以逆向式光聲影像進行光吸收係數重建及血流量測

Optical absorption reconstruction and blood flow measurements using backward mode photoacoustic imaging

廖超康(Chao-Kang Liao)

指導教授 ： 李百祺

國立臺灣大學/電機資訊學院/電機工程學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.03266

摘要

光聲影像的構成結合了雷射能量的發射以及超音波的接收，因此光聲可以在優於其他純光學系統的探測深度條件下提供光學參數(即光吸收係數)。目前研究中的光聲成像依照掃描架構的不同分成三個形式，包含斷層模式、逆向模式以及正向模式。其中，逆向式光聲成像法是將超音波探頭與雷射照射配置於同側來進行光聲訊號的偵測。在這樣的架構下，逆向式光聲成像相較於其他形式的成像方式具有更簡單的機構配置以及更彈性的掃描範圍。不過，在現有的逆向式光聲成像方法中，只能侷限於觀測光吸收係數的梯度分佈的重建。光吸收物體內的吸收係數分佈卻無法真實的呈現。在這篇論文中，我們提出三個連續的重建演算法來取得光吸收係數的分佈，其中包含聚焦的可適性權重法、光能量吸收分佈重建法以及遞迴式吸收係數重建法。首先，由於光聲無方向性傳遞的特性以及探頭較大的偵測區間會造成影像的橫向解析度較差，而聚焦的可適性權重法則是用來提升橫向的影像解析度。較好聚焦品質的影像可以讓我們進一步藉由光能量吸收分佈重建法得到物體所吸收的光能量分佈。此重建法是以光聲的線性波動方程式中推導所得。最後，我們使用遞迴式吸收係數重建法，使用上個步驟所得到的光吸收能量分佈進而計算出光吸收係數的分佈。研究中，我們以模擬以及實驗的方式驗證這些演算法在逆向式光聲影像的重建效果。實驗結果證實了此重建方法的可行性以及影像的改善程度。組織內的光學參數(包含光吸收係數及光能量累積分佈)都可以經由我們提出的方法測量出。模擬結果與理論值影像的相似度經過計算後有相當大的一致性。除了結構性影像的重建，我們另外驗證了以高速光聲影像配合金奈米桿進行流速測量的可能性。研究中，我們使用單能量與雙能量灌入流速測量法進行測量。由於灌入流速測量法需要連續並且快速的追蹤光聲訊號，因此我們建立一套高速的光聲成像系統。此系統包含Nd:YAG雷射、自製光聲線形陣列探頭、及超音波前端擷取系統。超音波擷取系統具有同時接收64通道光聲訊號的能力，使得高速光聲成像系統的幈數僅侷限於現有之雷射系統的脈衝產生率。實驗使用雞胸組織作為仿體，並且以人體血液加入金奈米粒子作為流體溶液。實驗所擷取的影像可以同時呈現逆向式的光聲影像以及提供流體的流速資訊，所計算出的流速與理論值有極佳的線性相關度。

關鍵字

光聲影像 ； 逆向式 ； 光吸收能量分佈 ； 光吸收係數 ； 金奈米粒子 ； 非侵入式 ； 流速估算

並列摘要

Photoacoustic imaging combines laser irradiation and ultrasound detection and offers the advantage of visualizing optical properties (e.g., the optical absorption) with a better penetration depth as compared with all-optical imaging techniques. Three major scanning modes, including the tomographic, the backward, and the forward modes, have been used. In the backward mode, photoacoustic signals are measured by using an ultrasound transducer placed at the same side as the laser irradiation. Such a setup makes the backward mode more flexible and easier to be integrated. However, performance of image reconstruction in backward mode has suffered from the small angular extent during the data acquisition. In most situations, only gradients of the optical absorption can be visualized. In this thesis, sequential steps of a new reconstruction algorithm including the adaptive weighting method, reconstruction of energy deposition (RED), and the iterative recovery of absorption (IRA) are introduced for reconstruction of the absorption coefficient. First of all, the adaptive weighting was utilized to improve the lateral resolution that was degraded due to the nondirective photoacoustic wave and the broad radiation pattern of photoacoustic detector. Secondly, the RED was used to obtain the deposited energy based on the photoacoustic wave equation. Finally, the resultant energy deposition was used to reconstruct the absorption coefficient by applying the IRA. Simulations and experiments were performed to evaluate the efficacy of the proposed reconstruction algorithm. Optical parameters, including the deposited energy and the absorption coefficient, were accurately obtained. The results also agree well to the theory.
In addition to the reconstruction algorithm, we also performed flow estimation by using a high-speed backward mode photoacoustic imaging system with gold nanorods as the contrast agent. Two wash-in flow estimation methods were developed by measuring intensities from a sequence of photoacoustic images. A system consisted of a Q-switch Nd:YAG laser, a photoacoustic transducer array, and an ultrasound front-end subsystem that allows photoacoustic signals to be acquired simultaneously from 64 transducer elements. Currently, the frame rate of this system is only limited by the pulse repetition rate of the laser. Experimental results from a chicken breast tissue show that both the structural image and the flow velocities can be measured simultaneously. The measured flow rates are in linear proportion to the theoretical values.

並列關鍵字

Photoacoustic imaging ； backward mode ； optical energy deposition ； optical absorption coefficient ； gold nanorods ； noninvasive ； flow estimation

參考文獻

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