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Biomedical subjects

A J Welch

Publications and source records attributed to A J Welch.

At least 19 recordsLinked to original sources
PubMed1992

Dynamic optical property changes: implications for reflectance feedback control of photocoagulation.

During laser treatment, coagulation affects the optical properties of the tissue. In particular, the formation of a white lesion significantly increases the scattering coefficient. This change in the optical properties in turn affects the laser light distribution in the tissue. The white lesion formed during photocoagulation of the retina has a dynamic effect upon reflection and fluence rate. This problem has been simulated on a model medium consisting of a thin absorbing layer covered with a 1 cm thick layer of albumin. The albumin layer is subdivided into coagulated (white) and uncoagulated (clear) layers. The optical properties of each layer have been determined and these values have been used to model light distribution in the medium. One-dimensional adding-doubling and three-dimensional Monte Carlo methods have provided light distributions in the medium for varying thicknesses of the coagulated albumin. Computed fluence reaching the absorbing layer decreased in the presence of a 275 microns or thicker coagulated layer. The coagulated layer attenuates light because it is highly scattering; however, this scattering also leads to a sub-surface peak in fluence rate at a level higher than the incident fluence. The latter effect outweighed the former for coagulated layer thicknesses less than 275 microns. Computed reflectance of argon laser light from a semi-infinite coagulated region initially increased linearly as a function of thickness. As the coagulation thickness increased beyond 4-5 optical depths, the reflectance approached a constant value, R infinity, at 9 optical depths (2 mm). Experimentally measured total reflectance is shown to be an inadequate indicator of the thickness of a lesion (finite coagulated volume); however, central reflectance from a lesion measured with a CCD camera confirmed the computed trends. These results provide a theoretical foundation for control of lesion thickness using reflectance images.

Argon↗
PubMed1991

Rate process parameters of albumen.

Both egg white and egg yolk are excellent media for studying photocoagulation due to thermal response of tissue to laser light. They are readily available, consistently the same, and provide a standard biological medium for different labs to compare results. The irradiation parameters for laser coagulation of these media depend upon their optical properties and their temperature-dependent rate reactions. Estimates of the rate process damage coefficients were obtained from constant-temperature water bath experiments. The activation energy E for egg white was estimated to be 92,000 (cal/M*K] and the integration constant A was found to be 3.8 x 10(57) (second-1).

Egg White↗
PubMed1991

Laser thermal ablation.

Continuous wave and pulsed laser ablation of tissue is described as an explosive event. A subsurface temperature maximum and superheated tissue produce high pressures that eject fragments from the tissue. Decreased water content due to dehydration and vaporization decreases thermal conductivity which reduces heat conduction. Also, a decrease in water content dramatically alters the local rate of heat generation of laser radiation above 1.3 microns since water is the primary absorber. In contrast, at UV wavelengths protein and DNA are the primary absorbers so destruction of tissue bonds is due to direct absorption of the laser light rather than heat transfer from water.

Animals↗
PubMed1991

Microscopic instrumentation and analysis of laser-tissue interaction in a skin flap model.

A dorsal skin flap model for microcirculatory studies has been modified for "in vivo" studies of laser-tissue interaction with microcirculation. An experimental apparatus has been built implementing a laser delivery system, video microscopy during irradiation, and thermal recordings. This model has been used to study irradiation effects on microcirculation using the argon laser (488 and 514.5 nm) and the argon pumped dye laser at 577 nm. The results include: measurements of the optical properties of the model; dosimetry measurements for the production of embolized and stationary coaguli in arterioles and venules; and focal vessel disappearance of venules irradiated with the argon or the argon pumped dye laser at 577 nm; a method to determine light attenuation in the model; a unique method for measurements of blood flow velocity in arterioles and venules and measurements obtained with this method; measurements of transient and steady state temperatures during irradiation and a study of laser induced photorelaxation phenomena in venules.

Animals↗
PubMed1991

Laser balloon angioplasty.

