Researcher:
Ceyhan, Elvan

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Elvan

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Ceyhan

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Ceyhan, Elvan

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2006 - 2009122010 - 201735

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    Performance evaluation of an exact method for the obstacle neutralization problem
    (IEOM Society, 2016) Alkaya, Ali Fuat; Algin, Ramazan; Oz, Dindar; Aksakalli, Vural; Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    The Obstacle Neutralization Problem (ONP) is an NP-Hard path planning problem wherein an agent needs to swiftly navigate from a given start location to a target location through an arrangement of disc-shaped obstacles on the plane. The agent has a limited neutralization capability in the sense that it can neutralize an obstacle after which it can safely traverse through. A neutralization can only be performed at a cost, which is added to the overall traversal length. The goal is to find the optimal neutralization sequence that minimizes the agent's total traversal length. In this study, we compare the performance of a recently proposed exact algorithm for ONP against a conventional solution obtained via an integer programming formulation. This exact algorithm consists of two phases. In Phase I, an effective and fast algorithm is used to obtain a suboptimal solution. In the Phase II, a k-th shortest path algorithm is used to close any gaps. The integer programming formulation is solved via the popular SCIP solver. We present computational experiments conducted on synthetic problem instances on a discrete plane with varying resolutions. Our results indicate that the exact algorithm provides an almost 10-fold improvement in execution time when compared against the integer programming approach.
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    Exploratory and inferential methods for spatio-temporal analysis of residential fire clustering in urban areas
    (Elsevier, 2013) Ertugay, Kıvanç; Düzgün, Şebnem; Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    The spatio-temporal analysis of residential fires could allow decision makers to plan effective resource allocations in fire management according to fire clustering levels in space and time. In this study, we provide guidelines for the use of various methods in detecting the differences in clustering patterns of fire and non-fire (i.e., background residential) locations and how these patterns change over time. As a preliminary analysis step, various exploratory data analysis methods, such as, intensity plots (i.e., kernel density estimates) are used. Moreover, the use of Diggle's-function (a second order analysis technique) is proposed for detecting the clustering of residential fire locations (if any) and whether there is additional clustering (or regularity) in the locations of the fires compared to background residential pattern. A test for trend over time (in years, months, and weeks) of the fire location patterns are provided with a space-time interaction analysis by the spatio-temporal-function. Residential fire data from Cankaya Municipality of Ankara, Turkey is used as an illustrative example. The presented methodology is also applicable to residential fire data from similar urban settings. (C) 2013 Elsevier Ltd. All rights reserved.
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    Cerebral microhemorrhages detected by susceptibility-weighted imaging in amateur boxers
    (Amer Soc Neuroradiology, 2011) Hasiloğlu, Z. I.; Albayram, S.; Selçuk, H.; Delil, S.; Arkan, B.; Başköy, L.; Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Background and Purpose: SWI is a new technique for evaluating diffuse axonal injury associated with punctate hemorrhages. The aim of our study was to determine the prevalence of cerebral microhemorrhages in amateur boxers compared with nonboxers by using SWI and to evaluate the sensitivity of SWI compared with T2 FSE and T2*GE sequences. MATERIALS and METHODS: We performed cranial MR imaging with a 1.5T scanner in 21 amateur boxers and 21 control subjects. The study protocol included conventional MR images, T2 FSE, T2*GE, and SWI sequences. The proportions of boxers and controls having CSP, DPVS, cerebral atrophy, cerebellar atrophy, ventricular dilation. PSWMD, and microhemorrhages were computed and were compared by using the chi(2) test of proportions. The relationship between microhemorrhages and boxing-related covariates was assessed by using the Wilcoxon rank sum test. The association between the categories was tested by using the Fisher exact test. RESULTS: Using SWI, microhemorrhages were found in 2 (9.52%) of 21 boxers. The microhemorrhages were not visible on T2 FSE or T2*GE images. The proportion of subjects with microhemorrhages did not differ significantly between the boxers and control subjects (chi(2) = 0.525, df = 1, P = .4688). The prevalence of CSP and DPVS was significantly higher in the boxers than in the control subjects. CONCLUSIONS: More microhemorrhages were detected in amateur boxers than in controls, but this difference was not statistically significant.
