1,720,996 research outputs found
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
Quantitative amyloid PET imaging in Alzheimer’s disease:Walking the line between accuracy and simplicity
Get PDFPositron emission tomography (PET) allows for in vivo visualisation and quantification of amyloid-β (Aβ) plaques, which are known to play a major role in the development of Alzheimer’s disease (AD). However, it remains unclear whether these plaques have a causal relationship with AD dementia. Quantitatively tracking of Aβ plaques using PET may 1) enhance our understanding of AD, 2) improve our ability to predict who will develop AD dementia and 3) improve participant selection in secondary prevention trials. Typically, quantification requires a dynamic scan and measurement of the available PET tracer in plasma. As these are time consuming and complex procedures, widespread implementation in clinical or research practise is not feasible and warrants simplified approaches. The overall aim of this thesis was to evaluate to what extent approaches for quantification of Aβ pathology using PET can be simplified, within the context of different applications. In short, the main findings of this thesis were that, in case of the [18F]flutemetamol, [18F]florbetaben and [11C]PiB amyloid tracers, the semi-quantitative standardized uptake value ratio (SUVR), calculated from a static PET scan, showed a bias that was proportional to underlying Aβ pathology compared with the quantitative distribution volume ratio (DVR). The bias was smallest in participants (or regions) with very low levels of Aβ pathology and the effect of cerebral blood flow changes on bias in SUVR appeared to be minimal in cognitively unimpaired individuals. The proportional bias in SUVR suggests that for highly accurate quantification of Aβ pathology, such as when measuring small changes in longitudinal studies, dynamic or validated dual-time window scanning protocols may be required. More specifically, the present work showed that dual-time window protocols can substitute dynamic protocols with only limited loss of accuracy, while increasing patient comfort and throughput. Clearly, highly accurate and robust quantification can only be achieved if these scanning protocols are used in combination with a validated pharmacokinetic model and reference tissue, if applicable. Another advantage of dynamic and dual-time window scanning protocols is that a measure of relative tracer delivery (R1) can be extracted. In terms of its methodological characteristics, it was demonstrated that R1 is a stable parameter and, it has the potential to serve as an additional biomarker for measuring disease progression in AD. Finally, considering that Aβ plaque accumulation starts focally, research aiming to measure small changes in Aβ pathology over time should focus on regional instead of global measures of Aβ pathology. This strategy has the potential to improve participant selection in clinical trials and predict cognitive decline. In addition, regional quantification of amyloid burden and the use of dynamic scanning protocols can reduce the required sample sizes in primary and secondary AD prevention trials
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
Challenges and opportunities in quantitative brain PET imaging
No full textEen van de voornaamste voordelen voor de beeldvorming van Positron Emissie Tomografie (PET) is het verkrijgen van kwantitatieve informatie omtrent fysiologische processen. Helaas vereisen de wiskundige modellen voor het kwantificeren over het algemeen lange acquisitieprotocollen en potentiele invasieve procedures, welke zorgen voor uitdagingen in termen van patiënt comfort en de bezetting van de gezondheidszorg. Als gevolg, de kwantitatieve kwaliteiten van PET komen niet terecht in de klinische toepassing en veel preklinische studies (kleine dierlijke studies) onderzoeken de potentie van kwantificatiemodellen niet volledig. In deze context onderzoekt deze thesis een aantal kansen om 1) het gat te overbruggen tussen het kwantitatieve potentieel en de noodzaak van simpele methodes voor klinische toepassing en 2) het optimaliseren van kwantitatieve methodologieën voor de preklinische omgeving. Ten eerste zijn er twee nieuwe gesimplificeerde methodes ontwikkeld en gevalideerd. Deze methodes pasten wiskundige benaderingen toe om modellen vast te stellen en waren succesvol in het significant reduceren van PET-scan tijd zonder nadelig te zijn voor de kwantitatieve accuraatheid. Als consequentie is het mogelijk volledige kwantitatieve informatie te verkrijgen door middel van korte, simpele, patiëntvriendelijke scans. Vervolgens richtte het onderzoek zich op het optimaliseren van kwantitatieve methodologie voor PET-beeldvorming van het rattenbrein. Hier werd een gevestigde tracer uit menselijk onderzoek geëvalueerd op specifieke toepassing in de preklinische omgeving en optimale kwantitatieve methodes vastgesteld. De bevindingen van dit werk lieten zien dat het niet mogelijk is de methodologie van mensen direct toe te passen op onderzoek bij ratten. Het markeerde het belang van model validatie, ook in het geval van radiotracers welke vastgesteld zijn voor een ander soort.One of the main advantages of Positron Emission Tomography (PET) imaging is the potential to obtain quantitative information about physiological processes. Unfortunately, the mathematical models that allow for quantitation generally require long image acquisition protocols and potentially invasive procedures, which poses challenges in terms of patient comfort and healthcare resource utilization. As a consequence, the quantitative character of PET does not find space in the clinical practice, and many preclinical studies (small animal imaging) also do not explore quantification methods to the fullest. In this context, this thesis explored a number of opportunities to 1) bridge the gap between quantitative potential and the necessity of simple methods for clinical practice and to 2) optimize quantitative methodologies for the preclinical setting. First, two novel simplified methods were developed and validated. The methods applied mathematical approximations to established models and were successful in significantly reducing PET scan time without compromising quantitative accuracy. As a consequence, full quantitative information might be obtained from simple, short, patient-friendly scans. Next, the thesis focused on optimizing quantitative methodology for PET imaging of the rat brain. There, a well-established radiotracer in human studies was assessed for its specific use in the preclinical setting, and optimal quantitative methods were determined. The findings of this work showed it is not possible to directly translate methodology from human to rat studies, and highlighted the importance of model validation, also in the case of radiotracers which have been established in a different species
Supplementary material for "Multi-tracer model for staging cortical amyloid deposition using PET imaging"
No full textObjective: To develop and evaluate a model for staging cortical amyloid deposition using PET with high generalizability.
