The aim of this research work is to develop a Photoacoustic-Ultrasound (PA-US) imaging system that uses pulsed laser diode (PLD) as an alternate source of illumination to the conventionally used solid state Nd:YAG laser source. This thesis demonstrates the use of sub-pitch translation approach to improve resolution, contrast and depth of penetration for the PLD based imaging system. It further aims to investigate a fast ω-k reconstruction algorithm as an alternate method of reconstruction to the conventionally employed delay and sum method.
Photoacoustic (PA) imaging is a hybrid imaging modality that captures both the functional molecular information and the structural anatomical information about the underlying tissue by combining benefits of light and sound. In conventional PA imaging, solid state lasers such as the Nd:YAG are used as optical sources of excitation. These sources are capable of producing high energy per pulse (~tens of mJ) with narrow pulse duration (few nanoseconds), required for effective photoacoustic signal generation and optimal depth of light penetration. However, these sources are bulky, expensive and have very low pulse repetition frequency (10-200Hz), which limits their utility for real time imaging in a clinical setup. On the other hand, Pulsed Laser Diodes(PLD) are compact, relatively cheaper and can provide very high pulse repetition frequency (order of kHz) which makes them attractive to be explored as an alternative. However, the PLDs energy per pulse is only in the order of μJ which directly affects the image quality and depth of penetration (DoP) within the tissue. Novel methods to increase the image quality and depth sensitivity in PLD-based PAT imaging system are developed and reported in this thesis. It is demonstrated that for a given source settings, medium properties and receiver settings, it is feasible to achieve 7dB improvement in image contrast and 20-23% improvement in DoP compared to other state of the art methods reported in literature.
In addition to source-side modification in the system, a fast frequency-wavenumber (ω-k) reconstruction algorithm was developed for diverging beam synthetic aperture-based ultrasound data acquisition scheme. The results showed that it is feasible to achieve several orders of magnitude faster reconstruction in comparison to standard delay and sum (DAS) beamforming. DAS reconstruction takes as long as 2931.48±70.84 seconds (48.84±1.18 mins) whereas the ω-k algorithm takes only 5.437±2.56 seconds when programmed and executed in MATLAB®.
The work reported in this PhD thesis demonstrates that by using sub-pitch translation approach on the receive-side of the PLD-PAT imaging system and employing a fast ω-k reconstruction algorithm it is feasible to achieve improved contrast, DoP and reduced computational load. These findings not only suggest the feasibility of developing a practical PLD-based PAT system, but also a less bulky and portable imaging system with anticipated use in clinical point of care applications.