Temperature feedback-controlled photothermal treatment with diffusing applicator: theoretical and experimental evaluations

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.

Theoretical Estimation of Tissue Thermal Response and Associated Thermal Damage During Gold Nanorod-enhanced Photothermal Therapy of Tumors

In the present work, we implemented a computational framework of in vivo gold nanorod (GNR)-enhanced photothermal therapy (PTT) for tumor treatment. The temperature-dependent thermophysical properties of biological tissue and the optical properties of both GNRs and the biological media were included. The latter were modulated during the treatment simulation to account for their variation, from the native to the coagulated state. The contribution of tissue injury-dependent blood perfusion was also considered. The developed model allowed for the estimation of temperature distribution during the photothermal procedure at different procedural settings and amounts of GNRs embedded in the tumor region (i.e., 12.5 μg, 25 μg, and 50 μg). Furthermore, the influence of GNRs on thermal injury, estimated with different damage models, was assessed. The inclusion of GNRs in the tumor entailed an increment of maximum tissue temperature, and faster heating kinetics, as witnessed by the lower time needed to reach complete thermal damage at the tumor center. The percentage of tumor thermal damage evaluated at the end of the simulated treatment was 48%, 69%, and 90%, for PTT in the presence of 12.5 μg, 25 μg, and 50 μg of GNRs, respectively.Clinical Relevance-This establishes that simulation-based tools, modeling the tissue properties variation during the photothermal treatment, can serve as promising preplanning platforms for nanoparticle-assisted light therapies.

Real-time automatic temperature regulation during in vivo MRI-guided laser-induced thermotherapy (MR-LITT)

Precise control of tissue temperature during Laser-Induced Thermotherapy (LITT) procedures has the potential to improve the clinical efficiency and safety of such minimally invasive therapies. We present a method to automatically regulate in vivo the temperature increase during LITT using real-time rapid volumetric Magnetic Resonance thermometry (8 slices acquired every second, with an in-plane resolution of 1.4 mmx1.4 mm and a slice thickness of 3 mm) using the proton-resonance frequency (PRF) shift technique. The laser output power is adjusted every second using a feedback control algorithm (proportional-integral-derivative controller) to force maximal tissue temperature in the targeted region to follow a predefined temperature-time profile. The root-mean-square of the difference between the target temperature and the measured temperature ranged between 0.5 °C and 1.4 °C, for temperature increases between + 5 °C to + 30 °C above body temperature and a long heating duration (up to 15 min), showing excellent accuracy and stability of the method. These results were obtained on a 1.5 T clinical MRI scanner, showing a potential immediate clinical application of such a temperature controller during MR-guided LITT.

Estimation of porcine pancreas optical properties in the 600-1100 nm wavelength range for light-based therapies

This work reports the optical properties of porcine pancreatic tissue in the broad wavelength range of 600–1100 nm. Absorption and reduced scattering coefficients (µ_a and µ_s′) of the ex vivo pancreas were obtained by means of Time-domain Diffuse Optical Spectroscopy. We have investigated different experimental conditions—including compression, repositioning, spatial sampling, temporal stability—the effect of the freezing procedure (fresh vs frozen-thawed pancreas), and finally inter-sample variability. Good repeatability under different experimental conditions was obtained (median coefficient of variation less than 8% and ~ 16% for µ_a and µ_s′, respectively). Freezing–thawing the samples caused an irreversible threefold reduction of µ_s′ and no effect on µ_a. The absorption and reduced scattering spectra averaged over different samples were in the range of 0.12–0.74 cm⁻¹ and 12–21 cm⁻¹ with an inter-sample variation of ~ 10% and ~ 40% for µ_a and µ_s′, respectively. The calculated effective transport coefficient (µ_eff) for fresh pancreatic tissue shows that regions between 800–900 nm and 1050–1100 nm are similar and offer the lowest tissue attenuation in the considered range (i.e., µ_eff ranging from 2.4 to 2.7 cm⁻¹). These data, describing specific light-pancreas interactions in the therapeutic optical window for the first time, provide pivotal information for planning of light-based thermotherapies (e.g., laser ablation) and instruction of light transport models for biophotonic applications involving this organ.

