Non-contact laser ultrasound concept for biomedical imaging

Abstract

The potential of a fully noncontact, standoff, laserultrasound system that acquires ultrasonic images within biological tissue is examined. A pulsed laser converts optical energy into ultrasound via photoacoustic mechanisms, while laser Doppler vibrometry measures emerging ultrasonic waves at the tissue surface. Differing from photoacoustic tomography (PAT), which maps spatial variations in tissue-optical absorptivity in the acoustic near field, the laser ultrasound (LUS) approach developed here, is driven by shallow, non-varying optical absorptivity that creates a laterally consistent acoustic source enabling ultrasound propagation well into the far field. LUS acoustic wave generation is explored in tissue and bone including longitudinal, shear, and Rayleigh wave components. Using information from LUS wave types can yield 1) tissue and bone anatomical images and 2) mechanical property distributions that apply to the emerging field of medical elastography. Imaging capabilities using a demonstration LUS system are also presented for complex bio-tissues. Ultrasonic images compare well with ground truth geometries, orientation, and depth of staged samples. 2D cross-sectional echo reflection images are generated for a phantom limb containing muscle and bone materials and use data inversion techniques to yield the elastic moduli distributions in the specimen.