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Aims The FLXibility Post-Approval Study collected data on unselected patients implanted with a WATCHMAN FLX in a commercial clinical setting. Methods and results Patients were implanted with a WATCHMAN FLX per local standard of care, with a subsequent first follow-up visit from 45 to 120 days post-implant and a final follow-up at 1-year post-procedure. A Clinical Event Committee adjudicated all major adverse events and TEE/CT imaging results were adjudicated by a core laboratory. Among 300 patients enrolled at 17 centres in Europe, the mean age was 74.6 ± 8.0 years, mean CHA2DS2-VASc score was 4.3 ± 1.6, and 62.1% were male. The device was successfully implanted in 99.0% (297/300) of patients. The post-implant medication regimen was DAPT for 87.3% (262/300). At first follow-up, core-lab adjudicated complete seal was 88.2% (149/169), 9.5% (16/169) had leak <3 mm, 2.4 (4/169) had leak ≥3 mm to ≤5 mm, and 0% had >5 mm leak. At 1 year, 93.3% (280/300) had final follow-up; 60.5% of patients were on a single antiplatelet medication, 21.4% were on DAPT, 5.6% were on direct oral anticoagulation, and 12.1% were not taking any antiplatelet/anticoagulation medication. Adverse event rates through 1 year were: all-cause death 10.8% (32/295); CV/unexplained death 5.1% (15/295); disabling and non-disabling stroke each 1.0% (3/295, all non-fatal); pericardial effusion requiring surgery or pericardiocentesis 1.0% (3/295); and device-related thrombus 2.4% (7/295). Conclusion The WATCHMAN FLX device had excellent procedural success rates, high LAA seal rates, and low rates of thromboembolic events in everyday clinical practice. [ABSTRACT FROM AUTHOR]

Objectives: The PROGRESS PVL registry evaluated transcatheter aortic valve implantation (TAVI) in patients treated with ACURATE neo, a supra-annular self-expanding bioprosthetic aortic valve.Background: While clinical outcomes with TAVI are comparable with those achieved with surgery, residual aortic regurgitation (AR) and paravalvular leak (PVL) are common complications. The ACURATE neo valve has a pericardial sealing skirt designed to minimize PVL.Methods: The primary endpoint was the rate of total AR over time, as assessed by a core echocardiographic laboratory. The study enrolled 500 patients (mean age: 81.8 ± 5.1 years; 61% female; mean baseline STS score: 6.0 ± 4.5%) from 22 centers in Europe and Canada; 498 patients were treated with ACURATE neo.Results: The rate of ≥ moderate AR was 4.6% at discharge and 3.1% at 12 months; the rate of ≥ moderate PVL was 4.6% at discharge and 2.6% at 12 months. Paired analyses showed significant improvement in overall PVL between discharge and 12 months (P < 0.001); 64.6% of patients had no change in PVL grade, 24.9% improved, and 10.5% worsened. Patients also exhibited significant improvement in transvalvular gradient (P < 0.001) and effective orifice area (P=0.01). The mortality rate was 2.2% at 30 days and 11.3% at 12 months. The permanent pacemaker implantation (PPI) rate was 10.2% at 30 days and 12.2% at 12 months.Conclusions: Results from PROGRESS PVL support the sustained safety and performance of TAVI with the ACURATE neo valve, showing excellent valve hemodynamics, good clinical outcomes, and significant interindividual improvement in PVL from discharge to 12-month follow-up. [ABSTRACT FROM AUTHOR]

