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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
PubMed1995

Wavelengths for laser treatment of port wine stains and telangiectasia.

BACKGROUND AND OBJECTIVE: This report presents analytical modelling of the influence of wavelength on the amount of volumetric rate of heat produced in dermal blood vessels by millisecond laser radiation. STUDY DESIGN/MATERIALS AND METHODS: A new anatomical model is proposed that represents port wine stains as well as telangiectatic lesions. It consists of a target blood vessel, representing the deepest dermal blood vessel that requires irreversible injury, and a layer of whole blood, representing all other dermal blood vessels above the target vessel. The laser light that interacts with the blood vessels is assumed to be diffuse. Selective photothermolysis is the basis for the analysis. We consider wavelengths between 577 nm and 600 nm, the argon laser wavelengths at 488/515 nm, and the frequency doubled Nd:YAG laser wavelength at 532 nm. RESULTS: The rate of volumetric heat production of absorbed laser light in the target blood vessel is expressed analytically as a function of blood absorption, the concentration of additional dermal blood, and the depth of the target vessel. CONCLUSION: The model explains why 585 nm is a good compromise for treating port wine stains that vary widely in number of dermal blood vessels. It predicts that wavelengths between 577 nm and 582 nm are excellent for the treatment of port wine stains in young children, and it suggests a possible explanation as to why the argon laser is sometimes said to be capable of treating dark mature port wine stains. The copper vapour laser wavelength at 578 nm, and the frequency doubled Nd:YAG laser wavelength at 532 nm, are predicted to be suitable for the treatment of port wine stains that contain, respectively, a small to moderate and a moderate number of dermal blood vessels. When laser beam spotsize becomes smaller, the best wavelength for producing maximal rate of heat in the target vessel is predicted to shift to 577 nm.

Animals↗
PubMed1995

Lifetime testing of sapphire and sculpted silica fiber scalpels.

BACKGROUND AND OBJECTIVE: Sapphire and sculpted silica fiber scalpels were evaluated for performance as they aged. Performance was determined by measuring their useful lifetime, forward transmission, incision depth, and the thermal coagulation thickness at the sides and bottom of the incision. STUDY DESIGN/MATERIALS, AND METHODS: Aging was performed with a chicken breast model. Performance measurements were made at periodic intervals. Sapphire scalpels were cooled with air or saline. RESULTS: The air-cooled, frosted sapphire scalpels had the longest useful lifetime, whereas the saline-cooled, frosted sapphire scalpels had the shortest. Aging deteriorated the forward transmission of the sculpted silica fiber scalpels the most. Little difference was found between the averages of all incision measurements. Two of the saline-cooled, frosted sapphire scalpels fractured during the aging process. CONCLUSION: Aging influences the scalpel lifetime, but there was no evidence that the aging process significantly affected the incision size.

Aluminum Oxide↗
PubMed1995

Combination sensor for measurement of light and temperature.

This report describes light and temperature measurements made in a gel cell using a single probe. Glass pipettes were drawn to an outer tip diameter of 50 microns and a thermocouple was vapor deposited at the tip of the pipette. Direct absorption of light by the thermocouple produced near step changes in temperature proportional to the fluence rate of the light.

Absorption↗
PubMed1995

Automated lesion placement in the rabbit eye.

BACKGROUND AND OBJECTIVE: The objective of this research was to build a prototype feedback control system to precisely place argon laser lesions on the retina for treatment of retinal disorders. STUDY DESIGN/MATERIALS, AND METHODS: The prototype feedback control system was tested by placing lesions at specific locations on the retina of pigmented rabbits to simulate the treatment of diabetic retinopathy, retinal breaks or tears, and a pre-programmed, two-dimensional array of lesions was placed at a specific site. RESULTS: Results of feedback-controlled lesion placement performed in vivo on pigmented rabbits are presented. The ability to place lesions with automated feedback control is demonstrated. CONCLUSION: Automated feedback control placement of argon laser lesions is possible at a reasonable cost and has numerous therapeutic and safety benefits over current ballistic delivery.

Animals↗
PubMed1995

Corneal photocoagulation with continuous wave and pulsed holmium: YAG radiation.

In this study, the effectiveness of pulsed and continuous wave (CW) holmium: YAG lasers in coagulating in vitro pig corneas was analyzed. With the CW laser, irradiance and exposure time were varied; irradiance, from 162 to 324 W/cm2 and exposure time, from 200 to 800 ms. With the pulsed laser, number of pulses and radiant exposure were varied; number of pulses per lesion, from 4 to 30 and radiant exposure, from 10 to 25 J/cm2. Laser-induced corneal damage was determined by analyzing histological cross sections of each lesion. Depth and diameter of the lesions were plotted against the varying laser parameters. Light and birefringent photomicrographs of typical lesion histology show that the pulsed laser significantly damaged superficial layers of the cornea and could not achieve the coagulation depths produced by the CW laser. Additional histology demonstrates that minimal surface damage (intrastromal coagulation) occurred when the CW laser beam was delivered with a sapphire-tipped contact probe. The results provide empirical data on the sensitivity of each parameter in producing a range of coagulation end points. In addition, the experimental results describe trends between the parameters of either laser and the extent of coagulation.

