Photoacoustic imaging is an emerging hybrid imaging modality that can provide multicontrast, multiscale imaging of biological features ranging from organelles to organs. The three major embodiments of photoacoustic imaging are microscopy, endoscopy, and computed tomography. Photoacoustic tomography (PAT) or photoacoustic computed tomography allows deep-tissue imaging, and hence it is more suitable for whole body preclinical/clinical imaging applications. Due to fast-growing laser technology and ultrasound detector technology, PAT is evolving rapidly, leading to a quicker translation into clinical trials. We review the recent developments of PAT systems and their applications in preclinical and clinical practices.

In biomedical imaging, there is a huge demand for a noninvasive imaging modality that can provide high spatiotemporal resolution, high penetration depth, and anatomical and functional contrasts at an affordable price. Imaging modalities such as magnetic resonance imaging (MRI), x-ray computed tomography (CT), single-photon emission tomography (SPET), positron emission tomography (PET), and ultrasound (US) imaging, have been widely used for clinical and preclinical imaging.^{1 – 5} Although they provide deeper penetration, leading to whole-body imaging, they have their own drawbacks. High-resolution MRI systems are expensive and slow for dynamic imaging.^6,7 The x-ray CT and PET/SPECT use ionizing radiation and hence may not be suitable for frequent imaging.⁸ US imaging cannot measure functional (physiological) changes, including metabolism, chemical composition, and absorption.⁹