1,720,991 research outputs found
Adaptive parameter selection scheme for nonlinear simulations using field II with operator splitting
No full textObjectives: Understanding and simulating nonlinear pulse propagation is of increasing interest in current ultrasound research. This article gives an adaptive parameter selection scheme for selecting parameters needed when the Field II linear propagation simulator is combined with operator splitting to enable nonlinear propagation simulations.Methods: The general Field II/operator splitting technique was introduced in [1, 2], requiring the selection of various geometrical parameters associated with virtual propagating planes. At that stage these needed to be selected essentially by an iterative trial-and-error technique for a particular transducer geometry, frequency and focussing scheme. Whilst careful parameter selection could provide satisfactory results over a limited propagation range for a given transducer, excitation and focussing scheme, an automated parameter selection method for the general case was still lacking.Results: The paper provides an adaptive method for selecting the required parameters as a function of the varying harmonic and spatial properties of the nonlinear pulse as it propagates through the medium.Conclusions: The method is fully derived and the significant improvement in the simulator’s performance is demonstrated by comparison to previous results in [1, 2]
System identification approach to contrast imaging with single pulse emission
No full textObjectives: Contrast imaging methods such as pulse inversion exploit the difference in scattering responses between linear tissue and nonlinear contrast agents. Such methods emit two or more pulses separated either temporally or spatially, at the cost of either reduced frame rate or reduced spatial resolution as compared to the theoretical values possible from a single pulse emission. This article proposes a method for imaging contrast agents specifically using single pulse emission only.Methods: A system identification approach is taken, first fitting a least squares linear transfer function model between emitted and received pulses, and then obtaining a set of predicted received pulse values based on the emitted pulse and the linear least squares model.Results: In the case of the reflector being linear (tissue), the linear least squares model accurately predicts the received pulse and the pulse prediction error is insignificant. In the case of a nonlinear reflector (contrast agent), the linear model cannot accurately predict the received pulse and the pulse prediction error is significant.Conclusions: These results are parallel to the insignificant and significant residual pulses obtained respectively in pulse inversion, but the method here requires only a single emission to obtain the discrimination. Results for typical contrast agent responses are given
Computation of steered nonlinear fields using offset KZK axes
No full textIntroduction/Background: Commercial medical scanners utilise electronic beam steering and the second harmonic signals generated by tissue nonlinear processes to form images at the second harmonic frequency. Furthermore, characteristics of nonlinearly generated harmonics within the beam itself contribute to improvements in lateral resolution and quality of the reconstructed image. Fully understanding these harmonic features offers new possibilities for redesigning or optimising array transducers to attain better imaging performances, as well as in other areas such as contrast agent responses and
microbubble destruction strategies for drug delivery. However, such an understanding is hindered at present by the computational difficulty in accurately predicting all the nonlinear characteristics of steered beams. This paper addresses this issue, presenting a KZK based method for analysing beams
steered at arbitrary angles.Method: The parabolic KZK equation is often used to study nonlinear characteristics of medical ultrasound beams. This equation is traditionally applied in a propagation direction perpendicular to the surface of the transducer, and has been shown to model the pulse propagation well along the central axis of the transducer [1]. However, its accurate angle of application is restricted to within approximately only 16 degrees away from the central transducer axis due to the underlying approximations employed in the derivation of the KZK equation.
This restriction causes a problem for studying steered beams, since typical steering ranges up to the order of 45 degrees away from the transducer centreline. To overcome this problem whilst at the same time exploiting the traditional KZK benefits, we develop an iterative method for displacing the KZK axes away from the central transducer axis to investigate arbitrary beam or field angles of interest. Results/Conclusions: The implemented algorithm is based on a time domain solution of the KZK equation, on a standard 3GHz PC with 2GB RAM producing runtimes in the order of a few hours per investigation angle. The transducer used is a 64 element linear phased array operating at 1.7MHz (height 10.5 mm, width 0.27mm, kerf 0.065mm) with beam steering at 0 and 45 degrees. Validation of the basic KZK algorithm without steering has been given previously at linear pressures (54kPa) against both experimental measurements and Field II and at nonlinear pressures (700kPa) against measurements described previously in [1]. The KZK axes are swept over a range of angles covering the full spectrum 499 of -90 to +90 degrees relative to transducer centreline, in order to investigate the entire emitted field for both steered and unsteered beams. Element pitch is then increased to investigate grating lobes. All results are decomposed into first and second harmonic fields to compare relative frequency domain properties. The overall contribution is a better characterisation of linear and nonlinear field characteristics at high angles than is possible with the traditional KZK approach
Connection between X-Waves, Fourier-Bessel series and optimal modelling aperture for circular symmetric arrays
No full textThis paper addresses various unresolved issues raised in publications [1], [2], and [3], in connection with the study and application of limited-diffraction and non-diffracting beams. Nondiffracting beams have the property of a constant radial profile with propagation distance, subject to an infinite aperture source, and the related theories advanced in the context of medical imaging have resulted in the possibility of extremely high fame rates. However, all the fundamental theory assumes an infinite-aperture source being available. In reality this is not possible, and when nondiffracting beams are implemented on finite source apertures they become limited diffraction beams. [1], [2], [3] have studied the use of and numerical differences between nondiffracting theory and limited diffraction implementation of such beams for field computation and tuning. In [1], [2] it was shown that an iterative technique could be applied to extend the effective modelling aperture from the physical limits of the transducer (limited diffraction basis functions) towards an infinite modelling aperture (Bessel beams
/ X-waves), to both tune and compute the emitted field from any given circular symmetric flat array with linear propagation conditions. This technique involved the concept of a modelling aperture spanning the gap between the physical limit of the transducer and infinity, which, when increased iteratively resulted in convergence of the corresponding computed field and source driving function. However, the technique relied on a combination of iterative combinations and 137 numerical field convergence to within a pre-selected limit in order to terminate the computations at an appropriate point.
In this paper, a formal mathematical connection between the limited-aperture (limited diffraction) basis functions and the nondiffracting infinite aperture theory (Bessel beams / X-Waves) is established as a function of the increasing modelling aperture. The result is that a specific optimal modelling aperture may then be specified as a function of frequency spectrum, spatial field extent to be investigated, and pulse repetition frequency. Consequently the previous iterative technique may be replaced by a single one-shot computation to achieve the same result. As a result, the new technique is significantly more efficient than the previous technique and the specific saving in computation depends on the particular transducer considered, but typically computational reductions are in the order of 50%. The global contribution of the paper is twofold : firstly a formal mathematical connection between limited diffraction beams and nondiffracting beams as function of increasing modelling aperture, and secondly the derivation of the optimal modelling aperture required for computation and tuning of circular symmetric fields with minimal computational demands
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Full text linkWe 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
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