Proceedings Volume 3601

Laser-Tissue Interaction X: Photochemical, Photothermal, and Photomechanical

Steven L. Jacques, Gerhard J. Mueller, Andre Roggan, et al.
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Proceedings Volume 3601

Laser-Tissue Interaction X: Photochemical, Photothermal, and Photomechanical

Steven L. Jacques, Gerhard J. Mueller, Andre Roggan, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 14 June 1999
Contents: 11 Sessions, 59 Papers, 0 Presentations
Conference: BiOS '99 International Biomedical Optics Symposium 1999
Volume Number: 3601

Table of Contents

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Table of Contents

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  • Ophthalmic Effects of Ultrashort Pulses
  • Immunologic Responses to Thermal and PDT Effects
  • Photochemical and Photo-Oxidative Mechanisms
  • Photothermal Interactions
  • Photomechanical Interactions
  • Optoacoustic Diagnostics and Therapy
  • Optical Dosimetry and Monitoring of Therapy
  • Laser Ablation
  • Laser-Tissue Interaction
  • Basic Investigations
  • Simulation, Monitoring, and Clinical Applications
  • Photochemical and Photo-Oxidative Mechanisms
  • Ophthalmic Effects of Ultrashort Pulses
  • Photomechanical Interactions
  • Optical Dosimetry and Monitoring of Therapy
Ophthalmic Effects of Ultrashort Pulses
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Retinal damage mechanisms and safety for ultrashort laser exposure
For the past several years the US Air Force has led a research effort to investigate the thresholds and mechanisms for retinal damage from ultrashort laser pulses [i.e. nanosecond (10-9 sec) to femtosecond (10-15 sec) pulse widths]. The goal was to expand the biological database into the ultrashort pulse regime and thus to allow establishment of maximum permissible exposure limits for these lasers. We review the progress made in determining trends in retial damage by ultrashort laser pulses in the visible and near infrared, including variations in spot size and number of pulses. We also discuss the most likely damage mechanisms operative in this pulse width regime and discuss relevance to laser safety.
Methods of achieving three-dimensional reconstruction of tissue at the ultrastructural level demonstrating the distribution of melanosomes within retinal pigment epithelium
Cynthia A. Toth M.D., Ewa Worniallo, Sarah F. Bailey, et al.
In predicting and measuring laser effect on retinal tissue for most of the visible to near infrared spectrum, one is concerned with the melanosome as the major absorber of incident energy. Differences in the location and density of melanosomes in the retinal pigment epithelium may have an impact on the effect of laser energy delivered to those tissues. Current models use estimates of numbers of melanosomes usually in an even distribution across a 5 - 8 micrometer deep volume. The goal of our study is to identify the three-dimensional distribution of melanosomes within the retinal pigment epithelium (RPE) for the use of those modeling laser tissue effects. We examined normal retinal pigment epithelium using three-dimensional (3-D) reconstruction from images obtained by transmission electron microscopy (TEM), light microscopy (LM) and confocal microscopy. Images were captured on a digital camera system attached to the microscope for both the transmission electron and light microscopy. Three-dimensional reconstruction was performed after digital deconvolution of microscopic images (Vaytek, Inc.). Three- dimensional images were then utilized for analysis of distribution of melanosomes and organelles within the pigment epithelial block. The distribution of melanosomes will be useful for accurate mathematical modeling of laser impact on the retina.
Multiple pulse thresholds in live eyes for ultrashort laser pulses in the near infrared
Damage thresholds using multiple laser pulses to produce minimum visible lesions (MVL) in rhesus monkey eyes are reported for near-infrared (800 nm) at 130 femtoseconds. Previous studies by our research group using single pulses in the near-infrared (1060 nm) have determined damage thresholds and retinal spot size dependence. We report the first multiple pulse damage thresholds using femtosecond pulses. MVL thresholds at 1 hour and 24 hours postexposure were determined for 1, 100 and 1,000 pulses and we compare these with other reported multiple pulse thresholds. These new data will be added to the databank for retinal MVL's as a function of pulse repetition rate for this pulsewidth and a comparison will be made with the ANSI standard for multiple pulse exposures. Our measurements show that the retinal ED50 threshold/pulse in the paramacula decreases for increasing number of pulses. The MVL-ED50 at the threshold/pulse decreased by a factor of 4 (0.55 (mu) J to 0.13 (mu) J/pulse) for an increase from 1 to 100 pulses.
Cavitation thresholds in the rabbit retinal pigmented epithelium
Dale J. Payne, Thomas R. Jost, James J. Elliot, et al.
We performed measurements to examine retinal injury from laser pulses in the sub-nanosecond time regime. Both ex-vivo porcine and Dutch belted rabbit retinal pigment epithelium (RPE) models were used in conjunction with time resolved imaging to observe cavitation bubble formation. Included in this study are 3 ps, 300 fs, and 100 fs pulses at a wavelength of 580 nm, as well as 70 ps and 5 ns pulses at 532 nm. Threshold values varied between 37 mJ/cm2 and 50 mJ/cm2 across the range of pulse widths. Following laser irradiation cell viability was checked using a fluorescent dye marker (calcein). Our current results are compared to an earlier investigation using the artificial retina model.
Histopathology of ultrashort pulsed laser retinal damage: changing retinal pathology with variation in spot size for near-infrared laser lesions
Cynthia A. Toth M.D., Katrina P. Winter, Michelle L. Norton McCall, et al.
We wish to identify the change in extent of retinal tissue injury due to varying the spot size at the retina of ultrashort laser pulses. We compared the effects of delivery of near infrared (1060 nm) single laser pulses to an 800 micron diameter retinal spot to previously reported laser retinal effects. We examined macular lesions 24 hours after delivery of near-infrared (1060 nm wavelength) ultrashort laser to 804 micron spot-size, using fundus examination, fundus photographs and fluorescein angiograms. Using light microscopy, we examined sections of these lesions obtained 24 hours after laser delivery. The degree of retinal damage was compared to our data published previously by using a modified version of our previous grading scale. The 150 fs near infrared, large spot laser lesions were remarkable in their clinical and pathological appearance. The lesions, rather than centering on a single focal spot of pallor as typically seen in pulsed laser lesions of the retina, demonstrated a spotted pattern of multiple focal lesions across the area of laser delivery. There was also choroidal damage in several eyes but the Bruch's membrane remained intact. Although there was choroidal damage in the 150 fs near infrared wavelength small spot laser lesions there was not significant thermal spread. The small spot ultrashort visible wavelength showed no significant thermal spread and no choroidal damage. Larger spot-size demonstrated a broader area of damage than that of the smaller spot-size and different choroidal effect when compared to smaller sized lesions.
Comparison of retinal damage thresholds of laser pulses in the macula/paramacula regions of the live eye
Single pulses in the near-infrared (800 nanometers) were used to measure retinal minimum visible lesion (MVL) thresholds in rhesus monkey eyes at a pulse width of 130 femtoseconds (fs) within both the macula and paramacula regions. We report the MVL thresholds, determined at 1 hour and 24 hours post exposure, which were obtained within the macula and adjacent paramacula. This data will provide a direct comparison of the sensitivities of different retinal areas to laser injury and provide additional insight to laser damage. These new data points will be added to the databank for MVLs for single pulses. The MVL-ED50 threshold for the macula was measured to be 0.35 (mu) J at 24 hours postexposure, which compares with 0.43 (mu) J measured at 580 nm and the 0.17 (mu) J measured at 532 nm in our laboratory. Our measurements show that the retinal ED50 threshold in the paramacula was larger by a factor of 1.6 than in the macula. This factor of 1.6 is in good agreement with the factor of 1.1 to 2.5 reported in previous studies.
Modeling of pressure generation by lasers in melanosomes: how to avoid stress confinement and blow up a melanosome
Jinming Sun, Bernard S. Gerstman
We have made a detailed theoretical investigation of the pressure generated in melanosomes upon absorption of laser energy. Our model treats the melanosome as a homogeneous absorber surrounded by a transparent water medium. The mechanical impedance mismatch between the absorbing melanosome and the surrounding medium is taken into account and has important ramifications. We have calculated the pressure profiles for pulses of microsecond duration down to picosecond duration. We show that the concept of stress confinement is not valid for this system. Though pressure amplitudes generated at the surface do reach a limit as the pulse duration is shortened below a nanosecond, no such pressure confinement occurs at the center of the absorber. As the pulse duration is shortened below a nanosecond, the tensile pressure at the center continues to rise without limit. This implies that explosive fracturing of the melanosome can occur due to the large tensile stresses, and accomplishing this fracturing requires smaller fluences as the pulse duration is shortened. We present quantitative results exhibiting how the core's tensile stress increases as the pulse duration is shortened.
