Multispectral photoacoustic coded excitation using orthogonal unipolar Golay codes

Abstract

In photoacoustics (PA) ultrasound is generated by laser irradiation of tissue. Commonly, Nd:YAG lasers are used as PA sources but it would be desirable to use less expensive laser diodes instead, which can be used for multispectral applications. Laser diodes combine low pulse energy with high pulse repetition frequency (PRF) compared to Nd:YAG lasers, averaging procedures can be employed to increase the SNR. In order to prevent range ambiguities the PRF of the laser diodes must be limited to the inverse of the acoustical propagation time. We propose to use coded excitation for pulsed PA imaging to increase the PRF beyond this limit, leading to higher SNR. In particular, by using orthogonal unipolar Golay codes (OUGC) it would be possible to acquire photoacoustic data generated by two laser diodes at two different wavelengths simultaneously and separate them afterwards. Thus, we derive an estimation of the SNR gain and coding gain compared to averaging for multispectral photoacoustic coded excitation (MS-PACE) using OUGC. The SNR gains range from 6.0dB to 30.1dB for code lengths between 4bit and 1024bit, respectively. A coding gain of 9.8dB, compared to a time equivalent averaging procedure, can be achieved for realistic parameters (absorber depth: 3cm, code length: 512bit, laser PRF: 1MHz). In a second step, we evaluate the feasibility of MS-PACE by simulations based on experimental data. Using a reconfigured clinical ultrasound system with a 128 element linear array, we acquire single channel data of photoacoustic signals generated by irradiating a metal slab with pulsed laser diodes emitting light at 850nm and 905nm. The acquired data are rearranged according to the OUGC sequences and decoded. The resulting data are used for image reconstruction employing synthetic aperture focusing techniques. These images allow a clear discrimination of both wavelengths, suggesting the feasibility of MS-PACE using OUGC. © 2009 Springer-Verlag.