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The study of the colonic volume is a procedure with strong relevance to gastroenterologists. Depending on the clinical protocols, the volume analysis has to be performed on MRI of the unprepared colon without contrast administration. In such circumstances, existing measurement procedures are cumbersome and time-consuming for the specialists. The algorithm presented in this paper permits a quasi-automatic segmentation of the unprepared colon on T2-weighted MRI scans. The segmentation algorithm is organized as a three-stage pipeline. In the first stage, a custom tubularity filter is run to detect colon candidate areas. The specialists provide a list of points along the colon trajectory, which are combined with tubularity information to calculate an estimation of the colon medial path. In the second stage, we delimit the region of interest by applying custom segmentation algorithms to detect colon neighboring regions and the fat capsule containing abdominal organs. Finally, within the reduced search space, segmentation is performed via 3D graph-cuts in a three-stage multigrid approach. Our algorithm was tested on MRI abdominal scans, including different acquisition resolutions, and its results were compared to the colon ground truth segmentations provided by the specialists. The experiments proved the accuracy, efficiency, and usability of the algorithm, while the variability of the scan resolutions contributed to demonstrate the computational scalability of the multigrid architecture. The system is fully applicable to the colon measurement clinical routine, being a substantial step towards a fully automated segmentation.

Mobile devices have experienced an incredible market penetration in the last decade. Currently, medium to premium smartphones are relatively affordable devices. With the increase in screen size and resolution, together with the improvements in performance of mobile CPUs and GPUs, more tasks have become possible. In this paper we explore the rendering of medium to large volumetric models on mobile and low performance devices in general. To do so, we present a progressive ray casting method that is able to obtain interactive frame rates and high quality results for models that not long ago were only supported by desktop computers.

This is the peer reviewed version of the following article: Bendezú, Á., Mego, M., Monclús, E., Merino, X., Accarino, A., Malagelada, J., Navazo, I., Azpiroz, F. Colonic content: effect of diet, meals, and defecation. "Neurogastroenterology and motility", Febrer 2017, vol. 29, núm. 2, which has been published in final form at [http://onlinelibrary.wiley.com.recursos.biblioteca.upc.edu/doi/10.1111/nmo.12930/full]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. The metabolic activity of colonic microbiota is influenced by diet; however, the relationship between metabolism and colonic content is not known. Our aim was to determine the effect of meals, defecation, and diet on colonic content. Methods: In 10 healthy subjects, two abdominal MRI scans were acquired during fasting, 1 week apart, and after 3 days on low- and high-residue diets, respectively. With each diet, daily fecal output and the number of daytime anal gas evacuations were measured. On the first study day, a second scan was acquired 4 hours after a test meal (n=6) or after 4 hours with nil ingestion (n=4). On the second study day, a scan was also acquired after a spontaneous bowel movement. Results: On the low-residue diet, daily fecal volume averaged 145 ± 15 mL; subjects passed 10.6 ± 1.6 daytime anal gas evacuations and, by the third day, non-gaseous colonic content was 479 ± 36 mL. The high-residue diet increased the three parameters to 16.5 ± 2.9 anal gas evacuations, 223 ± 19 mL fecal output, and 616 ± 55 mL non-gaseous colonic content (P

Throughout the years, many shading techniques have been developed to improve the conveying of information in volume visualization. Some of these methods, usually referred to as realistic, are supposed to provide better cues for the understanding of volume data sets. While shading approaches are heavily exploited in traditional monoscopic setups, no previous study has analyzed the effect of these techniques in virtual reality. To further explore the influence of shading on the understanding of volume data in such environments, we carried out a user study in a desktop-based stereoscopic setup. The goals of the study were to investigate the impact of well-known shading approaches and the influence of real illumination on depth perception. Participants had to perform three different perceptual tasks when exposed to static visual stimuli. 45 participants took part in the study, giving us 1152 trials for each task. Results show that advanced shading techniques improve depth perception in stereoscopic volume visualization. As well, external lighting does not affect depth perception when these shading methods are applied. As a result, we derive some guidelines that may help the researchers when selecting illumination models for stereoscopic rendering.

This is the peer reviewed version of the following article: Bendezú, R. A., Barba, E., Burri, E., Cisternas, D., Accarino, A., Quiroga, S., Monclus, E., Navazo, I., Malagelada, J.-R. and Azpiroz, F. (2016), Colonic content in health and its relation to functional gut symptoms. Neurogastroenterol. Motil., 28: 849–854, which has been published in final form at [doi:10.1111/nmo.12782]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving Gut content may be determinant in the generation of digestive symptoms, particularly in patients with impaired gut function and hypersensitivity. Since the relation of intraluminal gas to symptoms is only partial, we hypothesized that non-gaseous component may play a decisive role. Methods: Abdominal computed tomography scans were evaluated in healthy subjects during fasting and after a meal (n = 15) and in patients with functional gut disorders during basal conditions (when they were feeling well) and during an episode of abdominal distension (n = 15). Colonic content and distribution were measured by an original analysis program. Key results: In healthy subjects both gaseous (87 ± 24 mL) and non-gaseous colonic content (714 ± 34 mL) were uniformly distributed along the colon. In the early postprandial period gas volume increased (by 46 ± 23 mL), but non-gaseous content did not, although a partial caudad displacement from the descending to the pelvic colon was observed. No differences in colonic content were detected between patients and healthy subjects. Symptoms were associated with discrete increments in gas volume. However, no consistent differences in non-gaseous content were detected in patients between asymptomatic periods and during episodes of abdominal distension. Conclusions & inferences: In patients with functional gut disorders, abdominal distension is not related to changes in non-gaseous colonic content. Hence, other factors, such as intestinal hypersensitivity and poor tolerance of small increases in luminal gas may be involved.

