Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Medical Biophysics

Supervisor

Carson, Jeffrey J.L.

Abstract

Photoacoustic tomography (PAT) is a widely explored hybrid imaging modality combining advantages of ultrasound and optical imaging. However, on the acoustic detection side, limited-view angle coverage and limited-detector bandwidth are common key issues in PAT systems that result in unwanted artifacts. While analytical and simulation studies of limited-view artifacts are extensive, experimental setups capable of comparing limited-view to an ideal full-view case are lacking. Due to the lack of PAT systems capable of artifact-free full-view imaging, applications for such a system have also been left unexplored.

A custom ring-shaped detector array was assembled and mounted to a 6-axis robot, which was used to rotate and translate the array to achieve up to 3.8π steradian view angle coverage of an imaged object. To optimize the system, a method to minimize negativity artifacts and phantom imaging were used, followed by demonstrative imaging of a star contrast phantom, a synthetic breast tumor specimen phantom, and a vascular phantom. Finally, this system was applied towards imaging of the relative optical fluence distribution emitted by a fiber bundle into a series of two-layered absorbing and scattering agarose phantoms. Resulting 3D images were compared to Monte Carlo simulations using two software packages, the diffusion equation, and extracted reduced scattering coefficient values.

Optimization of the angular scans found ~4000 effective detectors spread over a spherical surface were needed for high quality images, while 15-mm steps were used to increase the field of view. Example phantoms were clearly imaged with all discerning features visible and minimal artifacts. When measuring fluence distributions, reasonable visual correspondence was found compared to simulated distributions. Image volumes compared well with similar trends, while shape analysis revealed a better match to TracePro than Monte Carlo eXtreme. Reduced scattering coefficient recovery appeared successful for values below 0.5 mm-1.

A near full-view closed spherical system has been developed, paving the way for future work demonstrating experimentally the significant advantages of using a full-view PAT setup. When applied to one previously unexplored application, initial results to measure fluence distribution appeared to have potential, although some barriers remain and further development is needed.

Summary for Lay Audience

Photoacoustic tomography (PAT) is a widely explored hybrid imaging modality combining advantages of ultrasound and optical imaging. Briefly, laser light is used to illuminate an object, which causes it to emit its own ultrasound waves. By detecting these waves and counting how long it takes each signal to reach a detector, three-dimensional images can be formed. Ideally, numerous detectors fully surrounding the imaged object (full-view angle coverage) are required for high-quality images. However, due to practical, financial, and technological constraints, previously developed PAT systems typically only have detectors covering up to approximately half the possible angles. This thesis focuses on the development of an imaging system approaching the ideal full-view angle coverage, how it was optimized, and then how it was validated using simple objects and geometries that emulated common applications of PAT. In addition, this new full-view system was applied to measure how light travels in a medium that both scatters and absorbs light, such as in human soft tissue.

The new PAT system was found to provide up to 95% of the ideal full-view angle coverage. This was tested successfully on a star-shaped object, an object imitating a breast tumor after it had been removed from a breast, and on a model of a blood vessel network. When applied to measure light distribution, the experimental results were compared to computer-generated models of light distribution and found to be similar, although not identical.

In conclusion, a new imaging system has been developed that can now address both clinical and technical applications that were not previously possible. While further development and improvements are still needed, we look forward to seeing where this technology can take us.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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