Medial Axis Research Articles

Medial-axis-driven shape deformation with volume preservation

The medial axis is a natural skeleton for shapes. However, it is rarely used in the existing skeleton-based shape deformation techniques. In this paper, we propose a novel medial-axis-driven skin surface deformation algorithm with volume preservation property. Specifically, an as-rigid-as-possible deformation scheme is used to deform the medial axis so that its local transform is as close as possible to a rigid transform. We maintain surface features of the deformed shape based on an implicit skinning method. Our experiments show that the proposed algorithm effectively preserves the volume of deformed shape and addresses the bending and twisting problems associated with traditional skeleton-based shape deformation techniques.

NC process reuse oriented effective subpart retrieval approach of 3D CAD models

As a huge number of 3D CAD models with NC process are generated each year, retrieval of 3D CAD models for achieving NC process reuse is becoming an effective strategy to improve the efficiency of NC programing. However, there has been little research on how to discover similar subparts for NC process reuse. In this paper, a NC process reuse oriented effective subpart retrieval approach of 3D CAD models is proposed. Firstly, the medial axis transform (MAT) of a pocket is introduced and adopted as the reference tool paths. The association mechanism between MAT and NC machining is established to reveal the association between geometry and NC process for features. Then the rapid filtering for accelerating subpart retrieval is presented to filter out infeasible subparts quickly under the requirements of NC process reuse. Moreover, a multilevel feature descriptor capturing different levels of NC process information for reuse is elaborated to construct the feature similarity assessment model. Finally, the matched feature pairs similarity and feature layout information similarity are presented to calculate the similarity between subparts. In the experiment, a prototype based on CATIA is developed and the experimental results are analyzed at length to verify the effectiveness of the proposed approach.

Rational adaptive blends among obstacles in 3D by contour method

In this paper we will continue in investigating ‘contour method’ and its using for the computation of rational parameterizations of canal surfaces without a need of sum of squares (SOS) decomposition. Further approaches for constructing flexible smooth transitions between canal surfaces will be presented. Mainly, we focus on one particular application of recently introduced rational envelope curves, newly constructed over an arbitrary planar rational curve in space. Using this type of curves significantly simplifies the previous methods discussed in Bizzarri (2015), and mainly new situations, which could not have been handled with the previous setup, are successfully solved, now. Especially a method for constructing rational adaptive blends which bypass a given obstacle (or more given obstacles when needed) is thoroughly discussed and its functionality is demonstrated on a number of examples. The designed approach works not only for simple obstacles represented by one-dimensional medial axis transforms but also for more general obstacles described by two-dimensional medial surface transforms.

MRI-Based Medial Axis Extraction and Boundary Segmentation of Cranial Nerves Through Discrete Deformable 3D Contour and Surface Models

This paper presents a segmentation technique to identify the medial axis and the boundary of cranial nerves. We utilize a 3-D deformable one-simplex discrete contour model to extract the medial axis of each cranial nerve. This contour model represents a collection of two-connected vertices linked by edges, where vertex position is determined by a Newtonian expression for vertex kinematics featuring internal and external forces, the latter of which include attractive forces toward the nerve medial axis. We exploit multiscale vesselness filtering and minimal path techniques in the medial axis extraction method, which also computes a radius estimate along the path. Once we have the medial axis and the radius function of a nerve, we identify the nerve surface using a two-simplex deformable model, which expands radially and can accommodate any nerve shape. As a result, the method proposed here combines the benefits of explicit contour and surface models, while also achieving a cornerstone for future work that will emphasize shape statistics, static collision with other critical structures, and tree-shape analysis.