Animals↗
PubMed1995

Combined ultrasound and fluorescence spectroscopy for physico-chemical imaging of atherosclerosis.

This paper describes a combined ultrasonic and spectroscopic system for remotely obtaining physico-chemical images of normal arterial tissue and atherosclerotic plaque. Despite variations in detector-tissue separation, R, fluorescence powers corresponding to pixels in the image are converted to the same set of calibrated units using distance estimations from A-mode ultrasound reflection times. An empirical model, validated by Monte Carlo simulations of light propagation in tissue, is used to describe changes in fluorescence power as a function of R. Fluorescence spectra of normal and atherosclerotic human aorta obtained with this system are presented as a function of R. To compensate for changes in fluorescence power with R, the empirical model was used in each case to calculate the fluorescence power at a constant reference value of R(Rref = 1.67 mm). Prior to compensation, tissue fluorescence power decreased more than a factor of two as R was increased from 2.5 to 5 mm. Following compensation, the fluorescence power varied less than +/- 10% of the average compensated peak. The chemical composition of each sample was determined by fitting its fluorescence spectrum (in calibrated units) to a model of tissue fluorescence incorporating structural protein and ceroid fluorescence, as well as structural protein and hemoglobin attenuation. Parameters of the fit were used to classify tissue type. Without compensation for distance variation, classification of tissue type was frequently incorrect; however, with compensation, predictive value was high. A 1-D chemical image of a section of human aorta containing both normal and atherosclerotic regions obtained with this system is also presented. After compensation for detector-sample separation, tissue classifications along the cross-section closely resemble those obtained from histology. Regions of elevated ceroid concentration and intimal thickening are clearly observable in the resultant chemical image. The potential value of this type of system in the diagnosis and treatment of coronary artery disease is discussed.

Aorta↗
PubMed1995

Automated retinal robotic laser system.

Researchers at the University of Texas and the USAF Academy have worked toward the development of a retinal robotic laser system. The overall goal of this ongoing project is to precisely place and control the depth of laser lesions for the treatment of various retinal diseases such as diabetic retinopathy and retinal tears. Separate low speed prototype subsystems have been developed to control lesion depth using lesion reflectance feedback parameters and lesion placement using retinal vessels as tracking landmarks. Both subsystems have been successfully demonstrated in vivo on pigmented rabbits using an argon continuous wave laser. Recent efforts have concentrated on combining the two subsystems into a single prototype capable of simultaneously controlling both lesion depth and placement. We have designated this combined system CALOSOS for Computer Aided Laser Optics System for Ophthalmic Surgery. Following the dual-use concept, this system is being adapted for clinical use as a retinal treatment system as well as a research tool for military laser-tissue interaction studies.

Animals↗
PubMed1994

Comparison of the thermal tissue effects produced by aged sapphire and silica hemispherical tips.

This study evaluated the performance of sapphire and fused silica hemispherical tips under the same exposure conditions. Lesions produced in the chicken breast and a blood field were sectioned for light and transmission polarizing microscopy. Lesion size and thermal damage area were recorded as a function of the tips accumulated exposure. The tips transmission was measured after every 1,000 J of exposure. Fused silica tips lasted for approximately 5,000 J and experienced significant surface and transmission deterioration. The sapphire hemispherical tips lasted for > 12,000 J with no surface and transmission deterioration. Lesions produced with the fused silica tips generally increased in depth with use, and depths of 6 mm were common. Lesions produced by the sapphire tips were subsurface spherical areas of coagulation with the tissue surface relatively intact. This difference in resulting lesions may be attributed to the higher thermal conductivity of sapphire.

Aluminum Oxide↗
PubMed1994

Tissue optical property measurements: overestimation of absorption coefficient with spectrophotometric techniques.

This study examines the validity of optical property measurements by comparing surface temperatures rises predicted by a well-tested mathematical model with temperatures measured experimentally during laser irradiation. Analysis is based on the early temperature response that is proportional to the absorption coefficient. The results of the investigation suggest that values for tissue absorption coefficient can be greatly overestimated when current spectrophotometric techniques are used. This seems especially true when a broad light beam and a low port-to-beam-size ratio are used for transmission and reflection measurements with an integrating sphere.

Absorption↗
PubMed1994

Temperature dependence of the absorption coefficient of water for midinfrared laser radiation.

