Purpose: Optical stimulation methods in development aim to provide high spatial selectivity of target tissue, overcoming a critical limitation of contemporary neural prostheses. The purpose of this study is to determine if tapered fibers are capable of delivering the minimum necessary power density 1W/mm2 within a 0.050mm spot size to induce focused infrared neural stimulation (INS). Materials: A numerical simulation program based on equations derived from Snell’s law was developed in MATLAB to predict the energy emitted from a tapered fiber coupled to a Capella laser (λ=1863nm, Lockheed Martin Aculight). Energy predictions were compared to emittance from a tapered fiber (core diameter = 200µm, tapered output face = 50µm, NA=0.22) to determine its accuracy. Energy measurements were made at 17.8, 41.6, 65.4, 89.3, and 113.1µJ output energy and at distances between 0-2 mm from the fiber-tip with a Coherent FieldMax Energy meter coupled to a detector with a 2.1 mm aperture. Results: Mean difference between the predicted and measured energy ranged from 4.3±1.9µJ (17.8µJ) to 16.3±11.3 µJ (113.1µJ). Minimum required power density within a 0.05 mm spot size was predicted to be achieved at 0 mm for all energies, at 2 mm for 41.6µJ, and at distances ≥ 1.0 mm for 17.8 µJ. Conclusion: A numerical simulation program was developed that accurately predicts within minimal error the emittance from a tapered fiber. The predicted results indicate feasibility of tapered optical fibers to provide a more efficient and selective means of delivering the minimum power density necessary to achieve INS.
Purpose: To evaluate the feasibility to induce lens epithelial cell death with intraoperative hyperthermia for prevention of secondary cataract. Methods: A prototype miniature resistive hyperthermia probe was designed. The probe contained a thermocouple for temperature feed-back. A timer allowed monitoring of the electrical driving of the hyperthermia probe and the temperature induced as a function of time. To model the heating response, a simple model of the lens capsule was constructed using a thin acrylic plastic shell embedded in a sponge immersed in a water bath at 37°C. The shell was filled with sodium hyaluronate. The probe was positioned at the center of the shell with the thermocouple next to the wall. An experimental protocol was developed to assess the feasibility of increasing the temperature of the human lens to hyperthermia levels in fresh cadaver eyes: An annular metal ring was bonded just below the limbus, the cornea and iris were sectioned, the lens material was removed through a central 5mm diameter capsulorhexis, the capsule was filled with SHA and the globe was set on a temperature-controlled cylindrical vial. Preliminary Results: At 3.3W (2.2V, 1.5A) the shell's content increases from 37°C to 51°C in 30s. At that temperature, LEC death is expected to occur within 1sec. Conclusion: This preliminary study demonstrates the feasibility of increasing the temperature of the capsular bag to kill LECs by hyperthermia.
The purpose of these preliminary experiments was to calculate the activation energy and the frequency factor constants of the Arrhenius equation for prediction of the denaturation zone produced by laser interstitial thermotherapy in female breast tissue models. In a first step, pairs of parameters consisting of the activation energy (ΔE) and frequency factor (A) were calculated for different half denaturation times at 50°C and 60°C. Pairs of parameters were eliminated if the calculated time of denaturation at 70°C was excessively short (<0.1s). In a second step, denaturation was calculated over time for each remaining pair of parameters using temperature measurements performed during a laser heating experiment in ex-vivo porcine mammary chain tissue. The zone of denaturation in the tissue after laser heating was 2.0 cm x 3.5 cm. One pair of Arrhenius parameters was chosen whose denaturation plot showed no denaturation outside the experimentally-observed denaturation zone, 50% denaturation at the border of the experimental denaturation zone, and 100% denaturation within the experimental denaturation zone after 1200 seconds. The activation energy and frequency factor which best fit our experimental data were ΔE=3.02x105 J mol-1 and A=1.18x1044 s-1, respectively. These values fall within the general range given in the literature.
Purpose: The purpose of this work was to develop a model to predict and compare laser-trabecular meshwork interactions during laser trabeculoplasty with three different lasers. Methods: A qualitative multilayer optical-thermal model of the trabecular meshwork was developed. The model was used to estimate the penetration depth and heat diffusion volume during laser trabeculoplasty with a continuous wave argon laser (ALT), a Q-switched frequency-doubled Nd:YAG laser (SLT), and a flashlamp-pumped near-infrared alexandrite or titanium sapphire laser emitting microsecond pulses (μs-IRLT). Results: The model predicts that both SLT and μs-IRLT produce selective heating of pigmented trabecular meshwork cells with negligible heat diffusion to surrounding structures and with a deeper penetration at the IR wavelength. A preliminary quantitative analysis indicates that selective targeting of spherical pigments is achieved as long as the pulse duration remains less than approximately 1μs. Conclusion: The qualitative model indicates that infrared laser trabeculoplasty with microsecond pulses can produce selective targeting of pigemented trabecular meshwork cells with a deeper penetration than SLT.
The potential of ns pulses of 266 nm from a frequency shifted Nd:YAG laser for corneal ablation was tested on human eye bank eyes. Scanning electron microscopy demonstrated that consecutive pulses in the same spot induced thermal damage that was related to the pulse energy. The ablation threshold was estiamted to 4.5 J/cm2. The ablation per pulse slightly above threshold was 0.2-0.6 μm. It is concluded that the precision of the ablation is relevant for refractive ablations of the cornea. The higher radiation exposure required for corneal ablation at 266 nm compared to that for shorter wavelengths induces higher energy load on the cornea. To avoid heat build-up, a random flying spot is probably needed.
