Effect of optical energy modulation on the thermal response of biological tissue: computational and experimental validations

Transient simulation of laser ablation based on Monte Carlo light transport with dynamic optical properties model

Laser ablation is a minimally invasive therapeutic technique to denature tumors through coagulation and/or vaporization. Computational simulations of laser ablation can evaluate treatment outcomes quantitatively and provide numerical indices to determine treatment conditions, thus accelerating the technique's clinical application. These simulations involve calculations of light transport, thermal diffusion, and the extent of thermal damage. The optical properties of tissue, which govern light transport through the tissue, vary during heating, and this affects the treatment outcomes. Nevertheless, the optical properties in conventional simulations of coagulation and vaporization remain constant. Here, we propose a laser ablation simulation based on Monte Carlo light transport with a dynamic optical properties (DOP) model. The proposed simulation is validated by performing optical properties measurements and laser irradiation experiments on porcine liver tissue. The DOP model showed the replicability of the changes in tissue optical properties during heating. Furthermore, the proposed simulation estimated coagulation areas that were comparable to experimental results at low-power irradiation settings and provided more than 2.5 times higher accuracy when calculating coagulation and vaporization areas than simulations using static optical properties at high-power irradiation settings. Our results demonstrate the proposed simulation's applicability to coagulation and vaporization region calculations in tissue for retrospectively evaluating the treatment effects of laser ablation.

Simulation of thermal field distribution in biological tissue and cell culture media irradiated with infrared wavelengths

In recent years, there has been a growing interest in the singlet form of oxygen as a regulator of the physiological functions of cells. One of the ways to generate singlet oxygen is direct optical excitation of the triplet oxygen form. Since molecular oxygen weakly absorbs light, high power is required to obtain sufficient concentrations of singlet oxygen. However, the increase in the radiation power of laser can induce a local temperature increase around the laser spot. This may be critical considering the temperature governs every biological reaction within living cells, in particular. Here, the interaction of laser radiation of infrared wavelengths, generating singlet oxygen, with biological tissues and cell culture media was simulated. Using the COMSOL Multiphysics software, the thermal field distribution in the volume of skin, brain tissue and cell culture media was obtained depending on the wavelength, power and exposure time. The results demonstrate the importance of taking temperature into account when conducting experimental studies at the cellular and organismal levels.

Enhancement of gold nanorods-assisted photothermal treatment on cancer with laser power in stepwise modulation

OBJECTIVES

Photothermal therapy (PTT) is a minimally invasive or noninvasive method by destructing cancer cells through selective thermal decomposition. However, a long period of laser irradiation to achieve coagulative necrosis often causes unfavorable thermal damage to the surrounding healthy tissue. The current study aims to evaluate the feasibility of temporal power modulation to improve the treatment efficacy of gold nanorods-assisted PTT against tumor tissue.

MATERIALS AND METHODS

A total of 25 µg/ml of PEGylated gold nanorods (PEG-GNR) was used as an absorbing agent during 1064 nm laser irradiation for PTT. Temperature monitoring was conducted on the aqueous solution of PEG-GNR for dosimetry comparison. For in vivo tests, CT-26 tumor-bearing murine models with PEG-GNR injected were treated with three irradiation conditions: 3 W/cm² for 90 s, 1.5 W/cm² for 180 s, and 3 W/cm² for 60 s followed by 1.5 W/cm² for 60 s (modulated). Ten days after the treatments, histology analysis was performed to assess the extent of coagulation necrosis in the treated tissues.

RESULTS

The temporal power modulation maintained the tissue temperature of around 50°C for a longer period during the irradiation. Histology analysis confirmed that the modulated group entailed a larger coagulative necrosis area with less thermal damage to the peripheral tissue, compared to the other irradiation conditions.

CONCLUSION

Therefore, the power-modulated PTT could improve treatment efficacy with reduced injury by maintaining the constant tissue temperature. Further studies will examine the feasibility of the proposed technique in large animal models in terms of acute and chronic tissue responses and treatment margin for clinical translations.

Quantitative investigation of laser ablation based on real-time temperature variations and OCT images for laser treatment applications

BACKGROUND AND OBJECTIVE

Lasers are widely employed in clinical applications. In vivo monitoring of real-time information about different-wavelength laser surgeries would provide important surgical feedback for surgeons or clinical therapy instruments. However, the quantitative effect of laser ablation or vaporization still needs to be further explored and investigated. Here, we investigate and quantitatively evaluate the ablation variations and morphological changes of two laser ablation models: point- and sweeping-based models.

