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Basher five-two : the true story of F-16 fighter pilot Captain Scott O'Grady
The autobiography of Scott O'Grady, who was shot down while flying his F-16 on a mission to keep the peace in Bosnia.
Book
Broad plumes rooted at the base of the Earth's mantle beneath major hotspots
A whole-mantle seismic imaging technique, combining accurate wavefield computations with information contained in whole seismic waveforms, is used to reveal the presence of broad conduits beneath many of Earth’s surface hotspots, supporting the idea that these conduits are the source of hotspot volcanoes.
Plume-like conduits beneath surface hotspots
Scott French and Barbara Romanowicz use a whole-mantle seismic imaging technique, combining accurate wavefield computations with information contained in whole seismic waveforms, to reveal the presence of wide, quasi-vertical conduits beneath many of the Earth's surface hotspots. The conduits they image extend from the core–mantle boundary, where they are rooted in patches of strongly reduced shear velocity, and correspond to known locations of large ultralow-velocity zones beneath Hawaii, Iceland and Samoa, in support of the idea that they may be the source of hotspot volcanoes. As the conduits are broader than classical thermal plume tails, the authors suggest that they are long lived and may have a thermochemical origin.
Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate
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,
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. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide
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, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique—combining accurate wavefield computations with information contained in whole seismic waveforms
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—that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core–mantle boundary to about 1,000 kilometres below Earth’s surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones
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,
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. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived
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, and may have a thermochemical origin
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. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth’s mantle and guide further research into thermochemical convection.
Journal Article
Waveform Tomography Reveals Channeled Flow at the Base of the Oceanic Asthenosphere
Understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists. Using full-waveform seismic tomography, we imaged a pattern of horizontally elongated bands of low shear velocity, most prominent between 200 and 350 kilometers depth, which extends below the well-developed low-velocity zone. These quasi-periodic fingerlike structures of wavelength ~2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 kilometers depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plumelike features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer.
Journal Article
Biomarkers of blood–brain barrier and neurovascular unit integrity in human cognitive impairment and dementia
Blood–brain barrier (BBB) dysfunction is recognized as an early step in the development of Alzheimer's disease and related dementias (ADRD). Biomarkers are needed to monitor BBB integrity over time, better understand the role of the BBB in neurodegeneration, potentially help define long‐term ADRD risk, and monitor effects of therapeutics. In this review, we discuss the current biomarkers used to detect human BBB dysfunction in the context of cognitive decline and dementia. We also discuss promising candidate fluid biomarkers to detect BBB dysfunction in blood. Highlights BBB permeability occurs during normal aging and is further exacerbated in ADRD. In this review, we discuss in vivo imaging and CSF biomarkers of BBB dysfunction currently used in the setting of aging and ADRD in humans. We also review promising candidate blood‐based biomarkers that may represent BBB dysfunction.
Journal Article
Lithospheric Thinning Beneath Rifted Regions of Southern California
The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere.
Journal Article
North American lithospheric discontinuity structure imaged by Ps and Sp receiver functions
Sp and Ps converted seismic waves at 93 permanent seismic stations are used to image upper mantle velocity discontinuities across the contiguous United States and portions of southeast Canada and northwest Mexico. Receiver functions are calculated with frequency‐domain deconvolution and migrated with 1D models that account for variations in crustal structure and mantle velocities between stations. Strong positive Ps phases from the Moho are observed and agree well with previous crustal thickness estimates. In the tectonically active western U.S., high amplitude, negative Sp phases are interpreted as the lithosphere‐asthenosphere boundary (LAB) at depths of 51–104 km. These phases indicate a large and rapid LAB velocity gradient and are consistent with an anomalously hot asthenosphere that is rich in water or contains partial melt. In the regions of the Phanerozoic southern and eastern U.S where Sp phases are interpretable as the LAB, the discontinuity lies at depths of 75–111 km and is also too sharp to be explained by temperature alone. In contrast, no Sp phases are observed at depths comparable to the base of the thick high velocity lithosphere that lies beneath cratonic North America and certain portions of the Phanerozoic eastern U.S. At these stations, negative Sp phases occur at depths of 59–113 km and are interpreted as the top of a low velocity zone internal to the lithosphere. The absence of an observable LAB discontinuity in regions of thick lithosphere indicates that the LAB velocity gradient is distributed over more than 50–70 km in depth and is consistent with a purely thermal boundary.
Journal Article
Contrast input and manual interventions significantly affect FreeSurfer morphometry and clinical correlations
•T2-weighted MRI significantly alters FreeSurfer morphometry.•Contrasts alter cortical thickness associations to established biological factors.•Manual editing corrects contrast-related bias and improves segmentation quality.•Edited T1-only pipelines may yield the most biologically consistent morphometrics.•Findings were validated in both single-center and external ADNI aging cohorts.