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    A radiological study on the topographical relationships between the vestibular, cochlear and facial nerves
    (Atatürk Üniversitesi, 2012) Ünel, Sacide; Yılmaz, Mehmet; Albayram, Sait; Kiriş, Adem; Işık, Zehra; Işıldak, Hüseyin; Savaş, Yıldıray; Keser, Zafer; Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Objective: The purpose of our study was to investigate the topographical relationship between these nerves along their course from the brainstem through the internal acoustic canal IAC in the living human brain using MR imaging. Materials and Methods: We performed three-dimensional gradient echo balanced Fast Field Echo (3D bFFE) sequence oblique parasagittal MR imaging in 73 healthy subjects. The IACs were analyzed from the brainstem end of the IAC to the fundus in contiguous sections. At five levels, the topographical relationships between the facial and vestibulocochlear nerves (VCN) were recorded. In the lateral portions of the IACs where they separated from each other, the relative sizes of the individual nerves were examined. Results: In general, the facial nerve (FN), which is a round structure, is located anteriorly and superiorly to the vestibulocochlear nerve throughout its course. The vestibulocochlear nerve is usually rectangular; however, it was found to be round and at times triangular in shape near the brainstem, before it became crescent-shaped at the porus in 89% of the cases. The superior vestibular nerve kept its posterosuperior position in the canal, and the inferior vestibular nerve (IVN) and the cochlear nerve (CN) travelled inferior to it. The superior and inferior vestibular nerves were divided by the falciform crest in 53% of the cases. The inferior vestibular nerve was the smallest nerve in 52% of the cases, and the cochlear nerve was the largest in 36% of the cases. Conclusion: To the best of our knowledge, this study is the largest in vivo MR study, and most of our findings differ from previous cadaver studies. Determination of these topographical relationships may facilitate our understanding of the complicated physiological relationships between the 7th and 8th nerve complexes during surgery in this region. / Öz: Amaç: Bizim amacımız, yaşayan insanlarda MR görüntülemeyi kullanarak, bu sinirlerin beyin sapından, internal akustik kanala kadar olan kısmında topografik ilişkilerini araştırmaktır. Gereç ve Yöntem: Yetmiş üç sağlıklı bireyde 3D gradient echo balanced fast field echo (3D bFFE) sekans oblik parasagittal MR inceleme yaptık. İAK, beyin sapından İAK’ın bitimindeki fundusa kadar, ardışık kesitlerle analiz edildi. Beş seviyede fasiyal ve vestibülokoklear sinirin topografik ilişkileri kaydedildi. İAK’ın lateral bölümünde (sinirin kendi içinde bölümlere ayrıldığı kısımda) sinirlerin rölatif boyutları değerlendirildi. Bulgular: Genel olarak fasiyal sinir, yuvarlak bir yapıda, anterior yerleşimli ve tüm seyri boyunca vestibulokoklear sinirin süperiorundadır. Vestibulokoklear sinir ıklıkla dikdörtgen şeklindedir. Fakat bazen beyin sapına yakın kısımda yuvarlak ve nadiren üçgen şeklinde ve vakaların %89’unda porusda kresentrik yapıdadır. Superior vestibuler sinir kanal içinde posterosüperior pozisyonunu korur ve inferior vestibüler sinir ve koklear sinir superior vestibüler sinirin altından geçer. Süperior ve inferior vestibüler sinirler vakaların %53’ünde falsiform krest tarafından bölünür. İnferior vestibüler sinir vakaların %52’sinde en küçük, koklear sinir vakaların %36’sında en geniş olarak izlenmiştir. Sonuç: Bildiğimiz kadarıyla, bu en büyük in vivo MR çalışması olup, bulgularımızın çoğu önceki kadavra çalışmalarından farklıdır.
