Progress and challenges in microwave imaging and microwave induced thermoacoustic tomography

Breast cancer imaging by microwaves has been investigated intensively over the past two decades due to the potentially high contrasts in permittivity and conductivity between malignant tumors and normal breast tissue. In comparison with the conventional ultrasound imaging where the acoustic impedance contrast between malignant tumors and normal breast tissue is low (typically a few percent), the dielectric contrast is indeed one to two orders of magnitude higher. Nevertheless, progress toward a clinically mature system for microwave breast imaging is painfully slow, primarily due to the low resolution of microwaves that can provide adequate penetration only at a relatively low frequency. We will describe challenges in achieving such a system, and ways to improve the resolution of microwave imaging. In the meantime, recent progress in microwave induced thermoacoustic tomography (MITAT) provides a new impetus for combining microwave and ultrasound modalities. In MITAT, we use millisecond-pulsed microwaves to produce ultrasound through thermal expansion, thus the induced ultrasound source represents the high contrast in electrical conductivity, while the collected ultrasound signals provide the high resolution corresponding to a short wavelength of ultrasound. We will describe our recent progress in both microwave imaging and MITAT, in both computational methods and system development.