Hybrid three-dimensional full-view multi-wavelength photoacoustic and ultrasound breast tomography

Photoacoustic tomography is a contrast agent-free imaging technique capable of visualizing blood vessels and tumor-associated vascularization in breast tissue. While sophisticated breast imaging systems have been recently developed, there is yet much to be gained in imaging depth, image quality and tissue characterization capability before clinical translation is possible. In response, we have developed a hybrid photoacoustic and ultrasound tomographic system (PAM3). The photoacoustic component has for the first time, in a full-view hemispherical breast system, three-dimensional multi-wavelength imaging capability and implements substantial technical advancements in critical hardware and software sub-systems. The ultrasound component enables three-dimensional ultrasound (computed) tomography from both reflected and transmitted signals from which we currently extract an image of the sound speed. For the first time, in vivo sound speed images were reconstructed using a fully 3D full-waveform inversion (FWI) algorithm, which demonstrated excellent quantitative as well as spatial accuracy. The sound speed image of the breast was then incorporated in photoacoustic reconstruction to correct for acoustic inhomogeneities, enabling accurate target recovery. The results demonstrate identification of blood vessels to depths of up to 48 mm, with a more uniform field of view than hitherto, and an isotropic spatial resolution comparable to the in-plane resolution of clinical breast Magnetic Resonance Imaging. The in vivo performance achieved, and the complementary diagnostic value of interrogating angiogenesis-driven optical contrast as well as tumor mass sound speed contrast, gives confidence in the system's clinical potential.