Unlike conventional transluminal percutaneous angioplasty (PTCA), which applies only intraluminal pressure, laser balloon angioplasty (LBA) employs simultaneous heat and pressure to reopen heavily occluded arterial lumens. The circumferential irradiation of Nd:YAG (1.06 microns) laser light is directly absorbed by approximately 1 to 2 mm of arterial tissue immediately adjacent to the inflated balloon. Such heating by LBA is able to seal disrupted luminal flaps, thermally remodel the luminal surface topology, reduce arterial recoil, selectively (partially) dehydrate thrombus, and possibly even reduce thrombogenicity at atherosclerotic sites. Criteria for successful LBA are defined based on earlier fundamental in vitro experiments to determine effective welding temperature, laser power doses, and exposure period; in addition, the derivation and validity of a three-part optical-thermal model and its application in parametric dosimetry analysis are presented. Though the lumen remodeled by LBA is acutely satisfactory, recurrence of the lesion is problematic chronically. Because of this, LBA is currently most useful as an adjunctive procedure whenever PTCA fails to produce optimal results or causes acute vessel closure. Perhaps, another potential application of the LBA system is to aid localized delivery of pharmacologic agents and their thermal adhesion to superficial tissue at angioplastied sites.

Angioplasty, Laser↗
PubMed1990

Disparate absorption of argon laser radiation by fibrous versus fatty plaque: implications for laser angioplasty.

The thermal response of white fibrous atheromatous plaque to argon laser irradiation was compared with the thermal response of yellow fatty plaque and normal aortic wall to the same type of radiation. Samples of normal aorta, fibrous, and fatty plaque were irradiated in air with 3.5 Watts of laser power on a 2 mm spot for 5 and 10 seconds. Heterogeneous foci, each covering normal aorta and either fibrous or fatty plaque, were additionally irradiated with 7 Watts on a 1 cm spot for 30 seconds to 2.5 minutes. Tissue surface temperature was monitored during laser irradiation via a 3-5 micron infrared camera. For the 2 mm spot and 5 second exposure time, argon laser irradiation of normal aorta produced popping and surface tearing at a peak temperature of 145 +/- 10 degrees C. Irradiation of fatty plaque produced popping and crater formation at a peak temperature of 200 +/- 10 degrees C. However, fibrous plaque was nonablatively discolored by the same dose of laser radiation with a peak temperature of only 85 +/- 10 degrees C. Irradiation for 10 seconds caused crater formation and carbonization in the fatty plaque but failed to produce ablation in the fibrous plaque. Irradiation of the heterogeneous foci confirmed the disparity in the temperature attained by these two types of plaque and their degree of damage. Therefore, this study suggests that the ablation threshold for soft atheroma is strongly dependent on the optical properties of the particular type of tissue. Yellow fatty plaque preferentially absorbs argon laser radiation, but white fibrous plaque absorbs this radiation less readily than normal aortic wall.

Angioplasty, Balloon↗
PubMed1990

Limitations of a thermal camera in measuring surface temperature of laser-irradiated tissues.

Thermal cameras are used in research laboratories to measure tissue temperature during laser irradiation. This study was an evaluation of the accuracy of a 3-5 microns thermal camera and two 8-12 microns cameras in detecting the maximum temperatures of small targets. The size of the targets was within the range of laser spot diameters which are used for vessel welding, angioplasty, and dermatology. The response to a sharp thermal edge was measured and analyzed for the three cameras, which had a scanning rate of 30 frames per second. The response of the 3-5 microns camera to reference black body targets of different sizes was also studied. It was found that the detector system required an average of 2.44 microseconds to reach 90% of maximum step response for the 8-12 microns system and 5.85 microseconds for the 3-5 microns system. With a 3 x telescope and a 9.5 inch focal distance close-up lens, the 3-5 microns camera underestimated the temperature of targets smaller than 2.0 mm because of its slow detector response. Although the 8-12 microns camera provides more accurate measurements due to its faster detector response, it still underestimates the temperature of targets smaller than 900 microns, when similar magnification and focal distance are used. Methods to compensate for the inaccuracies are discussed, including empirical correction factors and the inverse filtering technique.

Calibration↗
PubMed1990

Laser-irradiation-induced relaxation of blood vessels in vivo.

The response of blood vessels to laser irradiation in vivo was studied in the dorsal skin flap glass window chamber model of hamsters. The vasodilatory response of venules was critically dependent on the wavelength of irradiating laser. Relaxation was not produced in arterioles, although it was tried repeatedly. Vessels were irradiated with the 514.5 nm single line argon laser with irradiances from 1 to 10 W/cm2 on a 1.2 mm-diameter spot. Irradiation of venules with 2.2 W/cm2 and 4.25 W/cm2 produced reversible relaxation. Venules relaxed initially and after the interruption of irradiation returned to their original diameter. At higher irradiances (8.5 W/cm2) an irreversible relaxation was observed. At irradiances of 10 W/cm2 and above initial relaxation was accompanied with constriction, focal coaguli, and hemostasis. Irradiation with the argon-pumped dye laser at 595 nm did not produce any significant relaxation.

