All-Optical Full-Field Photoacoustic Imaging System for Biomedical Applications

Abstract

Photoacoustic imaging technique differs from conventional optical imaging techniques; it measures the acoustic signal generated by pulsed laser onto the sample to obtain three-dimensional structural information of the target object. By using pulsed lasers at wavelengths with high optical absorption by the target object, selective imaging is possible, allowing for not only structural information but also functional analysis such as measuring oxygen saturation within blood vessels or detecting lipid plaque within samples. Ultrasound transducers, which converts pressure changes due to photoacoustic signals into electrical signals, are primarily used to acquire these photoacoustic signals. They have high sensitivity and are relatively easy to use, making them applicable not only to photoacoustic imaging but also to ultrasound imaging and non-destructive testing. However, contact is essential for impedance matching. For volumetric imaging, either time-consuming two-dimensional scanning or complex arrayed transducers with increased computational and system complexity are required. Additionally, they are limited by the bandwidth of measurable frequencies because of the presence of membranes capable of measuring physical pressure changes. Also, as the physical size of each transducer element is limited, lateral effective sampling decreases with an increasing number of elements. To address these limitations, photoacoustic imaging techniques based on non-contact measurements using optical interferometry and techniques that can measure two-dimensional photoacoustic signals at once using cameras have been developed. However, even for obtaining three-dimensional images of the target object, point-scanning optical interferometry-based photoacoustic imaging still require two-dimensional scanning, and camera-based photoacoustic imaging have not been able to perform effective three-dimensional reconstruction, thus failing to obtain in vivo results. In this thesis, a holography-based full-field photoacoustic tomography system capable of acquiring photoacoustic signals in a two-dimensional field of view at once and introducing in vivo imaging using a biocompatible cover layer was proposed. This thesis consists of six parts. Firstly, (1) the necessity and background of this research are introduced. Secondly, (2) the basic principles of photoacoustic imaging techniques and various methods for measuring photoacoustic signals are introduced. Thirdly, (3) the system configuration for developing full-field photoacoustic tomography system and key methods to improve image resolution are introduced. Fourthly, (4) the process and results of acquiring three-dimensional images of targets in PDMS phantoms and biological specimens to evaluate the performance of the proposed methods are presented. Fifthly, (5) the method of applying a biocompatible cover layer to obtain more effective three-dimensional images in biological specimens and in vivo imaging results are introduced. Lastly, (6) the entire summary of this research and future research directions are summarized.