FreeSurfer is a widely used software for quantification of brain morphometrics in studies of neurodegeneration and aging. However, choice of input MRI contrast(s) and manual editing are variable across studies and their influence on clinically relevant outcomes remains unclear. Using non-demented, older-aged participant data from the Carotid and Mind study (CAM; N = 123) and Alzheimer’s Disease Neuroimaging Initiative (ADNI, N = 143), FreeSurfer morphometrics from T1-MPRAGE, T1+T2-FLAIR, and T1+T2-SPACE were assessed with and without manual edits. In CAM and ADNI cohorts, input contrast significantly affected cortical thickness, surface area, and volume estimates across lobar regions. T1+T2-SPACE and T1+T2-FLAIR consistently produced greater cortical thickness estimates and smaller surface areas than T1-MPRAGE alone. These systematic differences altered the detection of expected age- and smoking-related associations to cortical thickness. Expert ratings indicated that T1-MPRAGE produced the highest baseline segmentation quality, while manual editing reliably improved this quality and reduced contrast-related morphometric biases across contrast types. These results suggest that choice of input contrast may introduce non-biological variation into FreeSurfer morphometrics, with T1-MPRAGE alone and manual editing yielding the most reliable outcomes. This indicates that careful consideration and reporting of post-processing protocols is critical for reproducibility and interpretation of morphometric outcomes across cohorts.
Journal Article
Carotid endarterectomy mitigates Alzheimer's disease and non–Alzheimer's disease dementia risk linked to asymptomatic carotid stenosis
INTRODUCTION Asymptomatic extracranial carotid artery disease (aECAD) is associated with increased Alzheimer's disease (AD) and non‐AD dementia risk. aECAD treatment includes carotid endarterectomy (CEA) and carotid artery stenting (CAS) for stroke prevention, but their impact on dementia incidence is poorly studied. METHODS Propensity score matching was used in a retrospective cohort study of United States–based insurance claims (2010–2022) in 487,676 patients with aECAD to evaluate the effect of CEA and CAS on AD and non‐AD dementia incidence. RESULTS After matching, 37,317 patients underwent CEA or CAS. CEA was associated with a significantly lower AD risk (relative risk = 0.93; 95% confidence interval, 0.86–0.99; P < 0.05), whereas CAS was associated with a slight but non‐significant increase. Similar trends were observed for non‐AD dementia. DISCUSSION CEA, but not CAS, may confer a protective effect against AD and non‐AD dementia in patients with aECAD, a common cerebrovascular disease affecting up to 15% of adults over age 60. Highlights Asymptomatic extracranial carotid artery disease (aECAD) is associated with increased Alzheimer's disease (AD) and non‐AD dementia risk. Limited studies have evaluated the role of carotid endarterectomy (CEA) and (carotid artery stenting (CAS) on dementia outcomes. Using United States–based insurance claims data, 487,676 patients with aECAD were evaluated. After propensity score matching, CEA was significantly associated with reduced AD risk. CAS was not significantly associated with a change in AD risk.
Journal Article
Cognitive impairment and p‐tau217 are high in a vascular patient cohort
INTRODUCTION Vascular comorbidities are modifiable contributors to cognitive impairment and Alzheimer's disease (AD), yet brain health outcomes are rarely evaluated in cardiovascular patients. METHODS This study prospectively evaluated cognition and AD pathology in 162 community‐dwelling adults with asymptomatic cardiovascular disease who did not have a clinical diagnosis of dementia or cognitive impairment. RESULTS Twenty‐nine percent of the cohort had Montreal Cognitive Assessment (MoCA) scores indicative of cognitive impairment or dementia after adjusting for age, sex, and education based on National Alzheimer's Coordinating Center normative data. AD blood biomarker phosphorylated tau217 was elevated in 55% of the cohort, significantly associated with decreased MoCA scores (β = −1.46, 95% confidence interval [CI] −2.53 to −0.39, p < 0.01), and accurately differentiated cognitive impairment (area under the curve 0.94, 95% CI 0.88–0.99). DISCUSSION This level of undiagnosed cognitive impairment and AD pathology exceeds what would be expected in the general population and highlights a potential need for screening and future work to better identify treatment options. Highlights Brain health outcomes are rarely evaluated in vascular patients. One hundred sixty‐two adults with asymptomatic cardiovascular disease but without diagnoses of cognitive impairment or dementia were evaluated. Phosphorylated tau217 accurately differentiated cognitive impairment in patients with cardiovascular disease. High levels of cognitive impairment and Alzheimer's disease pathology are greatly underdiagnosed in the cardiovascular population.
Journal Article
In an older‐age vascular cohort, carotid stenosis is associated with processing speed and executive function cognitive deficits, which correlate with p‐tau217
INTRODUCTION Asymptomatic extracranial carotid atherosclerotic disease (aECAD) has been associated with Alzheimer's disease (AD), but the earliest cognitive deficits in this hallmark vascular population are largely unknown. METHODS A total of 182 participants 50–85 years of age with ≥2 vascular comorbidities with/without aECAD were evaluated prospectively using a neurocognitive battery and compared to vascular comorbidities, percent stenosis, apolipoprotein E (APOE) ε4 status, and plasma phosphorylated tau (p‐tau217). Dementia, stroke < 6 months, and neurological disorders were exclusionary. RESULTS aECAD was associated with significantly worse scores in processing speed (β = −0.19, p = 0.004) and executive function (β = −0.20, p = 0.009) domains after controlling for age, sex, education, race, and ethnicity. From these, a carotid cognitive index was created, which correlated with p‐tau217 (β = −0.29, p < 0.001), accounting for demographics, vascular comorbidities, white matter lesions, and APOE ε4 status. DISCUSSION These data identify specific cognitive deficits associated with carotid stenosis and build further impetus to understand how vascular‐related cognitive deficits contrast and complement the classic memory deficits of AD.
Journal Article