The effects of dynamic optical properties during interstitial laser photocoagulation

A comparative study between conventional chemotherapy and photothermal activated nano-sized targeted drug delivery to solid tumor

Delivery of chemotherapeutic medicines to solid tumors is critical for optimal therapeutic success and minimal adverse effects. We mathematically developed a delivery method using thermosensitive nanocarriers activated by light irradiation. To assess its efficacy and identify critical events and parameters affecting therapeutic response, we compared this method to bolus and continuous infusions of doxorubicin for both single and multiple administrations. A hybrid sprouting angiogenesis approach generates a semi-realistic microvascular network to evaluate therapeutic drug distribution and microvascular heterogeneity. A pharmacodynamics model evaluates treatment success based on tumor survival cell percentage. The study found that whereas bolus injection boosted extracellular drug concentration levels by 90%, continuous infusion improved therapeutic response due to improved bioavailability. Cancer cell death increases by 6% with several injections compared to single injections due to prolonged chemotherapeutic medication exposure. However, responsive nanocarriers supply more than 2.1 times more drug than traditional chemotherapy in extracellular space, suppressing tumor development longer. Also, controlled drug release decreases systemic side effects substantial through diminishing the concentration of free drug in the circulation. The primary finding of this work highlights the significance of high bioavailability in treatment response. The results indicate that responsive nanocarriers contribute to increased bioavailability, leading to improved therapeutic benefits. By including drug delivery features in a semi-realistic model, this numerical study sought to improve drug-bio interaction comprehension. The model provides a good framework for understanding preclinical and clinical targeted oncology study outcomes.

3D modeling of vector/edge finite element method for multi-ablation technique for large tumor-computational approach

Microwave ablation (MWA) is a cancer thermal ablation treatment that uses electromagnetic waves to generate heat within the tissue. The goal of this treatment is to eliminate tumor cells while leaving healthy cells unharmed. During MWA, excess heat generation can kill healthy cells. Hence, mathematical models and numerical techniques are required to analyze the heat distribution in the tissue before the treatment. The aim of this research is to explain the implementation of the 3D vector finite element method in a wave propagation model that simulates the specific absorption rate in the liver. The 3D Nedelec elements from H(curl; Ω) space are used to discretize the wave propagation model, and this implementation is helpful in solving many real-world problems that involve electromagnetic propagation with perfect conducting and absorbing boundary conditions. One of the difficulties in ablation treatment is creating a large ablation zone for a large tumor (diameter greater than 3 cm) in a short period of time with minimum damage to the surrounding tissue. This article addresses the aforementioned issue by introducing four antennas into the different places of the tumor sequentially and producing heat uniformly over the tumor. The results demonstrated that 95.5% of the tumor cells were killed with minimal damage to the healthy cells when the heating time was increased to 4 minutes at each position. Subsequently, we studied the temperature distribution and localised tissue contraction in the tissue using the three-dimensional bio-heat equation and temperature-time dependent model, respectively. The local tissue contraction is measured at arbitrary points in the domain and is more noticeable at temperatures higher than 102°C. The thermal damage in the liver during MWA treatment is investigated using the three-state cell death model. The system of partial differential equations is solved numerically due to the complex geometry of the domain, and the results are compared with experimental data to validate the models and parameters.

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.

Use of microwave ablation for thermal treatment of solid tumors with different shapes and sizes-A computational approach

Microwave Ablation (MWA) is one of the most recent developments in the field of thermal therapy. This approach is an effective method for thermal tumor ablation by increasing the temperature above the normal physiological threshold to kill cancer cells with minimum side effects to surrounding organs due to rapid heat dispersive tissues. In the present study, the effects of the shape and size of the tumor on MWA are investigated. To obtain the temperature gradient, coupled bio-heat and electromagnetic equations are solved using a three-dimensional finite element method (FEM). To extract cellular response at different temperatures and times, the three-state mathematical model was employed to achieve the ablation zone size. Results show that treatment of larger tumors is more difficult than that of smaller ones. By doubling the diameter of the tumor, the percentage of dead cancer cells is reduced by 20%. For a spherical tumor smaller than 2 cm, applying 50 W input power compared to 25 W has no significant effects on treatment efficiency and only increases the risk of damage to adjacent tissues. However, for tumors larger than 2 cm, it can increase the ablation zone up to 21%. In the spherical and oblate tumors, the mean percentage of dead cells at 6 GHz is nearly 30% higher than that at 2.45GHz, but for prolate tumors, treatment efficacy is reduced by 10% at a higher frequency. Moreover, the distance between two slots in the coaxial double slot antenna is modified based on the best treatment of prolate tumors. The findings of this study can be used to choose the optimum frequency and the best antenna design according to the shape and size of the tumor.

Stereo-crossed ablation guided by stereoelectroencephalography for epilepsy: comprehensive coagulations via a network of multi-electrodes

Background:

Introducing multiple different stereoelectroencephalography electrodes in a three-dimensional (3D) network to create a 3D-lesioning field or stereo-crossed radiofrequency thermocoagulation (scRF-TC) might create larger lesioning size; however, this has not been quantified to date. This study aimed to quantify the configurations essential for scRF-TC.

