430 research outputs found
Comparison of magnetic resonance with computed tomography angiography for preoperative localization of the Adamkiewicz artery in thoracoabdominal aortic aneurysm patients
ObjectivePreoperative localization of the Adamkiewicz artery and its segmental supplier in advance of thoracic aortic aneurysm (TAA) and thoracoabdominal aortic aneurysm (TAAA) repair is proposed to be useful to prevent postoperative paraplegia. The diagnostic potential of magnetic resonance angiography (MRA) and computed tomography angiography (CTA) was evaluated for the preoperative localization of the Adamkiewicz artery in white TAAA patients.MethodsThirty-nine consecutive patients with a TAA(A) scheduled for elective open surgical aortic repair preoperatively underwent MRA and CTA. Objective image quality was assessed by measuring the signal-to-noise ratio and contrast-to-noise ratio of the Adamkiewicz artery and was related to patient thickness. Two independent observers scored the location of the Adamkiewicz artery and the subjective image quality of vessel-background contrast of the Adamkiewicz artery, image noise, spinal cord tissue enhancement, epidural venous enhancement, and overall image quality.ResultsAverage detection rate for Adamkiewicz artery localization was 71% (67% to 74%) for CTA and 97% (94% to 100%) for MRA. Interobserver agreement was 82% for CTA and 94% for MRA. Signal-to-noise ratio was significantly higher (P < .001) and contrast-to-noise ratio was significantly (P < .001) lower for CTA than for MRA. Contrast of the Adamkiewicz artery (P < .001) and overall image quality (P < .004) were judged to be significantly better for MRA. Spinal cord tissue enhancement was judged stronger at CTA (P < .03), with significantly less epidural venous enhancement (P < .001). No significant difference was found in image noise. Signal-to-noise and contrast-to-noise decreased significantly (P < .001) with increasing patient thickness for CTA but not for MRA.ConclusionsLocalization of the Adamkiewicz artery in white TAAA patients is possible with both CTA and MRA. Compared with CTA, MRA is more favorable because of the higher Adamkiewicz artery detection rate, the higher contrast-to-noise ratio, and its independence of patient thickness
A Comprehensive View on MRI Techniques for Imaging Blood-Brain Barrier Integrity
The blood-brain barrier (BBB) is the interface between the blood and brain tissue, which regulates the maintenance of homeostasis within the brain. Impaired BBB integrity is increasingly associated with various neurological diseases. To gain a better understanding of the underlying processes involved in BBB breakdown, magnetic resonance imaging (MRI) techniques are highly suitable for noninvasive BBB assessment. Commonly used MRI techniques to assess BBB integrity are dynamic contrast-enhanced and dynamic susceptibility contrast MRI, both relying on leakage of gadolinium-based contrast agents. A number of conceptually different methods exist that target other aspects of the BBB. These alternative techniques make use of endogenous markers, such as water and glucose, as contrast media. A comprehensive overview of currently available MRI techniques to assess the BBB condition is provided from a scientific point of view, including potential applications in disease. Improvements that are required to make these techniques clinically more easily applicable will also be discussed.</p
Second harmonic generation in SiC polytypes
LMTO calculations are presented for the frequency dependent second harmonic
generation (SHG) in the polytypes 2H, 4H, 6H, 15R and 3C of SiC. All
independent tensor components are calculated. The spectral features and the
ratios of the 333 to 311 tensorial components are studied as a function of the
degree of hexagonality. The relationship to the linear optical response and the
underlying band structure are investigated. SHG is suggested to be a sensitive
tool for investigating the near band edge interband excitations.Comment: 12 pages, 10 figure
Quantifying blood-brain barrier leakage in small vessel disease: Review and consensus recommendations
Cerebral small vessel disease (cSVD) comprises pathological processes of the small vessels in the brain that may manifest clinically as stroke, cognitive impairment, dementia, or gait disturbance. It is generally accepted that endothelial dysfunction, including blood-brain barrier (BBB) failure, is pivotal in the pathophysiology. Recent years have seen increasing use of imaging, primarily dynamic contrast-enhanced magnetic resonance imaging, to assess BBB leakage, but there is considerable variability in the approaches and findings reported in the literature. Although dynamic contrast-enhanced magnetic resonance imaging is well established, challenges emerge in cSVD because of the subtle nature of BBB impairment. The purpose of this work, authored by members of the HARNESS Initiative, is to provide an in-depth review and position statement on magnetic resonance imaging measurement of subtle BBB leakage in clinical research studies, with aspects requiring further research identified. We further aim to provide information and consensus recommendations for new investigators wishing to study BBB failure in cSVD and dementia. (C) 2019 The Authors. Published by Elsevier Inc. on behalf of the Alzheimer's Association. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
Evaluation of miCRovascular rarefaction in vascUlar Cognitive Impairment and heArt faiLure (CRUCIAL): Study protocol for an observational study
