Biomedical ultrasound imaging has been widely used as an imaging technology based on ultrasound signals for viewing the internal structure of the body and finding the source of diseases. In recent decades, owing to the development of ultrasonic transducers, ultrasound imaging has made significant progress in obtaining important diagnostic information using rapid and noninvasive methods. Traditional transducers are excited by electricity and take advantage of the piezoelectric effect to achieve a transformation between electricity and ultrasound. However, considering the demanding requirements of application environments, the primary restriction is the limited bandwidth of traditional transducers.
Laser-generated ultrasound, a novel technology based on photoacoustic effect, is excited by a laser instead of electricity. Ultrasound pulses are generated by the absorption of a short-pulse laser, thus leading to elastic thermal expansion caused by the transient temperature increase. In this process, the time-varying laser acts as the only excitation source. The upper limits of the energy and frequency of the ultrasound are restricted by the laser. Compared with piezoelectric transducers, the ultrasound generated by laser-generated ultrasound transducers has the characteristics of high frequency and large bandwidth, which are necessary for sensing and imaging.
With the breakthrough of laser-generated ultrasound transducers in the structural simplification and excitation of large-bandwidth ultrasound, laser-generated ultrasound technology has been gradually applied in various fields where traditional piezoelectric ultrasound methods cannot be applied, essentially providing a novel idea for high-precision and high-resolution biomedical applications.
The amplitude of ultrasound produced by laser-generated ultrasound technology is related to various characteristics, such as laser energy, transducer absorbance, thermal expansion coefficient, and shape. Moderately high-energy laser, highly absorbing nano-scale light absorbers, and expanders with high thermal expansion coefficients positively affect the ultrasound amplitude generated by laser-generated ultrasound transducers. Meanwhile, the ultrasound frequency domain generated by photoconductive ultrasound technology is related to parameters such as the excitation light pulse width, transducer material, and transducer thickness (Figs. 2?4). For example, under test conditions in which the imaging depth is small but the imaging resolution is very high, an ultra-narrow pulse width laser with a nanoscale metal layer can be used as an optical ultrasound transducer (Table 1 and Fig. 2). If the test environment has high requirements for imaging depth and imaging speed but low requirements for imaging resolution, a common nanosecond transducer is suitable. If the test environment has high requirements for imaging depth and speed and low requirements for resolution imaging, a common nanosecond-pulsed laser with the carbon-based polymer material is suitable as a solution for ultrasound.
Moreover, the less complicated structure of the laser-generated ultrasound transducer promises a large amplitude of the ultrasound at the focal point, with a self-focusing effect when using a concave transducer (Figs. 1 and 5). Furthermore, the ultrasound generated by a laser-generated ultrasound transducer has a high frequency and large bandwidth, thereby contributing to a smaller sound field at the focal point (Fig. 6).
This study summarizes the mechanism of action, transducer system, performance characterization, and application areas of phototransduction ultrasound technology, as well as the applications of concave transducers in neural stimulation, ultrasonic cavitation, and ultrasound imaging, and describes the advantages and disadvantages of piezoelectric-based and photoacoustic effect-based transducers by comparing them with conventional ultrasound transducers. With the continuous development of theoretical systems of laser-generated ultrasound and precision processing technology, the advancement of laser-generated ultrasound technology has led to new opportunities for the development of biomedical ultrasound.