Determination of single-pulse exposure threshold (ED50) for cornea for 1400- to 2000-nm lasers
Thomas E. Johnson, Thomas A. Eggleston, David J. Fletcher D.V.M., et al.
We report on preliminary work undertaken to determine ED50 thresholds for both skin and cornea exposure from 1400 to 2000 nm laser light. Work presented here is focused initially on 1540 nm exposures to both human skin and cornea. Light microscopy and confocal microscopy used to help understand the type of photon-tissue interactions responsible for skin and corneal injury are discussed along with preliminary results from these techniques. Further, we report on in vivo models which are considered to best represent human skin for laser tissue interaction studies. Additionally, in vitro models for corneal exposure are discussed as replacement models for in vivo corneal exposures.
Immunologic Responses to Thermal and PDT Effects
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Antitumor immune reaction elicited by photodynamic therapy
This work examines why photodynamic therapy (PDT) is capable of eliciting a strong immune reaction against treated solid tumors. It is postulated that this phenomenon originates from the basic charter of the insult inflicted by the photodynamic treatment, which is dominated by singlet oxygen-mediated oxidative stress. The early event associated with this initial impact, which is of major relevance for the development of immune response, is the generation of photo-oxidative lesions responsible for the activation of cellular signal transduction pathways and consequent induction of stress proteins. Importantly, these lesions, as well as other types of PDT mediated oxidative injury, have a strong pro-inflammatory character. It is suggested that the antitumor immune response is primed and propagated by the PDT-induced inflammatory process. Of critical importance for the immune recognition of treated tumor is the generation of large amounts of cancer cell debris that occurs rapidly following PDT treatment.
Photothermal and immunological reactions against metastatic tumors using laser photosensitizer immunoadjuvant
Wei R. Chen, Ahmad El-Samad, Robert E. Nordquist
Photothermal tissue interaction is the most common phenomenon when laser energy is deposited in tissue. Because of the sensitivity of cancer cells to temperature increase, photothermal reaction can be an effective mechanism of direct cancer destruction using lasers. Tumor-specific immune response is crucial in achieving systemic and long-term cures for cancers, particularly for metastatic cancers. Can photothermal interaction induce sufficient immunological reaction when the local destruction of tumor cells occurs? To achieve selective photothermal destruction, indocyanine green as a photosensitizer was directly injected into rat mammary tumors, followed by irradiation of 805 nm laser light. Although extensive photothermal tumor killing was achieved and tumor growth was slowed down immediately following the treatment, photothermal reaction alone was shown not sufficient in controlling the treated primary tumors and their metastases. When an immunoadjuvant was used with the indocyanine green, however, the same laser treatment not only could eventually eradicate the treated primary tumors but also eradicate the untreated metastases at remote sites. The tumor eradication went through a growth-regression process over a period of six to nine weeks post-treatment, indicating an induced immune response. The Western Blot analysis using the serum from a laser-immunotherapy cured rat showed that the tumor-specific antibody induced by the treatment had a long- lasting effect. Our experimental data indicated that photothermal interaction alone was not sufficient to slow and eventually reverse tumor growth. However, it can reduce the tumor burden and at the same time release tumor antigens to be recognized by the host immune system. Therefore, in conjunction with specific immunological stimulation using in situ immunoadjuvants, the selective thermal injury to tumors plays an important and a direct role in this laser immunotherapy.
Photodynamic therapy and immune response in tumor-bearing mice
Since immune response of the host is important in the control of tumor growth and spreading, and the Photodynamic therapy (PDT) is able to increase the antitumor immunity, in our laboratory we examine the effect of PDT on immune compartment of tumor bearing mice. Lymphocytes and macrophages collected from tumor bearing mice pretreated with PDT are cytotoxic in vitro and in vivo against the parental tumor lines, in contrast the same immune cells population collected from tumor bearing mice pretreated only with laser light are unable to lyse the parental tumor cells. In adoptive immunotherapy experiments, treatment of mice bearing MS-2 tumor with adoptive transfer of immune lymphocytes collected from mice pretreated with PDT is able to significantly increase the survival time; in contrast the lymphocytes collected from mice pretreated only with laser light were not able to modify the survival time suggesting that the laser treatment alone did not increase the immune response of the host. In conclusion these results demonstrate that the PDT induce a strong immune response on the host and the stimulated lymphocytes generated could be used for an adoptive immunotherapy approach; moreover laser treatment alone (thermal effect) is unable to modulate the immune response of the host.
Photochemical and Photo-Oxidative Mechanisms
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Ultraviolet light attenuates vasospasm development: low-intensity 325-nm illumination in animal model
Yuji Morimoto, Kanji Nakai, Tsunenori Arai, et al.
We studied whether prophylactic low-intensity ultraviolet light (UV) irradiation reduces development of delayed cerebral vasospasm in rabbit model. Eleven rabbits had carotid angiography and placement of silicon sheaths around the right common carotid artery (CCA). Just before the placement, CCAs of 6 in the 11 animals were adventitiously exposed to UV (10 mJ/mm2) emitted from a helium-cadmium laser (wavelength equals 325 nm). Sheaths were filled with autologous blood in all of 11 animals. Vasospasm developed 24 to 48 hours (Day 2) later. The CCA luminal diameter without UV irradiation was 77% of the Day 0 state. The prophylactic laser treatment reduced the development of vasospasm, 90% of the Day 0 state. Histological examination of the treated CCA revealed extended smooth muscle cells. There was no endothelial damage. These results suggest that UV is effective for prophylaxis of cerebral vasospasm.
Intracellular photochemical reactions in the RPE cell exhibit a wavelength dependence that resembles the action spectrum of melanin
Randolph D. Glickman, Meena Vendal, Mary Ann Gonzalez, et al.
Melanosomes isolated from retinal pigment epithelial (RPE) cells support photochemical oxidation of cellular components when excited by visible light. These reactions have an action spectrum peaking between 450 and 500 nm. We now report that similar, wavelength-dependent reactions occur within intact RPE cells. The chemical probes, 2', 7'-dichlorofluorescein and dihydrorhodamine 123, are non-fluorescent when reduced and fluorescent when oxidized. Cultured bovine and baboon RPE cells were labeled with these probes, and then exposed to quantum-equivalent, 488, 514.5 or 647.1 nm emissions from Argon and Krypton ion CW lasers. The probes were isolated from the cells by solid phase extraction, and the amount of oxidized probe quantified by HPLC with fluorescence detection. Alternatively, cells were imaged with a fluorescence microscope. Images were acquired at various intervals after the cells were exposed to blue ((lambda) max equals 490 nm) and yellow ((lambda) max equals 582 nm) wavelengths derived from the microscope exciter lamp. The kinetics and amplitude of the fluorescence change in the cells were quantified with image processing software. Both types of experiments yielded the conclusion that blue-green wavelengths, on a quantal basis, most efficiently induced photo-oxidative stress in the pigmented cells. The microscopy also indicated that fluorescence was restricted to the cytoplasm. These findings are consistent with the involvement of melanosomes in photo- oxidative reactions.
Photodynamic processes of cholesteryl oleate accumulated on the arterial wall using IR free electron lasers
Kunio Awazu, Katsuo Aizawa
The selective removal of cholesterol esters accumulated in the intra-cellular or extra-cellular spaces has clinical significance. In the present work we investigate the removal of cholesterol esters by using a free electron laser (FEL) in an arteriosclerotic region. Thin films of cholesteryl oleate and albumin, and the cross section of a rabbit artery were placed on an inverted microscope stage and the changes caused by the FEL irradiation of 5.75 micrometer and 6.1 micrometer with 1.5 - 3 mW in average were monitored continuously by a CCD camera in real time. FEL irradiation at a wavelength of 5.75 micrometer, which is a stretching vibrational mode of the ester, was able to ablate cholesterol esters without affecting albumin and that it can also remove cholesterol esters from rabbits' arteriosclerotic arterial walls.
Photodynamic treatment of the RIF-1 tumor with verteporfin with online monitoring of tissue oxygen using electron paramagnetic resonance oximetry
Brian W. Pogue, Julia A. O'Hara, Ke Jian Liu, et al.
