Photoacoustic signal generation with Golay coded excitation

Photoacoustic (PA) imaging has the potential to image soft tissue with high contrast and high spatial resolution. Conventionally, a Q-switched Nd:YAG laser providing ns pulse duration and mJ pulse energy is suitable for PA applications. However, such a laser is typically bulky and expensive. On the other hand, a small, relative low-cost diode laser is potentially useful to achieve PA imaging as it provides PRF up to kHz but the pulse energy of such a laser is generally too low for effective PA generation. In this study, we proposed Golay coded excitation using a diode laser to generate PA signal. A high frequency 20-MHz PA transducer integrated an optical fiber was proposed to enable backward mode ultrasound and PA detection. The signal-to-noise ratio (SNR) of the generated PA signals of different Golay code lengths (i.e., 2, 4, 16 and 256 bit) and pulse durations (i.e., 25, 50, 100 and 200 ns) were evaluated. Results show that the SNR increased with the code length and achieved from 30.8 to 37.0 dB of 256-bit code length with different pulse durations. In addition, the SNR improvement was also shown. Results indicate that the highest SNR improvement is 23.5 dB. Finally, an ultrasound/PA fusion image of an agar phantom was shown.