ISEP - DM – Engenharia Biomédica
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- Transmissão de feixes laser de intensidade modulada em tecidos biológicos e a sua optimizaçãoPublication . RODRIGUES, GONÇALO OLIVEIRA; Oliveira, Luís Manuel Couto deThis work focuses on the study of the transmission of intensity-modulated laser beams in biological tissues, with an emphasis on skeletal muscle tissue, and on how the application of optical clearing treatments can improve such transmission. This work arises in the context of the growing interest in non-invasive optical techniques in the medical field, with safer and more effective alternatives to methods that use ionizing radiation, such as computed tomography. Biophotonics, a research field that combines principles of physics, optics, and engineering to develop innovative medical applications, has enabled significant advances in diagnostic and therapeutic techniques based on light. In the case of lasers, they offer clear advantages such as emission at specific wavelengths, collimated beams, high coherence, and the possibility of intensity modulation or pulsed mode emission. These attributes make lasers powerful tools for both diagnostic and treatment procedures, such as photocoagulation and tumor ablation in tissues. However, due to the absorption and scattering properties that are characteristic of tissues, laser light loses intensity and beam collimation with increasing light-probing depth in the tissues. In the vast majority of biological tissues, scattering is always greater than absorption, and this difference is even greater at shorter wavelengths. Such strong scattering is largely due to the heterogeneous composition of tissues and the localized refractive index mismatch. The optical clearing technique emerges as a solution to minimize light scattering and increase tissue transparency, through the partial or total replacement of interstitial and cellular water with one or more clearing agents, which promotes refractive index matching, making the tissue optically more homogeneous and, consequently, more transparent. In this study, four optical clearing agents were used: sucrose, glycerol, electronic cigarette fluid, and tartrazine. Each of these agents was tested for its ability to create temporary transparency in samples of rabbit skeletal muscle, with the aim of evaluating the improvement in the transmission of intensity-modulated laser beams, with wavelengths between 400 and 900 nm. The experimental methodology used in this study consisted of two experimental setups for measuring transmittance. The first of these setups was used to measure spectra between 200 and 1000 nm, which allowed the calculation of the main spectral optical properties of muscle tissue, as well as the absorption spectra of the solutions of optical clearing agents used. The spectral optical properties obtained were the absorption coefficient and the anomalous dispersion of the muscle and the solutions of optical clearing agents , as well as the scattering coefficient of the muscle. The second experimental setup was used to measure the transmittance of intensity-modulated laser beams in muscle tissue, before and after clearing, in order to quantify that transparency at each laser wavelength used. The lasers used with this experimental setup had wavelengths between 400 and 900 nm, and the intensity modulation was performed with an optical chopper, being the transmitted beam detected using an oscilloscope. By measuring the spectra of the tissue and the optical clearing agents solutions, it was possible to calculate their spectral optical properties, which were then used to explain the transparencies observed in muscle after each treatment. The results obtained revealed that all the clearing agents used produced visible transparency effects, but with different efficiencies. The maximum increase in muscle transparency was created by tartrazine, with an intermediate increase observed for treatments with the electronic cigarette fluid and glycerol, and a lower transparency increase obtained for the treatment with sucrose. In the treatment with tartrazine, no transparency was observed for lasers emitting at 405 and 532 nm. This observation is due to the strong absorption that tartrazine presents in this spectral range. From an experimental point of view, it was possible to quantify the optical clearing efficiency created by each agent, allowing a comparative analysis between the different transparency agents and the identification of relevant spectral transparency windows for clinical applications. Changes in anomalous dispersion profiles were also observed, as a result of the structural reorganization of the tissues and interactions with the clearing agents. This study provides important information for the development of innovative and less invasive clinical methodologies, through the use of intensity-modulated laser beams when combined with optical clearing treatments, which will allow the optimization of diagnostic and treatment procedures. In short, this work highlights the fundamental role of the transparency effects in biological tissues to allow higher transmission of intensity-modulated laser beams to bigger tissue depths, paving the way for safer, faster, and more effective applications in modern clinical practice.