Methods:

By using polyacrylamide gel (PAG), we investigated the effect of electrode conformation (angled/parallel/multiple edges) and electrode distance of creating an electrode network. Volume, time, and temperature were analyzed quantitatively with magnetic resonance imaging, video analysis, and machine learning. A network of electrodes to the pathological left area 47 was created in a patient; the seizure outcome and coverage range were further observed.

Results:

After the compatibility test between the PAG and brain tissue, the sufficient distance of contacts (from different electrodes) for confluent lesioning was 7 mm with the PAG. Connection to the lesioning field could be achieved even with a different arrangement of electrodes. One contact could achieve at least six connections with different peripheral contacts. Coagulation with a network of electrodes can create more significant lesioning sizes, 1.81–2.12 times those of the classic approaches. The confluent lesioning field created by scRF-TC had a volume of 38.7 cm³; the low metabolic area was adequately covered. The representative patient was free of seizures throughout the 12-month follow up.

Conclusion:

Lesioning with electrodes in a network manner is practical for adequate 3D coverage. A secondary craniotomy could be potentially prevented by combining both monitoring and a large volume of lesions.

Characteristics of temperature changes in photothermal therapy induced by combined application of indocyanine green and laser

Photothermal therapy, a type of laser application, has the ability to eradicate tumor cells by a local thermal effect and elicit a tumor specific immune response. Indocyanine green (ICG), a photosensitizer, can effectively elevate the local temperature by absorbing energy from the laser. The present study aimed to investigate the characteristics of temperature changes during photothermal therapy with an infrared thermometer in an ICG solution and in tumor-bearing mice treated with a combination of laser and ICG. Additionally, the present study observed the morphological changes of tumor tissue by hematoxylin-eosin staining following photothermal therapy. In the solution experiment, when the laser power density was 1 W/cm² and the concentration of ICG was 0 or 0.0187 mg/ml, the temperature of the water was elevated by 3 and 28°C, respectively. In the tumor-bearing mice experiment, when the laser power density was 1 W/cm² and the concentration of ICG was 0 and 0.1 mg/ml, the temperature of the tumor-bearing mice was elevated by 6.9 and 28.5°C, respectively. With an increase in laser power density, including 0.6, 0.8 and 1.0 W/cm², the temperature was 23.3, 26.7 and 28.5°C, respectively. Pathological tissue sections demonstrated that a large number of tumor cells experienced necrosis, and the envelope of the tumor was destroyed. Numerous inflammatory cells, in particular lymphocytes, infiltrated into the tumor tissue following tumor tissue treatment with a combination of laser and ICG. These results indicated that a combination treatment with laser and ICG may significantly increase the temperature of the water solutions and in the tumor-bearing mice. The concentration of ICG and laser power density contributed to the temperature elevation, in particular to the concentration of ICG.

Analysis of Thermally Denatured Depth in Laser Vaporization for Benign Prostatic Hyperplasia using a Simulation of Light Propagation and Heat Transfer (secondary publication)

Background and Aims: Laser vaporization of the prostate is expected as a less invasive treatment for benign prostatic hyperplasia (BPH), via the photothermal effect. In order to develop safer and more effective laser vaporization of the prostate, it is essential to set optimal irradiation parameters based on quantitative evaluation of temperature distribution and thermally denatured depth in prostate tissue. Method: A simulation model was therefore devised with light propagation and heat transfer calculation, and the vaporized and thermally denatured depths were estimated by the simulation model. Results: The results of the simulation were compared with those of an ex vivo experiment and clinical trial. Based on the accumulated data, the vaporized depth strongly depended on the distance between the optical fiber and the prostate tissue, and it was suggested that contact laser irradiation could vaporize the prostate tissue most effectively. Additionally, it was suggested by analyzing thermally denatured depth comprehensively that laser irradiation at the distance of 3 mm between the optical fiber and the prostate tissue was useful for hemostasis. Conclusions: This study enabled quantitative and reproducible analysis of laser vaporization for BPH and will play a role in clarification of the safety and efficacy of this treatment.

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

To minimize thermal injury, the current study evaluated the real-time temperature monitoring with a proportional-integrative-derivative (PID) controller during 980-nm photothermal treatment with a radially-diffusing applicator. Both simulations and experiments demonstrated comparable thermal behaviors in temperature distribution and the degree of irreversible tissue denaturation. The PID-controlled application constantly maintained the pre-determined temperature of 353 K (steady-state error = < 1 K). Due to constant energy delivery, coagulation volumes linearly increased up to 1.04 ± 0.02 cm(3) with irradiation time. Integration of temperature feedback with diffuser-assisted photothermal treatments can provide a feasible therapeutic modality to treat pancreatic tumors in an effective manner.