INTRODUCTION: Microvascular rarefaction, the functional reduction in perfused microvessels and structural reduction of microvascular density, seems to be an important mechanism in the pathophysiology of small blood vessel related disorders including vascular cognitive impairment (VCI) due to cerebral small vessel disease and heart failure with preserved ejection fraction (HFpEF). Both diseases share common risk factors including hypertension, diabetes mellitus, obesity, and ageing; in turn, these co-morbidities are associated with microvascular rarefaction. Our consortium aims to investigate novel non-invasive tools to quantify microvascular health and rarefaction in both organs, as well as surrogate biomarkers for cerebral and/or cardiac rarefaction (via sublingual capillary health, vascular density of the retina, and RNA content of circulating extracellular vesicles), and to determine whether microvascular density relates to disease severity. METHODS/DESIGN: The clinical research program of CRUCIAL consists of four observational cohort studies. We aim to recruit 75 VCI patients, 60 HFpEF patients, 60 patients with severe aortic stenosis (AS) undergoing surgical aortic valve replacement as a pressure overload HFpEF model, and 200 elderly participants with mixed comorbidities to serve as controls. Data collected will include medical history, physical examination, cognitive testing, advanced brain and cardiac MRI, ECG, echocardiography, sublingual capillary health, optical coherence tomography angiography (OCTa), extracellular vesicles RNA analysis and myocardial remodelling-related serum biomarkers. The AS cohort undergoing surgery will also have myocardial biopsy for histological microvascular assessment. DISCUSSION: CRUCIAL will examine the pathophysiological role of microvascular rarefaction in VCI and HFpEF using advanced brain and cardiac MRI techniques. Furthermore, we will investigate surrogate biomarkers for non-invasive, faster, easier, and cheaper assessment of microvascular density since these are more likely to be disseminated into widespread clinical practice. If microvascular rarefaction is an early marker of developing small vessel diseases, then measuring rarefaction may allow pre-clinical diagnosis, with implications for screening, risk stratification, and prevention. Further knowledge of the relevance of microvascular rarefaction and its underlying mechanisms may provide new avenues for research and therapeutic targets
FACT -- The G-APD revolution in Cherenkov astronomy
Since two years, the FACT telescope is operating on the Canary Island of La
Palma. Apart from its purpose to serve as a monitoring facility for the
brightest TeV blazars, it was built as a major step to establish solid state
photon counters as detectors in Cherenkov astronomy. The camera of the First
G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes
(G-APD), equipped with solid light guides to increase the effective light
collection area of each sensor. Since no sense-line is available, a special
challenge is to keep the applied voltage stable although the current drawn by
the G-APD depends on the flux of night-sky background photons significantly
varying with ambient light conditions. Methods have been developed to keep the
temperature and voltage dependent response of the G-APDs stable during
operation. As a cross-check, dark count spectra with high statistics have been
taken under different environmental conditions. In this presentation, the
project, the developed methods and the experience from two years of operation
of the first G-APD based camera in Cherenkov astronomy under changing
environmental conditions will be presented.Comment: Proceedings of the Nuclear Science Symposium and Medical Imaging
Conference (IEEE-NSS/MIC), 201
White Matter Connectivity Abnormalities in Prediabetes and Type 2 Diabetes:The Maastricht Study
OBJECTIVE: Prediabetes and type 2 diabetes are associated with structural brain abnormalities, often observed in cognitive disorders. Besides visible lesions, (pre)diabetes might also be associated with alterations of the intrinsic organization of the white matter. In this population-based cohort study, the association of prediabetes and type 2 diabetes with white matter network organization was assessed. RESEARCH DESIGN AND METHODS: In the Maastricht Study, a type 2 diabetes-enriched population-based cohort study (1,361 normal glucose metabolism, 348 prediabetes, and 510 type 2 diabetes assessed by oral glucose tolerance test; 52% men; aged 59 ± 8 years), 3 Tesla structural and diffusion MRI was performed. Whole-brain white matter tractography was used to assess the number of connections (node degree) between 94 brain regions and the topology (graph measures). Multivariable linear regression analyses were used to investigate the associations of glucose metabolism status with network measures. Associations were adjusted for age, sex, education, and cardiovascular risk factors. RESULTS: Prediabetes and type 2 diabetes were associated with lower node degree after full adjustment (standardized [st]βPrediabetes = -0.055 [95% CI -0.172, -0.062], stβType2diabetes = -0.256 [-0.379, -0.133], Ptrend < 0.001). Prediabetes was associated with lower local efficiency (stβ = -0.084 [95% CI -0.159, -0.008], P = 0.033) and lower clustering coefficient (stβ = -0.097 [95% CI -0.189, -0.005], P = 0.049), whereas type 2 diabetes was not. Type 2 diabetes was associated with higher communicability (stβ = 0.148 [95% CI 0.042, 0.253], P = 0.008). CONCLUSIONS: These findings indicate that prediabetes and type 2 diabetes are associated with fewer white matter connections and weaker organization of white matter networks. Type 2 diabetes was associated with higher communicability, which was not yet observed in prediabetes and may reflect the use of alternative white matter connections
FACT - The First G-APD Cherenkov Telescope: Status and Results
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). It is built on the mount of the
HEGRA CT3 telescope, still located at the Observatorio del Roque de los
Muchachos, and it is successfully in operation since Oct. 2011. The use of
Silicon devices promises a higher photon detection efficiency, more robustness
and higher precision than photo-multiplier tubes. The FACT collaboration is
investigating with which precision these devices can be operated on the
long-term. Currently, the telescope is successfully operated from remote and
robotic operation is under development. During the past months of operation,
the foreseen monitoring program of the brightest known TeV blazars has been
carried out, and first physics results have been obtained including a strong
flare of Mrk501. An instantaneous flare alert system is already in a testing
phase. This presentation will give an overview of the project and summarize its
goals, status and first results
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