2012 Proceedings - International Tissue Elasticity Conference

054 EXPERIMENTAL EVALUATION OF SIMULTANEOUS MULTISPECTRAL CODED EXCITATION
FOR PHOTOACOUSTIC IMAGING.
Haichong Zhang 1 , Kengo Kondo 1 , Makoto Yamakawa 2 , Tsuyoshi Shiina 1 .
1 Graduate School of Medicine, 2 Advanced Biomedical Engineering Research Unit, Kyoto
University, Kawahara–cho, Shogoin, Kyoto 606–8507, JAPAN.
Background: Research on tissue elasticity using ultrasound has been done in many centers for many
applications. Photoacoustic imaging, which is expected to be combined with ultrasonic imaging, can
expand the capacity of ultrasound by showing functional imaging of tissue [1].
Aims: The aim of this study is to acquire multispectral information for functional images with a high
signal–to–noise ratio (SNR) and a high frame rate by using coded excitation.
Methods: We propose coded excitation using m–sequences and related sequences that have good
autocorrelation and cross–correlation properties. In this study, we demonstrate the feasibility through
experimental evaluation. Preferred pairs of m–sequences and related sequences such as Gold codes are
binary sequences having good cross–correlation. Thus, we investigated the coded excitation method to
separate multispectral components, which are irradiated with coded laser pulses from different laser
sources, into separate spectral components. The performance increases as the pulse repetition frequency
(PRF) and code length increase. Theoretical consideration in previous research using Gold codes has
showed a clear SNR improvement compared with averaging [2]. Here, we conducted an experiment to
verify the separation of spectral components through two–wave simultaneous sending. We compared the
separated signals with the signals acquired by single–wavelength irradiation using the same codes to
examine the reproducibility of separation. Two laser modules (532nm Nd: YVO4 and 700nm OPO–YAG)
were used, and the data were captured by a 9mm hydrophone. Two strings (red and green) and a black
rubber wire were used as the absorber to contrast wavelength dependency (Figure 1a).
Results: As a result, the separated signals were almost equivalent to the signals acquired by a
single–wavelength irradiation. For instance, the SNR improvement of separated signals of 532nm over
non–coded signals was 5.00dB (Figure 1d), and the improvement of single–wavelength signals after
63 averages was 5.57dB (Figure 1b) when the PRF was 10kHz and a 63–bit preferred pair of m–sequence was
used. The PRF in this experiment was 10kHz because of the performance limitation of our laser system, but a
faster laser module will allow faster PRF. At the same time, it was also confirmed that the proposed method
can reduce the sending time cost for multispectral information. The 11.69dB improvement of SNR compared
with averaging was achieved by simulation under the conditions that four 1023–bit Gold codes were used.
Conclusions: The experiment successfully validated the feasibility of simultaneous multispectral coded
excitation. The advantage over the previous study is that only one sequence is needed to be irradiated
compared with orthogonal Golay codes [3] that must be as many sequences as the number of
components. The proposed method can provide a huge impact since the demand for multispectral
photoacoustic imaging is expected to increase more and more.
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Figure 1: (a) Experimental setup; (b) 532nm laser single irradiation result; (c) 700nm laser single irradiation result;
Two wavelengths simultaneous irradiation results separated into (d) 532nm and (e) 700nm.
Acknowledgements: This work was supported by a Grant–in–Aid for Scientific Research A (No. 22240063) from the
Japan Society for the Promotion of Science.
References:
[1] L. V. Wang: Medical Physics, Vol. 35, pp. 5758–5767, 2008.
[2] H. Zhang , K. Kondo, M. Yamakawa, T. Tsuyoshi: Jpn Journal of Applied Physics, Vol. 51, No. 7, p. 07GF03, 2012.
[3] M. P. Mienkina, C. S. Friedrich, N. C. Gerhardt, M. F. Beckmann, M. F. Schiffner, M. R. Hofmann, and G.
Schmitz: Optics Express, Vol. 18, p. 9076, 2010.
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