Multispectral photoacoustic coded excitation using pseudorandom codes

Abstract

Photoacoustic imaging (PAI) combines high ultrasound resolution with optical contrast. Laser-generated ultrasound is potentially beneficial for cancer detection, blood oxygenation imaging, and molecular imaging. PAI is generally performed using solid state Nd:YAG lasers in combination with optical parametric oscillators. An alternative approach uses laser diodes with higher pulse repetition rates but lower power. Thus, improvement in signal-to-noise ratio (SNR) is a key step towards applying laser diodes in PAI. To receive equivalent image quality using laser diodes as with Nd:YAG lasers, the lower power must be compensated by averaging, which can be enhanced through coded excitation. In principle, perfect binary sequences such as orthogonal Golay codes can be used for this purpose when acquiring data at multiple wavelengths. On the other hand it was shown for a single wavelength that sidelobes can remain invisible even if imperfect sequences are used. Moreover, SNR can be further improved by using an imperfect sequence compared to Golay codes. Here, we show that pseudorandom sequences are a good choice for multispectral photoacoustic coded excitation (MSPACE). Pseudorandom sequences based upon maximal length shift register sequences (m-sequences) are introduced and analyzed for the purpose of use in MSPACE. Their gain in SNR exceeds that of orthogonal Golay codes for finite code lengths. Artefacts are introduced, but may remain invisible depending on SNR and code length.