Image guided photothermal therapy
Photothermal therapy involves the use to continuous wave lasers and targeted photoabsorbers to carry out minimally invasive cancer treatment. However, the tumor size and the presence of photoabsorbers need to be identified before therapy, the temperature rise needs to be monitored during therapy and finally tumor needs to be evaluated for necrosis after therapy. Combined ultrasound and photoacoustic imaging can be used to guide and monitor photothermal therapy.
(a) Ultrasound image of porcine muscle tissue. (b) Photoacoustic image displaying the change in optical contrast. All images cover a 20 mm by 20 mm field of view.
Photoacoustic imaging can detect the presence of exogenous photoabsorbers in the tumor due to enhanced optical contrast. Ultrasound imaging can estimate the temperature increase during therapy by monitoring the thermally induced differential speckle motion. In addition, photoacoustic signal amplitude depends on temperature. Photoacoustic imaging can thus also be used to perform thermal imaging.
A continuous wave therapeutic laser and metal nanoparticles were used to carry out photothermal therapy on ex vivo porcine tissue. An ultrasound imaging system was interfaced with a tunable pulsed laser for imaging during the procedure.
The ultrasound and photoacoustic images in Fig. 1 cover a 20 mm by 20 mm region of a tissue sample injected with gold nanoparticles. As expected, the photoabsorbers are not visible in the ultrasound image (Fig. 1a). The spatial location of the nanoparticles is visible in the photoacoustic image (Fig. 1b). Thus, ultrasound imaging displays the structure of the tissue while photoacoustic imaging identifies the changes in optical contrast (Fig. 1).
Before performing photothermal therapy, the tissue samples were calibrated to measure temperature. For a 10°C change in temperature, the photoacoustic signal increased by close to 42%. For ultrasound thermal imaging, the apparent time shifts in ultrasound RF signals were converted to strain. The measured strain for the same temperature change was 0.8%.
The ultrasound image before therapy and thermal images computed during therapy are displayed in Fig. 2. The thermal images (Fig. 2 b-d) after 1, 2 and 4 minutes of photothermal therapy show the gradual increase in temperature. The temperature at the injection site reaches an elevation of more than 8°C while the surrounding temperature is less than 2°C.
(a) Ultrasound image of porcine tissue before therapy. Thermal images after (b) 60, (c) 120 and (d) 240 seconds of therapy. All images cover a 20 mm by 20 mm field of view.
Thermal imaging can be performed by both ultrasound and photoacoustic imaging. Furthermore, the thermally damaged tissue has different mechanical properties compared to normal and cancerous tissue; elasticity imaging can be employed to identify the size and location of thermal lesion.
(a) Ultrasound image of PVA phantom before photothermal therapy. (b) Thermal image after 3 minutes showing temperature increase in the inclusion. (c) Strain image displaying the harder inclusion having lower strain .All images cover a 20 mm by 20 mm field of view.
Thus a combination of ultrasound, photoacoustic and elasticity imaging has the potential to provide anatomical, functional and mechanical information to guide and monitor photothermal cancer therapy.
Further Readings:
[1] J. Shah, S. R. Aglyamov, K. Sokolov, T. E. Milner, and S. Y. Emelianov, "Ultrasound-based thermal and elasticity imaging to assist photothermal cancer therapy - preliminary study," Proceedings of the 2006 IEEE Ultrasonics Symposium, pp. 1029-1032, (2006). PDF
[2] S. Y. Emelianov, S. R. Aglyamov, A. B. Karpiouk, S. Mallidi, S. Park, S. Sethuraman, J. Shah, R. W. Smalling, J. M. Rubin, and W. G. Scott, "Synergy and applications of combined ultrasound, elasticity, and photoacoustic imaging," Proceedings of the 2006 IEEE Ultrasonics Symposium, pp. 405-415, (2006). PDF
[3] J. Shah, S. R. Aglyamov, K. Sokolov and S. Y. Emelianov, “Photoacoustic imaging to assist photothermal therapy,” Abstract, 2007 Biomedical Engineering Society Annual Fall Meeting, (2007).
[4] J. Shah, S.R. Aglyamov, K. Sokolov, T. Milner, S.Y. Emelianov, “Temperature monitoring during photothermal therapy using ultrasound-based thermal imaging,” Abstract of the 24nd Annual Houston Conference on Biomedical Engineering Research, The Houston Society for Engineering in Medicine and Biology, 8-9 February 2007, Houston, TX, 91 (2007)
[5] J. Shah, J. Mendeloff, S.Y. Emelianov, "Temperature monitoring using ultrasound and photoacoustic imaging during laser therapy," Abstract of the 25th Annual Houston Conference on Biomedical Engineering Research, The Houston Society for Engineering in Medicine and Biology, 7-8 February 2008, Houston, TX, p. 53 (2008) PDF
[6] J. Shah, S.R. Aglyamov, K. Sokolov, T.E. Milner, and S.Y. Emelianov, "Ultrasound imaging to monitor photothermal therapy - feasibility study," Optics Express, vol. 16, pp. 3776-3785 (2008) PDF
[7] J. Shah, S. Park, S.R. Aglyamov, T. Larson, L. Ma, K. Sokolov, K. Johnston, T. Milner and S.Y. Emelianov, "Photoacoustic imaging and temperature measurement for photothermal cancer therapy," accepted for publication in Journal of Biomedical Optics (2008)
[8] J. Shah, S. Park, S. Aglyamov and S. Emelianov, “Role of Photoacoustic and Ultrasound Imaging in Photothermal Therapy,” in Photoacoustic imaging and spectroscopy, L.V. Wang, editor, Taylor & Francis Group/CRC press (in press) 2008.