In this study, treatment of the RIF-1 tumor was examined with photodynamic therapy using Verteprofin (formerly benzoporphyrin derivative, BPD). The effects of two different optical dose rates were examined, with no detectable difference in the tumor regrowth time. Oxygen consumption during PDT could reliably be monitored with electron paramagnetic resonance (EPR) oximetry using an implanted paramagnetic material within the tumor. A reduction of the tumor pO2 was detected in the animals that were followed after treatment, suggesting that there was a compromise to the tumor vasculature that persisted throughout the measurements. At high total doses some of the tumors did not regrow. Altogether these results are indicative of the tumor destruction being caused by destruction of the blood vessels from the treatment.
Photothermal Interactions
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Effect of wavelength on threshold and kinetics of tissue denaturation under laser radiation
Emil N. Sobol, Alexander P. Sviridov, Moishe S. Kitai, et al.
We consider the denaturation process as an alteration in ordered organization of tissue structure and study the threshold and kinetics of laser-induced denaturation in cartilage and cornea undergoing irradiation from a free electron laser (FEL) in the wavelength range 2.2 - 8.5 (mu) . Light-scattering by cartilage samples was measured in real- time during FEL irradiation using a 630-nm diode laser and a diode array with time resolution of 10 ms. We found that denaturation threshold is slightly lower than that for cartilage, and both depend on laser wavelength. A strong inverse correlation between denaturation thresholds and the absorption spectrum of the tissue is observed. Only for the wavelength region near the 3 (mu) water absorption band was the denaturation threshold not inversely proportional to the absorption coefficient. We believe this was because the radiation penetration depth was very small in this high- absorption region, so tissue denaturation occurred only in a layer too thin to produce significant light scattering. ATR spectra of 2.4 mm thick cartilage samples was measured before and after irradiation at 6.0 and 2.2 (mu) . At 6.0 (mu) , where the absorption is high, the spectrum of the irradiated (front) surface showed changes, while the spectrum of the back surface was identical to that before irradiation. This difference results from dramatic denaturation (with chemical bond breaking) at the front surface due to laser heating in a small absorption depth. For 2.2 (mu) irradiation, where the absorption is small, the spectra of the front and back of the irradiated sample were unchanged from before irradiation, while light scattering alteration shown the denaturation process began, for laser fluences above the denaturation threshold. This indicates that the absorption is too small to produce deep denaturation of the tissue with dramatic alteration of structure. Thus, we have shown that light scattering is useful for measuring denaturation thresholds and kinetics for biotissues except where the initial absorptivity is very high.
Comparison of cooling criteria with a cryogen spray and water/air spray
Jonathan Exley, Mark Russell Dickinson, Terence A. King, et al.
Skin cooling using a cryogen spray (tetrafluoroethane) has been shown to dramatically reduce the skin surface temperature whilst predictions show that the underlying dermal tissue is unaffected. This technique is repeated with a chilled water spray, along with a continuous airflow to enhance evaporation. Radiometric skin surface temperature measurements are recorded during trials utilizing this technique and the results are compared with theoretical predictions in order to determine the mechanism by which the heat is removed from the skin. The optimum spray conditions are achieved when the water is chilled to around 2 degrees Celsius with a continuous airflow of 50 liters/minute. Under these conditions skin surface temperature reduction is about 8 degrees Celsius - 10 degrees Celsius. The measured radiometric skin surface temperature change indicates that the mechanism by which this process removes heat from the skin is predominantly evaporation. Predictions of skin temperature change with varying skin depth indicate that the optimum spray time is around 100 ms.
Numerical simulations of a diode laser BPH treatment system
Numerical simulations are presented of the laser-tissue interaction of a diode laser system for treating benign prostate hyperplasia. The numerical model includes laser light transport, heat transport, cooling due to blood perfusion, thermal tissue damage, and enthalpy of tissue damage. Comparisons of the stimulation results to clinical data are given. We report that a reasonable variation from a standard set of input data produces heating times which match those measured in the clinical trials. A general trend of decreasing damage volume with increasing heating time is described. We suggest that the patient-to-patient variability seen in the data can be explained by differences in fundamental biophysical properties such as the optical coefficients. Further work is identified, including the measurement and input to the model of several specific data parameters such as optical coefficients, blood perfusion cooling rate, and coagulation rates.
Thermal imaging of laser-tissue interaction using color Schlieren techniques quantified by ray-tracing simulation
In various studies the use of a color Schlieren technique to visualize the dynamic thermal effects of lasers in an aqueous environment has proven to be very useful. Besides for the research, this setup has also proven to be very successful for education and demonstration purposes. This 'pseudo thermal imaging' technique has also been applied to study the behavior of diathermia devices and ultrasound resectors used in surgery. Since, the images reflect gradients in refractive index induced by thermal gradients, they can not simply be converted and interpreted as thermal images. In order to understand the pathway of light through thermal gradients, a ray-trace program was developed. The program is capable of visualizing the path of light rays through simulated thermal gradients as well as generating an image, which can be compared with the 'real' images from the color Schlieren setup. For calibration, the program was successfully tested on well defined optical configurations such as spherical and index-gradient lenses. Images calculated using data from a temperature profile measured with a small thermocouple appeared to be almost similar to the actual Schlieren image. Matching the calculated and actual image was possible by either assuming a minimal error in the temperature measurements or in the temperature dependence of the refractive index. The ray-trace program has been a helpful tool to quantify the absolute temperatures in color images from simple geometries. Expanding the code might enable the quantification of more complex temperature gradients. Such information is valuable for the clinical application of energy source such as lasers.
Interaction of radiation with microparticles for enhancement of drug delivery in tumors
Various promising anti-cancer drugs have been proposed for cancer therapy for the past two decades. However, limited success has been achieved in cancer therapy. Slow diffusion of the drugs (especially macromolecular ones with molecular weight greater than 5,000) in the interstitium and their poor penetration through tumor vasculature and cancer cell membrane explain partially this paradox. A novel technique is proposed to alter properties of these barriers and enhance drug delivery in cancer cells with minimal damage to normal tissues. This technique is based on interaction of exogenous nano- and microparticles selectively delivered in tumor blood vessels with radiation resulting in cavitation near the particles. Cavitation may perforate tumor blood vessels and cancer cell membranes and induce microconvection in the interstitium. This yields increased penetration of anti-cancer drugs from blood into cancer cells. Preliminary experiments were performed with various tissues (liver, muscle, kidney, lung) to demonstrate penetration and migration of particles and macromolecules in the interstitium upon irradiation by laser pulses or ultrasound. Q-switched Nd:YAG and Alexandrite laser pulses and ultrasound with the frequency of 50 kHz were used in these studies. Particle penetration was 30 - 180 micrometer upon 10-min irradiation by laser pulses or ultrasound that is comparable to the average distance of 200 micrometer between tumor capillaries. Considerable enhancement of delivery of FITC-dextran (M.W. equals 12,000) simulating macromolecular anti-cancer drugs in these tissues was obtained. Our results suggest that the proposed technique can potentially enhance delivery of anti-cancer drugs in tumors of various organs.
Photomechanical Interactions
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Self-consistent evolution of tissue damage under stress wave propagation
Moshe Strauss, Yitzhak Kaufman, Micha Sapir, et al.
Laser-initiated stress waves are reflected from tissue boundaries, thereby inducing tensile stresses, which are responsible for tissue damage. A self-consistent model of tissue failure evolution induced by stress wave propagation is considered. The failed tissue is represented by an ensemble of spherical voids and includes the effect of nucleation, growth and coalescence of voids under stress wave tension. Voids nucleate around impurities and grow according to an extended Rayleigh model that includes the effects of surface tension, viscosity and acoustic emission at void collapse. The damage model is coupled self-consistently to a one-dimensional planar hydrodynamic model of stress waves generated by a short pulse laser. We considered the problem of a bipolar wave generated by a short pulse laser absorbed on a free boundary of an aqueous system. The propagating wave includes a tensile component, which interacts with the impurities of exponential distribution in dimension, and an ensemble of voids is generated. For moderate impurity density (approximately 108 cm-3) void growth reduces the tensile wave component and causes the pressure to oscillate between tension and compression. For low impurity density (approximately 106 cm-3) the bubbles grown on a long time scale (5 - 10 microseconds) relative to the wave interaction time (approximately 100 nsec). At later times the growing bubbles interact with each other causing pressure oscillations between tension and compression, with an average compression pressure below 1 bar. This effect increases considerably the bubble lifetime consistent with experiments. At the collapse stage small bubbles collapse earlier and induce pressures, which reduce the collapse time of the larger bubbles.
