Developments and Applications of Laser-Based and X-Ray-Based Biomedical Thermoacoustic Imaging Techniques

Abstract

Thermoacoustic imaging (TAI) is one class of biomedical imaging techniques that share the same physical basis, called the thermoacoustic effect (TAE). The TAE phenomenon can be categorized as sonic waves generated following the absorption of energy/heat. In recent decades, as a result of the continuous development of radiation sources such as masers and lasers, the TAE phenomenon has been extensively utilized to achieve biomedical imaging. The hybrid modality offers high contrast and spectroscopic-based specificity image with ultrasonic spatial resolution. It shows great potential for preclinical research and clinical practice in achieving anatomical, functional, and molecular images. So far, however, TAI has not been widely adopted in clinic. The major challenges include 1) the limited imaging depth due to the applied radiation and 2) the difficulty in achieving quantitative image. The purpose of this research is to further investigate the fundamental mechanism of TAI and to broaden its applications. In the first part of this study, laser-based TAI technique, also known as photoacoustic (PA) imaging, is implemented to improve the diagnosis of Crohn’s disease, especially solving the challenge of characterizing the intestinal strictures in bowel. The feasibility of assessing the spatially varying molecular components in ex vivo intestinal strictures by obtaining PA molecular component images using a developed acoustic resolution PA microscopy system is validated. Then, the microscopy system is miniaturized to a prototype side-view scanning capsule-shaped probe and its practicability in quantitatively differentiate the intestinal disease conditions is proved by performing in vivo colonoscopy in the rabbit disease model. In the second part of this study, the potential applications of x-ray-based TAI technique, named x-ray induced acoustic (XA) imaging, are evaluated. Based on soft-tissue phantom studies, the feasibility in monitoring the position of the x-ray beam and measuring the spatially varying dose deposition is validated. These results suggested a potential application of XA imaging method as a novel in vivo dosimetric tool in external beam radiotherapy. Furthermore, an XA and ultrasound (US) dual-modality imaging system is established utilizing a commercial ultrasound unit, aiming to obtain XA image and US image simultaneously, both in real time. As demonstrated by the experiments on soft-tissue phantoms, the XA image showing the deposited radiation dose and the US image capturing the motion of target tissue can be naturally co-registered, offering a potential approach for image-guided radiotherapy.