Feedback-controlled laser ablation for cancer treatment: comparison of On-Off and PID control strategies. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2022;2022:5012-5015. [PMID: 36085688 DOI: 10.1109/embc48229.2022.9871972]

Laser ablation is a rising technique used to induce a localized temperature increment for tumor ablation. The outcomes of the therapy depend on the tissue thermal history. Monitoring devices help to assess the tissue thermal response, and their combination with a control strategy can be used to promptly address unexpected temperature changes and thus reduce unwanted thermal effects. In this application, numerical simulations can drive the selection of the laser control settings (i.e., laser power and gain parameters) and allow evaluating the thermal effects of the control strategies. In this study, the influence of different control strategies (On-Off and PID-based controls) is quantified considering the treatment time and the thermal effect on the tissue. Finite element model-based simulations were implemented to model the laser-tissue interaction, the heat-transfer, and the consequent thermal damage in liver tissue with tumor. The laser power was modulated based on the temperature feedback provided within the tumor safety margin. Results show that the chosen control strategy does not have a major influence on the extent of thermal damage but on the treatment duration; the percentage of necrosis within the tumor domain is 100% with both strategies, while the treatment duration is 630 s and 786 s for On-Off and PID, respectively. The choice of the control strategy is a trade-off between treatment duration and unwanted temperature overshoot during closed-loop laser ablation. Clinical Relevance-This work establishes that different temperature-based control of the laser ablation procedure does not have a major influence on the extent of thermal damage but on the duration of treatment.

Collective bacterial disinfection by opto-chemical treatment on mature biofilm in clinical endoscope

The present study proposes an innovative opto-chemical treatment using a basket-integrated optical device (BIOD) to disinfect mature bacterial biofilm on endoscope channels. A BIOD was designed to position an optical diffuser on the central axis of an endoscope channel and to distribute laser light concentrically to the bacterial biofilm on the channel surface. To apply thermal damage and oxidative stress to the bacterial biofilm, a low concentration of a crosslinking agent (glutaraldehyde ~0.5%) was combined with 808 nm infrared (IR) and 405 nm blue (BL) laser lights. The applied irradiances of IR and BL were 10 W/cm² and 1.6 W/cm² for Teflon channel model and 20 W/cm² and 3.2 W/cm² for a clinical model, respectively. Individual irradiation of either IR or BL for 180 s induced the maximum temperatures of 62 ± 2 °C and 53 ± 3 °C on the biofilm, respectively. The simultaneous opto-chemical treatment reduced a significant population of the bacterial biofilms (7.5-log₁₀ for Staphylococcus aureus and 7.1-log₁₀ for Pseudomonas aeruginosa), which were 2.9-fold and 3.9-fold higher than that of the standard treatment with 2% glutaraldehyde (GA) solution, respectively. The proposed opto-chemical disinfection method can help reduce multi-drug resistant bacteria and prevent cross-infection during the clinical usage of a flexible endoscope.

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.

Thermomechanical Modeling of Laser Ablation Therapy of Tumors: Sensitivity Analysis and Optimization of Influential Variables

In cancer treatment, laser ablation is a promising technique used to induce localized thermal damage. Different variables influence the temperature distribution in the tissue and the resulting therapy efficacy; thus, the optimal therapy settings are required for obtaining the desired clinical outcome. In this work, thermomechanical modeling of contactless laser ablation was implemented to analyze the sensitivity of independent variables on the optimal treatment conditions. The Finite Element Method was utilized to solve the governing equations, i.e., the bioheat, mechanical deformation, and the Navier-Stokes equations. Validation of the model was evaluated by comparing experimental and simulated temperatures, which indicated high accuracy for estimating temperature. In particular, the results showed that the model is capable of estimating temperature with a good correlation factor (R=0.98) and low Mean Absolute Error (3.9 C). A sensitivity analysis based on laser irradiation time, power, beam distribution, and the blood vessel depth on temperature distribution and fraction of necrotic tissue was performed. Based on the most significant variables i.e., laser irradiation time and power, an optimization process was performed. This resulted into an indication of the optimal therapy settings for achieving maximum procedure efficiency i.e., the required fraction of necrotic tissue within the target volume, constituted by tumor and safety margins around it.