Pore network extraction from pore space images of various porous media systems

AbstractPore network extraction, which is defined as the transformation from irregular pore space to a simplified network in the form of pores connected by throats, is significant to microstructure analysis and network modeling. A physically realistic pore network is not only a representation of the pore space in the sense of topology and morphology, but also a good tool for predicting transport properties accurately. We present a method to extract pore network by employing the centrally located medial axis to guide the construction of maximal‐balls‐like skeleton where the pores and throats are defined and parameterized. To validate our method, various rock samples including sand pack, sandstones, and carbonates were used to extract pore networks. The pore structures were compared quantitatively with the structures extracted by medial axis method or maximal ball method. The predicted absolute permeability and formation factor were verified against the theoretical solutions obtained by lattice Boltzmann method and finite volume method, respectively. The two‐phase flow was simulated through the networks extracted from homogeneous sandstones, and the generated relative permeability curves were compared with the data obtained from experimental method and other numerical models. The results show that the accuracy of our network is higher than that of other networks for predicting transport properties, so the presented method is more reliable for extracting physically realistic pore network.

Multiregion Inspection by Combining Clustered Traveling Salesman Tours With Sampling-Based Motion Planning

This paper develops an efficient approach to generate a collision-free and dynamically feasible trajectory that enables a robotic vehicle to inspect the entire workspace or a subset consisting of one or several regions. The approach makes it possible to specify constraints on the order in which the regions should be inspected by using colors to ensure that regions with the same color are inspected as a group. A key aspect is the transformation of the multiregion inspection into a clustered traveling salesman problem (CTSP). This is achieved by generating several inspection points on the medial axis of each region to increase the visibility and by grouping the inspection points into clusters. We also develop a fast branch-and-bound CTSP solver to find low-cost clustered tours. These tours guide sampling-based motion planning, as it expands a motion tree in search for a collision-free and dynamically feasible trajectory to carry out the multiregion inspection. The approach is evaluated in simulation using a snake-like robot model to carry out inspection tasks in complex environments. Comparisons to related work show significant speedups.

Medial Axis and Singularities

This paper is devoted to the study of the medial axes of sets definable in polynomially bounded o-minimal structures, i.e. the sets of points with more than one closest point with respect to the Euclidean distance. Our point of view is that of singularity theory. While trying to make the paper self-contained, we gather here also a large bunch of basic results. Our main interest, however, goes to the characterization of those singular points of a definable, closed set Xsubset {mathbb {R}}^n, which are reached by the medial axis.

Reduction in stray radiation dose using a body-shielding device during external radiation therapy.

With the purpose of reducing stray radiation dose (SRD) in out‐of‐field region (OFR) during radiotherapy with 6 MV intensity‐modulated radiation therapy (IMRT), a body‐shielding device (BSD) was prepared according to the measurements obtained in experimental testing. In experimental testing, optimal shielding conditions, such as 1 mm lead, 2 mm lead, and 1 mm lead plus 10 mm bolus, were investigated along the medial axis of a phantom using thermoluminescent dosimeters (TLDs). The SRDs at distances from field edge were then measured and analyzed for a clinical IMRT treatment plan for nasopharyngeal carcinoma before and after shielding using the BSD. In addition, SRDs in anterior, posterior, left and right directions of phantom were investigated with and without shielding, respectively. Also, the SRD at the bottom of treatment couch was measured. SRD decreased exponentially to a constant value with increasing distance from field edge. The shielding rate was 50%–80%; however, there were no significant differences in SRDs when shielded by 1 mm lead, 2 mm lead, or 1 mm lead plus 10 mm bolus (P>0.05). Importantly, the 10 mm bolus absorbed back‐scattering radiation due to the interaction between photons and lead. As a result, 1 mm lead plus 10 mm bolus was selected to prepare the BSD. After shielding with BSD, total SRDs in the OFR decreased to almost 50% of those without shielding when irradiated with IMRT beams. Due to the effects of treatment couch and gantry angle, SRDs at distances were not identical in anterior, posterior, left and right direction of phantom without BSD. As higher dose in anterior and lower dose in posterior, SRDs were substantial similarities after shielding. There was no significant difference in SRDs for left and right directions with or without shielding. Interestingly, SRDs in the four directions were similar after shielding. From these results, the BSD developed in this study may significantly reduce SRD in the OFR during radiotherapy, thus decreasing the risk of secondary cancers.

