Optical excitation and acoustic detection enable PAI to combine the high contrast available from optical imaging with the spatiotemporal resolution of ultrasound imaging. Several comprehensive reviews on PAI can be found in the literature [19; 22; 33; 34; 36]. There are three types of PAI systems: photoacoustic microscopy (PAM) [34; 37; 38], photoacoustic endoscopy (PAE) [39], photoacoustic tomography (PAT), or photoacoustic-computed tomography (PACT) [26; 33; 40]. The interest in PAT is mostly connected with the successful results in tumor diagnostics obtained over the past years [16; 22; 26; 33; 34; 41]. Time-resolved detection of a PA signal excited by a laser pulse in the investigated medium provides information on the spatial distribution of the optical properties of tissue. Laser pulses may be effectively used to produce acoustic waves in tissue with different optical absorption. Ultrasonic waves can propagate in biological tissue with minimal distortion and deliver diagnostic information to the surface of the tissue, where they may be detected with high temporal resolution by piezoelectric or optical detectors. Hence, PAT uses the inherent optical contrast of tissue structure. PAT is applicable to almost any problem where the absorption of light occurs [19]. For example, in the task of tumor detection, on the one hand, the contrast of a PA image depends on the large difference in optic absorption between a tumor and healthy tissue. On the other hand, high resolution can be achieved in a PA image. In this case the penetration depth can reach several centimeters, since ultrasonic waves are attenuated much less then light (\(\sim\)1 dB/MHz in the range from 20 kHz to 2 MHz) in soft biological tissues [41]. PAI found applications outside biomedicine. In [42], PAT was proposed as a method for the study of femtosecond filaments in air, and the spatial resolution was studied numerically for this case. In [43], the waveforms calculated numerically were compared with those obtained in experiments. Kerr self-focusing, in the medium, causes a high-power femtosecond laser pulse to overcome the diffraction and collapse, ionizing the medium. Filamentation is a unique optical phenomenon where Kerr self-focusing is balanced with plasma defocusing, so that an elongated plasma column (the core of the filament) is formed. When the plasma recombines, the medium is heated, due to non-radiative de-excitation, and an ultrasonic pulse is generated as in traditional PAI scenarios. The core of the filament is surrounded by laser radiation (the reservoir of the filament), which contains \(\sim\)90% of the laser pulse energy. In [44], the authors reported a first experimental PAT of a femtosecond filament in water. The position of the filament core was determined with 10 \(\upmu\)m accuracy. The authors observed \"the halo\" around the core in the experimental images and interpreted it as the reservoir of the filament. Thus, PAT has a potential of being a direct high-resolution reservoir imaging technique. PAT does not replace any current biomedical imaging methods; however, it can complement them. Since PAT combines optics and ultrasound, combining PAT with either optical or ultrasound imaging methods in one complex system is of interest. Since PAT already widely utilizes piezoelectric transducers, PAT systems can be combined with conventional pulse-echo ultrasound. Therefore, such systems can potentially provide not only PA images of endogenous absorbers, such as small blood vessels, but also ultrasound images of mechanical structures, indicating the boundaries of organs. In addition, combined PA and LU imaging can be used to guide invasive procedures, such as needle insertion into blood vessels [45], where the blood vessel is highlighted by PAT, and LUT is used to image the needle. As mentioned, commercial ultrasound detectors have been applied in PAT, such as hand-held arrays, as the PA detector, while optical fibers are used to deliver light [16; 46].