ABSTRACT
Primarily since the 1990s, a novel biomedical imaging modality called photoacoustic tomography (PAT) has gained signicant progress. PAT is based on the photoacoustic effect (Bell 1880; Oraevsky and Karabutov 2003; Xu and Wang 2006; C. Li and Wang 2009a), which refers to the generation of sound waves after the item absorbing intensity-varying electromagnetic (EM) waves. The photoacoustic effect is highly sensitive to the optical absorption properties of tissues. As one of the major categories of PAT, photoacoustic microscopy (PAM) breaks the diffusion barrier and opens a new window for optical microscopy. Different from traditional pure optical microscopic methods, where the multiscattering mechanism can signicantly impact the imaging quality, PAM can take advantage of those diffused photons to achieve deeper imaging, because PAM detects signals from much less scattered ultrasound waves instead of light. In general, the scattering coefcient for ultrasound in soft tissue is two to three orders less than light (L.V. Wang 2009). Thus, the soft tissue is nearly “transparent” to ultrasound. By detecting much less scattered ultrasound waves, PAM can therefore conserve high-resolution imaging of optically absorbing targets in deep tissue. PAM uniquely combines the optical contrast with ultrasonic detection, emerging as a novel hybrid biomedical imaging method. During the past decade, PAM has successfully imaged multiscale tissues, from submolecular organelles to subcutaneous cancer tissues in vivo (L.V. Wang 2008, 2009; C. Li and Wang 2009a; L.V. Wang and Hu 2012; J. Yao and Wang 2013).
