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Yilmaz O.,Ankara University | Doerschner K.,National Magnetic Resonance Research Center | Doerschner K.,Bilkent University
Machine Vision and Applications | Year: 2014

Successful identification of specularities in an image can be crucial for an artificial vision system when extracting the semantic content of an image or while interacting with the environment. We developed an algorithm that relies on scale and rotation invariant feature extraction techniques and uses motion cues to detect and localize specular surfaces. Appearance change in feature vectors is used to quantify the appearance distortion on specular surfaces, which has previously been shown to be a powerful indicator for specularity (Doerschner et al. in Curr Biol, 2011). The algorithm combines epipolar deviations (Swaminathan et al. in Lect Notes Comput Sci 2350:508-523, 2002) and appearance distortion, and succeeds in localizing specular objects in computer-rendered and real scenes, across a wide range of camera motions and speeds, object sizes and shapes, and performs well under image noise and blur conditions. © 2014 Springer-Verlag Berlin Heidelberg. Source


Doerschner K.,Bilkent University | Doerschner K.,National Magnetic Resonance Research Center | Yilmaz O.,Aselsan | Yilmaz O.,Ankara University | And 2 more authors.
Journal of Vision | Year: 2013

Surface specularity distorts the optic flow generated by a moving object in a way that provides important cues for identifying surface material properties (Doerschner, Fleming et al., 2011). Here we show that specular flow can also affect the perceived rotation axis of objects. In three experiments, we investigate how threedimensional shape and surface material interact to affect the perceived rotation axis of unfamiliar irregularly shaped and isotropic objects. We analyze observers' patterns of errors in a rotation axis estimation task under four surface material conditions: shiny, matte textured, matte untextured, and silhouette. In addition to the expected large perceptual errors in the silhouette condition, we find that the patterns of errors for the other three material conditions differ from each other and across shape category, yielding the largest differences in error magnitude between shiny and matte, textured isotropic objects. Rotation axis estimation is a crucial implicit computational step to perceive structure from motion; therefore, we test whether a structure from a motion-based model can predict the perceived rotation axis for shiny and matte, textured objects. Our model's predictions closely follow observers' data, even yielding the same reflectance-specific perceptual errors. Unlike previous work (Caudek & Domini, 1998), our model does not rely on the assumption of affine image transformations; however, a limitation of our approach is its reliance on projected correspondence, thus having difficulty in accounting for the perceived rotation axis of smooth shaded objects and silhouettes. In general, our findings are in line with earlier research that demonstrated that shape from motion can be extracted based on several different types of optical deformation (Koenderink & Van Doorn, 1976; Norman & Todd, 1994; Norman, Todd, & Orban, 2004; Pollick, Nishida, Koike, & Kawato, 1994; Todd, 1985). © 2013 Arvo. Source


Acikel V.,Bilkent University | Acikel V.,National Magnetic Resonance Research Center | Uslubas A.,MR Computer GmbH | Atalar E.,Bilkent University | Atalar E.,National Magnetic Resonance Research Center
Medical Physics | Year: 2015

Purpose: The authors purpose is to model the case of an implantable pulse generator (IPG) and the electrode of an active implantable medical device using lumped circuit elements in order to analyze their effect on radio frequency induced tissue heating problem during a magnetic resonance imaging (MRI) examination. Methods: In this study, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the modified transmission line method (MoTLiM) and the method of moments (MoM) simulations. Once the parameter values of an electrode/IPG case model are determined, they can be connected to any lead, and tip heating can be analyzed. To validate these models, both MoM simulations and MR experiments were used. The induced currents on the leads with the IPG case or electrode connections were solved using the proposed models and the MoTLiM. These results were compared with the MoM simulations. In addition, an electrode was connected to a lead via an inductor. The dissipated power on the electrode was calculated using the MoTLiM by changing the inductance and the results were compared with the specific absorption rate results that were obtained using MoM. Then, MRI experiments were conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead was connected to the case and placed inside a uniform phantom. During a MRI scan, the temperature rise at the lead was measured by changing the lead length. The power at the lead tip for the same scenario was also calculated using the IPG case model and MoTLiM. Then, an electrode was connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan, the temperature rise at the electrode was measured by changing the inductance and compared with the dissipated power on the electrode resistance. Results: The induced currents on leads with the IPG case or electrode connection were solved for using the combination of the MoTLiM and the proposed lumped circuit models. These results were compared with those from the MoM simulations. The mean square error was less than 9%. During the MRI experiments, when the IPG case was introduced, the resonance lengths were calculated to have an error less than 13%. Also the change in tip temperature rise at resonance lengths was predicted with less than 4% error. For the electrode experiments, the value of the matching impedance was predicted with an error less than 1%. Conclusions: Electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The concept of matching of the electrode to the lead is clarified using the defined electrode impedance and the lead Thevenin impedance. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. With these models, understanding the tissue heating due to the implants becomes easier. Also, these models are beneficial for implant safety testers and designers. Using these models, worst case conditions can be determined and the corresponding implant test experiments can be planned. © 2015 American Association of Physicists in Medicine. Source


