## 3 Combining PAI and LUI in Biomedicine
### Combined PA and LUI Pulse-Echo Imaging
The narrow laser  
Figure 5: **(Left)** Schematic of the 2D combined imaging setup by [PERSON] et al. in the needle imaging scenario. **(Right)** Combined “PA + LU” image of a needle (0.7 mm in diameter) inside a blood vessel model (2.4 mm in diameter): red—PA image, cyan—LU image. Adapted with permission from [45], CC BY, 2017.  
beam was scanned over the imaging area using a rotating galvo-mirror, and the 256-element linear US array was used for both modalities. The combined PA/US imaging system achieved 30 frames (2.8 \(\times\) 2.8 cm) per second. Another approach to needle guidance is integrating the PA generator and ultrasound sensor into the needle tip [116; 117; 118]. In [117], the needle contained three fibers. The authors used a Fabry-Perot cavity (two dielectric mirror coatings with an epoxy spacer in between) manufactured on the end face of the fiber as a receiver, a composite coating (polydimethylsiloxane with integrated multiwalled carbon nanotubes) on the end face of another fiber. The third fiber was supposed to be used for PA illumination; it was not used in [117], but was used in [116]. The needle tip tracking was implemented in [116]. The linear US array, at the tissue surface, transmitted acoustic pulses which were received by the fiber-optic sensor in the needle tip. The accuracy was better than 1 mm in both axial and lateral dimensions. One of the main challenges was crosstalk, since the generator and the receiver were situated at a very close proximity inside the needle. In [118], a custom needle with a metallic septum between the two fibers for acoustic isolation was used. The combined \"PA + LU + SoS\" setup, by [PERSON] et al. [57], employed optical detection of ultrasonic waves. This work expanded on the authors' earlier setup [81; 103] that combined PA and pulse-echo LU imaging. The same laser pulse was used for illuminating the object in PA mode, the PEs in SoS mode, and the PA generator in LU mode. The same laser beam was used for ultrasound detection, making the setup all-optical. The PA signals from the object, LU probe pulse, LU echo signals, and LU transmission signals all arrived at different times, allowing their separation. This non-overlapping condition limited the maximum diameter of the sample object. Focusing into the 2D plane was achieved by a concave cylindrical acoustic mirror coated with 100 nm thick Cr-layer, used as a PA generator for pulse-echo LU imaging. The spatial resolution of the optical detector is limited by the beam waist (\(\sim\)35 \(\upmu\)m). The overlaid \"PA + LU\" images of transparent zebrafish embedded in gelatin and \"PA + LU + SoS\" images of phantoms were presented. [PERSON] et al. [58] reported a hand-held combined real-time 3D \"PA + LU\" imaging system. The authors earlier [43] presented a custom-made sensor array (256 3 \(\times\) 3 mm piezocomposite elements) with a shape of a spherical cap covering an angle of 90\({}^{\circ}\). The curvature radius was 40 mm. On the axis of the array, the fiber bundle for illumination was embedded. On the same axis a carbon microsphere (PE) \(\sim\)400 \(\upmu\)m in diameter was placed embedded in agar \(\sim\)1.5 cm from the focal point. Thus, the illumination was not blocked completely, and both the object and the microsphere were illuminated simultaneously. Back-projection algorithm, implemented on a GPU, was used for real-time PA and LU image reconstruction (10 volumetric frames per second). It was noted that the emitted spectrum was significantly broader than the detection bandwidth of the piezocomposite transducers, and a band-pass filter from 2 MHz to 4 MHz was applied. Authors presented the \"PA + LU\" images of phantoms and a human finger. The achieved spatial resolution in LU mode was \(\sim\)500 \(\upmu\)m, due to the size of the PE, and a smaller PE could not be used due to image contrast deterioration. Later, in [119], the authors presented a hemispherical array with 510 broadband piezocomposite elements, 1 mm in diameter and with a curvature radius of 15 mm.