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Influence of laser pulse width to the photoacoustic temporal waveform and the image resolution with a solid-state excitation laser

Properties of excitation laser are the important parameters that affect the photoacoustic image quality. As for the pulse width, it is closely related to signal strength and image resolution, which reported as a result of an experiment using a laser diode that can control the pulse width easily1. However, though a solid-state laser is promising for a medical application due to its high pulse energy creating high photo acoustic signal, its influence on waveform or the image quality has not been discussed in detail because the pulse width is hardly changeable in a solid-state laser. We use two kinds of solid-state lasers, i.e., Q-switched Nd:YAG and Ti-Sapphire Laser, in this study and generate different pulse width between 4.5 and 45 ns by changing wavelength and excitation energy. These laser pulses are entered into a silicon tube composed of carbon-particle suspension as absorber whose wavelength dependence for absorption is small. We detect the generated laser-induced photoacoustic waves by hydrophone. The photoacoustic temporal waveform shows sharper as the pulse width is shorter, which also indicates high frequency signal components increase. The width of the first peak on the temporal waveform is corresponding to the pulse width. Additionally, as a result of the photoacoustic imaging experiment preformed with 192-channel PZT linear array probe to image a thin wire, the modulation transfer function shows that the narrower the pulse width, the slightly better the image resolution.