Navigation: Whence Our Sense of Direction?

Behavioral data have long implied our sense of direction derives from global environmental shape; electrophysiological evidence, however, has seemed to imply it derives from salient non-geometric landmarks. Experiments on the re-establishment of place fields in disoriented mice now align the electrophysiological data with the behavioral data.

Early secondary bone grafting in children with alveolar cleft does not modify the risk of maxillary permanent canine impaction at the age of 10 years

ObjectiveThe risk of maxillary permanent canine retention is elevated in children with cleft lip and palate (CLP). This study compared this risk in 10 ± 1 year-old patients with unilateral CLP (UCLP) who underwent or not early maxillary bone grafting at the age of 6 ± 1 years. MethodsRetrospective, controlled monocentric and observational study with the following inclusion criteria: patients with UCLP, isolated cleft, pre-surgery orthodontic maxillary expansion, same surgical protocol, same surgical team. The risk of maxillary canine retention was evaluated using a dental panoramic radiograph, on the basis of the canine root development stage and of the angle (> or <45°) between the main axis of the maxillary canine and a virtual reference axis parallel to the medial sagittal axis. ResultsThe impaction risk at the age of 10 was of 14.29% (4/28) for the group who underwent early bone grafting and 21.88% (7/32) for the group without early grafting (p = 0.45). Permanent lateral incisor agenesis and the patient's sex did not change this risk. ConclusionsEarly alveolar bone grafting does not significantly change the risk of permanent maxillary canine retention at 10. Long-term followup of a larger population is needed to confirm this result.

Aggressive Spiral Toolpaths for Pocket Machining Based on Medial Axis Transformation

High-speed machine tools typically provide high spindle speeds and feedrates to achieve an effective material removal rate (MRR). However, it is not possible to realize the full extent of their high-speed capabilities due to the sharp corners of toolpaths which are introduced by conventional machining strategies, such as contour- and direction-parallel toolpaths. To address this limitation, spiral toolpaths that can reduce the magnitude of sudden direction changes have been developed in previous researches. Nevertheless, for some pockets, the average radial cutting width is significantly decreased while the total length of the toolpath is significantly increased as compared to contour- and direction-parallel toolpath. In this situation, spiral toolpath may take more machining time. To overcome these drawbacks, an aggressive spiral toolpath generation method based on the medial axis (MA) transformation is proposed in machining pocket without islands inside, which refers to no additional material inside the counter. The salient feature of this work is that it integrates the advantages of both conventional contour-parallel machining strategy and the existing spiral toolpath machining strategy. The cutting width at each MA point is determined based on the diameter of the locally inscribed circle (LIC) of the MA point and the topological structure of MA. A distance-constrained contour determination algorithm is utilized to calculate the toolpath for each pass. Finally, a circular arc transition strategy is used to transform all the isolated passes into a spiral toolpath. Experiments are conducted to show the effectiveness of the proposed method.

Use of medial axis to find optimal channel designs to reduce mold filling time in resin transfer molding

In Resin Transfer Molding (RTM) process, pre-designed channels branching out like runners from the injection port can be used for faster resin distribution and impregnation as compared to traditional single gate injection to reduce the mold filling time. To search for the optimal channel design for maximum fill time reduction, the Medial Axis (MA) of the part surface is defined. Next, a methodology is developed to search for the topology of the MA using Finite Element based mold filling simulation software. The MA location is corrected for a part with non-homogeneous fabric permeability. Finally, some case studies are presented to illustrate the effectiveness and accuracy of the channel design approach for RTM with the objective of minimizing fill time, when fabricating composite parts that contain complexities in both the geometric features such as compound curvatures and corners and in material properties such as non-homogeneous and highly anisotropic fiber preform permeability.

