Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Supervisor

Carson, Jeffrey J.L.

2nd Supervisor

Diop, Mamadou

Abstract

The pursuit of non-invasive, high accuracy biomedical imaging modalities has prompted the exploration of innovative methods such as photoacoustic tomography (PAT). The photoacoustic wave generated from an optical absorber shows promise for measuring physiological processes and diagnostic imaging of medical conditions. However, the application and adoption of PAT for clinical use face challenges. Non-contact interferometric optical techniques have shown potential in addressing these challenges by improving resolution and usability. Nevertheless, there remain gaps in our understanding of the performance of non-contact detection methods. Furthermore, these techniques require further optimization related to faster recording speed, larger field of view (FoV), and broader frequency detection range.

This thesis first presents a digital holographic (DH) instrument that incorporates several modifications to improve performance. The DH setup was assembled on an air-floated optical table and shielded from stray light to effectively isolate it from external disturbances. A large sensor (36.4 mm × 27.6 mm) with matched detection beam size resulted in an enlarged FoV compared to similar systems reported in the literature. Static measurements on a resolution target revealed a resolution of 10 nm axially (out-of-plane). The DH system was also capable of tracking dynamic surface events in both solid and liquid materials.

The thesis also presents a double exposure electronic speckle pattern interferometry (ESPI) system designed with higher imaging speed for acquiring a broad spectrum of surface vibrations. This instrument utilized precise timing to coordinate multiple subsystems to capture signals at microsecond timescales. The ESPI system was tested on phantoms designed to mimic biological materials. Custom ESPI analysis software revealed nanometer-scale surface topography and confirmed the capability of the system to provide broadband detection of frequencies from 1 Hz up to 150 kHz. Additionally, the system was successfully applied to non-contact measurement of photoacoustic waves by capturing the surface motion on phantoms of varying thickness, optical density, geometry, and absorber arrangement.

The findings demonstrated the ability of DH and ESPI to capture out-of-plane nanoscale surface movements over a large field of view and at timescales down to 1 μs. Additionally, successful detection of photoacoustic waves motivates future research on testing ESPI as a possible detection method for non-contact photoacoustic imaging.

Summary for Lay Audience

Medical imaging plays a crucial part in diagnosing diseases and guiding treatments. A promising medical imaging method, called photoacoustic tomography, uses sound waves generated by the absorption of light in tissue to produce detailed images of the body's interior. However, using photoacoustic tomography in clinics is challenging because the current equipment is complicated and faces many technical barriers. To overcome the current limitations of the technique and make it more convenient and versatile, researchers are exploring non-contact detection methods to improve its adaption and flexibility. One such approach detects patterns of light that change with deformations of the surface, thus effectively enhancing sound wave detection.

This research explored optical methods that can measure small surface motions induced by internal sound waves. The obtained information, such as the appearance of displacement, its duration, and deformation amplitude, can be further used to better understand the sound wave source and strength. A novel optical approach was developed with a larger camera sensor to improve sensitivity. Experiments demonstrated that the system could detect vertical movements as small as 10 nanometers and accurately track surface motion in both solid and liquid materials. Additionally, another optical approach was developed to capture surface vibrations across a wide range of frequencies with high accuracy. This method successfully revealed small surface vibrations in various biomedical materials. Finally, the system was employed to measure sound waves generated by light absorption, proving that non-contact methods can accurately capture important details, which can further assist in the reconstruction of optical absorbers in tissue.

In conclusion, the research presented in this thesis introduces non-contact optical techniques that can record nanometer-level surface changes and vibrations over a wide frequency range. The successful detection of photoacoustic waves suggests the method has potential for non-contact photoacoustic imaging in clinical settings.

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Available for download on Friday, February 28, 2025

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