The effect of splitter plates (SP) on current interruption at medium voltage (MV) level is theoretically investigated in the framework of Mayr's arc model, which is generalized to take into account the electrode fall voltage, V0, of the SP. The model is analytically solved in the current zero (CZ) region where the current (before CZ) and the voltage (after CZ) can be linearized with respect to time. The current turns out to be successfully interrupted if the total electrode fall voltage, NV0, for N − 1 SP plus contact electrodes, is larger than twice the transient recovery voltage, ${\boldsymbol{U}} \approx {\dot{\boldsymbol{U}t}}$ , at the characteristic arc decay time, t = τ, after CZ: ${\boldsymbol{N}}{{\boldsymbol{V}}_0} > 2{\dot{\boldsymbol{U}\tau }}$. For ${\boldsymbol{N}}{{\boldsymbol{V}}_0} < 2{\dot{\boldsymbol{U}\tau }}$ , an additional thermal condition must be satisfied, where also the current rate, ${\dot{\boldsymbol{I}}} = {\boldsymbol{\ dI}}/{\boldsymbol{dt}}$ before CZ, and the arc cooling power, K, are involved. The intuitive results lead to a simple understanding of the electrical effect of SP on current interruption at MV and serve as an initial design rule for the number of SP in different gases. [ABSTRACT FROM AUTHOR]

Background: Transcatheter aortic valve implantation (TAVI) has become standard treatment for elderly patients with symptomatic severe aortic valve stenosis. The ACURATE neo AS study evaluates 30-day and 1-year clinical and hemodynamic outcomes in patients treated with the ACURATE neo2 valve. Methods: The primary endpoint of this single-arm multicenter study is 30-day all-cause mortality. Other key endpoints include device performance, echocardiographic measures assessed by an independent core laboratory, and VARC-2 clinical efficacy and safety endpoints through 12 months. Results: The study enrolled 120 patients (mean age 82.1 ± 4.0 years; 67.5% female, mean baseline STS score 4.8 ± 3.8%). The VARC-2 composite safety endpoint at 30 days occurred in 13.3% of patients. All-cause mortality was 3.3% at 30 days and 11.9% at 1 year. The 30-day stroke rate was 2.5% (disabling stroke 1.7%); there were no new strokes between 30 days and 12 months. The rate of permanent pacemaker implantation was 15.0% (18/120) at 30 days and 17.8% (21/120) at 1 year. No patients required re-intervention for valve-related dysfunction and there were no cases of valve thrombosis or endocarditis. Patients demonstrated significant improvement in mean aortic valve gradient (baseline 38.9 ± 13.1 mmHg, 1 year 7.8 ± 3.5 mmHg; P < 0.001 in a paired analysis). In the overall population, paravalvular leak was evaluated at 1 year as none/trace in 60.5%, mild in 37.0%, and moderate in 2.5%; no patients had severe PVL. Conclusions: One-year outcomes from the ACURATE neo AS study support the safety and performance of TAVI with the ACURATE neo2 valve. [ABSTRACT FROM AUTHOR]

Ionic wind devices or electrostatic fluid accelerators are becoming of increasing interest as tools for thermal management, in particular for semiconductor devices. In this work, we present a numerical model for predicting the performance of such devices; its main benefit is the ability to accurately predict the amount of charge injected from the corona electrode. Our multiphysics numerical model consists of a highly nonlinear, strongly coupled set of partial differential equations including the Navier-Stokes equations for fluid flow, Poisson's equation for electrostatic potential, charge continuity, and heat transfer equations. To solve this system we employ a staggered solution algorithm that generalizes Gummel's algorithm for charge transport in semiconductors. Predictions of our simulations are verified and validated by comparison with experimental measurements of integral physical quantities, which are shown to closely match. [ABSTRACT FROM AUTHOR]

This article presents an open source software able to convert, display, and process medical images. It differentiates itself from the existing software by its ability to design complex processing pipelines and to wisely execute them on a large databases. An MP3 pipeline can contain unlimited homemade or ready-made processes and can be carried out with a parallel execution system. As a viewer, MP3 allows display of up to four images together and to draw Regions Of Interest (ROI). Two applications showing the strengths of the software are presented as examples: a preclinical study involving Magnetic Resonance Imaging (MRI) data and a clinical one involving Computed Tomography (CT) images. MP3 is downloadable at https://github.com/nifm-gin/MP3. [ABSTRACT FROM AUTHOR]

