Super-resolution photoacoustic microscopy

Abstract

A novel super-resolution volumetric photoacoustic microscopy, based on the theory of structured illumination, is implemented in this thesis. The structured illumination is introduced to surpass the diffraction limit in an acoustic resolution PAM (AR-PAM) structure. Synthetic aperture focusing technique (SAFT) based on a virtual detector (VD) concept can compensate for the beam shape. Structured illumination can shift the spatial spectrum of an imaging target to low frequencies hence high-frequency contents can be preserved to overcome the diffraction limit. Structured illumination is equivalent to image with a system of two-fold bandwidth. The experimental results obtained from several tungsten filaments in the depth range of 1.2 mm, show an improvement of the lateral resolution from 55-287 µm to 25-29 µm. Then, a modified phase-shifting method is used to generate the second harmonic of the fringes and double the spectral shift. In this idea, higher frequency information compared to the three-phase shifting method can fall into the band-limited system response. The modified phase-shifting method expands the spatial bandwidth and improves the lateral resolution by five folds. The performance of the imaging system is validated with in vivo, and ex vivo targets. The in vivo experiment of modified structured illumination on mouse ear shows 21.9 dB enhancement in the image SNR. The third study proposes a modified Jerman filter for better extraction of vascular networks from medical images. The filter employs non-local means for denoising, adjusting the vesselness function to suppress blob-like structures and fill the gaps in wide vessels. The final work introduces a method to suppress artifacts in photoacoustic tomography systems. The study proposes optimized weights calculated using a Genetic algorithm to suppress side lobe level (SLL) by 9.9 dB compared to the delay-and-sum beamformer. These advancements in photoacoustic imaging techniques offer promising improvements in medical imaging quality and accuracy. Keywords: Volumetric photoacoustic microscopy, structured-illumination, super-resolution, Fourier-domain reconstruction, synthetic aperture focusing technique, virtual detector, second harmonic, photoacoustic tomography, array optimization, artifact, Genetic algorithm, Jerman filter.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhengtang LUO (Supervisor)