diagonal and off diagonal tensor components χ(2)(-2ω;ω,ω) at 15 mol % concentration and a rapid decrease above, with a stabilization. The origin of the observed concentration dependence is studied using the Monte Carlo (MC) modeling. We find that presence of maximum is conditioned by the pre-poling history of the sample, when entanglement of linear dipolar structures takes place. Length of the pre-poling interval is an important kinetic parameter which differentiates between various nonexponential kinetics of build-up of polar phase responsible for strong/weak SHG susceptibility.

Complex Dynamics of Photo-Switchable Guest Molecules in All-Optical Poling Close to the Glass Transition: Kinetic Monte Carlo Modeling.

We study theoretically the kinetics of noninteracting photoswitchable guest molecules (model azo-dye) dispersed at low concentration in host (model polymer matrix) in the all-optical poling process close to the glass transition temperature Tg. We modify kinetic Monte Carlo model used in our previous studies of nonlinear optical processes in host-guest systems. The polymer matrix is simulated using the bond-fluctuation model. The kinetics of multiple trans-cis-trans cycles is formulated in terms of transition probabilities which depend on local free volume in the matrix and its dynamics. Close to Tg, the buildup of polar order, monitored in terms of angular probability density functions, follows a power-law in time while the evolution of the nonlinear susceptibilities related to second harmonic generation effect follows the stretched-exponential law. This complex dynamics of guest molecules implies the presence of dynamic heterogeneities of the matrix in space and time which spread the complexity from the matrix to the otherwise simple dynamics of noninteracting guest molecules. A qualitative physical picture of mosaic-like states-intertwined areas of free- and hindered angular motion of guest molecules-is proposed and the role of related short and longer scales in space for the promotion of complex dynamics of guest molecules is discussed. A brief comparison of the theory to available experimental data is given.

Percolation limited emission intensity from upconverting NaYF4:Yb3+,Er3+ nanocrystals - a single nanocrystal optical study.

Upconverting nanocrystals (UCNC) have recently been subjected to intensive investigation due to their interesting optical properties and high potential for practical applications. Despite the level of attention paid to these materials, very low quantum yield is still an important issue. In order to break through this limitation, understanding of the emission intensity limitation is crucial. In this paper, we investigate the influence of percolation phenomena on the limitation of the emission intensity from NaYF4:Yb3+,Er3+ nanocrystals. We propose a numerical model and support this experimentally at the single nanocrystal level, explaining the influence of Yb3+ concentration on the optical properties of UCNC. Moreover, based on the experimental and numerical results, we explain the existence of the optimal Yb3+ concentration in the core architecture often reported in the literature. All the measurements have been performed using a custom-built wide-field fluorescence microscope to analyze the emission from hundreds of single nanocrystals and thus make analysis independent of UCNC concentration.

Thermodynamics of entropy-driven phase transformations.

Thermodynamic properties of one-dimensional lattice models exhibiting entropy-driven phase transformations are discussed in quantum and classical regimes. Motivated by the multistability of compounds exhibiting photoinduced phase transitions, we consider systems with asymmetric, double, and triple well on-site potential. One finds that among a variety of regimes, quantum versus classical, discrete versus continuum, a key feature is asymmetry distinguished as a "shift" type and "shape" type in limiting cases. The behavior of the specific heat indicates one phase transformation in a "shift" type and a sequence of two phase transformations in "shape"-type systems. Future analysis in higher dimensions should allow us to identify which of these entropy-driven phase transformations would evolve into phase transitions of the first order.

Ion-ion interactions in ß-NaGdF4:Yb(3+),Er(3+) nanocrystals--the effect of ion concentration and their clustering.

In this work we report co-thermolysis as a suitable method for nanomaterial synthesis which allows the creation of hexagonal upconverting nanocrystals, NaGdF4:Yb(3+),Er(3+), in a wide range of sizes (20-120 nm). Only a very high Yb(3+) concentration (above 70%) results in pure cubic-phase nanocrystals with irregular shape. Additionally, we showed that the impact of Yb(3+), Er(3+) and Gd(3+) ions on the size and optical properties of nanocrystals is significant. We found that the main changes in optical properties do not depend on the nanocrystal size mostly, but are determined by the ion-ion interactions which include both Er(3+)-Er(3+) and Er(3+)-Yb(3+) cross relaxation.

PET and functional testing in temporal lobe epilepsy.

Thirty-seven patients with severe temporal lobe epilepsy were studied interictally with [18F]fluorodeoxyglucose-PET in each of three conditions: resting, during emotional speech, and while performing a visual recognition task. In the resting state, each patient exhibited regional hypometabolism in agreement with his epileptic EEG focus, but that area was typically very large. The zone of maximum dysfunction was significantly better demarcated on activated scans showing an increase in whole-brain metabolism averaging 18%. Concurrently, the midtemporal-focus metabolic contrast was improved by 27%. This effect was more consistently produced by the speech paradigm that also induced significant amygdala recruitment and revealed the individual hemispheric speech dominance. Psychological trait factors played a role only for unspecific global activation.