Pure water is a weak electrolyte that dissociates into hydronium ions and hydroxide ions. In contact with a charged electrode a double layer forms for which neither experimental nor theoretical studies exist, in contrast to electrolytes containing extrinsic ions like acids, bases, and solute salts. Starting from a self-consistent solution of the one-dimensional modified Poisson–Boltzmann equation, which takes into account activity coefficients of point-like ions, we explore the properties of the electric double layer by successive incorporation of various correction terms like finite ion size, polarization, image charge, and field dissociation. We also discuss the effect of the usual approximation of an average potential as required for the one-dimensional Poisson–Boltzmann equation, and conclude that the one-dimensional approximation underestimates the ion density. We calculate the electric potential, the ion distributions, the pH-values, the ion-size corrected activity coefficients, and the dissociation constants close to the electric double layer and compare the results for the various model corrections. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper reports on a strongly physics-based minimal circuit model for polycrystalline cores, such as ferrites, including dimensional resonance (DR). The models and main properties of DR and hysteresis are recalled, a circuit model is described that incorporates the various phenomena, and it is mentioned how its parameters are extracted by broadband linear response and low-frequency nonlinear measurements. Finally, the simulations are validated with measurements for a specific example. The model turns out to predict accurately the magnetic behavior in the high-frequency nonlinear region. [ABSTRACT FROM AUTHOR]

A model of magnetic components is presented which includes saturation and static hysteresis and covers a wide frequency range. A dedicated setup measures the B–H curves for extraction of the model parameters and for model validation. The magnetic model is then represented as a spice circuit and is used in transient system simulations of power converters. Simulations and measurements of core losses and the filter performance of common mode chokes are discussed. [ABSTRACT FROM AUTHOR]

A model for the axially blown cylindrical arc is derived. In contrast to earlier theories, the model is gauge invariant with respect to energy, which is crucial for investigating current interruption. We determine from our model the dependence of the maximum interruptible current rate, (dI / dt)L, on the pressure, on the parallel capacitance, and on the line impedance for an SF6 arc. (dI / dt)L scales, approximately independent of the gas type, with the square root of the pressure. The arc resistance, at current zero with current rate equal to (dI / dt)L, is pressure independent. As a consequence, the arc resistance at current zero can serve as a figure of merit for the interruption performance of gas circuit breakers. [ABSTRACT FROM AUTHOR]

Purpose: To simultaneously assess reproducibility of three MRI transverse relaxation parameters ( R2', R2*, and R2 ) for brain tissue oxygenation mapping and to assess changes in these parameters with inhalation of gases that increase and decrease oxygenation, to identify the most sensitive parameter for imaging brain oxygenation.Materials and Methods: Forty-eight healthy subjects (25 male, ages 35 ± 8 years) were scanned at 3.0 Tesla, each with one of four gases (mildly and strongly hypercapnic and hypoxic) administered in a challenge paradigm, using a gas delivery setup designed for patient use. Cerebral blood flow mapping with arterial spin labeling, and simultaneous R2', R2*, and R2 mapping with gradient-echo sampling of free induction decay and echo (GESFIDE) were performed. Reproducibility in air and gas-induced changes were evaluated using nonparametric analysis with correction for multiple comparisons.Results: Our gas delivery setup achieved stable gas challenges as shown by physiological monitoring. Test-retest variability of R2', R2*, and R2 were found to be 0.24 s-1 (8.6% of mean), 0.24 s-1 (1.3% of mean), and 0.15 s-1 (1.0% of mean), respectively. Strong hypoxia produced the most conclusive oxygenation-driven relaxation change, inducing increases in R2' (25 ± 13%, P = 0.03), R2* (5 ± 2%, P = 0.02), and R2 (2 ± 2%, NS).Conclusion: We benchmarked the intra-scan test-retest variability in GESFIDE-based transverse relaxation rate mapping. Using a reliable framework for gas challenge paradigms, we recommend strong hypoxia for validating oxygenation mapping methods, and the use of tissue R2' change, instead of R2* or R2 , as a metric for studying brain tissue oxygenation using transverse relaxation methods.Level Of Evidence: 1 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:704-714. [ABSTRACT FROM AUTHOR]

