Kooperative Strategien im Gefangenendilemma: Computersimulation eines N-Personen-Spiels

Simulation studies in the context of Robert Axelrod's research on iterative prisoner's dilemma games focus nearly exclusively on the two-player-version of the game. In contrast, this article reports results of a simulation with an iterated N-prisoners' dilemma where group size N varies between 2 and 30. The simulation investigates the relative performance of conditional cooperative strategies with increasing group size. Results show that some 'nice' strategies like 'tit-for-tat' are relatively successful and robust even in larger groups and non-nice environments. However, this does not solve the cooperation problem. On the contrary, the relative success of some 'nice' conditional cooperative strategies is paralleled by a rapid decline of cooperation in large groups.Simulationsstudien im Kontext von Robert Axelrods Forschung über iterative Gefangenendilemma-Spiele konzentrieren sich nahezu ausschließlich auf die Zwei-Spieler-Version. Im Kontrast dazu zeigt dieser Artikel Ergebnisse einer Simulation mit einem wiederholt gespielten N-Gefangenendilemma, in der die Gruppengröße N variiert zwischen zwei und 30. Die Simulation erforscht die relative Durchführung von bedingt kooperativen Strategien mit zunehmender Gruppengröße. Die Ergebnisse zeigen, daß einige "freundliche" Strategien wie "Tit-for-Tat" relativ erfolgreich und robust sind und dies selbst in großen Gruppen und unfreundlicher Umgebung. Dies löst dennoch nicht das Kooperationsproblem. Im Gegensatz dazu wird der relative Erfolg einiger "freundlicher" bedingt kooperativer Strategien begleitet von einer rapiden Abnahme der Kooperation in großen Gruppen

Dynamic Reconstruction with Statistical Ray Weighting for C-Arm CT Perfusion Imaging

Abstract—Tissue perfusion measurement using C-arm angiography systems is a novel technique with potential high benefit for catheter-guided treatment of stroke in the interventional suite. However, perfusion C-arm CT (PCCT) is challenging: the slow C-arm rotation speed only allows measuring samples of contrast time attenuation curves (TACs) every 5 – 6 s if reconstruction algorithms for static data are used. Furthermore, the peaks of the tissue TACs typically lie in a range of 5 – 30 HU, thus perfusion imaging is very sensitive to noise. Recently we presented a dynamic, iterative reconstruction (DIR) approach to reconstruct TACs described by a weighted sum of linear spline functions with a regularization based on joint bilateral filtering (JBF). In this work we incorporate statistical ray weighting into the algorithm and show how this helps to improve the reconstructed cerebral blood flow (CBF) maps in a simulation study with a realistic dynamic brain phantom. The Pearson correlation of the CBF maps to ground truth maps increases from 0.85 (FDK), 0.87 (FDK with JBF), and 0.90 (DIR with JBF) to 0.92 (DIR with JBF and ray weighting). The results suggest that the statistical ray weighting approach improves the diagnostic accuracy of PCCT based on DIR. I

Double Your Views - Exploiting Symmetry in Transmission Imaging

For a plane symmetric object we can find two views - mirrored at the plane of symmetry - that will yield the exact same image of that object. In consequence, having one image of a plane symmetric object and a calibrated camera, we can automatically have a second, virtual image of that object if the 3-D location of the symmetry plane is known. In this work, we show for the first time that the above concept naturally extends to transmission imaging and present an algorithm to estimate the 3-D symmetry plane from a set of projection domain images based on Grangeat's theorem. We then exploit symmetry to generate a virtual trajectory by mirroring views at the plane of symmetry. If the plane is not perpendicular to the acquired trajectory plane, the virtual and real trajectory will be oblique. The resulting X-shaped trajectory will be data-complete, allowing for the compensation of in-plane motion using epipolar consistency. We evaluate the proposed method on a synthetic symmetric phantom and, in a proof-of-concept study, apply it to a real scan of an anthropomorphic human head phantom.Comment: Accepted for MICCAI 2018 (8 Pages