Derivation of the Mathematical Approach to the Radial Pump’s Meridional Channel Design Based on the Controlment of the Medial Axis

The meridional channel is the base for designing the radial pumps, and a new design approach is proposed here. Different from the previous studies, research here tries to establish the design model simply controlled with the radial coordinate. With the combination of a series of mathematical equations, the new design approach can shape the meridional contours directly by using the initial design variables. As for the mathematical constraint in the new design approach, it was presented in two forms, and each form had its corresponding solution. For the first form (Constraint I), the midpoints of the design points on the hub and shroud contours were thought to be located on the medial axis, and the PSO algorithm was adopted to search for the suitable results. Continually, to accelerate the design process, the second form (Constraint II) to simplify the mathematical constraint was added, and the explicit mathematical expressions calculating the coordinates on the hub and shroud contours were deduced. Finally, to check out the feasibility of the design approach in engineering, it was applied to redesign some typical meridional channels proposed by previous studies, and, through comparative analysis, the effectiveness of the new approach was evaluated and demonstrated.

Geometric Study of Surface Finishing of Selective Laser Melting Moulds

Selective laser melting, which is based on the principle of material incremental manufacturing, has been recognised as a promising additive manufacturing technology. The principle of additive manufacturing lies in fabricating a part or an assembly of parts, layer by layer through a bottom to top approach. The technology is suited for creating geometrically complex components that can not possibly or feasibly be made by any other means. This technique has a weak point related to the surface finishing. Therefore, during the construction of layer by layer, there is a need to use techniques such as milling to remove material. This hybrid approach allows the fabrication of parts with internal complex structures and very good surface finishing. To plan and optimize the successive additive and subtractive phases, we need a quick tool to determine when the geometry of a piece is suitable for surface finishing by a 3 axes milling machine. This problem can be reduced to a layer by layer subproblem of approximately covering a slice of the object by circles of the diameter of the smallest drill available that can reach its depth. This reduction to the plane allows us to use a medial axis approach. The medial axis of a planar domain, defined as the set of centers of maximal circles contained in the domain, relates very closely to the notion of generalized Voronoi diagram, and has been proposed in several milling applications that involve motion planning. We propose to use it, and certain extensions of it, as a practical way of determining the best possible finishing quality at a slice. To that end we have to find which of the available construction strategies best suits our needs to determine exactly or approximately the medial axis of a polygon and its extensions.

Subsurface damage in milling of lightweight open-cell aluminium foams

The present study examined the surface integrity of open cell aluminium foams at full volume using X-ray micro-computed tomography. The structural network was reconstructed in voxel models, for which watershed segmentation and medial axis extraction was utilized to identify interconnected pore (air) and strut (solid) phases. From these models, post-mortem surface integrity characteristics, including pore size, strut bending angle and preferential strut orientation were characterized as a function of controllable parameters of cutting speed, feed rate and depth of cut. Post-mortem characterization of the damage enabled mapping of fundamental failure mechanisms in the mechanics of the subsurface deformation.

Mechanical behaviour of weak snow layers: modelling a porous structure of sintered grains

Weak layers of snow are thin layers of low density and cohesion that consist of a complex network of sintered ice grains. These layers are very fragile and their collapse is considered to be the primary cause of dry slab snow avalanche release. This study reports on an attempt at modelling the mechanical response of these weak snow layers with discrete element (DEM) simulations, using x-ray tomographical images of real snow samples as input data on the microstructure of the material. An original method for representing irregular grain shapes in the DEM is first introduced. The method is based on utilizing the medial axis concept to represent an arbitrary grain shape with an optimal set of overlapping spheres. A thorough study of the effect of the grain approximating technique parameters on the key geometrical features of the grains is then carried out. Finally, the functionality of the model is demonstrated by performing an oedometric test on an image of a real snow sample.

Image Segmentation and Modeling of the Pediatric Tricuspid Valve in Hypoplastic Left Heart Syndrome.

