Optical absorption reconstruction and blood flow measurements using backward mode photoacoustic imaging
Date Issued
2007
Date
2007
Author(s)
Liao, Chao-Kang
DOI
en-US
Abstract
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.
Subjects
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
Photoacoustic imaging
backward mode
optical energy deposition
optical absorption coefficient
gold nanorods
noninvasive
flow estimation
Type
thesis
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