Purpose: the purpose of these experiments was to evaluate the temperature and predicted cell kill distribution during LITT of breast tissue phantoms at 940nm and compare with the results of previous experiments ar 830 nm and 980 nm. Material and Methods: A Dornier Medilas D Skinpulse 940 nm diode lase system coupled to a Dornier D-6111-T2 fiber (Dornier Surgical Products, Phoenix, AZ) was used to irradiate the porcine tissue three times at approximately 5 W for 10 minutes. Results: The initial rate of temperature increase at 940 nm for locations 5, 10, and 15 mm from the fiber axis ranged from 0.076 to 0.142 °C/s, 0.027 to 0.041 °C/s, and 0.008 to 0.013 °C/s, respectively, wile the maximum temperature increase ranged from 37.8 to 46.9 °C, 19.3 to 26.1 °C, 8.6 to 13.0 °C, respectively, temperature curves hed lower slopes at 940 nm than at 830 nm and 980 nm. the maximum temperature increase was higher at 940 nm than at 830 nm. Predicted area of 100% cell kill was approximately 2 cm by 2 cm Conclusion: Results of experiments at 940 nm were more comparable to those at 980 nm than at 830 nm.
The purpose of these experiments was to compare the temperature increase in ex-vivo porcine mammary chain tissue as a breast tissue model during interstitial laser irradiation with diode lasers emitting at 980 nm and 830 nm. Both wavelengths were delivered at 4.0 W for 10 minutes through a diffusing fiber inserted into ex-vivo porcine tissue. The temperature was measured with a set of 15 thermocouples placed 5, 10, and 15 mm from the fiber axis. The initial rate of temperature increase 5 mm away from the fiber tip was higher at 980 nm (0.12 to 0.20 degree(s)C/s) than at 830 nm (0.10 to 0.16 degree(s)C/s). At 10 mm and 15 mm (areas with less radiation), the rate was smaller than at 5 mm (less than 0.06 degree(s)C/s at 10 mm and less than 0.02 degree(s)C/s at 15 mm) for both wavelengths with no significant difference between the 980 nm and 830 nm radiation. The temperature increase at 5, 10 and 15 mm away from the fiber tip after 10 minutes of irradiation was higher at 980 nm (36 to 45 degree(s)C at 5 mm, 14 to 30 degree(s)C at 10 mm and 9 to 17 degree(s)C at 15 mm) than at 830 nm (27 to 33 degree(s)C at 5 mm, 11 to 17 degree(s)C at 10 mm and 8 to 9 degree(s)C at 15 mm) after 10 minutes. These results were found to be highly dependent on tissue composition (muscle vs. fatty tissue).
Experiments on the feasibility of x-ray monitoring of laser interstitial thermotherapy (LITT) of breast cancer are presented. Digital images of heated tissue phantoms (including fatty porcine tissue and polyacrylamide/acrylate hydrogel), obtained from an x-ray stereotactic biopsy system, were analyzed for changes in pixel intensity. Tissue phantoms were heated either by a 980 nm diode laser or by resistive heating elements in a custom-made chamber. Analysis was conducted by evaluating the changes in average pixel intensity of regions of interest. Image constancy and stability of the x-ray system were also investigated. Slight changes (less than 5%) dependent on the temperature of the tissue phantom were observed, while changes caused by heat- induced tissue displacement and/or shrinkage were more significant (greater than 10%).
A tissue phantom for experimental studies in Laser Interstitial ThermoTherapy (LITT) for the treatment of small breast tumors was developed and evaluated. The tissue phantom consists of a polyacrylamide/acrylate hydrogel matrix containing various concentrations of an absorber (Nigrosin) and a scatterer (Intralipid 10% solution), allowing the optical properties to be varied in accordance to experimental need. Temperature measurements in the phantom were performed with a thermocouple array placed symmetrically around the fiber axis during laser irradiation for different output powers, treatment duration, and different concentrations of absorber. A 980 nm diode laser system was used in conjunction with an REM LightStic 360 diffusing tip fiber. The polyacrylamide/acrylate matrix tissue phantom remained stable during laser irradiation and produced reproducible results. The behavior of the temperature curves produced by the phantom during laser irradiation was similar to the behavior of the temperature curves in ex-vivo tissue. Therefore, this tissue phantom can be used as a model for the thermal response of tissue during laser interstitial thermotherapy. The phantom will be used as an experimental model to determine a set of optimum laser treatment parameters for laser interstitial thermotherapy of breast cancer.
Purpose: Laser interstitial thermotherapy is a promising minimally- invasive technique for the treatment of small cancers of the breast that are currently removed surgically lumpectomy. The purpose of this work was to analyze in situ temperature fields recorded with stainless-steel thermocoupled probes during experimental laser interstitial thermo-therapy (LITT).
Methods: Both a CW Nd:YAG laser system emitting 20W for 25 to 30s and a 980 nm diode laser emitting 10 to 20 W for up to 1200s delivered through a fiber-optic probe were used to create localized heating in fatty cadaver pig tissue and milk as phantoms. To quantify an artifact due to direct heating of the thermocouple probes by laser radiation, experiments were also performed in air, water and intralipid solution. The temperature field around the fiber-optic probe during laser irradiation was measured every 0.3 s or 1 s with an array of up to fifteen needle thermocoupled probes. The effect of light absorption by the thermocouples probes was quantified and the time-dependence of the temperature distribution was analyzed.
Results: After removal of the thermocouple artifact, the temperature was found to vary exponentially with time with a time constant of 600 to 700 s.
Conclusions:The time-dependence of the interstitial temperature can be modeled by exponential functions both during ex vivo and in vivo experiments.
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