448 research outputs found
Square root singularity in the viscosity of neutral colloidal suspensions at large frequencies
The asymptotic frequency , dependence of the dynamic viscosity of
neutral hard sphere colloidal suspensions is shown to be of the form , where has been determined as a
function of the volume fraction , for all concentrations in the fluid
range, is the solvent viscosity and the P\'{e}clet time. For
a soft potential it is shown that, to leading order steepness, the asymptotic
behavior is the same as that for the hard sphere potential and a condition for
the cross-over behavior to is given. Our result for the hard
sphere potential generalizes a result of Cichocki and Felderhof obtained at low
concentrations and agrees well with the experiments of van der Werff et al, if
the usual Stokes-Einstein diffusion coefficient in the Smoluchowski
operator is consistently replaced by the short-time self diffusion coefficient
for non-dilute colloidal suspensions.Comment: 18 pages LaTeX, 1 postscript figur
High-order aberration compensation with Multi-frame Blind Deconvolution and Phase Diversity image restoration techniques
Context. For accurately measuring intensities and determining magnetic field
strengths of small-scale solar (magnetic) structure, knowledge of and
compensation for the point spread function is crucial. For images recorded with
the Swedish 1-meter Solar Telescope, restoration with Multi-Frame Blind
Deconvolution and Joint Phase Diverse Speckle methods lead to remarkable
improvements in image quality but granulation contrasts that are too low,
indicating additional stray light. Aims. We propose a method to compensate for
stray light from high-order atmospheric aberrations not included in MFBD and
JPDS processing. Methods. To compensate for uncorrected aberrations, a
reformulation of the image restoration process is proposed that allows the
average effect of hundreds of high-order modes to be compensated for by relying
on Kolmogorov statistics for these modes. The applicability of the method
requires simultaneous measurements of Fried's parameter r0. The method is
tested with simulations as well as real data and extended to include
compensation for conventional stray light. Results. We find that only part of
the reduction of granulation contrast in SST images is due to uncompensated
high-order aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging system
and to various high-altitude seeing effects. Conclusions. We conclude that
statistical compensation of high-order modes is a viable method to reduce the
loss of contrast occurring when a limited number of aberrations is explicitly
compensated for with MFBD and JPDS processing. We show that good such
compensation is possible with only 10 recorded frames. The main limitation of
the method is that already MFBD and JPDS processing introduces high-order
compensation that, if not taken into account, can lead to over-compensation.Comment: in press in Astronomy & Astrophysic
The value of multi-slice-computed tomography coronary angiography for risk stratification
Multi-slice-computed tomography coronary angiography (CTA) provides direct non-invasive anatomic assessment of the coronary arteries allowing for early identification of coronary artery disease (CAD). This information is useful for diagnosis of CAD, particularly the rule out of CAD. In addition, early identification of CAD with CTA may also be useful for risk stratification. The purpose of this review is to provide an overview of the current literature on the prognostic value of CTA and to discuss how the prognostic information obtained with CTA can be used to further integrate the technique into clinical practice. Non-invasive anatomic assessment of plaque burden, location, composition, and remodeling using CTA may provide prognostically relevant information. This information has been shown to be incremental to the Framingham risk score, coronary artery calcium scoring, and myocardial perfusion imaging. Characterization of atherosclerosis non-invasively has the potential to provide important prognostic information enabling a more patient-tailored approach to disease management. Future studies assessing outcome after CTA-based risk adjustments are needed to further understand the value of detailed non-invasive anatomic imaging
Incidence and predictive biomarkers of Clostridioides difficile infection in hospitalized patients receiving broad-spectrum antibiotics
Trial enrichment using gut microbiota derived biomarkers by high-risk individuals can
improve the feasibility of randomized controlled trials for prevention of Clostridioides difficile
infection (CDI). Here, we report in a prospective observational cohort study the incidence of
CDI and assess potential clinical characteristics and biomarkers to predict CDI in 1,007
patients ≥ 50 years receiving newly initiated antibiotic treatment with penicillins plus a beta lactamase inhibitor, 3rd/4th generation cephalosporins, carbapenems, fluoroquinolones or
