16 research outputs found
Contemporary frameless intracranial biopsy techniques: Might variation in safety and efficacy be expected?
Background: Frameless stereotactic neuronavigation has proven to be a feasible technology to acquire brain biopsies with good accuracy and little morbidity and mortality. New systems are constantly i
Outcomes Associated With Intracranial Aneurysm Treatments Reported as Safe, Effective, or Durable:A Systematic Review and Meta-Analysis
Importance: Testing new medical devices or procedures in terms of safety, effectiveness, and durability should follow the strictest methodological rigor before implementation. Objectives: To review and analyze studies investigating devices and procedures used in intracranial aneurysm (IA) treatment for methods and completeness of reporting and to compare the results of studies with positive, uncertain, and negative conclusions. Data Sources: Embase, MEDLINE, Web of Science, and The Cochrane Central Register of Clinical Trials were searched for studies on IA treatment published between January 1, 1995, and the October 1, 2022. Grey literature was retrieved from Google Scholar. Study Selection: All studies making any kind of claims of safety, effectiveness, or durability in the field of IA treatment were included. Data Extraction and Synthesis: Using a predefined data dictionary and analysis plan, variables ranging from patient and aneurysm characteristics to the results of treatment were extracted, as were details pertaining to study methods and completeness of reporting. Extraction was performed by 10 independent reviewers. A blinded academic neuro-linguist without involvement in IA research evaluated the conclusion of each study as either positive, uncertain, or negative. The study followed Preferring Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Main Outcomes and Measures: The incidence of domain-specific outcomes between studies with positive, uncertain, or negative conclusions regarding safety, effectiveness, or durability were compared. The number of studies that provided a definition of safety, effectiveness, or durability and the incidence of incomplete reporting of domain-specific outcomes were evaluated.Results: Overall, 12 954 studies were screened, and 1356 studies were included, comprising a total of 410 993 treated patients. There was no difference in the proportion of patients with poor outcome or in-hospital mortality between studies claiming a technique was safe, uncertain, or not safe. Similarly, there was no difference in the proportion of IAs completely occluded at last follow-up between studies claiming a technique was effective, uncertain, or noneffective. Less than 2% of studies provided any definition of safety, effectiveness, or durability, and only 1 of the 1356 studies provided a threshold under which the technique would be considered unsafe. Incomplete reporting was found in 546 reports (40%).Conclusions and Relevance: In this systematic review and meta-analysis of IA treatment literature, studies claiming safety, effectiveness, or durability of IA treatment had methodological flaws and incomplete reporting of relevant outcomes supporting these claims.</p
Single-DC-Input Multi-Level Envelope Tracking of a High-Efficiency X-band Power Amplifier
This paper addresses the efficiency enhancement of microwave power amplifiers (PAs) with discrete-level supply-modulation. We demonstrate an efficient modulator architecture that generates three levels of drain supply voltages from a single dc voltage. Each level is stored in a "flying" capacitor that acts as a temporary voltage supply and is dynamically regulated by feedback through reversal of the PA drain current. A hybrid modulator based on GaN-on-Si technology is tested with a single-stage 2-W X-band MMIC PA with a drain efficiency of 55% at the peak output power. The overall average drain efficiency (PA and modulator) of up to 43% is measured with Gaussian-like pulses for radar and a 5-MHz 6-dB PAPR OFDM signal, and is improved up to 14 percentage points over a constant supply case, with normalized root mean square error below 1.5% when pre-distortion is used
Assessing the Impact of Point Defects on Molecular Diffusion in ZIF‑8 Using Molecular Simulations
Because
defects are ubiquitous in materials, they may play an important
role in affecting the performance of materials in practical applications.
Here, we investigate the influence of point defects on the diffusion
of molecules including water, hydrocarbons, and acid gases in zeolitic
imidazolate framework-8 (ZIF-8) using molecular simulations. To make
these simulations possible, we introduce a force field that extends
previous descriptions of pristine ZIF-8 to include experimentally
relevant point defects. In general, the point defects we examined
increase the local hopping rate for molecular diffusion, suggesting
that low concentrations of these defects will not dominate long-range
molecular diffusion in ZIF-8
Lattice-Gas Modeling of Adsorbate Diffusion in Mixed-Linker Zeolitic Imidazolate Frameworks: Effect of Local Imidazolate Ordering
The
rates of adsorbate diffusion in zeolitic imidazolate frameworks
(ZIFs) can be varied by several orders of magnitude by incorporating
two different imidazolate linkers in the ZIF crystals. Although some
prior measurements of short-range order in these mixed-linker materials
have been reported, it is unclear how this short-range order impacts
the net diffusion of adsorbates. We introduce a lattice diffusion
model that treats diffusion in ZIF-8<sub><i>x</i></sub>-90<sub>100–<i>x</i></sub> crystals as a series of activated
hops between cages, allowing us to assess the effects of short-range
imidazolate order on molecular diffusion
Quantitative Predictions of Molecular Diffusion in Binary Mixed-Linker Zeolitic Imidazolate Frameworks Using Molecular Simulations
Experimental studies
have shown that adsorbate diffusion in zeolitic
imidazolate frameworks (ZIFs) can be tuned by incorporating two different
imidazolate linkers in the ZIF crystals. We demonstrate for the first
time that atomistic simulations are capable of quantitatively predicting
self-diffusion in binary mixed-linker ZIFs. Diffusion coefficients
of various adsorbates for which prior experimental data exist are
predicted in ZIF-8-90, ZIF-8/SALEM-2 materials composed of imidazolate
and 2-methylimidazolate linkers in the cubic SOD topology, and ZIF-7-90
materials over a composition range that is known experimentally to
be in the SOD topology. A combination of conventional and biased molecular
dynamics simulations as well as a previously developed lattice-diffusion
model allows us to access the full range of diffusion time scales
for adsorbates such as small hydrocarbons, alcohols, benzene, and
water
Impacts of Gas Impurities from Pipeline Natural Gas on Methane Storage in Metal–Organic Frameworks during Long-Term Cycling
The
development of adsorbed natural gas (ANG) technology creates
opportunities for use of pipeline natural gas as clean fuel in vehicles.
