Synthesis of Bessel Beam Using Time-Reversal Method Incorporating Metasurface

In this paper, a synthesis and implementation method for generating Bessel beams based on time-reversal theory incorporating electromagnetic (EM) meta-lens is proposed. As is known, time-reversal EM waves has the unique characteristic of adaptive backtracking. Therefore, with this characteristic, the EM characteristics of the radiation aperture can be obtained and further utilized to generate Bessel waves with any departure angle. Based on this concept, two meta-lenses for generating Bessel beams tilted in different directions were designed. Both meta-lenses were designed at the center frequency of 15 GHz, and the simulation results were consistent with the target expectation. A representative meta-lens was fabricated and measured. The final size of the meta-lens was 350 mm $\times \,\, 350$ mm, and a Bessel beam with a 30° emergence angle was generated by this structure. The experimental results were in good agreement with the simulation results and the theoretical derivation. This synthetic method of Bessel beam generation using the time-reversal operation may be of great use for the application of Bessel beams in microwave communications, and it can broaden the application scope of non-diffracting beams

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

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.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/150038/1/halei_1.pd