1,721,102 research outputs found
Impairment of cerebral autoregulation in the first hours after carotid endarterectomy (CEA)
No full textMeasurement of cerebral blood flow autoregulation from spontaneous changes in arterial blood pressure
No full textA parametric approach to measuring cerebral blood flow autoregulation from spontaneous variations in blood pressure
No full textAutoregulation maintains cerebral blood flow (CBF) almost constant in the face of changes in arterial blood pressure (ABP). Tests for impairment of this process using only spontaneous fluctuations in ABP, without provoking large variations, are of great clinical interest, and a range of different approaches have previously been applied. Extending earlier work based on linear filters, we propose a simple parametric method using a first order finite impulse response filter. We evaluate the method on ABP and CBF velocity [(CBFV), from trancranial Doppler ultrasound] signals collected in 60 patients with stenosis or occlusion of the carotid arteries. Data were collected during the inspiration of ambient air, a 5% CO2/air mixture, and finally the return to ambient air. Equivalent data were collected in 15 normal subjects. The filters estimated from the data segments with constant inspiratory pCO2 showed the expected high-pass characteristic, which was reduced during hypercapnia and also in patients. Highly significant correlation between the filter parameters and cerebrovascular reactivity (percent increase in CBFV per unit change in end-tidal pCO2) gives further evidence that the filters reflect autoregulation. The method allows simple parametrization of the dynamic autoregulatory responses in CBFV, and the analysis of short (1 min) data segments
Parametric modeling of arterial blood pressure and blood flow velocity, for measuring cerebral autoregulation
No full textEnhancement of effective electro-optic coefficient in domain engineered UV-written waveguides in LiNbO<sub>3</sub>
No full textUV laser-induced poling-inhibition produces inverted domains in LiNbO3 which overlap significantly with waveguide modes. We have observed a 26% enhancement of the effective electro-optic coefficient in such domain-engineered waveguides
Electro-optic coefficient enhancement in poled LiNbO<sub>3</sub> waveguides
No full textLithium niobate crystals (LN) show a significant electro-optic (EO) response which contributes to the fabrication of low-voltage operation, high speed integrated optical modulators routinely used in optical telecommunication and integrated optics [1]. A UV laser direct writing method for the fabrication of optical channel waveguides has been proposed and characterized recently [2-4]. Here we report on the enhancement of the electro-optic response of these UV laser-written LN waveguides as a result of a post-poling process. More specifically we have observed a 26% increase of the r33 coefficient compared to the bulk in LN waveguides, fabricated by direct UV writing, that have been subjected to poling inhibition [5]. Poling inhibition produces inverted ferroelectric domains which are only a few microns deep. These domains are formed exactly in the same place as the UV written tracks which are responsible for the waveguide formation, and they overlap significantly with the propagating waveguide mode as is illustrated schematically in Fig. 1. Due to the polarization-selective transmission in the UV-written waveguides only the r33 coefficient could be investigated. Fig. 1 Schematic of the cross section of a) a UV-written waveguide on a single domain substrate, and b) the tail-to-tail domain arrangement overlapping with the waveguide after poling-inhibition. Optical channel waveguides were fabricated by direct UV laser focused writing on the +z face of a z-cut undoped congruent LN substrate [4]. The sample was subsequently subjected to electric field poling using an externally applied electric field (~19.5 kV/mm) which resulted in local poling-inhibited domains of limited depth that overlap with the waveguides as shown in Fig. 1b [5,6]. The electro-optic response was evaluated interferometrically by placing the waveguides in one branch of a Mach-Zehnder interferometer [3]. A set of titanium in-diffused waveguides was used as a control sample to provide the background measurement of the bulk for the r33 coefficient. The measured values of the electro-optic coefficient (r33) in the poling-inhibited samples proved to be systematically higher than the value obtained with the control sample of unpoled titanium in-diffused waveguides which was 35 pm/V. The highest value of the r33 coefficient that was measured in the poling-inhibited waveguides was 44.2 pm/V, which corresponds to an enhancement of 26% as compared to the reference Ti indiffused waveguide sample. The observed enhancement in the value of the EO coefficient is attributed to the strain which is associated with the presence of a tail-to-tail domain boundary that surrounds the optical waveguide channel as illustrated in Fig. 1b. The enhancement of the EO coefficient varied for waveguides which were fabricated under different UV irradiation conditions. The irradiation conditions affect both the waveguide mode confinement and the depth of the poling-inhibited domains. This suggests that the enhancement can be further optimized and even applied to other waveguide systems such as titanium in-diffused and proton exchanged channel guides
Local electro-optic coefficient enhancement in LiNbO<sub>3</sub> channel waveguides by domain engineering
No full textUV laser-induced poling-inhibition produces opposite domains in LiNbO3 which overlap significantly with waveguide modes. A 55% enhancement of the effective electro-optic coefficient was observed in such domain-engineered waveguides
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