Tensile stress generation by optical breakdown in tissue: experimental investigations and numerical simulations
Alfred Vogel, Richard J. Scammon, Robert P. Godwin
Biological tissue is more susceptible to damage from tensile stress than to compressive stress. Tensile stress may arise through the thermoplastic response of laser-irradiated media. Optical breakdown, however, has to date been exclusively associated with compressive stress. We show that this is appropriate for water, but not for tissues for which the elastic-plastic material response needs to be considered. The acoustic transients follow optical breakdown in water and cornea were measured with a fast hydrophone and the cavitation bubble dynamics, which is closely linked to the stress wave generation, was documented by flash photography. Breakdown in water produced a monopolar acoustic signal and a bubble oscillation in which the expansion and collapse phases were symmetric. Breakdown in cornea produced a bipolar acoustic signal coupled with a pronounced shortening of the bubble expansion phase and a considerable prolongation of its collapse phase. The tensile stress wave is related to the abrupt end of the bubble expansion. Numerical simulations using the MESA-2D code were performed assuming elastic-plastic material behavior in a wide range of values for the shear modulus and yield strength. The calculations revealed that consideration of the elastic-plastic material response is essential to reproduce the experimentally observed bipolar stress waves. The tensile stress evolves during the outward propagation of the acoustic transient and reaches an amplitude of 30 - 40% of the compressive pulse.
Modeling optically generated focused stress waves
Anthony J. Makarewicz, Steven R. Visuri, Richard A. London, et al.
Stress waves generated at the end of optical fibers through thermoelastic expansion have been used for several purposes, including the destruction of blood clots, the destruction of kidney and gallstones, and the induction of cell permeability changes for drug delivery. We have undertaken the modeling of stress wave propagation in an effort to optimize the stress wave effects in these therapies. In particular, we have studied techniques to focus the stress wave in order to increase the pressure magnitude for a given pulse energy and to alter the compressive/tensile pressure ratio. This study includes the effects of acoustic wavelength and focusing fiber tip shape.
Two-dimensional Rayleigh model of vapor bubble evolution
Moshe Strauss, Menahem Friedman, Eitan Gurewitz, et al.
The understanding of vapor bubble generation in an aqueous tissue near a fiber tip has required advanced two dimensional (2D) hydrodynamic simulations. For 1D spherical bubble expansion a simplified and useful Rayleigh-type model can be applied. For 2D bubble evolution, such a model does not exist. The present work proposes a Rayleigh-type model for 2D bubble expansion that is faster and simpler than the 2D hydrodynamic simulations. The model is based on a flow potential representation of the hydrodynamic motion controlled by a Laplace equation and a moving boundary condition. We show that the 1D Rayleigh equation is a specific case of our model. The Laplace equation is solved for each time step by a finite element solver using a triangulation of the outside bubble region by a fast unstructured mesh generator. Two problems of vapor bubbles generated by short-pulse lasers near a fiber tip are considered: (1) the outside region has no boundaries except the fiber, (2) the fiber and the bubble are confined in a long channel, which simulates a fiber in a vessel wall. Our simulations for problems of type (1) include features of bubble evolution as seen in experiments, including a collapse away from the fiber tip. A different behavior was obtained for problems of type (2) when the channel boundary is close to the fiber. In this case the bubble's expansion and collapse are both extremely slow in the direction normal to this boundary and distortion of the bubble is observed.
Optoacoustic Diagnostics and Therapy
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Sound and light in turbid media
Lihong V. Wang, Geng Ku, Qimin Shen
Two imaging techniques combining ultrasound and light are reviewed. The motivation is to combine the advantages of optical information and acoustic imaging resolution. The first technique is sonoluminescence tomography, where a sonoluminescence signal generated internally in the media by continuous-wave ultrasound is used. Two-dimensional images can be produced for objects embedded in turbid media by raster scanning the media. The second technique is ultrasound- modulated optical tomography, where a frequency-swept ultrasonic wave was used to modulate the laser light passing through a scattering medium. Multiple 1D images obtained at various positions perpendicular to the ultrasonic axis were composed to obtain a 2D tomographic image of the medium.
Two-dimensional recording of optoacoustic waves
Guenther Paltauf, Heinz Schmidt-Kloiber, Kornel P. Koestli, et al.
An optical method is used to measure optoacoustic waves that are generated by irradiating absorbing targets with nanosecond-laserpulses. Pressure-induced changes of optical reflectance on the surface of a glass prism are used to capture the stress distribution in a plane at a certain time with a gated video camera. The temporal resolution is limited by the exposure time of the camera, which was 5 ns in our experiments. With this technique we imaged optoacoustic waves that were generated in an absorbing liquid in front of an optical fiber tip after transmission of pulses from an optical parametric oscillator (OPO, 6 ns pulse duration). The absolute pressure amplitudes and the temporal development of positive and negative stress in the detector plane could be obtained from the images. In a second series of experiments crossed hairs were irradiated with laser pulses passing through the detector plane. The optoacoustic waves traveling back in opposite direction of the laser radiation were recorded. These measurements yielded absolute pressure values in the detector plane and the location of the absorbing targets. Two- dimensional recording of acoustic waves can be used for the analysis of optoacoustic emission from small absorbing structures and for imaging of buried absorbers in tissue.
Two-dimensional optoacoustic tomography: transducer array and image reconstruction algorithm
Alexander A. Oraevsky, Valeri A. Andreev, Alexander A. Karabutov, et al.
Opto-acoustic tomography (OAT) utilizes laser pulses to create acoustic sources in tissue and wide-band detection of pressure profiles for the image reconstruction. A new laser optoacoustic imaging system (LOIS) for breast cancer detection and two-dimensional visualization is described. A Q-switched Nd:YAG laser was used for generation of opto-acoustic profiles in phantoms and tissues in vitro. Acoustic pulses were detected by a 12 element linear array of piezoelectric transducers. Each transducer was made of 0.5-mm thick PVDF slabs with dimensions of 4.3 mm X 12.5 mm. Signal-to-noise ratio was calculated and the sensitivity of optoacoustic system was evaluated. The axial (in-depth) resolution and the lateral resolution of the system were determined. The axial resolution of the receiving array was limited by its frequency band and was estimated to be approximately 1 mm. The lateral resolution was about 2.5 times the lateral dimension of the 'tumor' and defined by the finite aperture of the array and relatively large size of the single transducer. The time of full data acquisition was limited by the time allowed in clinical procedure of about 5 - 10 minutes. The procedure of signal processing is described. It includes high-pass signal filtering, compensation for acoustic diffraction, detection of the irradiated surface position and rejection of the reverberating signal. Radial back-projection algorithm for image reconstruction was developed and included in the computer code. Two-dimensional opto-acoustic images of simulated spheres and objects inside tissue phantoms are presented. The contrast of these images and limits of detection and localization of deeply embedded tumors are discussed.
Real-time optoacoustic monitoring of temperature in tissues
There is a need to monitor tissue temperature and optical properties during thermotherapy in real time in order to control the boundaries of hyperthermia or coagulation of diseased tissues and minimize the damage to surrounding normal tissues. We propose to use optoacoustic technique for monitoring of tissue temperature. Efficiency of thermoacoustic excitation in water is dependent on temperature. Therefore, the optoacoustic technique may be utilized to monitor tissue temperature if the efficiency of thermoacoustic excitation in tissue is temperature-dependent. We performed experiments on real-time optoacoustic monitoring of temperature in freshly excised canine tissues (liver and myocardium) and aqueous solution during conductive heating. Laser-induced optoacoustic pressure signals were recorded from tissues and the aqueous solution during heating with the use of sensitive wide-band acoustic transducers. Fundamental harmonic of Q-switched Nd:YAG lasers were used for pressure wave generation. Amplitude of the optoacoustic pressure induced in tissues increased linearly with temperature. Good agreement was obtained between the experimental data and theory for the aqueous solution. We demonstrated that laser optoacoustic technique is capable of measuring 1 - 2 degrees Celsius temperature change in tissue and at the distance of up to several centimeters between the investigated tissue volume and the transducer.