Interstitial photoacoustic sensor for the measurement of tissue temperature during interstitial laser phototherapy

Photothermal therapy is an effective means to induce tumor cell death, since tumor tissue is more sensitive to temperature increases than normal tissue. Biological responses depend on tissue temperature; target tissue temperature needs to be precisely measured and controlled to achieve desired thermal effects. In this work, a unique photoacoustic (PA) sensor is proposed for temperature measurement during interstitial laser phototherapy. A continuous-wave laser light and a pulsed laser light, for photothermal irradiation and photoacoustic temperature measurement, respectively, were delivered to the target tissue through a fiber coupler. During laser irradiation, the PA amplitude was measured. The Grüneisen parameter and the bioheat equation were used to determine the temperature in strategic positions in the target tissue. Our results demonstrate that the interstitial PA amplitude is a linear function of temperature in the range of 22 to 55 °C, as confirmed by thermocouple measurement. Furthermore, by choosing appropriate laser parameters, the maximum temperature surrounding the active diffuse fiber tip in tissue can be controlled in the range of 41 to 55 °C. Thus, this sensor could potentially be used for fast, accurate, and convenient three-dimensional temperature measurement, and for real-time feedback and control of interstitial laser phototherapy in cancer treatment.

InCVAX--a novel strategy for treatment of late-stage, metastatic cancers through photoimmunotherapy induced tumor-specific immunity

A novel, promising potential cancer vaccine strategy was proposed to use a two-injection procedure for solid tumors to prompt the immune system to identify and systemically eliminate primary and metastatic cancers. The two-injection procedure consists of local photothermal application on a selected tumor intended to liberate whole cell tumor antigens, followed by a local injection of an immunoadjuvant that consists of a semi-synthetic functionalized glucosamine polymer, N-dihydro-galacto-chitosan (GC), which is intended to activate antigen presenting cells and facilitate an increased uptake of tumor antigens. This strategy is thus proposed as an in situ autologous cancer vaccine (inCVAX) that may activate antigen presenting cells and expose them to tumor antigens in situ, with the intention of inducing a systemic tumor specific T-cell response. Here, the development of inCVAX for the treatment of metastatic cancers in the past decades is systematically reviewed. The antitumor immune responses of local photothermal treatment and immunological stimulation with GC are also discussed. This treatment approach is also commonly referred to as laser immunotherapy (LIT).

Tissue-mimicking gel phantoms for thermal therapy studies

Tissue-mimicking phantoms that are currently available for routine biomedical applications may not be suitable for high-temperature experiments or calibration of thermal modalities. Therefore, design and fabrication of customized thermal phantoms with tailored properties are necessary for thermal therapy studies. A multitude of thermal phantoms have been developed in liquid, solid, and gel forms to simulate biological tissues in thermal therapy experiments. This article is an attempt to outline the various materials and techniques used to prepare thermal phantoms in the gel state. The relevant thermal, electrical, acoustic, and optical properties of these phantoms are presented in detail and the benefits and shortcomings of each type are discussed. This review could assist the researchers in the selection of appropriate phantom recipes for their in vitro study of thermal modalities and highlight the limitations of current phantom recipes that remain to be addressed in further studies.

Automatic temperature controlled retinal photocoagulation

Laser coagulation is a treatment method for many retinal diseases. Due to variations in fundus pigmentation and light scattering inside the eye globe, different lesion strengths are often achieved. The aim of this work is to realize an automatic feedback algorithm to generate desired lesion strengths by controlling the retinal temperature increase with the irradiation time. Optoacoustics afford non-invasive retinal temperature monitoring during laser treatment. A 75 ns/523 nm Q-switched Nd:YLF laser was used to excite the temperature-dependent pressure amplitudes, which were detected at the cornea by an ultrasonic transducer embedded in a contact lens. A 532 nm continuous wave Nd:YAG laser served for photocoagulation. The ED50 temperatures, for which the probability of ophthalmoscopically visible lesions after one hour in vivo in rabbits was 50%, varied from 63°C for 20 ms to 49°C for 400 ms. Arrhenius parameters were extracted as ΔE=273 J mol(-1) and A=3 x 10(44) s(-1). Control algorithms for mild and strong lesions were developed, which led to average lesion diameters of 162 ± 34 μm and 189 ± 34 μm, respectively. It could be demonstrated that the sizes of the automatically controlled lesions were widely independent of the treatment laser power and the retinal pigmentation.

Modeling of laser coagulation of tissue with MRI temperature monitoring

Light energy from a laser source that is delivered into body tissue via a fiber-optic probe with minimal invasiveness has been used to ablate solid tumors. This thermal coagulation process can be guided and monitored accurately by continuous magnetic resonance imaging (MRI) since the laser energy delivery system does not interfere with MRI. This report deals with mathematical modeling and analysis of laser coagulation of tissue. This model is intended for "real-time" analysis of magnetic resonance images obtained during the coagulation process to guide clinical treatment. A mathematical model is developed to simulate the thermal response of tissue to a laser light heating source. For fast simulation, an approximate solution of the thermal model is used to predict the dynamics of temperature distribution and tissue damage induced by a laser energy line source. The validity of these simulations is tested by comparison with MRI-based temperature data acquired from in vivo experiments in rabbits. The model-simulated temperature distribution and predicted lesion dynamics correspond closely with MRI-based data. These results demonstrate the potential for using this combination of fast modeling and MRI technologies during laser heating of tissue for online prediction of tumor lesion size during laser heating.

