1,721,035 research outputs found
Radiofrequency coils for magnetic resonance applications: Theory, design, and evaluation
No full textMagnetic resonance imaging and magnetic resonance spectroscopy are noninvasive diagnostic techniques based on the phenomenon of nuclear magnetic resonance. Radiofrequency coils are key components in both the transmission and receiving phases of magnetic resonance systems. Transmitter coils have to produce a highly homogeneous alternating field in a wide field of view, whereas receiver coils have to maximize signal detection while minimizing noise. Development of modern magnetic resonance coils often is based on numerical methods for simulating and predicting coil performance. Numerical methods allows the behavior of the coil in the presence of realistic loads to be simulated and the coil’s efficiency at high magnetic fields to be investigated. After being built, coils have to be characterized in the laboratory to optimize their setting and performance by extracting several quality indices. Successively, coils performance has to be evaluated in a scanner using standardized image quality parameters with phantom and human experiments. This article reviews the principles of radiofrequency coils, coil performance parameters, and their estimation methods using simulations, workbench, and magnetic resonance experiments. Finally, an overview of future developments in radiofrequency coils technology is included
Estimation of losses in strip and circular wire conductors of radiofrequency planar surface coil by using the finite element method
No full textAn accurate coil design is a fundamental task to maximize signal-to-noise ratio in
magnetic resonance applications. Coil design techniques take advantage of computer
simulations especially when coil size is comparable to the radiofrequency
(RF) wavelength. In particular, the estimation of the losses within the conductors
as well as the radiative losses, both as a function of frequency, is instrumental to
a complete coil performance characterization. However, the cross-sectional shape
of the conductors strongly affects the radiofrequency coil’s performance, especially
at those frequencies where conductor losses represent the dominant power
dissipation mechanism. Indeed, at radiofrequencies, the current flowing in the
conductor is distributed in the proximity of its surface instead of being uniformly
distributed over the cross section; it follows that an accurate conductor losses estimation
can be performed only in the case of wire conductors by using analytical
formulations. For strip conductors, although different theoretical approaches have
been proposed in literature by taking into account the losses, no closed-form
expression for conductors resistance is available which takes into account both
classical and lateral skin effects. In this work, finite element method (FEM) simulations
have been performed for estimating conductor and radiative losses in planar
surface loops made of strips and circular wires; the results have been
compared against analytical formulations and literature data. Workbench tests performed
on two circular coil prototypes, the first one constituted by a strip and the
second one by circular wire conductors, tuned at 63.9 MHz and 127.8 MHz,
showed a good agreement with FEM simulations
Radiofrequency planar surface coil for magnetic resonance: When the use of a circular wire gives a noticeable advantage with respect to a flat strip conductor?
Full text linkDepending on their cross-sectional shape, commonly used conductors for radiofrequency (RF) Magnetic Resonance (MR) coils can be categorized into circular wires and flat strips. Due to a more symmetrical current distribution inside conductor volume, coils constituted by wire conductors provide better overall performance in unloaded conditions with respect to the ones made of strip conductor, although wire conductors are difficult to handle for coil manufacturing and additional mechanical competencies are required. Nevertheless, the accomplishment of the best coil performance during imaging, i.e. in the presence of a sample, remains the main issue in MRI. It follows that the use of wire conductors instead of strip ones is worthwhile only if the correspondent increase in coil quality factor with sample is substantial: this is related to the ratio between sample and coil resistance. This paper proposes the application of a finite element method (FEM)-based numerical approach for separately estimating the conductor and radiative losses in planar surface loops characterized by different cross-sectional shapes (circular wire and flat strip) in conjunction with a vector potential calculation-based method for sample-induced resistance estimation. Simulation data were acquired from 5.7 to 128 MHz, for four different size loops (from 2 to 15 cm diameters), with the scope of evaluating the region in the frequency-loop diameter plane where the use of a circular wire conductor gives a noticeable advantage with respect to the flat strip in maximizing signal-to-noise ratio (SNR) in MR applications
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
Computer tomography and magnetic resonance for multimodal imaging of fossils and mummies
No full textThe study of fossils and mummies has largely benefited from the use of modern noninvasive and nondestructive imaging technologies and represents a fast developing area. In this review, we describe the emerging role of imaging based on Magnetic Resonance (MR) and Computer Tomography (CT) employed for the study of ancient remains and mummies. For each methodology, the state of the art in paleoradiology applications is described, by emphasizing new technologies developed in the field of both CT, such as micro- and nano-CT, dual-energy and multi-energy CT, and MR, with the description of novel dedicated sequences, radiofrequency coils and gradients. The complementarity of CT and MR in paleoradiology is also discussed, by pointing out what MR provides in addition to CT, with an overview on the state of the art of emerging strategies in the use of CT/MR combination for the study of a sample following a multimodal integrated approach
An efficient algorithm for sample induced resistance estimation in Magnetic Resonance applications
No full textSviluppo di un tool per stima dell'esposizione professionale a campi elettromagnetici in risonanza magnetica
No full textLa Risonanza Magnetica (RM) è una tecnica diagnostica largamente utilizzata in diversi campi della medicina e per il suo corretto funzionamento ricorre a diverse tipologie di campi elettromagnetici (EM). Lo staff RM durante il lavoro giornaliero è esposto in maniera prolungata al campo magnetico statico e spazialmente eterogeneo presente in ogni momento nella sala RM. Muovendosi all’interno della stanza RM per espletare le loro funzioni, i tecnici sono esposti a campo magnetico variabile lentamente nel tempo che induce correnti elettriche nel corpo. Nonostante non ci siano ancora conferme sui possibili effetti nocivi sulla salute umana di questa corrente indotta [1], la Comunità Europea (CE) ha redatto una Direttiva (Direttiva 2004/40/EC) [2] nella quale si pone un limite di sicurezza a questa corrente. Tale Direttiva dovrà essere recepita da tutti gli stati membri entro aprile 2012. L’utilizzo di modelli matematici e di simulazioni al calcolatore permette di stimare la corrente indotta e di avere quindi informazioni importanti sull’esposizione degli operatori durante il turno giornaliero e in diverse condizioni operative.
In questo lavoro viene presentato un nuovo strumento per la stima della densità di corrente indotta a causa dei movimenti dei lavoratori nel campo magnetico statico di uno scanner MR. Il tool rappresenta un valido strumento per rivelare situazioni in cui i limiti di esposizione della direttiva possono essere superati, consentendo di addestrare gli operatori ad evitare, per quanto possibile, comportamenti a rischio di alta esposizione
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
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