Optical imaging and photothermal therapy have been applied in biomedical field for decades. However, the strong scattering of light in biological tissue hinders the focal light delivery and thus restricts their clinical applications because of the resultant limited penetration. We hypothesize that the photon scattering is reduced in the cylindrical heating zone of high intensity focused ultrasound (HIFU) and thus the efficiency of light delivery can be improved via transmission of light through the heating cylindrical tunnel, enabling photoacoustic signal enhancement at the targeted region. In this study, Monte Carlo simulation and intralipid-phantom experiments were used to verify our hypothesis. The thermal effect could increase the laser fluence at the targeted region by at least 10% no matter in the simulation or the experiment. Similar results were also presented in the measured photoacoustic signal. Note that special care had been taken to keep the Gruneisen coefficient at the targeted region constant so that the photoacoustic signal change solely depended on delivered laser fluence. In addition, the simulation results indicate that with the local cylindrical heating tunnel, the fluence at the targeted region is at least 10% higher than that with global heating, suggesting that HIFU heating tissue tunnel owns the potential in enhancing the light delivery efficiency, the light penetration and thus the photoacoustic signal at the targeted region as well. It is expected that our finding is not only applicable to photoacoustic imaging but also photothermal therapy which also requires more focal light delivery.
Zong Han Hsieh, Chih-Kuang Yeh, and Meng-Lin Li, "Improved light delivery in biological tissue using HIFU heating tunnel: photoacoustic verification (Conference Presentation)," Proc. SPIE 10878, Photons Plus Ultrasound: Imaging and Sensing 2019, 108782P (Presented at SPIE BiOS: February 06, 2019; Published: 4 March 2019); https://doi.org/10.1117/12.2507771.6008553307001.
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