Abstract
Photoacoustic (PA) imaging, also called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade. It is a hybrid modality, combining the high-contrast and spectroscopic-based specificity of optical imaging with the high spatial resolution of ultrasound imaging. In essence, a PA image can be regarded as an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but its optical properties, specifically optical absorption. As a consequence, it offers greater specificity than conventional ultrasound imaging with the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores, but with greater penetration depth than purely optical imaging modalities that rely on ballistic photons. As well as visualizing anatomical structures such as the microvasculature, it can also provide functional information in the form of blood oxygenation, blood flow and temperature. All of this can be achieved over a wide range of length scales from micrometres to centimetres with scalable spatial resolution. These attributes lend PA imaging to a wide variety of applications in clinical medicine, preclinical research and basic biology for studying cancer, cardiovascular disease, abnormalities of the microcirculation and other conditions. With the emergence of a variety of truly compelling in vivo images obtained by a number of groups around the world in the last 2-3 years, the technique has come of age and the promise of PA imaging is now beginning to be realized. Recent highlights include the demonstration of whole-body small-animal imaging, the first demonstrations of molecular imaging, the introduction of new microscopy modes and the first steps towards clinical breast imaging being taken as well as a myriad of in vivo preclinical imaging studies. In this article, the underlying physical principles of the technique, its practical implementation, and a range of clinical and preclinical applications are reviewed.
Highlights
Photoacoustic (PA) imaging, called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade
The combination of the strong spectral discrimination arising from optical tissue interactions and the high spatial resolution associated with ultrasound propagation lends PA imaging to a broad range of potential applications in clinical medicine, preclinical research
The aim of this paper is to provide an overview of the underlying physics of PA imaging, its practical implementation, the functional and molecular imaging capability it can provide and the biomedical applications it lends itself to
Summary
Research into the underlying physics of photoacoustic (PA) techniques has a relatively long, if sporadic, history dating back to 1880 when Alexander Graham Bell first discovered the PA effect following his observation of the generation of sound owing to the absorption of modulated sunlight [1]. The spectral dependence of optical absorption enables image contrast provided by specific tissue chromophores to be selectively enhanced by tuning the laser excitation wavelength to their peak absorption. As well as being able to image structure and function via optical absorption and its wavelength dependence, other capabilities of PA imaging include blood flow measurement by exploiting the acoustic Doppler effect in a manner analogous to conventional Doppler US and temperature sensing via the temperature dependence of the PA generation process. The combination of the strong spectral discrimination arising from optical tissue interactions and the high spatial resolution associated with ultrasound propagation lends PA imaging to a broad range of potential applications in clinical medicine, preclinical research.
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