Medical illustrations have been used for a long time for teaching and communicating information for diagnosis or surgery planning. Illustrative visualization systems create methods and tools that adapt traditional illustration techniques to enhance the result of renderings. Clipping the volume is a popular operation in volume rendering for inspecting the inner parts, though it may remove some information of the context that is worth preserving. In this paper we present a new editing technique based on the use of clipping planes, direct structure extrusion, and illustrative methods, which preserves the context by adapting the extruded region to the structures of interest of the volumetric model. We will show that users may interactively modify the clipping plane and edit the structures to highlight, in order to easily create the desired result. Our approach works with segmented volume models and non-segmented ones. In the last case, a local segmentation is performed on-the-fly. We will demonstrate the efficiency and utility of our method. © 2012 Wiley Periodicals, Inc.

Since the publication of the original Marching Cubes algorithm, numerous variations have been proposed for guaranteeing water-tight constructions of triangulated approximations of isosurfaces. Most approaches divide the 3D space into cubes that each occupy the space between eight neighboring samples of a regular lattice. The portion of the isosurface inside a cube may be computed independently of what happens in the other cubes, provided that the constructions for each pair of neighboring cubes agree along their common face. The portion of the isosurface associated with a cube may consist of one or more connected components, which we call sheets. The topology and combinatorial complexity of the isosurface is influenced by three types of decisions made during its construction: (1) how to connect the four intersection points on each ambiguous face, (2) how to form interpolating sheets for cubes with more than one loop, and (3) how to triangulate each sheet. To determine topological properties, it is only relevant whether the samples are inside or outside the object, and not their precise value, if there is one. Previously reported techniques make these decisions based on local —per cube— criteria, often using precomputed look-up tables or simple construction rules. Instead, we propose global strategies for optimizing several topological and combinatorial measures of the isosurfaces: triangle count, genus, and number of shells. We describe efficient implementations of these optimizations and the auxiliary data structures developed to support them.

Although 3D texture-based volume rendering guarantees image quality almost interactively, it is difficult to maintain an interactive rate when the technique has to be exploited on large datasets. In this paper, we propose a new texture memory representation and a management policy that substitute the classical one-texel per voxel approach for a hierarchical approach. The hier- archical approach benefits nearly homogeneous regions and regions of lower interest. The proposed algorithm is based on a simple traversal of the octree representation of the volume data. Driven by a user-defined image quality, defined as a combination of data homogeneity and importance, a set of octree nodes (the cut) is selected to be rendered. The degree of accuracy applied for the representation of each one of the nodes of the cut in the texture memory is set independently according to the user-defined parameters.The variable resolution texture model obtained reduces the texture memory size and thus texture swapping, improving rendering speed.

Level-of-detail occlusion culling is a novel approach to the management of occluders that can be easily integrated into most current visibility culling algorithms. The main contribution of this paper is an algorithm that automatically generates sets of densely overlapping boxes with enhanced occlusion properties from non-convex subsets. We call this method occluder synthesis because it is not sensitive to the way the objects are tesselated but to the space enclosed by them. The extension of this technique by allowing a bounded amount of image error is also discussed. We show that visibility computations can be based on a multiresolution model which provides several representations of these occluders with varying visibility accuracy. Our tests show that occlusion performance in tesselated scenes is improved severely even if no image-error is allowed.

Computer graphics education is becoming more and more relevant in technical studies in Spain. This positive evolution is partly due to the changes in the curricula structure in the undergraduate studies in Spain in recent years. New and updated curricula are being set up in most universities with a semester structure for their courses. As a consequence, a good number of computer graphics courses have been created, and the number of universities offering graphics courses has clearly increased.Due to the converging interests in computer graphics education, a Computer Graphics Education Group has existed within the Spanish chapter of the Eurographics Association since 1992. The main activities of the group include annual meetings to discuss curricula, covered topics, methodology and resources. Relevant information and support materials are accessible and shared through the Internet ( http://smagris.eleinf.uv.es/DOCEN-CIA_IG/docencia.html ).Undergraduate university studies in Spain are either technical (three years) or high (five years). Most of the computer graphics courses are offered in both curricula. Graduate courses are organized in Ph.D. programs. Ph.D. programs in computer graphics are emerging as the new universities settle their studies. In the next two sections we briefly summarize the organization and contents of the computer graphics courses in Spain.

This paper describes the authors experience in the organization and teaching of courses of Computer Graphics for both undergraduate and graduate students. Curriculum contents and teaching methodologies are discussed, taking into account practical constraints related to the facilities available at laboratories and curriculum structures.

Direct cell-to-cell volume visualization algorithms, also called projective rendering methods, present the advantage of allowing semi-transparencies and, additionally, as they project all the samples, they avoid the voxel-space aliasing. However, they are generally computationally expensive and artifacts may appear in the projection. In this paper, different projective strategies are reviewed and compared. A new algorithm, based on a back-to-front (BTF) voxel-to-voxel traversal, is proposed. It consists of the computation, in a preprocess, of 16 Extended Generalized Weight (EGW) matrices, based on the voxel's geometry and the viewpoint direction. Taking into advantage the voxel-to-voxel coherence, the EGW are applied incrementally to each voxel as footprints to be composited with the image buffer, according to the voxel shade and opacity values. It is shown that the method avoids artifacts, and that it has a low computational cost because it does not require re-voxellization and it does not perform any geometric computation in the projection process. It is specially suitable for voxel representations and image space resolutions such that the projection of a single voxel covers more than one pixel.