Measurement of the ability of blood vessels to dilate and constrict, known as vascular reactivity, is often performed with breath-holding tasks that transiently raise arterial blood carbon dioxide (PaCO2) levels. However, following the proper commands for a breath-holding experiment may be difficult or impossible for many patients. In this study, we evaluated two approaches for obtaining vascular reactivity information using blood oxygenation level-dependent signal fluctuations obtained from resting-state functional magnetic resonance imaging data: physiological fluctuation regression and coefficient of variation of the resting-state functional magnetic resonance imaging signal. We studied a cohort of 28 older adults (69±7 years) and found that six of them (21%) could not perform the breath-holding protocol, based on an objective comparison with an idealized respiratory waveform. In the subjects that could comply, we found a strong linear correlation between data extracted from spontaneous resting-state functional magnetic resonance imaging signal fluctuations and the blood oxygenation level-dependent percentage signal change during breath-holding challenge (R²=0.57 and 0.61 for resting-state physiological fluctuation regression and resting-state coefficient of variation methods, respectively). This technique may eliminate the need for subject cooperation, thus allowing the evaluation of vascular reactivity in a wider range of clinical and research conditions in which it may otherwise be impractical. [ABSTRACT FROM AUTHOR]

This work summarizes the results of a number of DC conductivity measurement studies on polypropylene (PP), low density polyethylene (LDPE), and cross linked polyethylene (XLPE). The main observation is that under apparently time-constant external conditions (voltage, temperature, etc.) no steady-state direct current (DC) was established even after very long measurement times. Nevertheless, this behavior seems to exhibit some common systematic features, and since the experiments were performed with different equipment at different R&D labs in different years by different teams, simple measurement artefacts can be excluded. One observation is that there are two electric field regimes with slightly different behavior, separated by crossover field of about 10–15 kV/mm. In this work we focus on the high-field region, where the main observation is that the conductance slowly decays sub-linearly with time, I∼ t−n, with 0.3 < n < 1, n mostly around 0.5. We provide experimental indications that this behavior is rather independent of the presence of 1) the voltage and 2) the peroxide decomposition products (in PE). The observations are in favor of an underlying thermally driven relaxation process related to structural changes (morphology, free volume) of the polymer. A main implication of the results is that the use of steady-state conductivity values for the characterization of certain polymer insulation is not appropriate and instead the decaying behavior of the conduction current must be considered. [ABSTRACT FROM PUBLISHER]

This study aimed to determine whether measurements of cerebrovascular reserve and oxygenation, assessed with spin relaxation rate R2', yield similar information about pathology in pre-operative Moyamoya disease patients, and to assess whether R2' is a better measure of oxygenation than other proposed markers, such as R2* and R2. Twenty-five preoperative Moyamoya disease patients were scanned at 3.0T with acetazolamide challenge. Cerebral blood flow mapping with multi-delay arterial spin labeling, and R2*, R2, and R2' mapping with Gradient-Echo Sampling of Free Induction Decay and Echo were performed. No baseline cerebral blood flow difference was found between angiographically abnormal and normal regions (49±12 vs. 48±11 mL/100 g/min, p=0.44). However, baseline R2' differed between these regions (3.2±0.7 vs. 2.9±0.6 s-1, p<0.001), indicating reduced oxygenation in abnormal regions. Cerebrovascular reserve was lower in angiographically abnormal regions (21±38 vs. 41±26%, p=0.001). All regions showed trend toward significantly improved oxygenation post-acetazolamide. Regions with poorer cerebrovascular reserve had lower baseline oxygenation (Kendall's s=-0.24, p=0.003). A number of angiographically abnormal regions demonstrated preserved cerebrovascular reserve, likely due to the presence of collaterals. Finally, of the concurrently measured relaxation rates, R2' was superior for oxygenation assessment. [ABSTRACT FROM AUTHOR]

