9 research outputs found

    Vascular Diseases of the Gastrointestinal Tract

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    Hepatology: A textbook of liver disease

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    Hepatic Manifestations of Systemic Diseases

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    Power estimation for non-standardized multisite studies

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    A concern for researchers planning multisite studies is that scanner and T1-weighted sequence-related biases on regional volumes could overshadow true effects, especially for studies with a heterogeneous set of scanners and sequences. Current approaches attempt to harmonize data by standardizing hardware, pulse sequences, and protocols, or by calibrating across sites using phantom-based corrections to ensure the same raw image intensities. We propose to avoid harmonization and phantom-based correction entirely. We hypothesized that the bias of estimated regional volumes is scaled between sites due to the contrast and gradient distortion differences between scanners and sequences. Given this assumption, we provide a new statistical framework and derive a power equation to define inclusion criteria for a set of sites based on the variability of their scaling factors. We estimated the scaling factors of 20 scanners with heterogeneous hardware and sequence parameters by scanning a single set of 12 subjects at sites across the United States and Europe. Regional volumes and their scaling factors were estimated for each site using Freesurfer's segmentation algorithm and ordinary least squares, respectively. The scaling factors were validated by comparing the theoretical and simulated power curves, performing a leave-one-out calibration of regional volumes, and evaluating the absolute agreement of all regional volumes between sites before and after calibration. Using our derived power equation, we were able to define the conditions under which harmonization is not necessary to achieve 80% power. This approach can inform choice of processing pipelines and outcome metrics for multisite studies based on scaling factor variability across sites, enabling collaboration between clinical and research institutions.publisher: Elsevier articletitle: Power estimation for non-standardized multisite studies journaltitle: NeuroImage articlelink: http://dx.doi.org/10.1016/j.neuroimage.2016.03.051 content_type: article copyright: © 2016 The Authors. Published by Elsevier Inc.status: publishe

    Power estimation for non-standardized multisite studies

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    AbstractA concern for researchers planning multisite studies is that scanner and T1-weighted sequence-related biases on regional volumes could overshadow true effects, especially for studies with a heterogeneous set of scanners and sequences. Current approaches attempt to harmonize data by standardizing hardware, pulse sequences, and protocols, or by calibrating across sites using phantom-based corrections to ensure the same raw image intensities. We propose to avoid harmonization and phantom-based correction entirely. We hypothesized that the bias of estimated regional volumes is scaled between sites due to the contrast and gradient distortion differences between scanners and sequences. Given this assumption, we provide a new statistical framework and derive a power equation to define inclusion criteria for a set of sites based on the variability of their scaling factors. We estimated the scaling factors of 20 scanners with heterogeneous hardware and sequence parameters by scanning a single set of 12 subjects at sites across the United States and Europe. Regional volumes and their scaling factors were estimated for each site using Freesurfer's segmentation algorithm and ordinary least squares, respectively. The scaling factors were validated by comparing the theoretical and simulated power curves, performing a leave-one-out calibration of regional volumes, and evaluating the absolute agreement of all regional volumes between sites before and after calibration. Using our derived power equation, we were able to define the conditions under which harmonization is not necessary to achieve 80% power. This approach can inform choice of processing pipelines and outcome metrics for multisite studies based on scaling factor variability across sites, enabling collaboration between clinical and research institutions

    Governance to governmentality: analyzing NGOs, states, and power

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    Studies of global governance typically claim that the state has lost power to nonstate actors and that political authority is increasingly institutionalized in spheres not controlled by states. In this article, we challenge the core claims in the literature on global governance. Rather than focusing on the relative power of states and nonstate actors, we focus on the sociopolitical functions and processes of governance in their own right and seek to identify their rationality as practices of political rule. For this task, we use elements of the conception of power developed by Michel Foucault in his studies of “governmentality.” In this perspective, the role of nonstate actors in shaping and carrying out global governance-functions is not an instance of transfer of power from the state to nonstate actors but rather an expression of a changing logic or rationality of government (defined as a type of power) by which civil society is redefined from a passive object of government to be acted upon into an entity that is both an object and a subject of government. The argument is illustrated by two case studies: the international campaign to ban landmines, and international population policy. The cases show that the self-association and political will-formation characteristic of civil society and nonstate actors do not stand in opposition to the political power of the state, but is a most central feature of how power, understood as government, operates in late modern society

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