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    An investigation of new graph invariants related to the domination number of random proximity catch digraphs
    (Springer, 2012) Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Proximity catch digraphs (PCDs) are a special type of proximity graphs based on proximity maps which yield proximity regions. PCDs are defined using the relative allocation of points from two or more classes in a region of interest and have applications in various fields. We introduce some auxiliary tools for PCDs and graph invariants related to the domination number of the PCDs and investigate their probabilistic properties. We consider the cases in which the vertices of the PCDs come from uniform and non-uniform distributions in the region of interest. We also provide some of the newly defined proximity maps as illustrative examples.
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    Reduced anterior cingulate gray matter volume and thickness in subjects with deficit schizophrenia
    (Elsevier, 2013) Takayanagi, Mizuho; Wentz, Jacqueline; Takayanagi, Yoichiro; Schretlen, David J.; Wang, Lei; Suzuki, Michio; Sawa, Akira; Barta, Patrick E.; Ratnanather, J. Tilak; Cascella, Nicola G.; Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Background: Patients with deficit schizophrenia (D-SZ) differ from patients with the non-deficit form of schizophrenia (ND-SZ) in several aspects such as risk factors, neurobiological correlates, treatment response and clinical outcome. It has been debated if brain morphology could differentiate D-SZ from ND-SZ. Anterior cingulate gyrus (ACG) region regulates cognitive and emotional processing and past studies reported structural changes in this region in patients with SZ. Methods: 1.5-T 3D MRI scans were obtained from 18 D-SZ patients, 30 ND-SZ patients and 82 healthy controls (HCs). We used FreeSurfer-initalized labeled cortical distance mapping (FSLCDM) to measure ACG gray matter volume, cortical thickness, and area of the gray/white interface. Furthermore, cortical thickness was compared among the 3 groups using the pooled labeled cortical distance mapping (LCDM) method. Results: The ACG cortex of the D-SZ group was thinner than the ND-SZ group. Pooled LCDM demonstrated that the ACG cortex was bilaterally thinner in both the ND-SZ group and the D-SZ group compared with the control group. The right ACG gray matter volume was significantly reduced in D-SZ patients as compared with healthy controls (p = 0.005 Conclusion: Our data suggest that qualitative, categorical differences in neuroanatomy may distinguish between deficit and non-deficit subtypes of schizophrenia. (C) 2013 Elsevier B. V. All rights reserved.
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    Segregation indices for disease clustering
    (Wiley-Blackwell, 2014) Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Spatial clustering has important implications in various fields. In particular, disease clustering is of major public concern in epidemiology. In this article, we propose the use of two distance-based segregation indices to test the significance of disease clustering among subjects whose locations are from a homogeneous or an inhomogeneous population. We derive the asymptotic distributions of the segregation indices and compare them with other distance-based disease clustering tests in terms of empirical size and power by extensive Monte Carlo simulations. The null pattern we consider is the random labeling (RL) of cases and controls to the given locations. Along this line, we investigate the sensitivity of the size of these tests to the underlying background pattern (e.g., clustered or homogenous) on which the RL is applied, the level of clustering and number of clusters, or to differences in relative abundances of the classes. We demonstrate that differences in relative abundances have the highest influence on the empirical sizes of the tests. We also propose various non-RL patterns as alternatives to the RL pattern and assess the empirical power performances of the tests under these alternatives. We observe that the empirical size of one of the indices is more robust to the differences in relative abundances, and this index performs comparable with the best performers in literature in terms of power. We illustrate the methods on two real-life examples from epidemiology. Copyright (c) 2013 John Wiley & Sons, Ltd.