The influence of the carrier injection properties of electrode contacts on unipolar space-charge limited currents (SCLC) in perfect insulators is theoretically studied. The usual electric transport model consisting of the charge continuity equation and the Poisson equation is supplemented by boundary conditions describing carrier injection. The dependence of the electric behavior on the saturation current density, which is the most important parameter of the boundary condition, is investigated, mainly with FEM simulations. The scenarios in the focus are transient DC current when a constant voltage is switched on, and dielectric (or impedance) spectroscopy (AC). For the latter, higher harmonics up to the 5th order are relevant due to the intrinsic nonlinearity of SCLC. A simplified ordinary differential equation model for transient contact charging is derived, in an analogous way as it is usually done for the early SCLC transient with ideal ohmic contacts. Finally, a specific case with traps that may quench the transit-time peak is briefly discussed to illustrate trapping effects. It turns out that the SCLC dynamics can be separated in three time (or frequency) regimes. Two of them are separated as usual by the time of flight, τtoƒ, of carriers through the sample. A third region at shorter times t<τc≪τtoƒ (or higher frequencies) exists, where τc characterizes the time for space charge formation at the injecting contact. The contact injection properties thus mainly influence the short-time and high-frequency behavior, where the characteristic time decreases with increasing saturation current. This behavior is reflected by the appearance of specific structures in the absorption current and the impedance spectrum. For instance, an additional current maximum associated with the fast diffusive expansion of the locally injected space charge may appear at early times. [ABSTRACT FROM AUTHOR]

This contribution discusses thermally stimulated current (TSC) measurements on low density polyethylene (LDPE) in the form of thin films and virtually without additives. The TSC responses are divided into different parts attributed to capacitive currents, currents due to release of trapped carriers, and normal conduction currents. The three contributions dominate at low, intermediate, and high temperatures, respectively. Dielectric spectroscopy measurements at different temperatures indicate the relevance of the capacitive currents for the measurement of thermally stimulated polarization currents (TSPC). The separation of the conduction current, by subtracting reference curves from the total current, allowed the investigation of trap and conduction properties for different experimental parameters (e.g., excitation levels, fields, heating rates). [ABSTRACT FROM PUBLISHER]

The aim of this study was to determine the ability of multiparametric MRI to identify the early effects of individual treatment, during combined chemo-radiotherapy on brain tumours. Eighty male rats bearing 9L gliosarcomas were randomized into four groups: untreated, anti-angiogenic therapy (SORA group), microbeam radiation therapy (MRT group) and both treatments (MRT+SORA group). Multiparametric MRI (tumour volume, diffusion-weighted MR imaging (ADC), blood volume fraction (BVf), microvessel index (VSI), vessel wall integrity (AUCP846) and tissue oxygen saturation (StO2)) was performed 1 day before and 2, 5 and 8 days after treatment initiation. Unpaired t-tests and one-way ANOVA were used for statistical analyses. Each MR parameter measured in our protocol was revealed to be sensitive to tumour changes induced by any of the therapies used (individually or combined). When compared with untreated tumours, SORA induced a decrease in BVf, VSI, StO2 and AUCP846, MRT generated an increase in ADC and AUCP846 and combined therapies yielded mixed effects: an increase in ADC and AUCP846 and a decrease in BVf, StO2 and AUCP846. MRT and MRT+SORA significantly slowed tumour growth. Despite these two groups presenting with similar tumour sizes, the information yielded from MR multiparameter assessment indicated that, when used concomitantly, each therapy induced distinguishable and appreciable physiological changes in the tumour. Our results suggest that multiparametric MRI can monitor the effects of individual treatments, used concomitantly, on brain tumours. Such monitoring would be useful for the detection of tumour resistance to drug/radiotherapy in patients undergoing concomitant therapies. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Purpose R2′, the reversible component of transverse relaxation, is an important susceptibility measurement for studies of brain physiology and pathologies. In existing literature, different R2′ measurement methods are used with assumption of equivalency. This study explores the choice of measurement method in healthy, young subjects at 3T. Methods In this study, a modified gradient-echo sampling of free induction decay and echo (GESFIDE) sequence was used to compare four standard R2′ measurement methods: asymmetric spin echo (ASE), standard GESFIDE, gradient echo sampling of the spin echo (GESSE), and separate R2 and R2* mapping. Results GESSE returned lower R2′ measurements than other methods ( P < 0.05). Intersubject mean R2′ in gray matter was found to be 2.7 s−1 using standard GESFIDE and GESSE, versus 3.4-3.8 s−1 using other methods. In white matter, mean R2′ from GESSE was 2.3 s−1 while other methods produced 3.7-4.3 s−1. R2 correction was applied to partially reduce the discrepancies between the methods, but significant differences remained, likely due to violation of the fundamental assumption of a single-compartmental tissue model, and hence monoexponential decay. Conclusion R2′ measurements are influenced significantly by the choice of method. Awareness of this issue is important when designing and interpreting studies that involve R2′ measurements. Magn Reson Med 73:1228-1236, 2015. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Purpose To investigate if delays in resting-state spontaneous fluctuations of the BOLD (sfBOLD) signal can be used to create maps similar to time-to-maximum of the residue function (Tmax) in Moyamoya patients and to determine whether sfBOLD delays affect the results of brain connectivity mapping. Materials and Methods Ten patients were scanned at 3 Tesla using a gradient-echo echo planar imaging sequence for sfBOLD imaging. Cross correlation analysis was performed between each brain voxel signal and a reference signal comprised of either the superior sagittal sinus (SSS) or whole brain (WB) average time course. sfBOLD delay maps were created based on the time shift necessary to maximize the correlation coefficient, and compared with dynamic susceptibility contrast Tmax maps. Standard and time-shifted resting-state BOLD connectivity analyses of the default mode network were compared. Results Good linear correlations were found between sfBOLD delays and Tmax using the SSS as reference (r2 = 0.8, slope = 1.4, intercept = −4.6) or WB (r2 = 0.7, slope = 0.8, intercept = −3.2). New nodes of connectivity were found in delayed regions when accounting for delays in the analysis. Conclusion Resting-state sfBOLD imaging can create delay maps similar to Tmax maps without the use of contrast agents in Moyamoya patients. Accounting for these delays may affect the results of functional connectivity maps. J. Magn. Reson. Imaging 2015;41:424-430.© 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Insulator boundaries like electrode contacts, interfaces, and surfaces play decisive roles for the electric field distributions in high-voltage direct-current (HVDC) insulation. Well-known examples are charging effects at injecting and blocking electrode contacts, and at interfaces or surfaces. Reliable modeling of boundaries and interfaces poses a challenge and can be crucial for the quality of the resulting field distributions in HVDC equipment, but is in practice often put in second place of priority in simulation models. This paper emphasizes the macroscopic boundary conditions and related issues for electrical transport and illustrates their relevance with simple examples. Importance is attached to an appropriate compromise between physical generality and practical usability for simulations in the development process of HVDC insulation systems. [ABSTRACT FROM AUTHOR]