Photoacoustic imaging of gelatin phantoms using matched-field processing
Matched Field Processing (MFP) has been used in the ocean acoustics community to localize acoustic sources by correlating experimental data with a modelled field based on a solution to the acoustic wave equation. Here we attempt to adapt the method of MFP to localize an acoustic source in a tissue phantom made from an acrylamide gel. An acrylamide gel in a cylindrical geometry was formed with a small optically absorbing sphere embedded within it. A Q-switched, frequency- doubled Nd:YAG laser operating at 532 nm coupled to an optical parametric oscillator (OPO) tuned to 726 nm was used to irradiate the absorbing sphere. The pulse duration was 4.75 ns and the absorption coefficient of the absorbing sphere was 15 cm-1. The stress confined laser energy resulted in an acoustic pulse radiating from the absorbing sphere. A piezoelectric transducer was used to detect the pulses at various locations on the gel. By vertically translating the transducer a virtual hydrophone array was constructed. The acoustic field was modeled using normal mode methods. A simulation was performed using the normal mode model as virtual data which was then correlated with the normal mode model itself. Finally, the experimental acoustic array data was correlated to the normal mode model.
Imaging of layered structures in biological tissues with optoacoustic front surface transducer
Optoacoustic tomography is a promising technique for imaging skin layered structures and vascular and pigmented lesions in vivo. The imaging of skin is complicated by necessity to perform laser irradiation and acoustic detection from the same site at the surface. We designed an opto-acoustic transducer incorporating a fiber-optic light delivery system and a LiNbO3 piezoelectric detector in one device. The results of test experiments in phantoms, chicken cockscomb, and human hand yielded the following parameters of the opto-acoustic transducer: (1) sensitivity of 0.98 V/bar, (2) temporal resolution of acoustic detection of 5 ns. This fast response time allows one to achieve an in-depth resolution of 10 - 15 micrometer. A small size of the detector provides lateral resolution of 200 micrometer. Feasibility studies demonstrated that the current design of the optoacoustic transducer permits monitoring of the light absorbing heterogeneities at the dept up to 4 mm. The principle scheme of the opto-acoustic transducer design, theoretical background and experimental testing is presented. Theoretical model of the wide-band ultrasonic detection is developed. The principles of the opto- acoustic image reconstruction under conditions of significant diffraction of acoustic wave are described. A basic algorithm for reconstruction of 3-D images from the laser-induced acoustic waves recorded in backward mode is also presented.
Laser-induced stress-wave propagation in the eye
Roberto Pini, Salvatore Siano, Renzo Salimbeni, et al.
The behavior of stress waves induced by Argon-Fluoride laser ablation of the cornea in the typical operative conditions of clinical laser keratectomy have been studied experimentally and analyzed in porcine eyes and in an artificial eye model. Laser-induced stresses with pressure peaks as high as 100 bar were measured in the anterior chamber of irradiated eyes. Analysis of stress wave propagation the eye evidenced diffraction effects modifying the temporal shape of the stress transient with the formation of a rarefaction phase. Besides, significant pressure enhancements caused by focusing of the stress wave front were observed to occur when the spot diameter exceeded 3 mm. For the maximum laser spot diameter of 6.5 mm, diffraction and focusing effects produced at the acoustic focus in the eye compression peaks of 250 and negative peaks of -90 bar, respectively. Implications to clinical procedures, as possible damages due to tissue stretching and cavitation formation are discussed.
Photoacoustic imaging of buried objects using an all-optical detection scheme
Photoacoustic imaging may become an important imaging tool for several diagnostic procedures, e.g. as an alternative to mammography. Imaging of large objects, sub-mm to mm sized, buried deep in optically turbid tissue is feasible. Our investigation is primarily concentrated on characterizing a photoacoustic imaging system based on an all-optical detection scheme experimentally. The optical detection scheme is based on a novel dual-beam common path interferometer. The experimental investigation is carried out using a phantom comprised of slices of chicken breast tissue and the absorbing object made of silicon rubber. The imaging capability of the system is investigated as a function of the thickness of the tissue sample. By irradiating the absorber directly with a short pulsed laser beam, the influence of changes in the stress wave profile on the detected signal is monitored. Finally, preliminary experiments imaging buried, absorbing objects in tissue are carried out demonstrating the feasibility of this imaging method.
Optoacoustic measurements of water, bone, and cartilage with an infrared OPO
Kornel P. Koestli, Martin Frenz, Heinz P. Weber, et al.
Optoacoustical signals generated with infrared laser pulses were studied. Thereby, the effective attenuation coefficient of cartilage and the absorption coefficient of water were measured for different wavelengths. The light-source used was an optical parametric oscillator (OPO) with a pulse duration of 5 ns. The OPO had a tuning range from 1500 to 3600 nm wavelength with a linewidth less than 30 nm. The absorption coefficient of water could be varied between 8 cm-1 and 104 cm-1 by tuning the wavelength of the OPO. The laser induced stress transients were measured with an optical transducer. Measurements in transmission mode and reflection mode (transducer and incident OPO pulse are at opposite sides or at the same side of the sample, respectively) were compared. In reflection mode an infrared- transparent liquid was used to uncouple acoustical and thermal effects. The attenuation coefficient was determined by an exponential fit to the pressure signal. Cartilage and water absorptions were compared between 1860 nm and 1935 nm wavelength. For bone optoacoustical signals were measured between 1835 nm and 1935 nm. Optoacoustic measurements with an infrared OPO give the possibility to obtain optical properties of tissues over a wide wavelength range. Especially in reflection mode in vivo tissue characterization and medical diagnostics are possible.
Optical Dosimetry and Monitoring of Therapy
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Three-dimensional Monte Carlo modeling of laser-tissue interaction
Beop-Min Kim, Nicholas A. Gentile, Kenneth B. Trauner M.D., et al.
A full three dimensional Monte-Carlo program was developed for analysis of the laser-tissue interactions. This project was performed as a part of the LATIS3D (3-D Laser-TISsue interaction) project. The accuracy was verified against results from a public domain two dimensional axisymmetric program. The code was used for simulation of light transport in simplified human knee geometry. Using the real human knee meshes which will be extracted from MRI images in the near future, a full analysis of dosimetry and surgical strategies for photodynamic therapy of rheumatoid arthritis will be followed.
Simulation of skin near-infrared reflectance spectrum
Cheng-Lun Tsai, Yi-Sheong Chou, Jen-Yuan Tsai
Light propagation and the intensity distribution in a biological tissue are characterized by its optical properties. The purpose of this study is to find out how the optical properties affect the profile of skin reflectance spectrum. A Monte Carlo simulation was performed to simulate the skin near-infrared reflectance spectrum in the spectral region between 900 nm and 1300 nm. The absorption coefficient of tissue was determined from the absorption properties and volume fractions of tissue constituents. The simulated reflectance spectra were compared with the skin reflectance spectra measured by a bifurcated fiber bundle. The simulation results show that most of the detected reflectance was scattered back by the dermis tissue. The scattering coefficient and anisotropy factor can be adjusted to modify the simulation result to match the measured spectrum. This inversion technique could be used to determine the scattering and anisotropy properties of skin tissue.
Temperature monitoring by infrared radiation measurements during ArF excimer laser ablation with cornea
We measured infrared thermal radiation from porcine cornea during various fluences ArF excimer laser ablations with 1 microsecond(s) rise time. To obtain absolute temperature by means of Stefan-Boltzman law of radiation, we carried out a collection efficiency and detective sensitivity by a pre-experiment using panel heater. We measured the time course of the thermal radiation intensity with various laser fluences. We studied the relation between the peak cornea temperature during the ablation and irradiation fluences. We found the ablation situations, i.e., sub-ablation threshold, normal thermal ablation, and over-heated ablation, may be judged by both of the measured temperature transient waveforms and peak temperature. The boundary fluences corresponding to normal thermal ablation were 90 and 160 mJ/cm2. Our fast remote temperature monitoring during cornea ablation might be useful to control ablation quality/quantity of the cornea ArF laser ablation, that is PRK.
Inverse method analysis in optothermal skin measurements
Peng Xiao, Robert E. Imhof
We present a new scheme for the inverse analysis of opto- thermal transients that is able to calculate initial temperature profiles from noisy data, despite the ill- conditioning. It is particularly useful for tackling problems where the inhomogeneities of the sample are insufficiently well understood, such as the diffusion of externally applied substances through skin or skin pigment depth profiling. We present the mathematical approach, supported with examples of analyses of in-vivo opto-thermal measurements.
Laser Ablation
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Tissue ablation with the free-electron laser: contributions of wavelength and pulse structure
Stephen R. Uhlhorn, Shannon Harrison, Hans Surya Pratisto, et al.
