Focused Ultrasound Thermal Therapy Monitoring using Ultrasound, Infrared Thermal, and Photoacoustic Imaging Techniques.

Abstract

Focused ultrasound (FUS) is a promising thermal treatment modality which deposits heat noninvasively in a confined tissue volume to treat localized diseased tissue or malignancy through hyperthermia or high temperature ablation. FUS compatible guiding and monitoring systems to provide real-time information on tissue temperature and/or status (e.g., native or necrotized) are important to ensure safe and effective treatment outcome; however, current development of such systems are restricted to ultrasound and magnetic resonance imaging (MRI). The work described in this dissertation represents efforts not only to explore new tools to evaluate current monitoring techniques but also to develop new FUS monitoring modalities. In the first study, a new evaluation platform for ultrasound thermometry using infrared (IR) thermography was developed and demonstrated using phantoms subjected to FUS heating, providing a fast calibration and validation tool with spatiotemporal temperature information unavailable with traditional thermocouple measurements. In the second study, IR thermography was investigated as a new tool for high temperature FUS ablation monitoring. The spatiotemporal temperature characteristics in correspondence to lesion formation and bubble activities were identified using simultaneous IR and bright-field imaging. Tissue-specific thermal damage threshold, which is critical for accurate estimation of tissue status based on temperature time history, was also obtained using the same system. In the final study, we developed a novel dual-wavelength photoacoustic (PA) sensing technique for monitoring tissue status during thermal treatments, which is capable of separating the two effects from temperature rise and changes in optical properties due to tissue alteration. Experimental validations of the theoretical derivation were carried out on ex-vivo cardiac tissue using water-bath heating on lesions generated by FUS. Future directions of research include in-vivo technique demonstration where effects such as blood perfusion on FUS heating need to be considered. When FUS operates in the non-ablative regime without causing irreversible changes in tissue, treatment monitoring techniques investigated in this study also have the potential to be translated into diagnostic tools.