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Simulations of Thermally Induced Photoacoustic Wave Propagation Using a Pseudospectral Time-Domain Method
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Numerical analysis and its applications in photoacoustic imaging
No full text本論文主體有二：其一研討光聲波在液體中之生成與傳播方程的數值解；其二乃探究傳統有限可視角影像重建方法在血管內光聲影像中之應用。
醫用光聲相關技術所採之簡易物理模型無法應用於任意光能照射時間與非線性且耗散之傳播。本論文第一部分由熱傳導、狀態、連續、推導與Navier-Stokes方程歸納出一般液態物經受熱轉形聲波的機制。此系統方程對任意初始光能分佈之解析解不易求得，故以數值偏微分之技巧，諸如時域有限差分和時域擬譜方式來計算光聲波生成與傳播。模擬架構為一柱對稱座標系，斷面之邊界則以理想匹配層(PMLs)吸收散射之光聲波。細微結構須隨不同差分技巧而調整。兩方法所得之數值結果符合系統方程之線性解析解。結果顯示擬譜方法之數值解的誤差優於有限差分方法之解。其中擬譜方法中只用了四個取樣格點，而有限差分則須至少15取樣格點方可免於數值頻散現象。
傳統光聲影像取得架構有三，為環型、柱型與平面掃描方式。今血管內光聲造影為一新興技術，其中所得掃得之光聲訊號唯以座標轉換重建，致使所建之影像與真實大有出入。論文第二部分將有限可視角影像重建方法中之濾波反投影(FBP)法、局部重建法與期望最大化(EM)演算法應用於血管內光聲影像架構。模擬與研究結果顯示濾波反投影與局部重建之法雖非此影像反問題之解析解，但可揭示部分物體形態；而期望最大化演算法藉由迭代計算物體影像所投射之光聲訊號與真實訊號之間的最大相似度來改善重建影像品質。Several photoacoustic (PA) techniques, such as photoacoustic imaging, spectroscopy, and parameter sensing, measure quantities that are closely related to optical absorption, position detection, and laser irradiation parameters. The photoacoustic waves in biomedical applications are usually generated by elastic thermal expansion, which has advantages of nondestructiveness and relatively high conversion efficiency from optical to acoustic energy. Most investigations describe this process using a heuristic approximation, which is invalid when the underlying assumptions are not met. This thesis developed a numerical solution of the general photoacoustic generation equations involving the heat conduction theory and the state, continuity, and Navier-Stokes equations in 2.5D axisymmetric cylindrical coordinates using a finite-difference time-domain (FDTD) and a pseudospectral time-domain (PSTD) scheme.
Simulation using the FDTD method included staggered grids and Berenger’s perfectly matched layers (PMLs). The spatial derivatives in the system of equations were approximated by the second-order-accurate central difference, and the time evolution was advanced by the method of lines. The numerical results were validated using the linear-perturbation analytical solutions and a heuristic model for generation and propagation of photoacoustic waves. In addition, as a free-space boundary, the 10-layer PML produced the lowest reflection error for commonly used boundary conditions.
Computation efficiency and accuracy can be further improved by the PSTD method. Derivatives in spatial domain were calculated using the differentiation theorem for Fourier transforms. Infinite order of accuracy is achieved with at least two sampling grids per minimum wavelength, as compared with the FDTD method, in which 15 or more sampling grids are necessary to reduce numerical dispersion. We developed an extended computation domain to avoid Gibbs phenomenon and wraparound effect associated with the periodicity inherent in the FFT operation by applying the concepts of PMLs and symmetry properties of boundary conditions in our simulation model. The accuracy of using four sampling grids was shown superior to those using 15 grids in FDTD method in the photoacoustic wave simulation. Furthermore, a nonlinear propagation of photoacoustic wave was demonstrated.
In the second part of the thesis, we explored feasibility and efficacy of applying limited-view reconstruction methods to intravascular photoacoustic (IVPA) imaging as a new type of scanning geometry in photoacoustic tomography. A very limited view angle can attribute to an enclosed scanning trajectory within the phantom. According to the data sufficiency condition, only a small portion of object interfaces can be reconstructed stably in such imaging configuration. To the present, intravascular images are obtained using only scan conversion, resulting in image distortion if a point-like detector is used. Analytical formulas such as the filtered backprojection (FBP) and the lambda-tomography method were employed to suggest object geometries. The reconstruction quality was further improved by the expectation maximization (EM) method which could minimize the error between the measured signals and those generated by a reconstructed image. Computer simulations and experiments were carried out to validate the methods
Photoacoustic image reconstruction for linear scanning geometry using particle swarm optimization with a K-space simulation scheme
No full textSimulations of photoacoustic wave propagation using a finite-difference time-domain method with Berenger’s perfectly matched layers
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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