Photoacoustic imaging and microwave-thermoacoustic imaging are innovative hybrid imaging techniques that have experienced rapid development in recent years. Photoacoustic imaging is based on the photoacoustic effect. When the laser pulses (the width of the laser pulse is usually several nanoseconds to tens of nanoseconds) irradiate the biological tissue, the absorbers in the tissue absorb the optical energy and then induce the instantaneous rise in temperature, and radiate the thermal energy in the form of mechanical energy, i.e. ultrasound signals. We can detect the ultrasound signals with ultrasound transducer and recover the absorption information of the absorbers in the tissue with different imaging algorithms. Photoacoustic imaging integrates the merit of high contrast of optical imaging and high imaging depth of ultrasound imaging. If the excitation source of the photoacoustic imaging is changed into the microwave (the width of the microwave pulse is usually from tens of nanoseconds to hundreds of nanoseconds), that is called thermoacoustic imaging, which can provide high-resolution imaging and imaging depth of more than ten centimeters. Meanwhile, photoacoustic imaging and thermoacoustic imaging have high molecular specificity and have already been widely used in the research of physics, chemistry, and biomedicine.

Photoacoustic imaging, an emerging biomedical imaging modality, holds great promise for preclinical and clinical researches. It combines the high optical contrast and high ultrasound resolution by converting laser excitation into ultrasonic emission. In order to generate photoacoustic signal efficiently, bulky Q-switched solid-state laser systems are most commonly used as excitation sources and hence limit its commercialization. As an alternative, the miniaturized semiconductor laser systemhas the advantages of being inexpensive, compact, and robust, which makes a significant effect on production-forming design. It is also desirable to obtain a wavelength in a wide range fromvisible to nearinfrared spectrumformultispectral applications. Focussing on practical aspect, this paper reviews the state-of-the-art developments of low-cost photoacoustic system with laser diode and light-emitting diode excitation source and highlights a few representative installations in the past decade.

Photoacoustic imaging (PAI) breaks through the optical diffusion limit by making use of the PA effect. By converting incident photons into ultrasonic waves, PAI combines high contrast of optical imaging and high spatial resolution in depth tissue of ultrasound imaging in a single imaging modality. This imaging modality has now shown potential for molecular imaging, which enables visualization of biological processes with systemically introduced functional nanoparticles. In the current review, the potentials of different optical nanoprobes as PAI contrast agents were elucidated and discussed.

Since changes in mechanical properties of biological tissues are often closely related to pathology, the viscoelastic properties are important physical parameters for medical diagnosis. A photoacoustic (PA) phase-resolved method for noninvasively characterizing the biological tissue viscoelasticity has been proposed by Gao et al. [G. Gao, S. Yang, D. Xing, “Viscoelasticity imaging of biological tissues with phase-resolved photoacoustic measurement,” Opt. Lett. 36, 3341–3343 (2011)]. The mathematical relationship between the PA phase delay and the viscosity–elasticity ratio has been theoretically deduced. Moreover, systems of PA viscoelasticity (PAVE) imaging including PAVE microscopy and PAVE endoscopy were developed, and high-PA-phase contrast images reflecting the tissue viscoelasticity information have been successfully achieved. The PAVE method has been developed in tumor detection, atherosclerosis characterization and related vascular endoscopy. We reviewed the development of the PAVE technique and its applications in biomedical fields. It is believed that PAVE imaging is of great potential in both biomedical applications and clinical studies.

All-optically integrated photoacoustic (PA) and optical coherence tomography (OCT) dualmode imaging technology that could offer comprehensive pathological information for accurate diagnosis in clinic has gradually become a promising imaging technology in the aspect of biomedical imaging during the recent years. This review refers to the technology aspects of alloptical PA detection and system evolution of optically integrated PA and OCT, including Michelson interferometer dual-mode imaging system, Fabry–Perot (FP) interferometer dualmode imaging system and Mach–Zehnder interferometer dual-mode imaging system. It is believed that the optically integrated PA and OCT has great potential applications in biomedical imaging.

Microwave-induced thermoacoustic imaging (TAI) is a noninvasive modality based on the differences in microwave absorption of various biological tissues. TAI has been extensively researched in recent years, and several studies have revealed that TAI possesses advantages such as high resolution, high contrast, high imaging depth and fast imaging speed. In this paper, we reviewed the development of the TAI technique, its excitation source, data acquisition system and biomedical applications. It is believed that TAI has great potential applications in biomedical research and clinical study.

Photoacoustic imaging (PAI), also known as optoacoustic imaging, is a rapidly growing imaging modality with potential in medical diagnosis and therapy monitoring. This paper focuses on the techniques of prostate PAI and its potential applications in prostate cancer detection. Transurethral light delivery combined with transrectal ultrasound detection overcomes light scattering in the surrounding tissue and provides optimal photoacoustic signals while minimizing invasiveness. While label-free PAI based on endogenous contrast has promising potential for prostate cancer detection, exogenous contrast agents can further enhance the sensitivity and specificity of prostate cancer PAI. Further in vivo studies are required in order to achieve the translation of prostate PAI to clinical implementation. The minimal invasiveness, relatively low cost, high specificity and sensitivity, and real-time imaging capability are valuable advantages of PAI that may improve the current prostate cancer management in clinic.

We present for the first time in vivo imaging of rat brain using microwave-induced thermoacoustic tomography (TAT). The in vivo imaging of rat brain was realized through an unconventional delivery of microwave energy from the front of rat brain (while the transducer was scanned along coronal plane of the animal brain), which maximized the microwave penetration into the brain. In addition, we found that the imaging contrast was highly dependent on the direction of the electric field polarization (EFP) and that more tissue structures/compositions could be revealed when both X- and Y -EFPs were used for TAT. The in vivo TAT images of rat brain obtained were compared with the 3.0 T MRI images and histological photographs, and numerous important brain anatomical structures were identified. An example of our TAT approach for imaging a foreign object embedded in a rat brain was also demonstrated. This study suggests that TAT has a great potential to be used in neuroscience studies and in noninvasive imaging of brain disorders.

Integrating photoacoustic (PA) and ultrasound (US) into a handheld probe to perform PA/US dual-modal imaging has been widely studied over the past few years. However, optical fluence decreases quickly in deeper tissue due to light scattering and absorption, which would significantly affect the quantitative PA imaging. In this paper, we performed a fluence compensation for a PA imaging study of human breast. The comparison of PA/US image with and without optical fluence compensation demonstrated that the fluence compensation could effectively improve imaging quality for handheld probe.