Antibacterial Activity of Porous Gold Nanocomposites via NIR Light-Triggered Photothermal and Photodynamic Effects

Phototherapeutic approaches, including photothermal therapy (PTT) and photodynamic therapy (PDT), have become a promising strategy to combat microbial pathogens and tackle the crisis brought about by antibiotic-resistant strains. Herein, porous gold nanoparticles (AuPNs) were synthesized as photothermal agents and loaded with indocyanine green (ICG), a common photosensitizer for PDT, to fabricate a nanosystem presenting near-infrared (NIR) light-triggered synchronous PTT and PDT effects. The AuPNs can not only convert NIR light into heat with a high photothermal conversion efficiency (50.6-68.5%), but also provide a porous structure to facilely load ICG molecules. The adsorption of ICG onto AuPNs was mainly driven by electrostatic and hydrophobic interactions with the surfactant layer of AuPNs, and the aggregate state of ICG significantly enhanced its generation of reactive oxygen species. Moreover, taking advantage of its synergistic PTT and PDT effect, the hybrid nanocomposites displayed a remarkable antibacterial effect to the gram-positive pathogen Staphylococcus aureus (S. aureus) upon 808 nm laser irradiation.

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).

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

This study develops an energy modulation technique to attain a constant interstitial tissue temperature and to induce the predetermined thermal coagulation without carbonization in tissue. An optical diffuser was employed to deliver 1064 nm light to the biological tissue. The combined mode maintained the interstitial temperature at 70 °C for longer durations compared to the continuous wave mode. Coagulation volumes increased linearly with the time and met the predetermined treatment volume range (0.32-0.52 cm³) after the combined treatment for 100 s. The combined modulation can be a feasible modality to induce the predetermined extent of thermal coagulation for treating papillary thyroid microcarcinoma.

Closed-Loop Temperature Control Based on Fiber Bragg Grating Sensors for Laser Ablation of Hepatic Tissue

Laser ablation (LA) of cancer is a minimally invasive technique based on targeted heat release. Controlling tissue temperature during LA is crucial to achieve the desired therapeutic effect in the organs while preserving the healthy tissue around. Here, we report the design and implementation of a real-time monitoring system performing closed-loop temperature control, based on fiber Bragg grating (FBG) spatial measurements. Highly dense FBG arrays (1.19 mm length, 0.01 mm edge-to-edge distance) were inscribed in polyimide-coated fibers using the femtosecond point-by-point writing technology to obtain the spatial resolution needed for accurate reconstruction of high-gradient temperature profiles during LA. The zone control strategy was implemented such that the temperature in the laser-irradiated area was maintained at specific set values (43 and 55 °C), in correspondence to specific radii (2 and 6 mm) of the targeted zone. The developed control system was assessed in terms of measured temperature maps during an ex vivo liver LA. Results suggest that the temperature-feedback system provides several advantages, including controlling the margins of the ablated zone and keeping the maximum temperature below the critical values. Our strategy and resulting analysis go beyond the state-of-the-art LA regulation techniques, encouraging further investigation in the identification of the optimal control-loop.

