1,720,981 research outputs found
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
Development of a software based automatic exposure control system for use in image guided radiation therapy
No full textModern image guided radiation therapy involves the use of an isocentrically mounted imaging system to take radiographs of a patient's position before the start of each treatment. Image guidance helps to minimize errors associated with a patients setup, but the radiation dose received by patients from imaging must be managed to ensure no additional risks. The Varian On-Board Imager (OBI) (Varian Medical Systems, Inc., Palo Alto, CA) does not have an automatic exposure control system and therefore requires exposure factors to be manually selected. Without patient specific exposure factors, images may become saturated and require multiple unnecessary exposures.
A software based automatic exposure control system has been developed to predict optimal, patient specific exposure factors. The OBI system was modelled in terms of the x-ray tube output and detector response in order to calculate the level of detector saturation for any exposure situation. Digitally reconstructed radiographs are produced via ray-tracing through the patients' volumetric datasets that are acquired for treatment planning. The ray-trace determines the attenuation of the patient and subsequent x-ray spectra incident on the imaging detector. The resulting spectra are used in the detector response model to determine the exposure levels required to minimize detector saturation.
Images calculated for various phantoms showed good agreement with the images that were acquired on the OBI. Overall, regions of detector saturation were accurately predicted and the detector response for non-saturated regions in images of an anthropomorphic phantom were calculated to generally be within 5 to 10 % of the measured values. Calculations were performed on patient data and found similar results as the phantom images, with the calculated images being able to determine detector saturation with close agreement to images that were acquired during treatment. Overall, it was shown that the system model and calculation method could potentially be used to predict patients' exposure factors before their treatment begins, thus preventing the need for multiple exposures.Graduate07600574075
Assessment of X-ray computed tomography dose in normoxic polyacrylamide gel dosimetry
Full text linkPolymer gel dosimetry, in conjunction with x-ray computed tomography (x-ray
CT) imaging, is a three-dimensional dosimetric tool that shows promise in the verification
of complex radiation therapy treatments. Previous studies have shown that
x-ray CT imaging of gel dosimeters is robust, easy-to-use, and has wide clinical accessibility.
The effects of x-ray CT dose imparted to the gel dosimeter, during imaging of
the delivered therapy dose distributions, is not well understood. This thesis quantifies
the effects of CT dose on normoxic polyacrylamide gel (nPAG) dosimeters.
The investigation is comprised of four parts. First, quantification of the x-ray
CT dose given during CT imaging of nPAG gels was measured using ion chamber
measurements and filmed dose profiles for a range of typical gel dosimetry imaging
protocols (200 mAs (current-time), 120-140 kVp (peak potential energy of photons),
2-10 mm slice thickness). It was found that CT doses ranged from 0.007 Gy/slice (120
kVp, 2 mm) to 0.021 Gy/slice (140 kVp, 10 mm) for volumetric phantoms. Second,
Raman spectroscopy was used to determine the effect of photon energy on the dose
response of nPAG dosimeters exposed to photon energies from a CT scanner (140 kVp photons) and from a Linac (6 MV photons). A weaker response was exhibited within
the gels irradiated with kV photons than MV photons. Thirdly, the measurements
of the given x-ray CT dose as established in the first study and the dose response of
the polymer gel to different photon energies in the second study were correlated to
estimate the induced changes of the nPAG CT number ("NCT ), caused by x-ray CT
imaging of the polymer gel. (CT number is defined to be the measured attenuation
coefficient normalized to water.) For typical gel imaging protocols (as above with
16-32 image averages), it was found that "NCT <0.2 H is induced in active nPAG
gel dosimeters. This "NCT is below the current threshold of detectability of CT
nPAG gel dosimetry. Finally, the traditional method of chemically fixing the dose
response mechanism of nPAG gels by passive oxygenation of the gel, is investigated
to determine if oxygenation would mitigate the changes caused by x-ray CT imaging of the gels. It was determined that oxygen diffusion was too slow to cause fixation
of nPAG dosimeters, as the diffusion constant was 1.2 ± 0.2 × 10−6cm2/s, or 25% of
the diffusion constant for anoxic PAG gel dosimeters.
In conclusion, it was found that x-ray CT dose in polymer gel dosimeters is not
a concern for standard gel imaging protocols. X-ray CT dose can potentially be a
concern when large numbers of image averages (e.g. >60 image averages) are utilized,
as in gel imaging protocols for high-resolution scans
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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