Simulation study of photoacoustic coded excitation using Golay Codes

Photoacoustics (PA) is a new imaging modality based on the generation of ultrasound due to laser irradiation. Instead of commonly used Q-switched Nd:YAG lasers the application of pulsed laser diodes as light sources for photoacoustic imaging is proposed. The high pulse repetition frequency of laser diodes enables the usage of coded excitation schemes for SNR improvement. Therefore, the performance of photoacoustic coded excitation (PACE) using Golay Codes is evaluated by a simulation study based on experimental data. Golay Codes consist of 2 bipolar, complementary code sequences. For PACE the codes must be split into 4 unipolar sequences and processed further. First, experiments are conducted to provide the PA impulse response of the system under investigation by irradiating a spherical absorber by a pulsed laser diode and acquiring the acoustic response. Then, based on these measurements, the acoustic response to a coded laser excitation is simulated by the superposition of PA impulse responses according to the coding scheme. For each code length the acquisition duration is computed, the equivalent number of possible averages is determined, and the coding gain compared to averaging is calculated. The SNR gain of the Golay Codes with respect to a single impulse response ranges from 5.87 dB for a 4 bit sequence to 26.97 dB for a 512 bit sequence. No range sidelobes are detectable and the acoustic response of the optical absorber is equivalent to the signal obtained from averaging. Assuming an acquisition depth of 9 cm, the comparison of the coding schemes with equivalent averaging procedures shows that for coding PRFs lower than 60 kHz Golay coding exhibits a lower SNR than averaging for sequences up to 512 bit. For a PRF of 250 kHz and a coding length of 512 bit the coding gain rises to 6.20 dB. Additionally, the coding gain is theoretically derived and is in good agreement with the simulations. Since state of the art high power laser diode drivers exhibit PRF up to 1 MHz, using coded excitation for photoacoustic imaging can improve the SNR in future set-ups.