The Vanderbilt free-electron laser provides a continuously tunable ((lambda) equals 2 - 10 micrometer) source of pulsed IR radiation with a pulse structure unlike those of conventional lasers (a macropulse of 4.5 ms consisting of a micropulse train of 1 ps pulses separated by 350 ps). Tuned to the vibrational mode of the amide-II band ((lambda) equals 6.45 micrometer), the laser is well suited for precise tissue ablation with reported minimal thermal and collateral tissue damage. However, the relative influences of the pulse structure and wavelength on tissue ablation is still not clear. The effects of different wavelengths ((lambda) equals 2.94, 3.36, and 6.45 micrometer) on tissue ablation were compared using pump-probe imaging of tissue phantoms while simultaneously laser-induced pressures were measured with a piezoelectric needle hydrophone. Bovine heart tissue was ablated in vitro using clinically relevant parameters and laser induced damage was examined histologically. The results of these experiments, and their implications will be discussed.
Selective photothermal ablation of tissue with a fiber-delivered Er:YAG laser
Mark C. Pierce, Mark Russell Dickinson, Hugh Devlin
The feasibility of using laser-induced photoemission signals to distinguish between hard and soft biological tissues during photothermal ablation with a pulsed Er:YAG laser has been investigated. Time-resolved emission spectroscopy indicated a threshold fluence of approximately 35 J/cm2 to regularly initiate photoemission from dental enamel, while no emission was detected from porcine muscle tissue with incident laser fluences of up to approximately 140 J/cm2. The delay time of an emission signal with respect to the incident, ablative Er:YAG laser pulse was found to decrease from approximately 150 microseconds near the emission threshold fluence to approximately 60 microseconds at the highest fluence level used. Optical multichannel analyzer spectroscopy of Er:YAG irradiated enamel demonstrated that photoemissions typically consisted of a broad, continuous background in the visible region, with superimposed peaks arising from the presence of elements including calcium, characteristic of plasma emission either from the sample surface or emission plume.
Chemical decomposition of urinary stones during holmium-laser lithotripsy: II. Evidence for photothermal breakdown
Randolph D. Glickman, Joel M. H. Teichman M.D., George J. Vassar, et al.
Because of the greater than or equal to 250 microsecond pulsewidth emitted by the Ho:YAG laser used in clinical lithotripsy, it is unlikely that stress confinement occurs within the irradiated stones. Experimental data supports a thermal mechanism for Ho:YAG laser stone ablation. Stone fragmentation occurs soon after the onset of the laser pulse, is uncorrelated to cavitation bubble formation or collapse, and is associated with low pressures (cf. part I). The mass- loss of desiccated calcium oxalate monohydrate (COM) stones exposed to 150 J from the Ho:YAG laser in air was 40 plus or minus 12 mg (mean plus or minus 1 s.d.); for hydrated stones in air was 25 plus or minus 9 mg; and for hydrated stones in water was 17 plus or minus 3 mg, p less than .001. These differences indicate that direct absorption of the laser radiation by the stone is required for the most efficient ablation. Lowering the initial temperature of COM or cystine stones also reduced the stone mass-loss following 20 J of delivered laser energy: 2.2 plus or minus 1.1 mg vs 5.2 plus or minus 1.6 mg for COM stones (-80 vs 23 degrees Celsius), and 0.8 plus or minus 0.4 mg vs 2.2 plus or minus 1.1 mg for cystine stones (-80 vs 23 degrees Celsius), p less than or equal to .05. Finally, chemical analysis of the laser-induced stone fragments revealed the presence of thermal breakdown products: CaCO3 from COM; free sulfur and cysteine from cystine; Ca2O7P2 from calcium hydorgen phosphate dihydrate, and cyanide from uric acid.
Chemical decomposition of urinary stones during holmium-laser lithotripsy: I. Lack of a photomechanical effect
Kin Foong Chan, George J. Vassar, T. Joshua Pfefer, et al.
The Ho:YAG laser commonly used for clinical lithotripsy of urinary stones typically emits 250-microsecond pulses at a wavelength of 2.12 micrometer and repetition rates of up to 10 Hz. This pulse duration is longer than the time required for a pressure wave to propagate beyond the optical penetration depth of this wavelength in water. Fast-flash photography was used to study the dynamics of urinary stone fragmentation by the Ho:YAG laser. Stone ablation began approximately 50 microseconds after the onset of the laser pulse, long before the collapse of the cavitation bubble at about 350 microseconds. Pressure measurements, made with a PVDF needle- hydrophone and correlated with the fast-flash images, indicated that the peak acoustical transient was less than 2 bars. Regardless of fiber orientation to the stone, no shockwaves were recorded at the beginning of the bubble, and the maximum pressure waves recorded at bubble collapse were approximately 20 bars. However, no fragmentation occurred during or subsequent to the bubble collapse. The measurements indicated that stone ablation was not due to a photomechanical effect.
Optimum Ho:YAG laser irradiation condition on percutaneous laser disk decompression (PLDD): in-vitro quantitive evaluation of influence on disk cells using three-dimensional cell culture system
The purpose of this study is to evaluate the influence on disc cells after laser irradiation using three-dimensional (3-D) culture system and to clarify the optimum Ho:YAG laser irradiation condition on percutaneous laser disc decompression (PLDD) therapy. Since the Ho:YAG laser ablation is characterized by water-vapor bubble dynamics with pressure wave, thermal effect on cell metabolism might occur in the intervertebral disc. We studied the disc cell damage on the metabolic point of view to investigate the optimum irradiation parameter of the Ho:YAG laser. We have developed the 3-D cultured disc cell system using agarose gel to investigate laser - disc cell interaction. This culture system provides a highly in vivo-like environment for disc cells in which cell- extracellular matrix interactions appear to be more important than contacts among cells. Intervertebral discs were obtained from Japanese white. The isolated disc cells were seeded in 96-well culture plates at the cell densities of 1 X 106 cells/ml, and incubated for 12 days. A pulsed Ho:YAG laser was delivered through a 200 micrometer-core diameter single silica glass fiber. On the agarose gel including the 3-D cultured disc cells, we used the Ho:YAG laser irradiation energy ranging from 40 to 180 mJ/pulse at the fiber end. Cytotoxicity and matrix synthesis after the laser irradiations were evaluated in time course to determine the optimum condition of laser irradiations. It was confirmed that laser irradiation causes necrosis of the cells and additionally produces apoptosis depending on the condition. The ability of matrix synthesis was maintained even after the irradiation, which differed depending on the irradiation conditions. The optimum irradiation conditions seemed related to the preservation of intact area and the acceleration of matrix synthesis in reactive area.
Acoustic online monitoring of IR laser ablation of burnt skin
In burn surgery necrotic tissue has to be removed prior to grafting. Tangential excision causes high blood loss and destruction of viable tissue. Pulsed infrared laser ablation can overcome both problems because of its high precision and the superficial coagulation of the remaining tissue. We investigated the ablation noise to realize an acoustic feedback system for a selective removal of necrotic tissue. We studied free-running Er:YAG laser ablation of gelatin and burnt skin. Acoustic signals were detected by a condenser microphone and a piezoelectric airborne transducer. Tissue discrimination was based on the evaluation of the normalized acoustic energy. The normalized acoustic energy differs significantly between gelatin samples of different water content and between necrotic and vital tissue. The normalized acoustic energy is a suitable parameter for the discrimination between necrotic and vital tissue.
Laser-Tissue Interaction
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Microheater as an alternative to lasers for in-vitro fertilization applications
Daniel V. Palanker, Igor Turovets, Rima Glazer, et al.
During the last decade various lasers have been applied to drilling of the micrometer-sized holes in the zona pellucida of oocytes for in-vitro fertilization applications. In this paper we describe an alternative approach to laser instrumentation based on microfabricated device capable of precise drilling of uniform holes in the zona pellucida of oocytes. This device consists of a thin (1 micrometer) film microheater built on the tip of glass capillary with a diameter varying between a few to a few tens of micrometers. Duration of the pulse of heat produced by this microheater determines the spatial confinement of the heat wave in the surrounding liquid medium. We have demonstrated that gradual microdrilling of the zona pellucida can be accomplished using a series of pulses with duration of about 300 microseconds when the microheater was held in contact with the zona pellucida. Pulse energy applied to 20 micrometer tip was about 4 (mu) J. In vitro development and hatching of 127 micromanipulated embryos was compared to 103 non-drilled control embryos. The technique was found to be highly efficient in creating round, uniform, well defined holes with a smooth wall surface, matching the size of the heating source. The architecture of the surrounding zona pellucida was unaffected by the drilling, as demonstrated by scanning electron microscopy. Micromanipulated embryos presented no signs of thermal damage under light microscopy. The rate of blastocyst formation and hatching was similar in the micromanipulated and control groups. Following further testing in animal models, this methodology may be used as a cost- effective alternative to laser-based instrumentation in clinical applications such as assisted hatching and embryo biopsy.