Hypoplastic left heart syndrome (HLHS) is a single-ventricle congenital heart disease that is fatal if left unpalliated. In HLHS patients, the tricuspid valve is the only functioning atrioventricular valve, and its competence is therefore critical. This work demonstrates the first automated strategy for segmentation, modeling, and morphometry of the tricuspid valve in transthoracic 3D echocardiographic (3DE) images of pediatric patients with HLHS. After initial landmark placement, the automated segmentation step uses multi-atlas label fusion and the modeling approach uses deformable modeling with medial axis representation to produce patient-specific models of the tricuspid valve that can be comprehensively and quantitatively assessed. In a group of 16 pediatric patients, valve segmentation and modeling attains an accuracy (mean boundary displacement) of 0.8 ± 0.2 mm relative to manual tracing and shows consistency in annular and leaflet measurements. In the future, such image-based tools have the potential to improve understanding and evaluation of tricuspid valve morphology in HLHS and guide strategies for patient care.

Optimal mesh topology generation for CFD

Multiblock structured mesh generation is amongst the most popular meshing techniques for flow simulations. However, the time consuming and user intensive process of blocking complex domains presents a major bottleneck in structured mesh generation. To this end, we have considered various automatic blocking strategies where some typical aeroengine domains have been showcased. An adjoint based error analysis procedure is then used to assess these methods. It is found that, in general, the medial axis based approaches provide optimal blocking and yields better accuracy in computing the functional of interest. This is because the medial axis based methods produce meshes which have better flow alignment especially in case of internal flows. A new hybrid blocking approach, combining the existing methods with the distance field isosurface is also presented to overcome the shortcomings of the current methods. Such a blocking technique has the potential to be applied to a wide range of flow domains.

Generation Method and Application of Product-Oriented Medial Axis

Abstract In this paper, the generation method of the medial axis in the arbitrary quadrilateral surface is proposed. It can provide a solution for the simplification of the complex fillet feature and the generation of the mesh in the model. By using the locus method associated with moving Frenet frame, we realize the simple and fast algorithm for generating the medial axis. As for the engineering problem, B-rep 3D solid models with clear boundary definition are mostly applied; the information of vertex, side and surface of the model, which is clearly stored in the model file, can be used to simplify the traditional locus method for generating the medial axis, in order to reduce the amount of data required by the generation. In this paper, we use the clear boundary information in the B-rep model as the condition for generating the medial axis and the characteristics of the bisector to eliminate the calculation of the branch points, reducing the factors affecting the accuracy of the medial axis. In order to ensure the accuracy of the medial axis, the density of the insertion points can be used for control.

Microarchicotylus ochroleucus gen. et sp. n. (Plathelminthes, Tricladida, Paludicola): A new genus and species of dwarf planarians in Lake Baikal

This paper gives an illustrated description of a new genus and species of endemic Baikal planarians, Microarchicotylus ochroleucus gen. et sp. n. The morphological similarity in the structure of the pharynx, male copulative organs, and external features allowed the authors to include two more species, M. stringulatus (Korotneff 1912) and M. elegans (Porfiriev et Timoshkin 2009), formerly belonging to a poorly investigated genus Archicotylus, into the new genus. Microarchicotylus gen. et sp. n. planarians are attributed to dwarf forms, since their maximal body length does not exceed 5–6 mm. One more distinctive external feature of these Microarchicotylus species is a bright and uneven coloration of the dorsal part of the body, sometimes being dark brown, light brown, or ochreous with distinct white transverse strokes or without them. The anterior part of the body is bright orange or blackish, its color always being different from the remaining body, separated by a white transverse collar behind the pair of eyes. The pharynx is either short cylindrical barrelshaped (length-to-width ratio of ~1) or elongated cylindrical (ratio, 3: 1). The copulative organ is compact and resides behind the pharynx, with its apical part directed downwards. The seminal receptacle, its canal, and the male and common atriums are asymmetrically located relative to the medial longitudinal body axis and centrally located gonopore.