Metal–organic frameworks (MOFs) are one class of materials
that have received considerable attention as possible adsorbents in
ANG applications. We examine how accumulation of trace components
from pipeline natural gas will impact the performance of MOFs in ANG
during long-term cycling. Our approach combines information from grand
canonical Monte Carlo (GCMC) simulations of single-component adsorption,
ideal adsorbed solution theory (IAST) of multicomponent adsorption,
and an isothermal model of tank cycling to assess accumulation of
heavy hydrocarbons and <i>tert</i>-butyl mercaptan (TBM).
In a series of MOFs, a reduction in deliverable energy up to 50% is
observed after 200 cycles. These results highlight the importance
of considering multicomponent effects during consideration of adsorbents
for ANG applications
Structure Elucidation of Mixed-Linker Zeolitic Imidazolate Frameworks by Solid-State <sup>1</sup>H CRAMPS NMR Spectroscopy and Computational Modeling
Mixed-linker
zeolitic imidazolate frameworks (ZIFs) are nanoporous
materials that exhibit continuous and controllable tunability of properties
like effective pore size, hydrophobicity, and organophilicity. The
structure of mixed-linker ZIFs has been studied on macroscopic scales
using gravimetric and spectroscopic techniques. However, it has so
far not been possible to obtain information on unit-cell-level linker
distribution, an understanding of which is key to predicting and controlling
their adsorption and diffusion properties. We demonstrate the use
of <sup>1</sup>H combined rotation and multiple pulse spectroscopy
(CRAMPS) NMR spin exchange measurements in combination with computational
modeling to elucidate potential structures of mixed-linker ZIFs, particularly
the ZIF 8-90 series. All of the compositions studied have structures
that have linkers mixed at a unit-cell-level as opposed to separated
or highly clustered phases within the same crystal. Direct experimental
observations of linker mixing were accomplished by measuring the proton
spin exchange behavior between functional groups on the linkers. The
data were then fitted to a kinetic spin exchange model using proton
positions from candidate mixed-linker ZIF structures that were generated
computationally using the short-range order (SRO) parameter as a measure
of the ordering, clustering, or randomization of the linkers. The
present method offers the advantages of sensitivity without requiring
isotope enrichment, a straightforward NMR pulse sequence, and an analysis
framework that allows one to relate spin diffusion behavior to proposed
atomic positions. We find that structures close to equimolar composition
of the two linkers show a greater tendency for linker clustering than
what would be predicted based on random models. Using computational
modeling we have also shown how the window-type distribution in experimentally
synthesized mixed-linker ZIF-8-90 materials varies as a function of
their composition. The structural information thus obtained can be
further used for predicting, screening, or understanding the tunable
adsorption and diffusion behavior of mixed-linker ZIFs, for which
the knowledge of linker distributions in the framework is expected
to be important
Growth of unruptured aneurysms: A meta-analysis of natural history and endovascular studies
The growth of unruptured intracranial aneurysms (UIAs) is a strong predictor of rupture. Clinical observations suggest that some UIAs might grow faster after endovascular treatment than untreated UIAs. There are no head-to-head comparisons of incidence rates of UIAs thus far. Methods: We searched PubMed, Embase and Google Scholar for relevant articles from the inception of the databases to March 2020. We pooled and compared the incidence rates for the growth of aneurysms from natural history studies and endovascular treatment studies. Generalized linear models were used for confounder adjustment for the prespecified confounders age, size and location. Results: Twenty-five studies (10 describing growth in natural history and 15 reporting growth after endovascular therapy) considering 6325 aneurysms were included in the meta-analysis. The median size of aneurysms was 3.7 mm in the natural history studies and 6.4 mm in endovascular treatment studies (p = 0.001). The pooled incidence rate (IR) of growth was significantly higher in endovascular treatment studies (IR 52 per 1000 person-years, with a 95% confidence interval (CI) 36–79) compared to natural history studies (IR 28 per 1000 person-years, 95% CI 17 – 46, p-value < 0.01) after adjustment for confounders. Conclusion: Our results suggest that the incidence rate of cerebral aneurysm growth might be higher after endovascular therapy than the incidence rates reported in natural history studies. These results should be viewed in light of the risk of bias of the individual studies and the risk of ecological bias