Preclinical assessment of a 980-nm diode laser ablation system in a large animal tumor model

PURPOSE

To characterize the performance of a 980-nm diode laser ablation system in an in vivo tumor model.

MATERIALS AND METHODS

This study was approved by the institutional animal care and use committee. The ablation system consisted of a 15-W, 980-nm diode laser, flexible diffusing-tipped fiber optic, and 17-gauge internally cooled catheter. Ten immunosuppressed dogs were inoculated subcutaneously with canine-transmissible venereal tumor fragments in eight dorsal locations. Laser ablations were performed at 79 sites where inoculations were successful (99%) at powers of 10 W, 12.5 W, and 15 W, with exposure times between 60 and 180 seconds. In 20 cases, multiple overlapping ablations were performed. After the dogs were euthanized, the tumors were harvested, sectioned along the applicator tract, measured, and photographed. Measurements of ablation zone were performed on gross specimen. Histopathology and viability staining was performed with hematoxylin and eosin and nicotinamide adenine dinucleotide hydrogen staining.

RESULTS

Gross pathologic examination confirmed a well circumscribed ablation zone with sharp boundaries between thermally ablated tumor in the center surrounded by viable tumor tissue. When a single applicator was used, the greatest ablation diameters ranged from 12 mm at the lowest dose (10 W, 60 seconds) to 26 mm at the highest dose (15 W, 180 seconds). Multiple applicators created ablation zones as large as 42 mm in greatest diameter (with the lasers operating at 15 W for 120 seconds).

CONCLUSIONS

The new 980-nm diode laser and internally cooled applicator effectively create large ellipsoid thermal ablations in less than 3 minutes.

Considerations for Thermal Injury Analysis for RF Ablation Devices

Background:

The estimation of lesion size is an integral part of treatment planning for the clinical applications of radiofrequency ablation. However, to date, studies have not directly evaluated the impact of different computational estimation techniques for predicting lesion size. In this study, we focus on three common methods used for predicting tissue injury: (1) iso-temperature contours, (2) Cumulative equivalent minutes, (3) Arrhenius based thermal injury.

Methods:

We created a geometric model of a multi-tyne ablation electrode and simulated thermal and tissue injury profiles that result from three calculation methods after 15 minutes exposure to a constant RF voltage source. A hybrid finite element technique was used to calculate temperature and tissue injury. Time-temperature curves were used in the assessment of iso-temperature thresholds and the method of cumulative equivalent minutes. An Arrhenius-based formulation was used to calculate sequential and recursive thermal injury to tissues.

Results:

The data demonstrate that while iso-temperature and cumulative equivalent minute contours are similar in shape, these two methodologies grossly over-estimate the amount of tissue injury when compared to recursive thermal injury calculations, which have previously been shown to correlate closely with in vitro pathologic lesion volume measurement. In addition, Arrhenius calculations that do not use a recursive algorithm result in a significant underestimation of lesion volume. The data also demonstrate that lesion width and depth are inadequate means of characterizing treatment volume for multi-tine ablation devices.

Conclusions:

Recursive thermal injury remains the most physiologically relevant means of computationally estimating lesion size for hepatic tumor applications. Iso-thermal and cumulative equivalent minute approaches may produce significant errors in the estimation of lesion size.

Considerations for thermal injury analysis for RF ablation devices

BACKGROUND

The estimation of lesion size is an integral part of treatment planning for the clinical applications of radiofrequency ablation. However, to date, studies have not directly evaluated the impact of different computational estimation techniques for predicting lesion size. In this study, we focus on three common methods used for predicting tissue injury: (1) iso-temperature contours, (2) Cumulative equivalent minutes, (3) Arrhenius based thermal injury.

METHODS

We created a geometric model of a multi-tyne ablation electrode and simulated thermal and tissue injury profiles that result from three calculation methods after 15 minutes exposure to a constant RF voltage source. A hybrid finite element technique was used to calculate temperature and tissue injury. Time-temperature curves were used in the assessment of iso-temperature thresholds and the method of cumulative equivalent minutes. An Arrhenius-based formulation was used to calculate sequential and recursive thermal injury to tissues.

RESULTS

The data demonstrate that while iso-temperature and cumulative equivalent minute contours are similar in shape, these two methodologies grossly over-estimate the amount of tissue injury when compared to recursive thermal injury calculations, which have previously been shown to correlate closely with in vitro pathologic lesion volume measurement. In addition, Arrhenius calculations that do not use a recursive algorithm result in a significant underestimation of lesion volume. The data also demonstrate that lesion width and depth are inadequate means of characterizing treatment volume for multi-tine ablation devices.

CONCLUSIONS

Recursive thermal injury remains the most physiologically relevant means of computationally estimating lesion size for hepatic tumor applications. Iso-thermal and cumulative equivalent minute approaches may produce significant errors in the estimation of lesion size.

