Stepped-frequency continuous-wave microwave-induced thermoacoustic imaging

Microwave-induced thermoacoustic (TA) imaging combines the dielectric contrast of microwave imaging with the resolution of ultrasound imaging. Prior studies have only focused on time-domain techniques with short but powerful microwave pulses that require a peak output power in excess of several kilowatts to achieve sufficient signal-to-noise ratio (SNR). This poses safety concerns as well as to render the imager expensive and bulky with requiring a large vacuum radio frequency source. Here, we propose and demonstrate a coherent stepped-frequency continuous-wave (SFCW) technique for TA imaging which enables substantial improvements in SNR and consequently a reduction in peak power requirements for the imager. Constructive and destructive interferences between TA signals are observed and explained. Full coherency across microwave and acoustic domains, in the thermo-elastic response, is experimentally verified and this enables demonstration of coherent SFCW microwave-induced TA imaging. Compared to the pulsed technique, an improvement of 17 dB in SNR is demonstrated.