Photoacoustic tomography: Ultrasonically breaking through the optical diffusion and diffraction limits

Abstract

We develop photoacoustic imaging technologies for in vivo early-cancer detection and functional, metabolic, molecular, and histologic imaging by physically combining non-ionizing electromagnetic and ultrasonic waves. Unlike ionizing x-ray radiation, non-ionizing electromagnetic waves - such as optical and radio waves - pose no health hazard and reveal new contrast mechanisms. Unfortunately, electromagnetic waves in the non-ionizing spectral region do not penetrate biological tissue in straight paths as x-rays do. Consequently, high-resolution tomography based on non-ionizing electromagnetic waves alone - such as confocal microscopy, two-photon microscopy, and optical coherence tomography - is limited to superficial imaging within approximately one optical transport mean free path (~1 mm in the skin) of the surface of scattering tissue. Ultrasonic imaging, on the contrary, provides good image resolution but suffers strong speckle artifacts as well as poor contrast in early-stage tumors. Ultrasound-mediated imaging modalities that combine electromagnetic and ultrasonic waves can synergistically overcome the above limitations. The hybrid modalities provide relatively deep penetration at high ultrasonic resolution and yield speckle-free images with high electromagnetic contrast.