Modeling of plasmonic heating from individual gold nanoshells for near-infrared laser-induced thermal therapy

Gold nanoparticles can be engineered to target cancerous cells and at the same time designed to absorb specific wavelengths of light. Consequently, with the presence of optically tunable gold nanoparticles such as gold nanoshells, light can be effectively converted to heat via photothermal effect well enough to raise the temperature of medium surrounding gold nanoshells for thermal ablation or hyperthermia treatments of cancers. In this study, the authors proposed a new computational method to estimate thermal response of gold nanoshells embedded in a tissue-like medium when illuminated by a near-infrared (NIR) laser. Specifically, the light transport theory with diffusion approximation was initially applied to model the temperature rise within a medium without gold nanoshells as a result of the dissipation of the NIR laser power throughout the medium. After then, the heat generated by individual gold nanoshells due to photothermal effect was calculated and combined with the results for the medium without gold nanoshells to estimate the global elevation of temperature within the gold nanoshell-laden medium. The current computational model was tested for its validity using two different phantom examples, one of which was similar to a previously reported phantom experiment. The test demonstrated the capability of the current model in terms of producing qualitatively reasonable results, while it also revealed a number of potential differences in the assumptions for the current model and previous experiment. After an adjustment in the model parameters to properly take into account such differences, the computational results and the experimental data matched reasonably well within the average percentage difference of 10%.

Mathematical modeling of laser lipolysis

Background and Objectives

Liposuction continues to be one of the most popular procedures performed in cosmetic surgery. As the public's demand for body contouring continues, laser lipolysis has been proposed to improve results, minimize risk, optimize patient comfort, and reduce the recovery period. Mathematical modeling of laser lipolysis could provide a better understanding of the laser lipolysis process and could determine the optimal dosage as a function of fat volume to be removed.

Study design/Materials and Methods

An Optical-Thermal-Damage Model was formulated using finite-element modeling software (Femlab 3.1, Comsol Inc). The general model simulated light distribution using the diffusion approximation of the transport theory, temperature rise using the bioheat equation and laser-induced injury using the Arrhenius damage model. Biological tissue was represented by two homogenous regions (dermis and fat layer) with a nonlinear air-tissue boundary condition including free convection.

Video recordings were used to gain a better understanding of the back and forth movement of the cannula during laser lipolysis in order to consider them in our mathematical model. Infrared video recordings were also performed in order to compare the actual surface temperatures to our calculations. The reduction in fat volume was determined as a function of the total applied energy and subsequently compared to clinical data reported in the literature.

Results

In patients, when using cooled tumescent anesthesia, 1064 nm Nd:YAG laser or 980 nm diode laser: (6 W, back and forth motion: 100 mm/s) give similar skin surface temperature (max: 41°C). These measurements are in accordance with those obtained by mathematical modeling performed with a 1 mm cannula inserted inside the hypodermis layer at 0.8 cm below the surface. Similarly, the fat volume reduction observed in patients at 6-month follow up can be determined by mathematical modeling. This fat reduction depends on the applied energy, typically 5 cm³ for 3000 J. At last, skin retraction was observed in patients at 6-month follow up. This observation can be easily explained by mathematical modeling showing that the temperature increase inside the lower dermis is sufficient (48–50°C) to induce skin tightening

Discussion and Conclusion

Laser lipolysis can be described by a theoretical model. Fat volume reduction observed in patients is in accordance with model calculations. Due to heat diffusion, temperature elevation is also produced inside the lower reticular dermis. This interesting observation can explain remodeling of the collagenous tissue, with clinically evident skin tightening.

In conclusion, while the heat generated by interstitial laser irradiation provides stimulate lipolysis of the fat cells, the collagen and elastin are also stimulated resulting in a tightening in the skin. This mathematical model should serve as a useful tool to simulate and better understand the mechanism of action of the laser lipolysis

An electrically-activated dynamic tissue-equivalent phantom for assessment of diffuse optical imaging systems

A novel design of solid dynamic phantom with tissue-like optical properties is presented, which contains variable regions of contrast which are activated electrically. Reversible changes in absorption are produced by localized heating of targets impregnated with thermochromic pigment. A portable, battery-operated prototype has been constructed, and its optical and temporal characteristics have been investigated. The phantom has been developed as a means of assessing the performance of diffuse optical imaging systems, such as those used to monitor haemodynamic changes in the brain and other tissues. Images of the phantom have been reconstructed using data acquired with a continuous wave optical topography system.

Characterization of tracked radiofrequency ablation in phantom

In radiofrequency ablation (RFA), successful therapy requires accurate, image-guided placement of the ablation device in a location selected by a predictive treatment plan. Current planning methods rely on geometric models of ablations that are not sensitive to underlying physical processes in RFA. Implementing plans based on computational models of RFA with image-guided techniques, however, has not been well characterized. To study the use of computational models of RFA in planning needle placement, this work compared ablations performed with an optically tracked RFA device with corresponding models of the ablations. The calibration of the tracked device allowed the positions of distal features of the device, particularly the tips of the needle electrodes, to be determined to within 1.4 +/- 0.6 mm of uncertainty. Ablations were then performed using the tracked device in a phantom system based on an agarose-albumin mixture. Images of the sliced phantom obtained from the ablation experiments were then compared with the predictions of a bioheat transfer model of RFA, which used the positional data of the tracked device obtained during ablation. The model was demonstrated to predict 90% of imaged pixels classified as being ablated. The discrepancies between model predictions and observations were analyzed and attributed to needle tracking inaccuracy as well as to uncertainties in model parameters. The results suggest the feasibility of using finite element modeling to plan ablations with predictable outcomes when implemented using tracked RFA.