Axially blown electric arcs in a model circuit breaker are investigated by electrical measurements and fast imaging with a high-speed camera for visualizing the arc structure. Parameters for a dynamic conductance model (known as the “black-box model” in the arc literature) are extracted from the electrical measurements for currents between a few tens of amperes and a few kiloamperes. The model is used for the prediction of the conductance-dependent cooling power and arc relaxation time, as well as the voltage-current characteristics of the arcs in our system. In order to work out the relevance of the different cooling mechanisms, an alternative and more physically based dynamic model for axially blown arcs is incorporated. It turns out that convective cooling predominates turbulent heat conduction and radiation cooling in the region of validity of the model. The result is consistent with the observed strong dependence of the cooling power on the flow conditions that are controlled by the blow-gas pressure and the geometry. Furthermore, the comparison between air and SF6 arcs indicates a considerable dependence of the conductance-cooling relation on the gas type. [ABSTRACT FROM PUBLISHER]

Purpose MRI is used to obtain quantitative oxygenation and blood volume information from the susceptibility-related MR signal dephasing induced by blood vessels. However, analytical models that fit the MR signal are usually not accurate over the range of small blood vessels. Moreover, recent studies have demonstrated limitations in the simultaneous assessment of oxygenation and blood volume. In this study, a multiparametric MRI framework that aims to measure vessel radii in addition to magnetic susceptibility and volume fraction was introduced. Methods The protocol consisted of gradient-echo sampling of the spin-echo, diffusion, T2, and B0 acquisitions. After correction steps, the data were postprocessed with a versatile numerical model of the MR signal. An important analytical model was implemented for comparison. The approach was validated in phantoms with coiling strings as proxy for blood vessels. Results The feasibility of the vessel radius measurement is demonstrated. The numerical model shows an improved accuracy compared with the analytical approach. However, both methods overestimate the radius. The simultaneous measurement of the magnetic susceptibility and the volume fraction remains challenging. Conclusion The results suggest that this approach could be interesting in vivo to better characterize the microvasculature without contrast agent. Magn Reson Med 72:646-658, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A quantitative estimate of cerebral blood oxygen saturation is of critical importance in the investigation of cerebrovascular disease. While positron emission tomography can map in vivo the oxygen level in blood, it has limited availability and requires ionizing radiation. Magnetic resonance imaging (MRI) offers an alternative through the blood oxygen level-dependent contrast. Here, we describe an in vivo and non-invasive approach to map brain tissue oxygen saturation (StO2) with high spatial resolution. StO2 obtained with MRI correlated well with results from blood gas analyses for various oxygen and hematocrit challenges. In a stroke model, the hypoxic areas delineated in vivo by MRI spatially matched those observed ex vivo by pimonidazole staining. In a model of diffuse traumatic brain injury, MRI was able to detect even a reduction in StO2 that was too small to be detected by histology. In a F98 glioma model, MRI was able to map oxygenation heterogeneity. Thus, the MRI technique may improve our understanding of the pathophysiology of several brain diseases involving impaired oxygenation. [ABSTRACT FROM AUTHOR]

Spontaneous fluctuations in blood oxygenation level-dependent (BOLD) images are the basis of resting-state fMRI and frequently used for functional connectivity studies. However, there may be intrinsic information in the amplitudes of these fluctuations. We investigated the possibility of using the amplitude of spontaneous BOLD signal fluctuations as a biomarker for cerebral vasomotor reactivity. We compared the coefficient of variation (CV) of the time series (defined as the temporal standard deviation of the time series divided by the mean signal intensity) in two populations: 1) Ten young healthy adults and 2) Ten hypertensive elderly subjects with chronic kidney disease (CKD). We found a statistically significant increase (P<0.01) in the CV values for the CKD patients compared with the young healthy adults in both gray matter (GM) and white matter (WM). The difference was independent of the exact segmentation method, became more significant after correcting for physiological signals using RETROICOR, and mainly arose from very low frequency components of the BOLD signal fluctuation (f<0.025 Hz). Furthermore, there was a strong relationship between WM and GM signal fluctuation CV's (R2 = 0.87) in individuals, with a ratio of about 1∶3. These results suggest that amplitude of the spontaneous BOLD signal fluctuations may be used to assess the cerebrovascular reactivity mechanisms and provide valuable information about variations with age and different disease states. [ABSTRACT FROM AUTHOR]

Cerebral blood volume maps are usually acquired using dynamic susceptibility contrast imaging which inherently limits the spatial resolution and signal to noise ratio of the images. In this study, we used ferumoxytol (AMAG Pharmaceuticals, Inc., Cambridge, MA), an FDA-approved compound, to obtain high-resolution cerebral blood volume maps with a steady-state approach in seven healthy volunteers. [ABSTRACT FROM AUTHOR]

A discrete-time audio \Delta \Sigma modulator for a MEMS microphone with digital output is presented that features a scalable signal bandwidth to also support ultrasonic frequencies for proximity sensing applications such as gesture recognition. The modulator achieves low power consumption and a small die area by using simple digital inverter instead of OTAs. To increase the robustness of the inverter approach, an LDO is used to provide an internally generated supply which regulates the inverter bias point. The LDO also improves the power supply rejection of the pseudo-differential architecture to above 78 dB. The presented \Delta \Sigma modulator supports a scalable signal bandwidth up to 100 kHz, by dynamically adjusting the internal supply voltage depending on the sampling frequency. In the nominal 20-kHz audio band, it dissipates 140 \mu\ W from a minimum 1.5-V supply and achieves a DR of 92.6 dB , a SNDR of 87.9 dB, and a THD of -102.7~\ dB, respectively. [ABSTRACT FROM AUTHOR]

