Special issue introduction: Photoacoustic microscopy

First proposed in the 1970s for non-destructive testing, the modern reincarnation of photoacoustic microscopy (PAM) has major impact in bio-medicine, spanning a wide range of applications from flow cytometry and cancer research to ophthalmology, neuroimaging and cardiovascular diagnostics. The hybrid nature of PAM combining optical excitation and acoustic detection provides it with a number of key advantages over both optical and acoustic microscopy, including versatile optical absorption contrast and high scalability of imaging depth and spatial resolution. The current special issue encompasses broad aspects of PAM with contributions from the photon plus ultrasound research community on new technical developments, biomedical applications and review of state-of-the-art. Photoacoustic microscopy (PAM) was first introduced in the 1970s for non-destructive testing of inorganic materials [1,2]. Its modern reincarnation for imaging biological tissues, however, has not been fully explored until 1990s. PAM harnesses both strong optical absorption contrast and much reduced ultrasonic scattering in biological tissue [3,4]. Compared with high resolution ultrasonic microscopy, PAM is much more versatile in offering physiology related information; compared with high resolution optical microscopy, PAM has the advantage in deeper penetration depth and more selections of contrast sources. Over the past two decades, PAM has drew an extremely broad spectrum of researchers with expertise from science and engineering (for example lasers, ultrasonic detectors, contrast agent, optical microscopy, and reconstruction algorithm) to biology and medicine (for example animal models, cardiology, oncology, and ophthalmology). As a result, an incredible growth in scientific publications that are related to PAM and its applications has been observed. In this special issue, we present papers from diverse disciplines, including new PAM system using novel Raman scattering fiber laser and beam deflection based optical detection of ultrasound, and PAM applications to comprehensive examination of human carotid atheroma and ophthalmology. Buma et al. [5] developed a laser system that takes advantage of multiple Raman scattering peaks in a graded-index multimode fiber to achieve multispectral PAM of lipid-rich tissues and tissue samples within the near-infrared spectral region. Maswadi et al. [6] reported an all-optical PAM system that relays on deflection a probing optical beam shining through the ultrasound propagation path as a result of refractive index modulation. Several advantages are demonstrated using this all-optics ultrasonic detection method as compared with traditional piezoelectric ultrasonic detection. Seeger et al. [7] developed a multimodal system with added