Feasibility study of multispectral photoacoustic coded excitation using orthogonal unipolar Golay codes

Photoacoustic (PA) imaging is a new imaging modality based on the generation of ultrasound in biological tissue by laser irradiation. It would be desirable to use laser diodes as excitation sources for PA since they are less expensive and more compact than commonly used Nd:YAG lasers. Laser diodes exhibit a higher pulse repetition frequency (PRF) and a lower pulse energy compared to Nd:YAG lasers. In order to increase the SNR of PA imaging using laser diodes, we propose multispectral photoacoustic coded excitation (MS-PACE) based on orthogonal unipolar Golay codes (OUGC). This concept allows acquiring photoacoustic data generated by two laser diodes at two different wavelengths simultaneously and separating them afterwards. The SNR gain and coding gain compared to time equivalent averaging is analytically derived. Furthermore, the feasibility of the concept is experimentally verified. Two laser diodes, emitting light at 850 nm and 905 nm, are triggered according the OUGC scheme. They irradiate a metal slab. The resulting PA signals are acquired using a reconfigured clinical ultrasound system with an 128 element linear array. The data are decoded and images are formed using a delay-and-sum algorithm. Using an 8 bit sequence the SNR is increased by 8.68 dB (905 nm) and 8.87 dB (850 nm). These results are in good agreement with the theoretical SNR gain of 9.03 dB. The resulting images allow a clear discrimination of both wavelengths, demonstrating the feasibility of MS-PACE using OUGC.