Animals↗
PubMed1990

Laser probe temperature control by measuring the returning infra-red radiation.

The metal-tipped fibre or 'laser probe' developed for angioplasty comprises a metallic probe at the end of an optical fibre. The probe is heated by an argon or Nd:YAG laser and applied against the tissue to be vapourized. The heated probe generates infra-red radiation which is proportional to the temperature of the probe. The paper investigates the feasibility of a feedback control system that measures the temperature of the probe by detecting the infra-red radiation transmitted back through the fibre. The probe was initially heated by physical contact with a hot surface, and then by an argon laser via the optical fibre. The returning IR radiation was sensed by a lead sulphide detector, while probe temperature was simultaneously measured by a thermocouple. Temperatures as low as 200 degrees C were measured through a 5 m long fibre during the laser heating of the probe. The detector signal increased in an exponential fashion as the probe temperature increased. A resolution of 1 degree C was obtained at a probe temperature of 400 degrees C. It can be concluded that, for the laser probe, it is feasible to use a feedback control system which measures the infra-red radiation transmitted back through the same fibre that carries the heating laser light.

Feasibility Studies↗
PubMed1990

Reflectance as an indirect measurement of the extent of laser-induced coagulation.

Two-dimensional reflectance images and surface thermal distributions were recorded during argon laser induced coagulation. During laser irradiation, coagulated egg yolk formed a white lesion. The whiteness, or reflectance caused by backscattering of light from the forming lesion, would be measured after a short delay from the onset of laser irradiation. In the experiments which covered exposure time from 4.5 to 17.0 s, we found that it started slowly, the reflectance increased rapidly once the surface temperature of the lesion reached approximately 90 degrees C. After this rapid rise, the reflectance began to taper off until no change in reflectance was recorded. There was a 0.98 correlation between lesion diameter measured from a two dimensional reflectance image and the lesion diameter that was measured microscopically. There was a 0.92 correlation between reflectance at the center of the lesion and microscopically measured depth of coagulation at the same point. However, the correlation between microscopically measured coagulation depth and width was only 0.88.

Egg Yolk↗
PubMed1989

Light distributions in artery tissue: Monte Carlo simulations for finite-diameter laser beams.

Finite-width light distributions in arterial tissue during Argon laser irradiation (476 nm) are simulated using the Monte Carlo method. Edge effects caused by radial diffusion of the light extend +/- 1.5 mm inward from the perimeter of a uniform incident beam. For beam diameters exceeding 3 mm the light distribution along the central axis can be described by the one-dimensional solution for an infinitely wide beam. The overlapping edge effects for beam diameters smaller than 3 mm reduce the penetration of the irradiance in the tissue. The beam profile influences the light distribution significantly. The fluence rates near the surface for a Gaussian beam are two times higher on the central axis and decrease faster radially than for a flat profile. The diverging light from a fiber penetrates tissue in a manner similar to collimated light.

Aorta↗
PubMed1989

Time constants in thermal laser medicine.

Temperature rise of laser-irradiated tissue due to direct absorption of laser light is related to laser parameters (power, spot size, irradiation time, and repetition rate) and tissue parameters (absorption and scattering coefficients, density, heat capacity, and thermal conductivity). Solutions to the bio-heat equation are approximated by introducing axial (z) and radial (r) time constants for heat conduction that represent two parallel channels for heat conduction. These axial (tau z) and radial (tau r) time constants are found proportional to squared distances (z02, r02) that represent the extent of axial and radial temperatures respectively. For convenience, z0 and r0 are approximated to the axial and radial extent of laser light in the tissue. The resulting solution of the bio-heat equation, expressed as temperature rise as function of time and position, is obviously exact for irradiation times short compared to ta z, tau r (adiabatic heating), but is also a quite reasonable approximation up to irradiation times three times the overall time constant. Comparison with (exact) numerical computations show that this holds for all ratios of (light) penetration depth to laser-beam radius; for strongly scattering materials, smaller laser beams give better predictions than do larger laser beams. Several examples of clinical relevance are discussed, such as multiple-laser-pulse irradiation of high- and low-absorbing tissues and laser treatment of port-wine stains, with some unexpected results that also show potential clinical relevance.

Body Temperature↗
PubMed1989

Evaluation of ocular protection filters in field situations.

