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Item Visual Assessment of Vascular Torsion using Ellipse Fitting(The Eurographics Association, 2018) Mistelbauer, Gabriel; Zettwitz, Martin; Schernthaner, Rüdiger; Fleischmann, Dominik; Teutsch, Christian; Preim, Bernhard; Puig Puig, Anna and Schultz, Thomas and Vilanova, Anna and Hotz, Ingrid and Kozlikova, Barbora and Vázquez, Pere-PauBlood vessels are well explored and researched in medicine and visualization. However, the investigation of vascular torsion has yet been unexplored. In order to understand the development and current state of a single blood vessel or even multiple connected ones, properties of vascular structures have to be analyzed. In this paper we assess the torsion of blood vessels in order to better understand their morphology. The aim of this work is to quantitatively gauge blood vessels by using an automated method that assumes an elliptical blood vessel model. This facilitates using state-of-the-art ellipse fitting algorithms from industrial measuring standards. In order to remove outliers, we propose a self-correcting iterative refitting of ellipses using neighboring information. The torsion information is visually conveyed by connecting the major and minor points of adjacent ellipses. Our final visualization comprises a visual representation of the blood vessel including four bands to outline the torsion.Item A Critical Analysis of the Evaluation Practice in Medical Visualization(The Eurographics Association, 2018) Preim, Bernhard; Ropinski, Timo; Isenberg, Petra; Puig Puig, Anna and Schultz, Thomas and Vilanova, Anna and Hotz, Ingrid and Kozlikova, Barbora and Vázquez, Pere-PauMedical visualization aims at directly supporting physicians in diagnosis and treatment planning, students and residents in medical education, and medical physicists as well as other medical researchers in answering specific research questions. For assessing whether single medical visualization techniques or entire medical visualization systems are useful in this respect, empirical evaluations involving participants from the target user group are indispensable. The human computer interaction field developed a wide range of evaluation instruments, and the information visualization community more recently adapted and refined these instruments for evaluating (information) visualization systems. However, often medical visualization lacks behind and should pay more attention to evaluation, in particular to evaluations in realistic settings that may assess how visualization techniques contribute to cognitive activities, such as deciding about a surgical strategy or other complex treatment decisions. In this vein, evaluations that are performed over a longer period are promising to study, in order to investigate how techniques are adapted. In this paper, we discuss the evaluation practice in medical visualization based on selected examples and contrast these evaluations with the broad range of existing empirical evaluation techniques. We would like to emphasize that this paper does not serve as a general call for evaluation in medical visualization, but argues that the individual situation must be assessed and that evaluations when they are carried out should be done more carefully.Item A Framework for Visual Comparison of 4D PC-MRI Aortic Blood Flow Data(The Eurographics Association, 2018) Behrendt, Benjamin; Ebel, Sebastian; Gutberlet, Matthias; Preim, Bernhard; Puig Puig, Anna and Schultz, Thomas and Vilanova, Anna and Hotz, Ingrid and Kozlikova, Barbora and Vázquez, Pere-PauFour-dimensional phase-contrast magnetic resonance imaging (4D PC-MRI) allows for the non-invasive acquisition of in-vivo blood flow, producing a patient-specific blood flow model in selected vascular structures, e.g. the aorta. In the past, many specialized techniques for the visualization and exploration of such datasets have been developed, yet a tool for the visual comparison of multiple datasets is missing. Due to the complexity of the underlying data, a simple side-by-side comparison of two datasets using traditional visualization techniques can only yield coarse results. In this paper, we present a toolkit that allows for an efficient and robust registration of different 4D PC-MRI datasets and offers a variety of both qualitative and quantitative comparison techniques. Differences in the segmentation and time frame can be amended semi-automatically using landmarks on the vessel centerline and flow curve of the datasets. A set of measures quantifying the difference between the datasets, such as the flow jet displacement or flow angle and velocity difference, is automatically computed. To support the orientation in the spatio-temporal domain of the flow dataset, we provide bulls-eye plots that highlight potentially interesting regions. In an evaluation with three experienced radiologists, we confirmed the usefulness of our technique. With our application, they were able to discover previously unnoticed artifacts occurring in a dataset acquired with an experimental MRI sequence.Item Automatic Generation of Web-Based User Studies to Evaluate Depth Perception in Vascular Surface Visualizations(The Eurographics Association, 2018) Meuschke, Monique; Smit, Noeska N.; Lichtenberg, Nils; Preim, Bernhard; Lawonn, Kai; Puig Puig, Anna and Schultz, Thomas and Vilanova, Anna and Hotz, Ingrid and Kozlikova, Barbora and Vázquez, Pere-PauUser studies are often required in biomedical visualization application papers in order to provide evidence for the utility of the presented approach. An important aspect is how well depth information can be perceived, as depth encoding is important to enable an understandable representation of complex data. Unfortunately, in practice there is often little time available to perform such studies, and setting up and conducting user studies may be labor-intensive. In addition, it can be challenging to reach enough participants to support the contribution claims of the paper. In this paper, we propose a system that allows biomedical visualization researchers to quickly generate perceptual task-based user studies for novel surface visualizations, and to perform the resulting experiment via a web interface. This approach helps to reduce effort in the setup of user studies themselves, and at the same time leverages a web-based approach that can help researchers attract more participants to their study. We demonstrate our system using the specific application of depth judgment tasks to evaluate vascular surface visualizations, since there is a lot of recent interest in this area. However, the system is also generally applicable for conducting other task-based user studies in biomedical visualization.