Photoacoustic Tomography: Beat Optical Diffusion and Diffraction

Abstract

We develop photoacoustic tomography technologies for in vivo functional, metabolic, molecular, and histologic imaging by physically combining non-ionizing electromagnetic and ultrasonic waves. Broad applications include early-cancer detection and brain imaging. 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.

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