clindamycin from 34 European hospitals. The estimated 90-day cumulative incidences of a
first CDI episode is 1.9% (95% CI 1.1-3.0). Carbapenem treatment (Hazard Ratio (95% CI):
5.3 (1.7-16.6)), toxigenic C. difficile rectal carriage (10.3 (3.2-33.1)), high intestinal abundance
of Enterococcus spp. relative to Ruminococcus spp. (5.4 (2.1-18.7)), and low Shannon alpha
diversity index as determined by 16 S rRNA gene profiling (9.7 (3.2-29.7)), but not nor malized urinary 3-indoxyl sulfate levels, predicts an increased CDI risk
Noninvasive Evaluation With Multislice Computed Tomography in Suspected Acute Coronary Syndrome Plaque Morphology on Multislice Computed Tomography Versus Coronary Calcium Score
ObjectivesOur aim was to evaluate the atherosclerotic plaque burden and morphology as determined by 64-slice multislice computed tomography (MSCT) coronary angiography in relation to the calcium score in patients presenting with suspected acute coronary syndrome (ACS).BackgroundThe absence of coronary calcium during coronary calcium scoring has been proposed to rule out significant coronary artery disease (CAD). However, data in patients presenting with suspected ACS are scarce.MethodsIn 40 patients (age 57 ± 11 years, 26 men) presenting with suspected ACS, MSCT coronary angiography in combination with coronary calcium scoring was performed before conventional coronary angiography. MSCT angiograms were evaluated for the presence or absence of coronary atherosclerotic plaque and the presence or absence of obstructive (≥50% luminal narrowing) CAD. In addition, plaque type was determined, and findings were related to the calcium score.ResultsCoronary artery disease was observed in 38 patients, of whom 10 patients had nonobstructive and 28 patients had obstructive CAD, confirmed by conventional coronary angiography in all patients. In patients with CAD, plaques were distributed as follows: 39% noncalcified plaques, 47% mixed plaques, and 14% calcified plaques. Coronary calcium was detected in 27 patients, of whom 10 had a score >400. In 13 (33%) patients, no coronary calcium was observed, but in 11 (85%), atherosclerotic plaques were detected on MSCT angiography.ConclusionsIn patients presenting with suspected ACS, noncalcified plaques are highly prevalent and the absence of coronary calcium does not reliably exclude the presence of (significant) atherosclerosis. This information may be of value to improve our understanding of the potential role of MSCT in this patient population
Practicalities in running early-phase trials using the time-to-event continual reassessment method (TiTE-CRM) for interventions with long toxicity periods using two radiotherapy oncology trials as examples
BACKGROUND: Awareness of model-based designs for dose-finding studies such as the Continual Reassessment Method (CRM) is now becoming more commonplace amongst clinicians, statisticians and trial management staff. In some settings toxicities can occur a long time after treatment has finished, resulting in extremely long, interrupted, CRM design trials. The Time-to-Event CRM (TiTE-CRM), a modification to the original CRM, accounts for the timing of late-onset toxicities and results in shorter trial duration. In this article, we discuss how to design and deliver a trial using this method, from the grant application stage through to dissemination, using two radiotherapy trials as examples. METHODS: The TiTE-CRM encapsulates the dose-toxicity relationship with a statistical model. The model incorporates observed toxicities and uses a weight to account for the proportion of completed follow-up of participants without toxicity. This model uses all available data to determine the next participant's dose and subsequently declare the maximum tolerated dose. We focus on two trials designed by the authors to illustrate practical issues when designing, setting up, and running such studies. RESULTS: In setting up a TiTE-CRM trial, model parameters need to be defined and the time element involved might cause complications, therefore looking at operating characteristics through simulations is essential. At the grant application stage, we suggest resources to fund statisticians' time before funding is awarded and make recommendations for the level of detail to include in funding applications. While running the trial, close contact of all involved staff is required as a dose decision is made each time a participant is recruited. We suggest ways of capturing data in a timely manner and give example code in R for design and delivery of the trial. Finally, we touch upon dissemination issues while the trial is running and upon completion. CONCLUSION: Model-based designs can be complex. We hope this paper will help clinical trial teams to demystify the conduct of TiTE-CRM trials and be a starting point for using this methodology in practice