Regional cerebral glucose metabolism in anorexia nervosa measured by positron emission tomography.

Regional cerebral glucose metabolism was measured in five female anorectic patients, during the anorectic state and after weight gain, using the fluorodeoxyglucose method and positron emission tomography. In addition, these results were compared with those of 15 young male normals. During the anorectic state, significant caudate hypermetabolism was found bilaterally, unlike the finding in repeat measurements or in male normals. In some other brain structures (temporal cortex, lentiform nucleus, thalamus, and brainstem), significant hypermetabolism was also found during the anorectic state, but these results were not concordant for both sides and in both comparisons. There was no difference between patients after improvement and young male normals.

Statistical analysis of functional neuroimaging data: exploratory versus inferential methods.

In the following, various image processing and analytical techniques, whose efficiency has been demonstrated empirically by comparison with expert readings of hundreds of positron emission tomography (PET) studies, will be outlined briefly.

The quantitative analysis of D2-dopamine receptors in baboon striatum in vivo with 3-N-[2'-18F]fluoroethylspiperone using positron emission tomography.

We used the ligand 3-N-[2'-18F]fluoroethylspiperone (FESP), which binds to D2-dopamine receptors in the striatum, and positron emission tomography (PET) to quantify striatal D2-dopamine densities (Bmax) and binding kinetics in baboon brain in vivo. Sequential PET scans were obtained for 4 h post injection. Various similar models based on a nonlinear kinetic four-compartment model that takes into account the effect of ligand specific activity were used. We investigated the effect of exact model configuration on the reliability of Bmax and other kinetic transfer coefficients. We found that with the ligand FESP and dynamic PET studies, the estimated values of Bmax and other model parameters are sensitive to the choice of model configuration, ligand specific activity, and data analysis technique. The limitations of the reliability of parameter estimates in a complex kinetic model for receptor ligands were studied in simulation calculations. Results showed that the accuracy of estimated values of Bmax is affected by both the ligand binding properties and the injected dose of ligand. The estimated average value of kinetic model parameters was as follows: ligand-receptor dissociation constant k4 = 0.0080 min-1; the product of ligand-receptor association constant and fraction of ligand available to bind to specific receptors f2ka = 0.0052 (min nM)-1; and D2-dopamine receptor density Bmax = 37.5 pmol g-1.

Effect of nimodipine on regional cerebral glucose metabolism in patients with acute ischemic stroke as measured by positron emission tomography.

In a randomized double-blind placebo-controlled study of 27 patients with acute ischemic stroke, the effect on regional CMRglc (rCMRglc) of the calcium channel blocking agent nimodipine administered in addition to routine treatment was investigated. Following computed tomography-supported diagnosis of focal ischemia in the middle cerebral artery territory, positron emission tomography (PET) of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) was performed, and the patients were entered into the study within 48 h after onset of symptoms, randomly receiving either nimodipine (2 mg/h constant i.v. infusion for 5 days, 120 mg/day orally for another 16 days) or carrier/placebo. FDG PET was repeated after completion of therapy. The clinical course was followed during the treatment period and for 6 months after the stroke, using the Mathew Score for early and the Barthel Index for late assessment. During that observation period, five patients died in the nimodipine group and four in the control group. Subsequently, the code was broken, and the clinical and PET data were analyzed in relation to treatment assignment, with the nimodipine group comprising 11 and the control group 12 eligible cases. The two groups were similar with respect to age and sex distribution, initial clinical deficit, and infarct size and location. While the infarct rCMRglc showed comparable slight increases over time in both groups, the metabolic changes in the other evaluated regions (contralateral infarct mirror region, ipsi- and contralateral cerebral gray matter, contra- and ipsilateral cerebellar hemispheres) differed significantly between treatment groups (side x region x treatment interaction p less than 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of blood-brain hexose transport with dynamic PET: comparison of [18F]2-fluoro-2-deoxyglucose and [11C]O-methylglucose.

Blood-to-tissue transport of [18F]2-fluoro-2-deoxyglucose (FDG) and [11C]O-methylglucose (CMG) was compared by dynamic positron emission tomography in four patients with recent ischemic infarcts and in three patients with intracerebral tumors. Local blood volume, tracer transport from tissue to blood, and FDG phosphorylation rates were also determined. A regional analysis of parametric images showed a close correlation of FDG and CMG transport rate constants in pathological tissue. Transport rates of FDG and CMG showed correspondingly less asymmetric remote effects than FDG phosphorylation rates. Transport rate constants were consistently higher for FDG than for CMG in pathological and normal tissue, in accordance with the higher affinity of carrier enzymes to FDG. There was a significant correlation between fitted regional blood volume values and correspondence of average absolute values with both tracers. It is concluded that dynamic FDG PET for measurement of cerebral glucose metabolism is also useful to measure alterations of hexose transport and local blood volume in pathological tissue.

