## 2 The Methods of PAI and LUI  
### Applications of PAI and LUI--Image Formation Principles  
Advantages of the PAI for characterization of optically heterogeneous tissue structures with high spatial resolution were demonstrated over the past several years [14; 15; 16; 62; 63]. One of the major merits of PAI is high sensitivity in detecting variations of laser-induced heating. Unique features of PAI found utility in various medical applications, including detection of early cancer [62], monitoring therapeutic effects of lasers in tissues [64; 65], and measuring tissue properties and molecular content [26; 30; 65]. Optimal design and development of PAI and monitoring systems requires, first of all, comprehensive analysis of capabilities and limitations in detection of wide-band ultrasonic transients.  
From an image formation standpoint, both conventional ultrasound and PAT can use delay-and-sum type algorithms. However, an important difference between conventional ultrasound and PAT is that ultrasonic systems normally operate in a pulse-echo mode, and it is possible to phase array elements for the radiation whereas, in PAT, the ultrasound sources are inside the investigated object and their radiation pattern is not known in advance. For this reason, the algorithm of image reconstruction is different than in classical ultrasound and, thus, the dependences of resolution and the contrast range of the obtained images differ from the array parameters, as well [66].  
The LU method uses sound generation by a laser pulse absorbed in a thin surface layer of the investigated medium, or a dedicated PA generator, the propagation of an ultrasonic pulse in the investigated medium and recording of the passed or scattered ultrasonic signals with high temporal resolution, like conventional ultrasonic method. This trend of object diagnostics is similar to conventional ultrasonic diagnostics and echoscopy [67]. Active research is being conducted to find better materials for PA generators [68]. The advantage of the LU method, compared with other methods, is laser generation of short and intense aperiodic ultrasonic pulses, inaccessible to the usual ultrasonic transducers [69; 70]. The method provides efficient excitation in a wide spectral range, from one to tens of megahertz. The short duration of the probing ultrasonic pulse results in an increase in longitudinal spatial resolution, while a reasonable width of the signal band is preserved. The small diameter of the probing beam serves to increase sensitivity; the aperiodic probing pulse almost completely avoids a \"dead\" zone.  
The temporal shape and the amplitude of the ultrasonic pulse depend on the light absorption and thermal conductivity of the medium, the temporal profile of laser pulse intensity, and the density of the absorbed laser energy [70; 71]. The pulses have a smooth bipolar profile free from reverberations (Figure 1a). The spectral band of such pulses lies within a wide frequency range, limited only by the spectrum width of the laser pulse (Figure 1b). In contact LUI scenarios, a dedicated PA generator, made of a light-absorbing material, is typically used to generate LU probe pulse [66; 70]. In this case, the temporal shape and the amplitude of the pulse are fixed, and can be optimized for the particular task. The surface absorption in the investigated medium can be used, for example, in non-contact LUI applications [55]. The temporal shape and the amplitude of the probe pulse here may depend on a particular point of the sample where the light pulse is absorbed.