Temporal modulation of optical energy for enhanced photothermal hemostasis of intraoperative bleeding during laser treatment

Intraoperative bleeding during laser treatment of benign prostate hyperplasia (BPH) often impedes cystoscopic vision, necessitating the use of conventional hemostatic devices. This study proposes an optical technique to improve the efficacy of photothermal hemostasis of bleeders during laser prostatectomy by temporally modulating a 532 nm laser beam. A perfused porcine kidney model is established to quantitatively investigate various optical pulse patterns and irradiation modes. Thermal simulations demonstrate a high success rate of complete hemostasis achieved by the modulated 532 nm pulse pattern. In comparison to the irradiation modes typically employed for hemostasis, the modulated 532 nm mode exhibits a short coagulation time and minimal thermal injury. ex vivo and in vivo cystoscopic observations validate the clinical feasibility of the proposed optical energy modulation method to regulate intraoperative bleeding.

Enhanced photothermal hemostasis using dual wavelengths in an in vivo leporine kidney model

The current study investigated the hemostatic effect of dual wavelengths on in vivo leporine kidney tissue using 532-nm and 980-nm laser systems. Three irradiation modes, 532 nm, 980 nm, and dual (532 and 980 nm) modes, were compared to test non-contact photothermal hemostasis on 36 bleeders in the kidney models. Each bleeder was flushed with saline during the irradiation. The dual mode achieved complete hemostasis more rapidly than the single modes (4.0 ± 1.4 s for dual vs. no hemostasis for 532 nm and 10.0 ± 1.3 s for 980 nm; p < 0.001). Application of 60 W from the dual wavelengths expanded the surface area of the thermal lesion (up to 60%). In vivo dual-wavelength irradiation achieved more rapid and complete hemostasis with ∼2 mm coagulation depth than the single-wavelength irradiation.

Computational analysis of linear energy modulation for laser thermal coagulation

Accurate treatment planning and monitoring are critical factors to ensure safe and effective outcomes of laser thermal coagulation (LTC). Computational and experimental models based upon linear energy modulation were deployed to predict temperature distribution and thermal damage within ex vivo porcine liver. 1470-nm Gaussian emission was confirmed by using digital imaging and the customized goniometry. The tissue temperature was maintained in the pre-determined range (65~75 °C) to induce thermally destructive volumes of 0.23 cm³ (simulation) and 0.17 ± 0.05 cm³ (experiment) once the applied power was linearly reduced from 3.5 W to 0.2 W in 50 s ("3.5 W fast slope" laser modulation mode). The proposed model may be a useful tool to predict thermal responses of the tissue during LTC.

Dual-wavelength-assisted thermal hemostasis for treatment of benign prostate hyperplasia

Laser treatment on a large size of prostate gland often encounters significant bleeding that can prolong the entire procedure and cause urinary complications. The current study investigates the feasibility of dual-wavelength (532 and 980 nm) application to achieve rapid hemostasis for 532-nm laser prostatectomy. Porcine kidney and bleeding phantom models were tested to quantify the degree of the irreversible tissue coagulation and to estimate the time for the complete hemostasis, respectively. The ex vivo kidney testing verifies that the dual-wavelength created up to 40% deeper and 25% wider coagulation regions than a single wavelength does. The bleeding phantom testing demonstrates that due to the enhanced thermal effects, the simultaneous irradiation yields the complete photocoagulation (~11 seconds) whereas 532 or 980 nm hardly stops bleeders. Numerical simulations validate that the combined optical-thermal characteristics of both the wavelengths account for the augmented thermal coagulation. The dual-wavelength-assisted coagulation can be a feasible treatment to entail the rapid hemostasis and to facilitate the laser prostatectomy in an effective manner.

Spatial effect of conical angle on optical-thermal distribution for circumferential photocoagulation

A uniformly diffusing applicator can be advantageous for laser treatment of tubular tissue. The current study investigated various conical angles for diffuser tips as a critical factor for achieving radially uniform light emission. A customized goniometer was employed to characterize the spatial uniformity of the light propagation. An ex vivo model was developed to quantitatively compare the temperature development and irreversible tissue coagulation. The 10-mm diffuser tip with angle at 25° achieved a uniform longitudinal intensity profile (i.e., 0.90 ± 0.07) as well as a consistent thermal denaturation on the tissue. The proposed conical angle can be instrumental in determining the uniformity of light distribution for the photothermal treatment of tubular tissue.