The design of compact microwave-induced thermoacoustic imaging systems for medical screening applications
Keywords: Electrical Engineering.

Medical imaging can create visual representations of the internal structure of a body for clinical analysis and therapeutic intervention. It has been successful in reducing the mortality rate of most diseases. However, the access to advanced imaging tools is limited to hospitals due to size, cost, and other constraints, which limits the screening frequency and widespread use, leading to missing some fast-growing and often fatal types of diseases. For example, the screening interval of mammography is limited by a desire to restrict the ionizing radiation due to X-ray, as well as secondary concerns of screening cost and the false positive rate. From a sampling point of view, a Nyquist screening is required to enable continuous monitoring and provide meaningful information for diagnosis. It needs significant innovations to scale the system into a compact dimension with low cost, to enable portable and even handheld operation without ionization radiation. The medical imaging community has long been in pursuit of such a suitable handheld imaging system which provides high contrast and resolution for point-of-care frequent screening and diagnostics. One such promising candidate is microwave-induced thermoacoustic (TA) imaging. As a multi-physics hybrid modality, TA imaging provides dielectric/conductivity contrast and ultrasound resolution at the same time. Ultrasound signals generated from thermal expansion differentials in soft tissue (the thermo-elastic response) are detected by a scanning single-element transducer or an ultrasonic array to form images. Combining microwave and acoustics provides the extra benefit of enabling a handheld and portable form factor, due to the integration potential of both modalities. This dissertation describes beamforming and coherent processing in TA imaging for improved signal generation and detection to enable handheld operations with low power and a small form factor. In TA beamforming, we increase the deposited radio frequency (RF) power to the target volume at depth and avoid excessive heating of the skin or surface by transmitting RF power from multiple locations instead of a single high-power element. With a phased array, we steer and control the RF focal point across the target region by tuning the phase of each channel. Spatial power combining can significantly improve TA signal generation at depth due to the coherent summation of E fields from excitation elements. In another direction, I perform coherent processing in TA imaging by exciting the target with the microwave of longer duration and much lower peak power compared to conventional pulse approaches. With matched-filter processing, we can reconstruct the target pulse response as well as achieving significant signal-to-noise ratio improvement by exploiting the amplitude and phase in the frequency domain. The coherent processing further reduces the requirements of the RF power source and enables fully solid-state implementation of TA imaging. The dissertation also presents a programmable integrated wideband RF transmitter for TA imaging based on the ST 55~nm BiCMOS technology. With the designed chip, the TA imaging system is scaled to a small form factor, while it can operate in both coherent mode and conventional pulse mode as well as simultaneous imaging and spectroscopy capability. By exploiting the different responses of tissues across microwave excitation frequency, TA spectroscopy provides another degree of freedom to enhance contrast, differentiate materials and help diagnosis. In addition, this dissertation demonstrates non-invasive temperature monitoring with TA imaging by exploiting temperature-dependent behavior, achieving degree accuracy in real time. The reconstruction algorithms in TA imaging are also discussed, including a proposed forward reconstruction algorithm which bypasses the ill-posed inverse problem by correlating the measured signals with pre-calculated point spread functions in an iterative manner.