The purpose of this study was to estimate magnetic resonance imaging-based brain perfusion parameters from combined multiecho spin-echo and gradient-echo acquisitions, to correct them for T1-, T2-, and -related contrast agent (CA) extravasation effects, and to simultaneously determine vascular permeability. Perfusion data were acquired using a combined multiecho spin- and gradient-echo (SAGE) echo-planar imaging sequence, which was corrected for CA extravasation effects using pharmacokinetic modeling. The presented method was validated in simulations and brain tumor patients, and compared with uncorrected single-echo and multiecho data. In the presence of CA extravasation, uncorrected single-echo data resulted in underestimated CA concentrations, leading to underestimated single-echo cerebral blood volume (CBV) and mean transit time (MTT). In contrast, uncorrected multiecho data resulted in overestimations of CA concentrations, CBV, and MTT. The correction of CA extravasation effects resulted in CBV and MTT estimates that were more consistent with the underlying tissue characteristics. Spin-echo perfusion data showed reduced large-vessel blooming effects, facilitating better distinction between increased CBV due to active tumor progression and elevated CBV due to the presence of cortical vessels in tumor proximity. Furthermore, extracted permeability parameters were in good agreement with elevated T1-weighted postcontrast signal values. [ABSTRACT FROM AUTHOR]

The quantification of bolus-tracking MRI techniques remains challenging. The acquisition usually relies on one contrast and the analysis on a simplified model of the various phenomena that arise within a voxel, leading to inaccurate perfusion estimates. To evaluate how simplifications in the interstitial model impact perfusion estimates, we propose a numerical tool to simulate the MR signal provided by a dynamic contrast enhanced (DCE) MRI experiment. Our model encompasses the intrinsic and relaxations, the magnetic field perturbations induced by susceptibility interfaces (vessels and cells), the diffusion of the water protons, the blood flow, the permeability of the vessel wall to the the contrast agent (CA) and the constrained diffusion of the CA within the voxel. The blood compartment is modeled as a uniform compartment. The different blocks of the simulation are validated and compared to classical models. The impact of the CA diffusivity on the permeability and blood volume estimates is evaluated. Simulations demonstrate that the CA diffusivity slightly impacts the permeability estimates ( for classical blood flow and CA diffusion). The effect of long echo times is investigated. Simulations show that DCE-MRI performed with an echo time may already lead to significant underestimation of the blood volume (up to 30% lower for brain tumor permeability values). The potential and the versatility of the proposed implementation are evaluated by running the simulation with realistic vascular geometry obtained from two photons microscopy and with impermeable cells in the extravascular environment. In conclusion, the proposed simulation tool describes DCE-MRI experiments and may be used to evaluate and optimize acquisition and processing strategies. [ABSTRACT FROM AUTHOR]

Quantitative blood oxygenation level dependent approaches have been designed to obtain quantitative oxygenation information using MRI. A mathematical model is usually fitted to the time signal decay of a gradient-echo and spin-echo measurements to derive hemodynamic parameters such as the blood oxygen saturation or the cerebral blood volume. Although the results in rats and human brain have been encouraging, recent studies have pointed out the need for independent estimation of one or more variables to increase the accuracy of the method. In this study, a multiparametric quantitative blood oxygenation level dependent approach is proposed. A combination of arterial spin labeling and dynamic susceptibility contrast methods were used to obtain quantitative estimates of cerebral blood volume and cerebral blood flow. These results were combined with T [ABSTRACT FROM AUTHOR]

Several MR methods have been proposed over the last decade to obtain quantitative estimates of the tissue blood oxygen saturation (StO2) using a quantification of the blood oxygen level dependent effect. These approaches are all based on mathematical models describing the time evolution of the MR signal in biological tissues in the presence of magnetic field inhomogeneities. Although the experimental results are very encouraging, possible biases induced by the model assumptions have not been extensively studied. In this study, a numerical approach was used to examine the influence on T2*, blood volume fraction, and StO2 estimates of possible confounding factors such as water diffusion, intravascular signal, and presence of arterial blood in the voxel. To evaluate the impact of the vessel geometry, straight cylinders and realistic data from two-photon microscopy for microvascular geometry were compared. Our results indicate that the models are sufficiently realistic, based on a good correlation between ground truth and MR estimates of StO2. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]