Experimental evaluation of photoacoustic coded excitation using unipolar golay codes

Abstract

Q-switched Nd:YAG lasers are commonly used as light sources for photoacoustic imaging. However, laser diodes are attractive as an alternative to Nd:YAG lasers because they are less expensive and more compact. Although laser diodes deliver about three orders of magnitude less light pulse energy than Nd:YAG lasers (tens of microjoules compared with tens of millijoules), their pulse repetition frequency (PRF) is four to five orders of magnitude higher (up to 1 MHz compared with tens of hertz); this enables the use of averaging to improve SNR without compromising the image acquisition rate. In photoacoustic imaging, the PRF is limited by the maximum acoustic time-of-flight. This limit can be overcome by using coded excitation schemes in which the coding eliminates ambiguities between echoes induced by subsequent pulses. To evaluate the benefits of photoacoustic coded excitation (PACE), the performance of unipolar Golay codes is investigated analytically and validated experimentally. PACE imaging of a copper slab using laser diodes at a PRF of 1 MHz and a modified clinical ultrasound scanner is successfully demonstrated. Considering laser safety regulations and taking into account a comparison between a laser diode system and Nd:YAG systems with respect to SNR, we conclude that PACE is feasible for small animal imaging.

Coded excitation scheme for acoustic radiation push pulse compression

“…However, an ARF pushes only; it does not pull. Therefore, a bipolar sequence is divided into a positive sequence {a p [n]} and a negative sequence {a n [n]} as in unipolar Golay codes [3].…”

Real-time Processing of Coded Photoacoustic Signals

“…Golay codes were analyzed in detail in [6]. Using Golay codes, it is possible to achieve perfect reconstruction of the original image except for small deviations e. g. due to jitter.…”

“…UGC G is also referred to as coding gain [6]. Golay codes permit perfect reconstruction of the original sample, but for a positive coding gain relatively long codes A and B are necessary.…”

“…The decoding process performed after acquisition of the samples will permit the reconstruction of the original image up to small errors depending on the code that was used even though the PRF might exceed the fundamental limitation. So far, various PACE code sequences have been thoroughly analysed [3][4][5][6][7][8], but none of them was set up for realtime array imaging. In this work, we present a system setup utilizing three different classes of PACE code sequences: perfectly reconstructing codes based on four transmitted sequences (1), perfectly reconstructing periodic codes based on three repetitions of a difference set code (2), and imperfectly reconstructing codes based on two transmitted sequences (3).…”

Multispectral photoacoustic coded excitation with low PRF high power laser diodes

“…While many groups focus on solid state laser systems as light sources, the flexibility, price and size of smaller light sources such as laser diodes or LED renders these sources an attractive alternative [4], [5]. The lower power that these devices offer still remains a problem and must be compensated by averaging or photoacoustic coded excitation (PACE) [6], even though some research is directed at increasing the output laser power as well. While solid state laser systems typically can be equipped with an optical parametric oscillator to achieve a tunable wavelength or are inherently tunable, laser diodes operate at fixed wavelengths.…”