Laser-induced thermal response and characterization of nanoparticles for cancer treatment using magnetic resonance thermal imaging

Spherical nanoparticles with a gold outer shell and silica core can be tuned to absorb near-infrared light of a specific wavelength. These nanoparticles have the potential to enhance the treatment efficacy of laser-induced thermal therapy (LITT). In order to enhance both the potential efficacy and safety of such procedures, accurate methods of treatment planning are needed to predict the temperature distribution associated with treatment application. In this work, the standard diffusion approximation was used to model the laser fluence in phantoms containing different concentrations of nanoparticles, and the temperature distribution within the phantom was simulated in three-dimensions using the finite element technique. Magnetic resonance temperature imaging was used to visualize the spatiotemporal distribution of the temperature in the phantoms. In most cases, excellent correlation is demonstrated between the simulations and the experiment (<3.0% mean error observed). This has significant implications for the treatment planning of LITT treatments using gold-silica nanoshells.

Perturbative diffusion theory formalism for interpreting temporal light intensity changes during laser interstitial thermal therapy

In an effort to understand dynamic optical changes during laser interstitial thermal therapy (LITT), we utilize the perturbative solution of the diffusion equation in heterogeneous media to formulate scattering weight functions for cylindrical line sources. The analysis explicitly shows how changes in detected interstitial light intensity are associated with the extent and location of the volume of thermal coagulation during treatment. Explanations for previously reported increases in optical intensity observed early during laser heating are clarified using the model and demonstrated with experimental measurements in ex vivo bovine liver tissue. This work provides an improved understanding of interstitial optical signal changes during LITT and indicates the sensitivity and potential of interstitial optical monitoring of thermal damage.

An investigation of the use of transmission ultrasound to measure acoustic attenuation changes in thermal therapy

The potential of using a commercial ultrasound transmission imaging system to quantitatively monitor tissue attenuation changes after thermal therapy was investigated. The ultrasound transmission imaging system used, the AcoustoCam (Imperium Inc., MD) allows ultrasonic images to be captured using principles similar to that of a CCD-type camera that collects light. Ultrasound energy is focused onto a piezoelectric array by an acoustic lens system, creating a gray scale acoustic image. In this work, the pixel values from the acoustic images were assigned acoustic attenuation values by imaging polyacrylamide phantoms of varying known attenuation. After the calibration procedure, data from heated polyacrylamide/bovine serum albumin (BSA) based tissue-mimicking (TM) phantoms and porcine livers were acquired. Samples were heated in water at temperatures of 35, 45, 55, 65, and 75 degrees C for 1 h. Regions of interest were chosen in the images and acoustic attenuation values before and after heating were compared. An increase in ultrasound attenuation was found in phantoms containing BSA and in porcine liver. In the presence of BSA, attenuation in the TM phantom increased by a factor of 1.5, while without BSA no significant changes were observed. The attenuation of the porcine liver increased by up to a factor of 2.4, consistent with previously reported studies. The study demonstrates the feasibility of using a quantitative ultrasound transmission imaging system for monitoring thermal therapy.

Photothermally induced delayed tissue death

We report pronounced delayed tissue death in photothermal surgery performed on the livers of live healthy rats with highly concentrated sunlight (ultrabright noncoherent light). Exposure times and power levels were selected to produce immediate necroses of the order of hundreds of cubic millimeters. Pathology reveals that lesion volumes increase by up to a factor of 5 within approximately 24 h after surgery, and then stabilize. Islands of viable cells can persist within damaged tissue, in the immediate vicinity of blood vessels, but also necrose within about 48 h.

Mathematical modeling of endovenous laser treatment (ELT)

Background and objectives

Endovenous laser treatment (ELT) has been recently proposed as an alternative in the treatment of reflux of the Great Saphenous Vein (GSV) and Small Saphenous Vein (SSV). Successful ELT depends on the selection of optimal parameters required to achieve an optimal vein damage while avoiding side effects. Mathematical modeling of ELT could provide a better understanding of the ELT process and could determine the optimal dosage as a function of vein diameter.

Study design/materials and methods

The model is based on calculations describing the light distribution using the diffusion approximation of the transport theory, the temperature rise using the bioheat equation and the laser-induced injury using the Arrhenius damage model. The geometry to simulate ELT was based on a 2D model consisting of a cylindrically symmetric blood vessel including a vessel wall and surrounded by an infinite homogenous tissue. The mathematical model was implemented using the Macsyma-Pdease2D software (Macsyma Inc., Arlington, MA, USA). Damage to the vein wall for CW and single shot energy was calculated for 3 and 5 mm vein diameters. In pulsed mode, the pullback distance (3, 5 and 7 mm) was considered. For CW mode simulation, the pullback speed (1, 2, 3 mm/s) was the variable. The total dose was expressed as joules per centimeter in order to perform comparison to results already reported in clinical studies.