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    Extension of one-dimensional proximity regions to higher dimensions
    (Elsevier, 2010) Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Proximity regions (and maps) are defined based on the relative allocation of points from two or more classes in an area of interest and are used to construct random graphs called proximity catch digraphs (PCDs) which have applications in various fields. The simplest of such maps is the spherical proximity map which gave rise to class cover catch digraph (CCCD) and was applied to pattern classification. In this article, we note some appealing properties of the spherical proximity map in compact intervals on the real line, thereby introduce the mechanism and guidelines for defining new proximity maps in higher dimensions. For non-spherical PCDs, Delaunay tessellation (triangulation in the real plane) is used to partition the region of interest in higher dimensions. We also introduce the auxiliary tools used for the construction of the new proximity maps, as well as some related concepts that will be used in the investigation and comparison of these maps and the resulting PCDs. We provide the distribution of graph invariants, namely, domination number and relative density, of the PCDs and characterize the geometry invariance of the distribution of these graph invariants for uniform data and provide some newly defined proximity maps in higher dimensions as illustrative examples. (C) 2010 Elsevier B.V. All rights reserved.
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    On the use of nearest neighbor contingency tables for testing spatial segregation
    (Springer, 2010) Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    For two or more classes (or types) of points, nearest neighbor contingency tables (NNCTs) are constructed using nearest neighbor (NN) frequencies and are used in testing spatial segregation of the classes. Pielou's test of independence, Dixon's cell-specific, class-specific, and overall tests are the tests based on NNCTs (i.e., they are NNCT-tests). These tests are designed and intended for use under the null pattern of random labeling (RL) of completely mapped data. However, it has been shown that Pielou's test is not appropriate for testing segregation against the RL pattern while Dixon's tests are. In this article, we compare Pielou's and Dixon's NNCT-tests; introduce the one-sided versions of Pielou's test; extend the use of NNCT-tests for testing complete spatial randomness (CSR) of points from two or more classes (which is called CSR independence, henceforth). We assess the finite sample performance of the tests by an extensive Monte Carlo simulation study and demonstrate that Dixon's tests are also appropriate for testing CSR independence; but Pielou's test and the corresponding one-sided versions are liberal for testing CSR independence or RL. Furthermore, we show that Pielou's tests are only appropriate when the NNCT is based on a random sample of (base, NN) pairs. We also prove the consistency of the tests under their appropriate null hypotheses. Moreover, we investigate the edge (or boundary) effects on the NNCT-tests and compare the buffer zone and toroidal edge correction methods for these tests. We illustrate the tests on a real life and an artificial data set.
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    Overall and pairwise segregation tests based on nearest neighbor contingency tables
    (Elsevier, 2009) Department of Mathematics; Ceyhan, Elvan; Faculty Member; Department of Mathematics; College of Sciences; N/A
    Multivariate interaction between two or more classes (or species) has important consequences in many fields and may cause multivariate clustering patterns such as spatial segregation or association. The spatial segregation occurs when members of a class tend to be found near members of the same class (i.e., near conspecifics) while spatial association occurs when members of a class tend to be found near members of the other class or classes. These patterns can be studied using a nearest neighbor contingency table (NNCT). The null hypothesis is randomness in the nearest neighbor (NN) structure, which may result from - among other patterns - random labeling (RL) or complete spatial randomness (CSR) of points from two or more classes (which is called the CSR independence, henceforth). New versions of overall and cell-specific tests based on NNCTs (i.e., NNCT-tests) are introduced and compared with Dixon's overall and cell-specific tests and various other spatial clustering methods. Overall segregation tests are used to detect any deviation from the null case, while the cell-specific tests are post hoc pairwise spatial interaction tests that are applied when the overall test yields a significant result. The distributional properties of these tests are analyzed and finite sample performance of the tests are assessed by an extensive Monte Carlo simulation study. Furthermore, it is shown that the new NNCT-tests have better performance in terms of Type I error and power estimates. The methods are also applied on two real life data sets for illustrative purposes. (c) 2008 Elsevier B.V. All rights reserved.