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UBC Theses and Dissertations

Investigating limited view problem in photoacoustic tomography Shu, Weihang


Photoacoustic (PA) imaging is a new biomedical imaging modality that is based on the PA effect. In the PA effect, nanosecond short pulsed laser light illuminates on tissue and is absorbed by optical absorbers, which undergo a temporary temperature rise. The illuminated region experiences thermo-elastic expansion and engenders an abrupt and localized pressure variation. This transient variation causes a PA wave to propagate from the absorber outward through the tissue to the surface for detection by an ultrasound transducer. The detected PA signal can then be used to estimate the initial pressure distribution. PA imaging is an absorption-based modality which is capable of providing the optical property of the illuminated tissue while achieving deep imaging penetration compared with conventional optical imaging. Meanwhile, the PA image can also provide a similar spatial resolution as ultrasound imaging. Photoacoustic tomography (PAT) is the most widely used imaging mode in PA imaging due to its simplicity and versatility. One major drawback in PAT is that the reconstruction algorithm required to estimate the initial pressure demands a large detection view angle for exact reconstruction, which is typically impractical in clinical applications. Our goal of this thesis is to develop a novel methodology to increase the detection view angle by using two linear array transducers at different orientations. The relative position between the two transducers needs to be calibrated in order to combine the received signal from the two transducers. A new calibration approach is developed by using the ultrasound modality. The efficacy of the calibration method is demonstrated by both simulation and experiment. With increased detection view angle, the reconstructed image indicates more complete tissue structures compared with the one acquired by a single transducer. By combining the PAT images from the two transducers, an improvement on the image quality through complementing the structural information is achieved. Our approach does not require a calibration phantom, which can largely simplify the calibration process and shorten the acquisition time. The use of linear array transducers and the flexibility of positioning the transducers to fit tissue geometry make this approach promising for clinical applications.

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