Corneal ablation with short-pulse mid-IR laser radiation
Hanna J. Hoffman, William B. Telfair, Henry G. Plaessmann
Photospallation is proposed as the mechanism behind recent animal studies involving corneal ablation by nanosecond-pulse mid-IR laser beams. Following a brief summary of earlier work directed to refractive procedures in the mid-IR, a preliminary analysis is performed based on simple 1D models of thermoelastic expansion developed previously. The results of the analysis indicate that front surface spallation is consistent with the striking tissue ablation characteristics observed in the most recent work with short pulse mid-IR radiation, including very small ablation rates and submicron thermal damage zones. This is because spallation is a mechanical -- rather than a thermal -- process, allowing tissue to be removed in thin layers at fluences far lower than those used in the earlier corneal studies with mid-IR beams, resulting in minimal heating. We conclude that the existing theoretical basis supports the use of nanosecond pulses as an effective approach to achieving controlled ablation in the presence of very high absorption. We further suggest that such domain of operation may in fact be preferred over shorter pulses, both from a practical standpoint and to mitigate against potential damage from shock waves. Additional validation of the precise nature of corneal ablation with mid- IR nanosecond pulses was obtained from recent ablation rate experiments conducted in gel models, which resulted in submicron ablation rates of magnitudes very similar to those achieved with excimer. A brief summary of these preliminary results is given.
Modeling of stress generation induced by laser ablation
Salvatore Siano, Roberto Pini, Renzo Salimbeni
In the present work we discuss in some details the development of the stress induced into the target by laser ablation in air. The analytical treatment based on the variable energy blast theory allowed to provide the scaling law of the peak and the temporal profile of the generated pressure wave. Measurements of the acoustic transient associated with excimer laser ablation of polyimide, obtained by means of polyvinyldenefloride (PVDF) sensors were successfully compared with the behaviors provided by the analysis. The description here proposed can be extended to pulsed laser ablation of soft tissues where the target material is removed in vapor or gaseous phases.
Temperature distributions in Nd:YAG (lambda=1.32 um) laser-irradiated cartilage grafts accompanied by cryogen spray cooling
Amir M. Karamzadeh, Brian Jet-Fei Wong M.D., Thomas E. Milner, et al.
Laser irradiation of mechanically deformed cartilage results in permanent shape change. Under optimal conditions, the cartilage graft must be rapidly heated to the critical transition temperature, required for reshaping, while maintaining a spatially uniform temperature distribution. These two conditions are not easily satisfied, as increasing irradiance results in larger temperature gradients between irradiated and non-irradiated surfaces. Moreover, such a gradient increases with cartilage thickness. Inasmuch as uncontrolled heating of the irradiated surface may result in overt chondrocyte death, the temperature in cartilage must be monitored and controlled. We propose feedback-controlled cryogen spray cooling as a technique to allow rapid laser heating within the cartilage while protecting the irradiated surface. In this investigation, we measured radiometric surface temperatures on irradiated and non-irradiated surfaces of porcine septal cartilage specimens (thickness 1.5, 2.0, 2.5 and 3 mm) during Nd:YAG laser exposure ((lambda) equals 1.32 micrometer, 50 pulse repetition rate, 5 - 50 W/cm2) with and without cryogen spray cooling. Along with surface temperatures, back-scattered light from an amplitude-modulated HeNe probe laser ((lambda) equals 632.8, 10 mW) was collected by a fiber and detected with a silicon photoreceiver using a lock-in amplifier. Previously, we have demonstrated that an increase followed by a sharp decline in the back-scattered light intensity signal correlates closely with characteristic alterations in cartilage thermal and mechanical properties accompanying the reshaping process, which suggest the occurrence of a phase transformation. The difference between irradiated and non-irradiated radiometric surface temperatures was determined as a function of time and related to incident laser irradiance and specimen thickness. To minimize excessive heating and produce a more uniform temperature distribution, a cryogen (10 ms) spurt was delivered at the surface coincident with the area of laser irradiation. The cryogen spurt was triggered whenever the radiometric surface temperature on the irradiated surface reached 50 degrees Celsius. Back-scattered HeNe light was noted to increase, peak, and subsequently decrease during sustained laser irradiation in the presence and absence of cryogen spray cooling (CSC). Laser irradiation was terminated after the peak in the back-scattered HeNe signal was observed on the lock-in amplifier. The critical transition radiometric surface temperature increased with laser irradiance. In the absence of CSC, temperatures on irradiated and non-irradiated surfaces of a 1.5 mm section of cartilage differed by 1.5, 7, and 10 degrees Celsius for 5, 15, and 30 W/cm2, respectively, at the peak in light scattering. During laser irradiation with CSC, the radiometric surface temperature on the irradiated surface varied between 25 - 50 degrees Celsius. Radiometric surface temperatures on the non-irradiated surface did not exceed 70 degrees Celsius. A peak in the light scattering curve was still consistently observed for CSC irradiated tissue and was noted to occur when the non-irradiated surface temperature reached 45 degrees Celsius. The use of CSC during laser irradiation permits rapid and homogeneous heating within the cartilage while potentially protecting the irradiated surface.
Basic Investigations
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Appropriate radiometric quantities in laser dosimetry
Unlike radiofrequency and ionizing radiation which can penetrate deeply into biological tissues, optical radiation is generally absorbed very superficially. Except for the relatively narrow band of visible and near-infrared (IR-A) radiation from approximately 400 - 1400 nm, skin and other biological tissues are nearly opaque to optical radiation. For this reason, the volumetric or mass-based concepts of absorbed dose (i.e., J/cm3 or J/gm) used in other areas of radiation biology are of little value. Additionally, the absorbed radiant energy is conducted out of the absorbing site and for this reason thermal effects depend largely upon the size and location of the absorbing site as well as exposure and exposure rate. Concepts of surface exposure dose are therefore most useful and practical. The concepts of fluence and fluence rate are shown to be useful for volume scattering in the visible spectral region where photochemical reactions are to be described.
Optical dosimetry in photodynamic therapy of human uterus and brain
Steen J. Madsen, Lars Othar Svaasand, Henry Hirschberg, et al.
Optical 'dose' is one of the fundamental parameters required in the design of an efficacious regimen of photodynamic therapy (PDT). The issues involved in delivering a sufficient optical dose to the human uterus and brain during PDT will be discussed. Specifically, measurements of optical properties and fluence rates in excised human uteri are presented. Measured fluence rates are compared to the predictions of a simple diffusion model and the clinical utility of the treatment is discussed. The delivery of light to brain tissue via a surgically implanted balloon applicator will also be considered. The time required to deliver and adequate dose is calculated based on known optical properties and diffusion theory.
Light dosimetry technique for endoscopic PDT using microlens optical fiber
A simple procedure is presented for determining the irradiance of light striking tissues during light delivery using a microlens optical fiber via an endoscope. The particular example is photodynamic therapy (PDT) of laryngeal cancer. A microlens optical fiber is an optical fiber with a small lens assembly at its terminus which magnifies the image of the end of the fiber yielding a uniform irradiance in a plane distant from the fiber. Prior to the therapy, the relation between fiber-target distance, h [cm], and the diameter of the uniform beam, d(h) [cm], was established and the area of the beam was calculated: A(h) equals (pi) d(h)2/4. The laser power P [W] required to achieve a desired irradiance E [W/cm2] was P(h) equals EA(h), and this relation was prepared as a simple graph for routine use in the clinic. During the therapy, the doctor advances the optical fiber through the working channel of the endoscope to touch the target tissue site while observing through the optical channel, and marks the fiber on the outside of the endoscope. The doctor then retracts the fiber until an aiming beam transmitted through the fiber fully illuminates the desired target area, and again marks the fiber on the outside of the endoscope. The difference in the two marks on the fiber outside the endoscope yields h, the height of the fiber above the target tissue. The laser operator then uses the P(h) equals EA(h) graph to select the proper laser power to achieve the desired E. Although trivially simple, this dosimetry procedure was critical to the proper implementation of PDT for laryngeal cancer.
Dosimetry for laser treatment of port-wine stains
Lars Othar Svaasand, Martin J. C. van Gemert, Wim Verkruysse, et al.
