Non-contact photoacoustic tomography and ultrasonography for biomedical imaging

Abstract

Photoacoustic tomography (PAT) and ultrasonography (US) of biological tissues usually rely on ultrasonic transducers for the detection of ultrasound. For an optimum sensitivity, transducers require a physical contact with the tissue using a coupling fluid (water or gel). Such a contact is a major drawback in important potential applications such as surgical procedures on human beings and small animal imaging in research laboratories. On the other hand, laser ultrasonics (LU) is a well established optical technique for the non-contact generation and detection of ultrasound in industrial materials. In this paper, the remote optical detection scheme used in industrial LU is adapted to allow the detection of ultrasound in biological tissues while remaining below laser exposure safety limits. Both non-contact PAT (NCPAT) and non-contact US (NCUS) are considered experimentally using a high-power single-frequency detection laser emitting suitably shaped pulses and a confocal Fabry-Perot interferometer in differential configuration. It is shown that an acceptable sensitivity is obtained while remaining below the maximum permissible exposure (MPE) of biological tissues. Results were obtained ex vivo on chicken breast specimens with embedded inclusions simulating blood vessels optical properties. Sub-mm inclusions are readily detected at depths approaching 1 cm. The method is expected to be applicable to living tissues.