Performance of a randomization test for single-subject (15)O-water PET activation studies.

We adapted and implemented a permutation test (Holmes 1994) to single-subject positron emission tomography (PET) activation studies with multiple replications of conditions. That test determines the experimentwise alpha error as well as location and extent of focal activations in each individual. Its performance was assessed in five normal volunteers, using (15)O-H2O-PET data acquired on a high-resolution scanner, with septa retracted (3D mode), during functional activation by repeating words versus resting (four replications each). Calculated alpha errors decreased and the size of activated tissue volumes (voxels with P < or = 0.05) increased with increasing filter kernel size applied to the difference images. At a filter kernel of 12 mm Gaussian full width at half maximum, significant focal activations were seen bilaterally in superior temporal cortex, including Brodmann's areas 41 and 42, in all five subjects. Additional foci were detected in the precentral gyrus, left inferior frontal gyrus, supplementary motor area, and cerebellum of several subjects. The average CBF increase in activated voxels ranged from 17.6% to 28.7%. Activated volumes were smaller than those detected with a standard parametric test procedure. We conclude that the permutation test is a less sensitive procedure, having the advantage of not depending on unproven distributional assumptions, that detects strong activation foci in individual subjects with high reproducibility.

Estimation of local cerebral glucose utilization by positron emission tomography of [18F]2-fluoro-2-deoxy-D-glucose: a critical appraisal of optimization procedures.

Various approaches estimating local cerebral glucose utilization by positron emission tomography of labeled deoxyglucose are compared. Autoradiographic methods that predict the glucose utilization rate from a single scan are unreliable in pathologic tissue because of abnormal values of the model rate constants. A normalization procedure using the ratio of measured tissue activity to activity calculated with standard rate constants is proposed to readjust the values of the rate constants. Reliable estimates of metabolic rates can be obtained from dynamic recordings of tracer uptake. In the graphic approach, metabolic rate can be derived from the slope of a segment of a transformed uptake curve, which becomes linear at 15-20 min after intravenous tracer injection, with an accuracy comparable with that in complete dynamic studies. However, by recording and analyzing full-length uptake curves, in addition to metabolic rate, the model rate constants can be determined regionally. The physiological significance of those parameters is demonstrated in crossed cerebellar deactivation in 30 patients with supratentorial infarcts. Mild hypometabolism both within the ischemic lesion and in the morphologically intact cerebellum is accompanied by a reduction of the phosphorylation rate only. Severe metabolic depression, by contrast, affects both cerebellar transport and phosphorylation processes, whereas in the cerebrum, only the rate constant k1 is significantly correlated with the degree of metabolic disturbance.

Neurologic deficit and cerebral ATP depletion after temporary focal ischemia in cats.

Focal ischemia was induced in 23 cats by occluding the left middle cerebral artery for 2 h. The animals were then divided into groups for unforced reperfusion of variable duration ranging from 2 to 48 h. Neurological ratings were obtained during both ischemia and reperfusion. Following planned sacrifice the regional ATP content was assessed by means of a bioluminescence method showing spatial distribution and degree of ATP depletion. All the animals developed a neurologic deficit, with a median of 6 points on a disability scale of 0-10. After reopening of the middle cerebral artery, neurologic recovery was quite variable depending on the initial neurologic deficit (partial phi = 0.67, p1 less than 0.05): Animals with mild initial functional impairment improved and those with severe neurologic disturbances either died early or developed a more severe neurologic deficit, irrespective of the duration of reperfusion. The degree of ATP depletion and the amount of brain tissue involved exhibited a significant correlation with the neurological outcome (tau = 0.50, p1 less than 0.05), but they were even more closely related to the initial neurologic deficit (partial phi = 1.00, p1 less than 0.001), suggesting an early definitive manifestation of deficiencies in regional energy metabolism.

Effect of piracetam on cerebral glucose metabolism in Alzheimer's disease as measured by positron emission tomography.

The effect of piracetam (a putative enhancer of cerebral metabolism) on regional CMRGlu was studied by positron emission tomography of 2[18F]-fluoro-2-deoxy-D-glucose in nine patients with Alzheimer's disease, and in seven cases with multiinfarct dementia or unclassified dementia. In Alzheimer's disease, i.v. administration of piracetam, 6 g b.i.d. for 2 weeks, significantly improved rCMRGlu in most cortical areas, whereas no effect on CMRGlu of the drug was observed in the multiinfarct dementia/unclassified dementia groups. These results lend further support to the notion that adjuvant piracetam treatment is of benefit in Alzheimer's disease. They may also indicate that the typical metabolic depression in Alzheimer's disease is caused by complex interaction of disturbed transmitter and cellular function rather than by a specific deficit in the cholinergic system alone.