The presently most promising treatment modality for port wine stains is selective photothermolysis. This technique is based on exposure of the lesion to yellow laser light of high energy density and long pulse-length, typically 5 - 10 J/cm2 and 0.5 ms, respectively. Although promising clinical results have been reported, only a small percentage of the patients obtains full fading of their lesions. This might in part be due to the fact that most clinicians treat port wine stains with rather standardized settings of the laser parameters. However, clinical response can, hopefully, be improved if these setting are adjusted to each individual lesion. This paper gives a brief discussion of the importance of parameters such as depth, diameter and vessel wall thickness, together with a presentation of non-invasive methods for determination of these values.
Simulation, Monitoring, and Clinical Applications
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Simulation of light scattering in biological tissues: the coherent component
Jun Q. Lu, Xin-Hua Hu, Zhi Song, et al.
Light propagating in turbid medium can be separated into two parts -- the unattenuated part and the scattered part. The two parts coexist everywhere in the medium and can be described by radiative transport theory. In an optically thick sample of turbid medium, very small portion of the light remain unattenuated and the diffusion model is widely used to describe the propagation of light. Monte Carlo method, having the potential of reaching high accuracy, can be used to account for both parts of the light distribution and has acquired considerable attention lately. We have developed a new method of Monte Carlo simulation and conducted large-scale simulation of converging laser beam propagating through tissue phantoms in slab configurations. In our cases, the unique geometry of the converging laser beams gathers the unattenuated photons together near the focal point and makes it significant even for optically thick samples in comparison with the scattered part. Numerical results were obtained for tissue slabs of different parameters to study the variation of the distribution of the transmitted light. We have also investigated the possibility of characterizing biological tissues through the measurement of unattenuated photons.
Investigations to monitor laser-induced thermotherapy (LITT) of breast tumors with photon density waves (PDW)
I. Mesecke-von Rheinbaben, Andre Roggan, Y. Helfman, et al.
An experimental setup will be presented which provides the detection of photon density waves (PDW) in transmission through tissue samples of up to 4 cm thickness. A laser diode (825 nm) which was modulated at frequencies between 40 - 1000 MHz served as light source. Amplitude and phase of the PDW were measured at various positions on tissue phantoms as well as during interstitial laser coagulation of tissue samples (porcine liver) by scanning source and detector. A Nd:YAG laser was used in combination with a scattering applicator to produce thermal lesions. Due to a change of the optical properties the actual size of the coagulated volume could be monitored during the therapy. Several phantom and in vitro experiments have been performed to show that this monitoring technique is capable of visualizing the coagulated region with a clinically relevant precision.
Time-resolved reflectance spectroscopy in tissues
A system for the measurement of absorption and scattering spectra of turbid media in a wide spectral range based on time-resolved reflectance spectroscopy is described. The system operates with tunable mode-locked lasers, and an electronic chain for time-correlated single-photon counting. Laser tuning and optimization, acquisition, and data analysis are fully automated so as to allow in vivo spectroscopy of human tissues within a reasonable measurement time. Absorption and transport scattering spectra were acquired from 610 nm up to 1010 nm every 5 nm on different tissues of healthy volunteers. Examples of the female breast as well as of the human forearm and abdomen are presented, showing a high inter- subject variability. Finally, the measured absorption and scattering values are used to calculate the penetration depth of light in the same tissues.
Dosimetry of laser radiation for immersed skin
Valery V. Tuchin, Svetlana A. Tatarkova, Yulia M. Proshina, et al.
We present experimental results of reflectance of immersed human skin. These results are used to analyze in vivo measurements on the human skin. In this paper, we will demonstrate near-infrared transmission and reflection measurements. The preliminary results of in vivo measurements of the reflectance at 830 nm of the human skin immersed by various lotions are described. A liquid suspension and lotions was used to study the effects of multiple scattering on the near-infrared (830 nm) spectrum. We explain these observations in the context of photon-diffusion theory.
Photochemical and Photo-Oxidative Mechanisms
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Laser-induced cartilage damage: an ex-vivo model using confocal microscopy
Martin Frenz, Benno J. Zueger, D. Monin, et al.
Although there is an increasing popularity of lasers in orthopedic surgery, there is a growing concern about negative side effects of this therapy e.g. prolonged restitution time, radiation damage to adjacent cartilage or depth effects like bone necrosis. Despite case reports and experimental investigations over the last few years little is known about the extent of acute cartilage damage induced by different lasers types and energies. Histological examination offers only limited insights in cell viability and metabolism. Ho:YAG and Er:YAG lasers emitting at 2.1 micrometer and 2.94 micrometer, respectively, are ideally suited for tissue treatment because these wavelengths are strongly absorbed in water. The Purpose of the present study is to evaluate the effect of laser type and energy on chondrocyte viability in an ex vivo model. Free running Er:YAG (E equals 100 and 150 mJ) and Ho:YAG (E equals 500 and 800 mJ) lasers were used at different energy levels using a fixed pulse length of 400 microseconds. The energy was delivered at 8 Hz through optical fibers. Fresh bovine hyaline cartilage samples were mounted in a water bath at room temperature and the fiber was positioned at 30 degree and 180 degree angles relative to the tissue surface. After laser irradiation the samples were assessed by a life-dead cell viability test using a confocal microscope and by standard histology. Thermal damage was much deeper with Ho:YAG (up to 1800 micrometer) than with the Er:YAG laser (up to 70 micrometer). The cell viability test revealed a damage zone about twice the one determined by standard histology. Confocal microscopy is a powerful tool for assessing changes in tissue structure after laser treatment. In addition this technique allows to quantify these alterations without necessitating time consuming and expensive animal experiments.
Ophthalmic Effects of Ultrashort Pulses
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Selective RPE photodestruction: mechanism of cell damage by pulsed-laser irradiance in the ns to um time regime
Ralf Brinkmann, Jan Roegener, Charles P. Lin, et al.
The subject of this study was to investigate the threshold radiant exposures for bubble formation at single porcine melanosomes in suspension and for porcine RPE cell damage when using pulse durations in the ns to microsecond(s) time regime. A frequency doubled Nd:YLF laser ((lambda) equals 527 nm) with adjustable pulse duration between 250 ns and 3 microsecond(s) and a Q- switched Nd:YAG laser ((lambda) equals 532 nm, (tau) equals 8 ns) were used for the single pulse irradiation. Fast flash photography was applied to probe vaporization around individual melanosomes while a fluorescence viability assay was used to probe cell vitality. Applying single ns laser pulses to RPE cells, an ED50 threshold radiant exposure of 84 mJ/cm2 was determined, which is close to the vaporization threshold around single melanosomes. When irradiating with pulse durations of 3 microsecond(s) , a threshold of about 223 mJ/cm2 was measured, which is only 40% lower of the vaporization threshold around the single melanosome at that pulse width. This can be explained with heat contribution from adjacent melanosomes, which increases towards longer pulse durations. Calculations are in good agreement with the experimental results when assuming a surface temperature at the melanosome of 140 degrees Celsius and an absorption coefficient of 8000 cm-1 to initiate vaporization. It can be concluded that the origin of RPE cell damage for single pulse irradiation with a duration of 8 ns results from transient microbubbles around the melanosomes, which lead to a transiently increased cell volume and subsequently a rupture of the cell structure. It is also likely that the same effect plays the major role when using pulse durations up to 3 microsecond(s) .
Photomechanical Interactions
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Aspherical bubble dynamics and oscillation times
Robert P. Godwin, Edward J. Chapyak, Joachim Noack, et al.
Novel features of aspherical bubble dynamics are explored. Time-resolved experimental photographs and simulations of large aspect ratio (length:diameter approximately 20) cylindrical bubble dynamics are presented. The experiments and calculations exhibit similar dynamics. A small high-pressure cylindrical bubble initially expands radially with hardly any axial motion. Then, after reaching its maximum volume, a cylindrical bubble collapses along its long axis with relatively little radial motion. The growth-collapse period of these very aspherical bubbles differs only slightly from twice the Rayleigh collapse time for a spherical bubble with an equivalent maximum volume. This fact justifies using the temporal interval between the acoustic signals emitted upon bubble creation and collapse to estimate the maximum bubble volume. As a result, hydrophone measurements can provide an estimate of the bubble energy even for aspherical bubbles. The prolongation of the oscillation period of bubbles near solids boundaries relative to that of isolated spherical bubbles is also discussed.
Optical Dosimetry and Monitoring of Therapy
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Optothermal skin pigment spectral depth profiling using an OPO laser
Peng Xiao, Xinxin Guo, Ioan Notingher, et al.
Abstract not available.