Novel clinical applications of state-of-the-art multi-slice computed tomography
Recent years have witnessed a rapid development of multi-slice computed tomography (MSCT) technology. The number of detector rows has increased from 4-slices to the current availability of 64-slice and even 320-slice systems. In addition, images are acquired with thinner slices and faster rotation times resulting in substantially improved image quality and diagnostic accuracy. Simultaneously, effective dose reduction acquisition techniques have been developed allowing considerable reduction of the radiation dose. Conceivably, these advancements may allow further expansion of the use of MSCT beyond the visual assessment of the presence or absence of significant coronary artery disease. Indeed, a particular advantage of the technique is that in addition to evaluation of the coronary arteries it also allows assessment of cardiac structures and function. The purpose of the current review is to discuss several novel applications of cardiac MSCT, including stenosis quantification, atherosclerotic plaque imaging and prognostification as well as imaging of left ventricular function, aortic and mitral valve anatomy using state-of-the-art technology
Reduced Left Ventricular Torsion Early After Myocardial Infarction Is Related to Left Ventricular Remodeling
Background—
Left ventricular (LV) torsion is emerging as a sensitive parameter of LV systolic myocardial performance. The aim of the present study was to explore the effects of acute myocardial infarction (AMI) on LV torsion and to determine the value of LV torsion early after AMI in predicting LV remodeling at 6-month follow-up.
Methods and Results—
A total of 120 patients with a first ST-segment elevation AMI (mean±SD age, 59±10 years; 73% male) were included. All patients underwent primary percutaneous coronary intervention. After 48 hours, speckle-tracking echocardiography was performed to assess LV torsion; infarct size was assessed by myocardial contrast echocardiography. At 6-month follow-up, LV volumes and LV ejection fraction were reassessed to identity patients with LV remodeling (defined as a ≥15% increase in LV end-systolic volume). Compared with control subjects, peak LV torsion in AMI patients was significantly impaired (1.54±0.64°/cm vs 2.07±0.27°/cm,
P
<0.001). By multivariate analysis, only LV ejection fraction (
β
=0.36,
P
<0.001) and infarct size (
β
=−0.47,
P
<0.001) were independently associated with peak LV torsion. At 6-month follow-up, 19 patients showed LV remodeling. By multivariate analysis, only peak LV torsion (odds ratio=0.77; 95% CI, 0.65–0.92;
P
=0.003) and infarct size (odds ratio=1.04; 95% CI, 1.01–1.07;
P
=0.021) were independently related to LV remodeling. Peak LV torsion provided modest but significant incremental value over clinical, echocardiographic, and myocardial contrast echocardiography variables in predicting LV remodeling. By receiver-operating characteristics curve analysis, peak LV torsion ≤1.44°/cm provided the highest sensitivity (95%) and specificity (77%) to predict LV remodeling.
Conclusions—
LV torsion is significantly impaired early after AMI. The amount of impairment of LV torsion predicts LV remodeling at 6-month follow-up
Impact of antimicrobial de-escalation on mortality: a literature review of study methodology and recommendations for observational studies
Introduction: The safety of de-escalation of empirical antimicrobial therapy is largely based on observational data, with many reporting protective effects on mortality. As there is no plausible biological explanation for this phenomenon, it is most probably caused by confounding by indication. Areas covered: We evaluate the methodology used in observational studies on the effects of de-escalation of antimicrobial therapy on mortality. We extended the search for a recent systematic review and identified 52 observational studies. The heterogeneity in study populations was large. Only 19 (36.5%) studies adjusted for confounders and four (8%) adjusted for clinical stability during admission, all as a fixed variable. All studies had methodological limitations, most importantly the lack of adjustment for clinical stability, causing bias toward a protective effect. Expert opinion: The methodology used in studies evaluating the effects of de-escalation on mortality requires improvement. We depicted all potential confounders in a directed acyclic graph to illustrate all associations between exposure (de-escalation) and outcome (mortality). Clinical stability is an important confounder in this association and should be modeled as a time-varying variable. We recommend to include de-escalation as time-varying exposure and use inverse-probability-of-treatment weighted marginal structural models to properly adjust for time-varying confounders
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