METHODS

An infrared thermal imager was used to monitor the temperature variations, and curve fitting was used to build the relationship between the laser radiation duration/sweeping speed and quantitative parameters of the ablated areas. Optical coherence tomography (OCT) images were used to visualize the inner structure and evaluate the depth of the ablated craters. Optical attenuation coefficients (OACs) were computed to characterize the normal and ablated tissues.

RESULTS

The results demonstrated that there was a good linear relationship between radiation duration and temperature variation. Similarly, a linear relationship was observed between the sweeping speed and quantitative parameters of craters or scratches (width and depth). The mean OAC of normal tissues was significantly distinguished from the mean OACs of the ablated craters or scratches.

CONCLUSION

Laser ablation was investigated based on a quantitative parameter analysis, thermal detection, and OCT imaging, and the results successfully demonstrated that there is a linear relationship between the laser parameters and quantitative parameters of the ablated tissues under the current settings. Such technology could be used to provide quantitative solutions for exploring the laser-tissue biological effect and improve the performance of medical image-guided laser ablation in the future.

Enhanced precision of real-time control photothermal therapy using cost-effective infrared sensor array and artificial neural network

Photothermal therapy (PTT) requires tight thermal dose control to achieve tumor ablation with minimal thermal injury on surrounding healthy tissues. In this study, we proposed a real-time closed-loop system for monitoring and controlling the temperature of PTT using a non-contact infrared thermal sensor array and an artificial neural network (ANN) to induce a predetermined area of thermal damage on the tissue. A cost-effective infrared thermal sensor array was used to monitor the temperature development for feedback control during the treatment. The measured and predicted temperatures were used as inputs of fuzzy control logic controllers that were implemented on an embedded platform (Jetson Nano) for real-time thermal control. Three treatment groups (continuous wave = CW, conventional fuzzy logic = C-Fuzzy, and ANN-based predictive fuzzy logic = P-Fuzzy) were examined and compared to investigate the laser heating performance and collect temperature data for ANN model training. The ex vivo experiments validated the efficiency of fuzzy control with temperature method on maintaining the constant interstitial tissue temperature (80 ± 1.4 ^°C) at a targeted surface of the tissue. The linear relationship between coagulation areas and the treatment time was indicated in this study, with the averaged coagulation rate of 0.0196 cm²/s. A thermal damage area of 1.32 cm² (diameter ∼1.3 cm) was observed under P-Fuzzy condition for 200 s, which covered the predetermined thermal damage area (diameter ∼1 cm). The integration of real-time feedback temperature control with predictive ANN could be a feasible approach to precisely induce the preset extent of thermal coagulation for treating papillary thyroid microcarcinoma.

Quasi-distributed fiber optic sensor-based control system for interstitial laser ablation of tissue: theoretical and experimental investigations

This work proposes the quasi-distributed real-time monitoring and control of laser ablation (LA) of liver tissue. To confine the thermal damage, a pre-planning stage of the control strategy based on numerical simulations of the bioheat-transfer was developed to design the control parameters, then experimentally assessed. Fiber Bragg grating (FBG) sensors were employed to design the automatic thermometry system used for temperature feedback control for interstitial LA. The tissue temperature was maintained at a pre-set value, and the influence of different sensor locations (on the direction of the beam propagation and backward) on the thermal outcome was evaluated in comparison with the uncontrolled case. Results show that the implemented computational model was able to properly describe the temperature evolution of the irradiated tissue. Furthermore, the realized control strategy allowed for the accurate confinement of the laser-induced temperature increase, especially when the temperature control was actuated by sensors located in the direction of the beam propagation, as confirmed by the calculated fractions of necrotic tissues (e.g., 23 mm³ and 53 mm³ for the controlled and uncontrolled LA, respectively).

Anisotropic scattering characteristics of nanoparticles in different morphologies: improving the temperature uniformity of tumors during thermal therapy using forward scattering

Precise control of the thermal damage area is the key issue during thermal therapy, which can be achieved by manipulating the light propagation in biological tissue. In the present work, a method is proposed to increase the uniformity of the specific absorption rate (SAR) distribution in tumors during laser-induced thermal therapy, which is proved to be effective in reducing the thermal damage of healthy tissue. In addition, a better way of manipulating light propagation in biological tissue is explored. It is found that the anisotropic scattering characteristics of nanoparticles are strongly dependent on their shapes, sizes, orientations, and incident wavelengths, which will strongly affect the light propagation in nanoparticle embedded biological tissue. Therefore, to obtain a better outcome from photothermal therapy, the scattering properties of nanoparticles are very important factors that need to be taken into consideration, along with the absorption efficiency. Further investigation finds that nanoparticles that predominantly scatter to the forward direction are favorable in obtaining a larger penetration depth of light, which will improve the uniformity of SAR and temperature distributions. This paper is meaningful for the application of nanoparticle-assisted laser-induced thermal therapy.