1,720,961 research outputs found
Numerical indicators of nonlinear propagation in acoustic fields
No full textMeasurements of acoustic exposure made in water and then derated to account for the attenuation of tissue can be distorted by nonlinear effects. We have evaluated numerical indicators of such distortion in diagnostic ultrasound fields.We evaluate the indicators of the degree of nonlinearity in diagnostic ultrasound fields that could be used to guarantee 'quasi-linearity' or to correct measurements made under nonlinear conditions. A new indicator, σi, is introduced which can be calculated using measurements currently being made in ultrasound test labs
The peak rarefactional pressure generated by medical ultrasound systems in water and tissue: a numerical study
No full textCurrent estimates of in-situ exposure are based on de-rating field measurements made in waterto allow for the attenuation of tissue, using a specific attenuation coefficient of 0.3 dB cm-1 MHz-1. This process assumes that the propagation process is linear. However for medical ultrasound systems nonlinear propagation effects can be significant. In order to explore improved methods of characterising finite amplitude fields an extensive programme of modelling has been performed with the aim of investigating the relationship between finite amplitude fields in tissue and water. This utilised a finite difference solution to the KZK equation to model 35 fields using starting conditions typical of medical ultrasound arrays. In each case the field was modelled in water and then in homogeneous tissue, assuming the specific attenuation coefficient of 0.3 dB cm-1 MHz-1. This enabled the de-rated peak rarefactional acoustic pressure (pr,α) at specific locations, derived from water predictions, to be compared with the corresponding predictions for the peak rarefactional pressure pr in tissue. The results show that a nonlinear propagation parameter and measurement range can be used to give a reasonably good indication of the extent to which pr,α underestimates pr in tissue. Corresponding results for the pulse intensity integral do not demonstrate such a simple relationship. (Work supportedby EPSRC under grant GR/R43747.
Numerical modeling of harmonic imaging and pulse inversion fields
No full textTissue Harmonic Imaging (THI) and Pulse Inversion (PI) Harmonic Imaging exploit the harmonics generated as a result of nonlinear propagation through tissue to improve the performance of imaging systems. A 3D finite difference model, that solves the KZK equation in the frequency domain, is used to investigate the finite amplitude fields produced by rectangular transducers driven with short pulses and their inverses, in water and homogeneous tissue. This enables the characteristic of the fields and the effective PI field to be calculated. The suppression of the fundamental field in PI is monitored, and the suppression of side lobes and a reduction in the effective beamwidth for each field are calculated. In addition, the differences between the pulse and inverse pulse spectra resulting from the use of very short pulses are noted, and the differences in the location of the fundamental and second harmonic spectral peaks observed
Measurement of the phase response of a membrane hydrophone and its application to ultrasonic field characterisation
No full textThe accurate measurement of acoustic waveforms containing multiple frequencies is complicated by the need to know the frequency dependent phase response of the measurement system. This is particularly relevant for high amplitude ultrasound propagation resulting in non-linear distortion. However, if the phase response of the system is known then the true acoustic waveform can be recovered. This work describes a means of obtaining the required phase response of a hydrophone and receiver system over a wide frequency range, its application, and impact on measurements
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
