Evaluation of Simplex Codes for Photoacoustic Coded Excitation

Abstract

In biomedical photoacoustics (PA) ultrasound is generated by laser irradiation of tissue. Usually, PA signals are generated by Nd:YAG lasers, but it would be attractive to use less expensive laser diodes instead. Although laser diodes exhibit low pulse energy, the pulse repetition frequency (PRF) of pulsed laser diodes is much higher than the PRF of Nd:YAG lasers thus averaging can be used to increase the SNR. The applied PRF for averaging is limited by the acoustical time-offlight. To further increase the SNR we propose to use coded excitation for pulsed PA imaging. In optical time-domain reflectometry, which is similar to PA imaging, Simplex codes are successfully used for coding. Therefore, we evaluate their performance for photoacoustic coded excitation by a simulation study based on experimental data.For Simplex coding a unipolar matrix S is derived from a normalized Hadamard matrix. Each row of S represents one temporal coding sequence of the laser diode excitation. The received acoustic signals are arranged to one matrix, multiplied with the inverted S-matrix and averaged. First, experiments are conducted to provide the PA impulse response of the system under investigation. A spherical absorber made of black plastic is irradiated by a pulsed laser diode. The resulting ultrasound waves are received by an ultrasound transducer, amplified and sampled by an A/D-card. To be able to flexibly vary the PRF over a broad range, the acoustic response to a coded laser excitation is simulated by the superposition of independent experiments according to the coding scheme.For a PRF of 350 kHz all matrices from 3 bit to 59 bit exhibit a positive coding gain compared to averaging ranging from 1.15 dB to 5.31 dB. Thus, since state-of-the-art laser diode drivers for high power laser diodes exhibit PRFs up to 1 MHz, improving SNR by coding is feasible.Keywordsphotoacousticscoded excitationsimplex codespulsed laser diodes