Results

In pulsed mode, for a 3 mm vein diameter, irrespective of the pullback distance (2, 5 or 7 mm), a minimum fluence of 15 J/cm is required to obtain a permanent damage of the intima. For a 5 mm vein diameter, 50 J/cm (15W-2s) is required. In continuous mode, for a 3 mm and 5 mm vein diameter, respectively 65 J/cm and 100 J/cm are required to obtain a permanent damage of the vessel wall. Finally, the use of different wavelengths (810 nm or 980 nm) played only a minor influence on these results.

Discussion and conclusion

The parameters determined by mathematical modeling are in agreement with those used in clinical practice. They confirm that thermal damage of the inner vein wall (tunica intima) is required to achieve the tissue alterations necessary in order to lead the vein to permanent occlusion. However, in order to obtain a high rate of success without adverse events, the knowledge of the vein diameter after tumescent anesthesia is recommended in order to use the optimal energy. As clearly demonstrated by our calculations, both pulsed and continuous mode operations of the laser can be efficient. An interesting observation in our model is that less amount of energy is required in pulsed mode than in continuous mode. Damaging the vein sequentially along its entire length may lead to permanent occlusion. However, the pulsed mode requires a very precise positioning of the fiber after each pullback and the duration of the treatment is much longer. For these reasons, continuous irradiation seems to be preferred by most clinicians. This model should serve as a useful tool to simulate and better understand the mechanism of action of the ELT

Dynamic modeling of photothermal interactions for laser-induced interstitial thermotherapy: parameter sensitivity analysis

A two-dimensional model was developed to model the effects of dynamic changes in the physical properties on tissue temperature and damage to simulate laser-induced interstitial thermotherapy (LITT) treatment procedures with temperature monitoring. A modified Monte Carlo method was used to simulate photon transport in the tissue in the non-uniform optical property field with the finite volume method used to solve the Pennes bioheat equation to calculate the temperature distribution and the Arrhenius equation used to predict the thermal damage extent. The laser light transport and the heat transfer as well as the damage accumulation were calculated iteratively at each time step. The influences of different laser sources, different applicator sizes, and different irradiation modes on the final damage volume were analyzed to optimize the LITT treatment. The numerical results showed that damage volume was the smallest for the 1,064-nm laser, with much larger, similar damage volumes for the 980- and 850-nm lasers at normal blood perfusion rates. The damage volume was the largest for the 1,064-nm laser with significantly smaller, similar damage volumes for the 980- and 850-nm lasers with temporally interrupted blood perfusion. The numerical results also showed that the variations in applicator sizes, laser powers, heating durations and temperature monitoring ranges significantly affected the shapes and sizes of the thermal damage zones. The shapes and sizes of the thermal damage zones can be optimized by selecting different applicator sizes, laser powers, heating duration times, temperature monitoring ranges, etc.

A finite element method model to simulate laser interstitial thermo therapy in anatomical inhomogeneous regions

BACKGROUND

Laser Interstitial ThermoTherapy (LITT) is a well established surgical method. The use of LITT is so far limited to homogeneous tissues, e.g. the liver. One of the reasons is the limited capability of existing treatment planning models to calculate accurately the damage zone. The treatment planning in inhomogeneous tissues, especially of regions near main vessels, poses still a challenge. In order to extend the application of LITT to a wider range of anatomical regions new simulation methods are needed. The model described with this article enables efficient simulation for predicting damaged tissue as a basis for a future laser-surgical planning system. Previously we described the dependency of the model on geometry. With the presented paper including two video files we focus on the methodological, physical and mathematical background of the model.

METHODS

In contrast to previous simulation attempts, our model is based on finite element method (FEM). We propose the use of LITT, in sensitive areas such as the neck region to treat tumours in lymph node with dimensions of 0.5 cm - 2 cm in diameter near the carotid artery. Our model is based on calculations describing the light distribution using the diffusion approximation of the transport theory; the temperature rise using the bioheat equation, including the effect of microperfusion in tissue to determine the extent of thermal damage; and the dependency of thermal and optical properties on the temperature and the injury. Injury is estimated using a damage integral. To check our model we performed a first in vitro experiment on porcine muscle tissue.

RESULTS

We performed the derivation of the geometry from 3D ultrasound data and show for this proposed geometry the energy distribution, the heat elevation, and the damage zone. Further on, we perform a comparison with the in-vitro experiment. The calculation shows an error of 5% in the x-axis parallel to the blood vessel.

CONCLUSIONS

The FEM technique proposed can overcome limitations of other methods and enables an efficient simulation for predicting the damage zone induced using LITT. Our calculations show clearly that major vessels would not be damaged. The area/volume of the damaged zone calculated from both simulation and in-vitro experiment fits well and the deviation is small. One of the main reasons for the deviation is the lack of accurate values of the tissue optical properties. In further experiments this needs to be validated.