Methods: 3027 subjects (1763 Cognitively Unimpaired (CU), 658 Impaired, 467 Alzheimer's disease (AD) dementia, 111 non-AD dementia, and 28 with missing diagnosis) from six cohorts (EMIF-AD, ALFA, ABIDE, ADC, OASIS-3, ADNI) who underwent amyloid PET were retrospectively included; 1049 subjects had follow-up scans. Applying dataset-specific cut-offs to global Standard Uptake Value ratio (SUVr) values from 27 regions, single-tracer and pooled multi-tracer regional rankings were constructed from the frequency of abnormality across 400 CU subjects (100 per tracer). The pooled multi-tracer ranking was used to create a staging model consisting of four clusters of regions as it displayed a high and consistent correlation with each single-tracer ranking. Relationships between amyloid stage, clinical variables and longitudinal cognitive decline were investigated.
Results: SUVr abnormality was most frequently observed in cingulate, followed by orbitofrontal, precuneal, and insular cortices, then the associative, temporal and occipital regions. Abnormal amyloid levels based on binary global SUVr classification were observed in 1.0%, 5.5%, 17.9%, 90.0%, and 100.0% of stage 0-4 subjects, respectively. Baseline stage predicted decline in MMSE (ADNI: N=867, F=67.37, p<0.001; OASIS: (N=475, F=9.12, p<0.001) and faster progression towards an MMSE≤25 (ADNI: N=787, HRstage1=2.00, HRstage2=3.53, HRstage3=4.55, HRstage4=9.91, p<0.001; OASIS: N=469, HRstage4=4.80, p<0.001).
Conclusion: The pooled multi-tracer staging model successfully classified the level of amyloid burden in >3000 subjects across cohorts and radiotracers, and detects pre-global amyloid burden and distinct risk profiles of cognitive decline within globally amyloid-positive subjects
Challenges and opportunities in quantitative brain PET imaging
Get PDFEen van de voornaamste voordelen voor de beeldvorming van Positron Emissie Tomografie (PET) is het verkrijgen van kwantitatieve informatie omtrent fysiologische processen. Helaas vereisen de wiskundige modellen voor het kwantificeren over het algemeen lange acquisitieprotocollen en potentiele invasieve procedures, welke zorgen voor uitdagingen in termen van patiënt comfort en de bezetting van de gezondheidszorg. Als gevolg, de kwantitatieve kwaliteiten van PET komen niet terecht in de klinische toepassing en veel preklinische studies (kleine dierlijke studies) onderzoeken de potentie van kwantificatiemodellen niet volledig. In deze context onderzoekt deze thesis een aantal kansen om 1) het gat te overbruggen tussen het kwantitatieve potentieel en de noodzaak van simpele methodes voor klinische toepassing en 2) het optimaliseren van kwantitatieve methodologieën voor de preklinische omgeving. Ten eerste zijn er twee nieuwe gesimplificeerde methodes ontwikkeld en gevalideerd. Deze methodes pasten wiskundige benaderingen toe om modellen vast te stellen en waren succesvol in het significant reduceren van PET-scan tijd zonder nadelig te zijn voor de kwantitatieve accuraatheid. Als consequentie is het mogelijk volledige kwantitatieve informatie te verkrijgen door middel van korte, simpele, patiëntvriendelijke scans. Vervolgens richtte het onderzoek zich op het optimaliseren van kwantitatieve methodologie voor PET-beeldvorming van het rattenbrein. Hier werd een gevestigde tracer uit menselijk onderzoek geëvalueerd op specifieke toepassing in de preklinische omgeving en optimale kwantitatieve methodes vastgesteld. De bevindingen van dit werk lieten zien dat het niet mogelijk is de methodologie van mensen direct toe te passen op onderzoek bij ratten. Het markeerde het belang van model validatie, ook in het geval van radiotracers welke vastgesteld zijn voor een ander soort
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