Monospectral photoacoustic imaging using Legendre sequences

Photoacoustic (PA) imaging is an imaging modality based on the generation of ultrasound using laser irradiation. Pulsed laser diodes are an attractive alternative to Q-switched Nd:YAG lasers since they are cheaper and handier. As acoustic time-of-flight limits the pulse repetition frequency (PRF) for averaging, photoacoustic coded excitation (PACE) can be used to enhance the diodes' low signal to noise ratio (SNR). Strategies exhibiting range side lobes can yield higher SNR than previously proposed perfect methods while using simpler code sending procedures. Here, we examine the performance of Legendre sequences (LGS) for PACE. The gain in SNR compared to time equivalent averaging (coding gain) is derived as a function of code length and verified experimentally. The main lobe to peak side lobe ratio (MPSR) of the codes' autocorrelation functions is used to quantify the artifacts introduced by the codes' range side lobes. The coding gain is asymptotically equal to that of previously proposed methods, such as Golay codes (GC) or Simplex codes (SC). For finite code sending time, it exceeds the gain of GC and SC. For a PRF of 500 kHz and an imaging depth of 4.5 cm, the coding gain ranges from about 2 dB (11 bit sequence) to about 5.8 dB (547 bit sequence). Range side lobes are introduced but remain invisible for sufficiently large code lengths, which are necessary for practical applications.