The aims of this study are: (a) to investigate the capability of photoacoustic (PA) method in detecting high-intensity focused ultrasound (HIFU) treatments in muscle tissues in vitro; and (b) to determine the optical properties of HIFU-treated and native tissues in order to assist in the interpretation of the observed contrast in PA detection of HIFU treatments.
A single-element, spherically concaved HIFU transducer with a centre frequency of 1 MHz was utilized to create thermal lesions in chicken breast tissues in vitro. To investigate the detectability of HIFU treatments photoacoustically, PA detection was performed at 720 and 845 nm on seven HIFU-treated tissue samples. Within each tissue sample, PA signals were acquired from 22 locations equally divided between two regions of interest within two volumes in tissue – a HIFU-treated volume and an untreated volume. Optical spectroscopy was then carried out on 10 HIFU-treated chicken breast specimens in the wavelength range of 500–900 nm, in 1-nm increments, using a spectrophotometer with an integrating sphere attachment. The authors’ optical spectroscopy raw data (total transmittance and diffuse reflectance) were used to obtain the optical absorption and reduced scattering coefficients of HIFU-induced thermal lesions and native tissues by employing the inverse adding-doubling method. The aforementioned interaction coefficients were subsequently used to calculate the effective attenuation coefficient and light penetration depth of HIFU-treated and native tissues in the wavelength range of 500–900 nm.
HIFU-treated tissues produced greater PA signals than native tissues at 720 and 845 nm. At 720 nm, the averaged ratio of the peak-to-peak PA signal amplitude of HIFU-treated tissue to that of native tissue was 3.68 ± 0.25 (mean ± standard error of the mean). At 845 nm, the averaged ratio of the peak-to-peak PA signal amplitude of HIFU-treated tissue to that of native tissue was 3.75 ± 0.26 (mean ± standard error of the mean). The authors’ spectroscopic investigation has shown that HIFU-treated tissues have a greater optical absorption and reduced scattering coefficients than native tissues in the wavelength range of 500–900 nm. In fact, at 720 and 845 nm, the ratio of the optical absorption coefficient of HIFU-treated tissues to that of native tissues was 1.13 and 1.17, respectively; on the other hand, the ratio of the reduced scattering coefficient of HIFU-treated tissues to that of native tissues was 13.22 and 14.67 at 720 and 845 nm, respectively. Consequently, HIFU-treated tissues have a higher effective attenuation coefficient and a lower light penetration depth than native tissues in the wavelength range 500–900 nm.
Using a PA approach, HIFU-treated tissues interrogated at 720 and 845 nm optical wavelengths can be differentiated from untreated tissues. Based on the authors’ spectroscopic investigation, the authors conclude that the observed PA contrast between HIFU-induced thermal lesions and untreated tissue is due, in part, to the increase in the optical absorption coefficient, the reduced scattering coefficient and, therefore, the deposited laser energy fluence in HIFU-treated tissues.
The authors would like to thank Dr. Alexandre Douplik for fruitful discussions during this project. Arthur Worthington, Aditya Pandya, Martin Hohmann, and Eno Hysi are gratefully acknowledged for assistance and technical support. This work was partially supported by the Ontario Research Fund-Research Excellence (ORF-RE) grant and the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery grant) that were awarded to Dr. J. Tavakkoli and Dr. M. C. Kolios. A Natural Sciences and Engineering Research Council of Canada Research Tools and Instrumentation (RTI) grant awarded to Dr. M. C. Kolios supported the purchase of the Shimadzu spectrophotometer used in this study.
II. MATERIALS AND METHODS
II.A. The HIFU transducer and its driving electronics
II.B. HIFU treatments
II.C. Photoacoustic detection
II.C.1. The PA system
II.C.2. Tissue sample preparation for PA detection
II.C.3. PA measurements
II.D. Optical spectroscopy
II.D.1. Tissue sample preparation for optical spectroscopy
II.D.2. Integrating-sphere measurements
II.D.3. Inverse adding-doubling method
II.E. Monte Carlo simulation of light distribution
III. RESULTS AND DISCUSSION
III.A. PA detection of HIFU-induced thermal lesions
III.B. Optical spectroscopy of HIFU-treated and native tissues
III.B.1. Diffuse reflectance and total transmittance
III.B.2. Optical absorption and reduced scattering coefficients
III.B.3. Effective attenuation coefficient and light penetration depth
III.C. Monte Carlo simulation of light distribution
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