Thermal modeling of lesion growth with radiofrequency ablation devices

BACKGROUND

Temperature is a frequently used parameter to describe the predicted size of lesions computed by computational models. In many cases, however, temperature correlates poorly with lesion size. Although many studies have been conducted to characterize the relationship between time-temperature exposure of tissue heating to cell damage, to date these relationships have not been employed in a finite element model.

METHODS

We present an axisymmetric two-dimensional finite element model that calculates cell damage in tissues and compare lesion sizes using common tissue damage and iso-temperature contour definitions. The model accounts for both temperature-dependent changes in the electrical conductivity of tissue as well as tissue damage-dependent changes in local tissue perfusion. The data is validated using excised porcine liver tissues.

RESULTS

The data demonstrate the size of thermal lesions is grossly overestimated when calculated using traditional temperature isocontours of 42 degrees C and 47 degrees C. The computational model results predicted lesion dimensions that were within 5% of the experimental measurements.

CONCLUSION

When modeling radiofrequency ablation problems, temperature isotherms may not be representative of actual tissue damage patterns.

Implementation of a practical model for light and heat distribution using laser-induced thermotherapy near to a large vessel

This paper introduces a simulation model for light and heat transport in tissues including perfusion effects. The model enables an efficient simulation of the damaged zone induced with an optical fibre for laser interstitial thermotherapy (LITT). It is designed specially for, but not limited to, tissue ablation in the neck region near to vessels. We describe in detail the effects of the rise in temperature caused by the absorption of light in tissue, using the heat equation and including the cooling effects of flow in vessels and of microperfusion in tissue in order to determine the extent of thermal damage. The extent of the necrosis zone is calculated with a damage function at each point of a finite element method (FEM) mesh. The FEM mesh is implemented with FEMLAB 2.3 as an add-on for finite element modelling for Matlab 6.5. LITT for tumour ablation in liver and some other anatomical regions is a well-known and established method (Bundesärztekammer und Kassenärztliche Bundesvereinigung 2002 Assessment der Bundesärztekammer und der Kassenärztlichen Bundesvereinigung, Köln). Investigations of treatments using LITT in the neck region are still in progress. We propose a refined model to validate the LITT method in the future in another anatomic region, e.g., in the highly sensitive region of the neck. Our calculations show that in order to induce a lesion with a maximum diameter of about 1 cm near to a large vessel, an application time between 3 and 4 min is needed using a laser power of about 10 W with a Nd:YAG 1064 nm radiation wavelength.

Models and measurements of light intensity changes during laser interstitial thermal therapy: implications for optical monitoring of the coagulation boundary location

We have developed a multi-region spherical Monte Carlo (MC) model to simulate the dynamic changes in light intensity measured during laser interstitial thermal therapy (LITT). Model predictions were validated experimentally in tissue-simulating albumen phantoms with well-characterized optical properties that vary dynamically with LITT in a way similar to tissue. For long treatments (2.5 W, approximately 1800 s), the transient light intensity changes demonstrated better qualitative agreement with a three-region MC model (with an inner layer of fully coagulated optical properties, a middle layer of partially coagulated properties and an outer region of native properties); for short treatments (4 W, approximately 240 s), better qualitative agreement was seen with a two-region MC model (with an inner layer of fully coagulated properties and outer region of native properties). These differences were attributed to differences in coagulation formation during low- and high-powered heating regimes, respectively. At the end of heating, a three-region coagulation zone was observed for both heating schemes. Quantitatively, final light intensity changes at the end of heating were compared with changes predicted by both two- and three-region MC for the same experimentally measured coagulation size and found to agree within approximately 30% for both models. The developed MC model helps lend insight into the nature of thermal coagulation events occurring for low and high power LITT irradiation schemes.

Changes in relative light fluence measured during laser heating: implications for optical monitoring and modelling of interstitial laser photocoagulation

Dynamic changes in internal light fluence were measured during interstitial laser heating of tissue phantoms and ex vivo bovine liver. In albumen phantoms, the results demonstrate an unexpected rise in optical power transmitted approximately I cm away from the source during laser exposure at low power (0.5-1 W), and a decrease at higher powers (1.5-2.5 W) due to coagulation and possibly charring. Similar trends were observed in liver tissue, with a rise in interstitial fluence observed during 0.5 W exposure and a drop in interstitial fluence seen at higher powers (1-1.5 W) due to tissue coagulation. At 1.5 W irradiation an additional, later decrease was also seen which was most likely due to tissue charring. Independent spectrophotometric studies in Naphthol Green dye indicate the rise in fluence observed in the heated albumen phantoms may have been primarily due to light exposure causing photobleaching of the absorbing chromophore. and not due to heat effects. Experiments in liver tissue demonstrated that the observed rise in fluence is dependent on the starting temperature of the tissue. Correlating changes in light fluence with key clinical endpoints/events such as the onset of tissue coagulation or charring may be useful for on-line monitoring and control of laser thermal therapy via interstitial fluence sensors.