Acikel V.,Bilkent University | Acikel V.,National Magnetic Resonance Research Center | Atalar E.,Bilkent University | Atalar E.,National Magnetic Resonance Research Center
Medical Physics | Year: 2011

Purpose: Metallic implants may cause serious tissue heating during magnetic resonance (MR) imaging. This heating occurs due to the induced currents caused by the radio-frequency (RF) field. Much work has been done to date to understand the relationship between the RF field and the induced currents. Most of these studies, however, were based purely on experimental or numerical methods. This study has three main purposes: (1) to define the RF heating properties of an implant lead using two parameters; (2) to develop an analytical formulation that directly explains the relationship between RF fields and induced currents; and (3) to form a basis for analysis of complex cases. Methods: In this study, a lumped element model of the transmission line was modified to model leads of implants inside the body. Using this model, leads are defined using two parameters: impedance per unit length, Z, and effective wavenumber along the lead, k t. These two parameters were obtained by using methods that are similar to the transmission line theory. As long as these parameters are known for a lead, currents induced in the lead can be obtained no matter how complex the lead geometry is. The currents induced in bare wire, lossy wire, and insulated wire were calculated using this new method which is called the modified transmission line method or MoTLiM. First, the calculated induced currents under uniform electric field distribution were solved and compared with method-of-moments (MoM) calculations. In addition, MoTLiM results were compared with those of phantom experiments. For experimental verification, the flip angle distortion due to the induced currents was used. The flip angle distribution around a wire was both measured by using flip angle imaging methods and calculated using current distribution obtained from the MoTLiM. Finally, these results were compared and an error analysis was carried out. Results: Bare perfect electric, bare lossy, and insulated perfect electric conductor wires under uniform and linearly varying electric field exposure were solved, both for 1.5 T and 3 T scanners, using both the MoTLiM and MoM. The results are in agreement within 10 mean-square error. The flip angle distribution that was obtained from experiments was compared along the azimuthal paths with different distances from the wire. The highest mean-square error was 20 among compared cases. Conclusions: A novel method was developed to define the RF heating properties of implant leads with two parameters and analyze the induced currents on implant leads that are exposed to electromagnetic fields in a lossy medium during a magnetic resonance imaging (MRI) scan. Some simple cases are examined to explain the MoTLiM and a basis is formed for the analysis of complex cases. The method presented shows the direct relationship between the incident RF field and the induced currents. In addition, the MoTLiM reveals the RF heating properties of the implant leads in terms of the physical features of the lead and electrical properties of the medium. © 2011 American Association of Physicists in Medicine. Source


Doerschner K.,Bilkent University | Doerschner K.,National Magnetic Resonance Research Center | Fleming R.W.,Justus Liebig University | Yilmaz O.,National Magnetic Resonance Research Center | And 4 more authors.
Current Biology | Year: 2011

Many critical perceptual judgments, from telling whether fruit is ripe to determining whether the ground is slippery, involve estimating the material properties of surfaces. Very little is known about how the brain recognizes materials, even though the problem is likely as important for survival as navigating or recognizing objects. Though previous research has focused nearly exclusively on the properties of static images [1-16], recent evidence suggests that motion may affect the appearance of surface material [17-19]. However, what kind of information motion conveys and how this information may be used by the brain is still unknown. Here, we identify three motion cues that the brain could rely on to distinguish between matte and shiny surfaces. We show that these motion measurements can override static cues, leading to dramatic changes in perceived material depending on the image motion characteristics. A classifier algorithm based on these cues correctly predicts both successes and some striking failures of human material perception. Together these results reveal a previously unknown use for optic flow in the perception of surface material properties. © 2011 Elsevier Ltd. All rights reserved. Source

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