Photoacoustic ultrasound: theory

Abstract

Localizing optical absorption within biologic tissue is compromised by the ubiquitous scattering of light that takes place within such tissues. As an alternative to purely optical detection schemes, regional absorption of optical radiation can be detected and localized within highly scattering tissues by detecting the acoustic waves that are produced whenever differential absorption of radiation takes place within such tissues. When the source of optical radiation is delivered in pulses of <EQ 1 microsecond(s) ec duration, the acoustic waves that are produced lie in the medical ultrasonic frequency range, and can be localized using conventional ultrasonic transducers and reconstruction methodology. Localizing such acoustic waves is not adversely affected by optical scattering. This paper introduces a simplistic theory of acoustic wave production within turbid media. The relationships among the irradiating optical pulse power, regional absorption, and strength of acoustic wave production are developed. Estimates of contrast and spatial resolution are presented, assuming a conventional, focused ultrasound transducer and translational scanning are used. Initial theoretical work indicates that optical absorption can be localized with millimeter spatial resolution for 10% absorption or less in biologic tissues as thick as 6.0 cm using safe levels of optical radiation.