A computer program has been developed to simulate eye movement during pursuit tracking to test the effectiveness of filters protecting the eyesight of individuals exposed to laser radiation. Two types of retinal damage are considered: macular lesions and retinal injury causing vitreous or subretinal hemorrhage. The retinal damage caused by optical radiation in the visible and near infrared of the spectrum differs from almost all other types of hazards in their proabilistic nature. That is, a small movement of the laser beam or change of eye position decreases or increases the probability of injury by several orders of magnitude. A laser beam combines the problem of a small source with extreme directionality with the probabilistic nature of this hazard location in space, and we now add the probabilistic nature of the action of the protective filter, i.e., a dependence upon angle of incidence. From this combination, an even more probabilistic picture of injury or disablement emerges. When it is necessary to plan the probabilities during military operations of mission fulfillment or injury with regard to various types of lasers in the battlefield ++environment, the problem must be analyzed very carefully. Our solution of the problem indicates the type of injuries to be expected and their dependence upon the various parameters of the angle of incidence of the lasers on the protective filters in the viewing system.

Computer Simulation↗
PubMed1989

A theoretical study of the effect of optical properties in laser ablation of tissue.

The role of optical properties in the distribution of laser light and the resulting thermodynamic processes in biological tissue is studied from a theoretical perspective. Light distribution is modeled by a discrete ordinate method and heat transfer and ablation is modeled by an immobilized finite element method. The effect of parametric variation of absorption, scattering anisotropicity on the dynamics of the ablation process is examined. The manifestation of higher than the ablation threshold temperature in the subsurface tissue is observed and discussed. Results indicate significant differences in the ablation behavior which may have important clinical implications.

Hot Temperature↗
PubMed1989

A model for optical and thermal analysis of laser balloon angioplasty.

Laser balloon angioplasty is modeled using an infinitely long cylinder possessing axisymmetry. The balloon surface is assumed to be uniformly irradiated by diffuse light at 1060 nm delivered from within the inner balloon core. The diffusion approximation to the radiative transport equation is solved for a single layer of homogeneous medium enclosing the transparent fluid-filled balloon. The computed light fluence rate (W.cm-2) just beneath the tissue surface is 4.7 times the primary irradiance, owing to scattering and secondary irradiance from the "integrating cylinder" effect of backscattered light into the inner core. The transient temperature response of the heated tissue is then calculated using an implicit finite difference solution of the heat conduction equation for concentric layers of varying thermal properties. Finally, the extent of damage is analyzed using the Arrhenius rate process model. Changes in optical and thermal properties with temperature and thermal phase transitions have been omitted in all our analyses. Irradiances which decrease with time can produce a "temperature plateau" for a longer time period than a constant irradiance of equal total energy output. This may be clinically important. Flexible boundary conditions at the balloon interface permit simulation of a "hot contact surface," such as a black balloon absorbing all incident laser power. In this situation, the computed surface damage is consistently higher than that obtained by LBA of equivalent energy output.

Angioplasty, Balloon↗
PubMed1989

Heat generation in laser irradiated tissue.

Many medical applications involving lasers rely upon the generation of heat within the tissue for the desired therapeutic effect. Determination of the absorbed light energy in tissue is difficult in many cases. Although UV wavelengths of the excimer laser and 10.6 microns wavelength of the CO2 laser are absorbed within the first 20 microns of soft tissue, visible and near infrared wavelengths are scattered as well as absorbed. Typically, multiple scattering is a significant factor in the distribution of light in tissue and the resulting heat source term. An improved model is presented for estimating heat generation due to the absorption of a collimated (axisymmetric) laser beam and scattered light at each point r and z in tissue. Heat generated within tissue is a function of the laser power, the shape and size of the incident beam and the optical properties of the tissue at the irradiation wavelength. Key to the calculation of heat source strength is accurate estimation of the light distribution. Methods for experimentally determining the optical parameters of tissue are discussed in the context of the improved model.

Hot Temperature↗
PubMed1988

Photodynamic assay of light distributions in tissue phantoms.

The distribution of light in tissues of varying blood content has been modeled using a gel agar system as a tissue phantom. Hematoporphyrin derivative (Photofrin I or HpD) was suspended in gels, along with a chemical indicator of its cytotoxic intermediate (singlet oxygen), and various amounts of red blood cells. The singlet oxygen detector used was tryptophan and its oxidation product. Qualitative analysis of the tryptophan photoproduct was determined using high-pressure liquid chromatography analysis of individual gel sections. The relative amount of photoproduct was used as a measure of the fluence of actinic light at a given depth within the tissue phantoms. In this manner, the relative activation efficiencies of HpD were determined for light from an argon laser (mainly 488 and 514.5 nm) and an argon-pumped dye laser (630 nm).

Hematoporphyrin Derivative↗