The dynamics of the water absorption peak around 1.94 microns was examined. This peak is important for the absorption of holmium and thulium laser radiation. To examine the effect of temperature on the absorption coefficient, the transmission of pulsed Ho:YAG, Ho:YAG, Ho:YSGG, and Tm:YAG laser radiation through water of 22 degrees C, 49 degrees C, and 70 degrees C was measured as a function of the thickness of the water layer. From these data the absorption coefficients were determined at the three wavelengths. We found that at all three wavelengths, the absorption coefficients decreased when increasing the temperature. Second, the absorption spectrum of water was measured from 1,850-2,150 nm with a spectrophotometer. It was found that the absorption peak at 1.94 microns (at 22 degrees C) shifts to shorter wavelengths with increasing temperatures, to 1.92 microns at 70 degrees C. A model was developed to predict the temperature distribution incorporating the dynamic change in absorption coefficient. The temperature distributions are compared to the predictions of a model assuming constant optical properties. It is shown in this study that the dynamics of the absorption coefficient has a significant influence on the expected zone of damage and ablation parameters in the 2-microns wavelength range.

Absorption↗
PubMed1994

Optical properties of human aorta: are they affected by cryopreservation?

This study was undertaken to investigate the effects of freezing upon the in vitro optical properties of human aorta from 300 nm to 800 nm. Freezing significantly decreased absorption coefficient over most of the spectrum from 300 nm to 800 nm. The only significant changes in the reduced scattering coefficient were from 300 nm to 335 nm.

Adult↗
PubMed1994

Rate process model for arterial tissue thermal damage: implications on vessel photocoagulation.

A numerical model for thermal damage to human arterial tissue is presented, based on protein denaturation kinetics. The model involves determination of coefficients of rate processes A & delta E, which are tissue type-dependent (arterial tissue in this study), and definition of threshold damage. A feedback-controlled constant surface temperature device was used to induce 80 coagulative lesions of arterial human tissue ranging in temperature from 66 degrees C to 76 degrees C and in duration from 15 to 1,500 seconds. The measured coefficients were determined to be A = 5.6 x 10(63) s-1 and delta E = 430 KJ mole-1. These numerical values closely approximate the coefficients of the rate process for denaturation of collagen molecules. These and other histological observations strongly suggest collagen to be the primary coagulating component of arterial tissue at the onset of thermal coagulative damage. The ability of this model to predict onset of tissue coagulation during laser coagulation was studied using 10 postmortem human arterial samples exposed to argon laser irradiation.

Algorithms↗
PubMed1994

Preliminary results on reflectance feedback control of photocoagulation in vivo.

The size of therapeutic laser-induced retinal lesions is critical for effective treatment and minimal complications. Due to tissue variability, the size of a lesion that results from a given set of laser irradiation parameters cannot be predicted. Real time feedback control of lesion size is implemented based on two-dimensional reflectance images acquired during irradiation. Preliminary results of feedback controlled lesions formed in pigmented rabbits demonstrate an ability to produce uniform lesions despite variations in tissue absorption or changes in laser power.

Algorithms↗
PubMed1993

Reflectance feedback control of photocoagulation in vivo.

OBJECTIVE: The objective of this research was to build a real-time feedback system that controlled lesion size. Two-dimensional reflectance images were acquired with a charge-coupled device camera during irradiation, and argon laser exposure was ended when parameters of the image reached prespecified values. METHODS: The real-time feedback control system was tested by creating lesions at different power levels in pigmented rabbits. Laser exposure time was controlled by monitoring the central reflectance. RESULTS: Results of feedback-controlled lesions formed in vivo in pigmented rabbits are presented. An ability to produce uniform lesions despite variation in tissue absorption or changes in laser power is demonstrated. CONCLUSIONS: Reflectance control of photocoagulation is possible; incorporation of feedback during photocoagulation has numerous therapeutic and safety benefits over current ballistic delivery.

Animals↗
PubMed1993

Limits of radial time constants to approximate thermal response of tissue.

The time constant model, as an approximation to the bio-heat equation, was tested for non-ablating, deep penetrating (argon) laser beams in absorbing tissue phantoms (polyacrylamide) in air. Temperature responses were measured with a thermal camera, both during laser irradiation as well as after the laser beam was switched off. The radial time constant model was found to give a reasonable prediction (within 5%) of temperature rise for irradiation times of up to the order of one time constant. The cooling behavior of tissue could not be described by one single time constant. The time constant model was found to fit the early cooling rate (cooling time about equal to irradiation time) for laser pulse durations of up to one time constant. The late cooling rate turned out to be much slower than predicted by the radial time constant model, yet it was faster than the cooling rate predicted by the one-dimensional instantaneous hot cylinder surface model.

Lasers↗
PubMed1993

Continuous wave laser ablation of tissue: analysis of thermal and mechanical events.

Thermal and mechanical events during continuous wave (CW) laser ablation of biological and phantom media were investigated. Porcine aortae, collagen fibers, and polyacrylamide control samples were subjected to argon laser irradiation while infrared and high-speed (240 images/s) video cameras were used to monitor their surfaces. Subsequent analysis of simultaneous changes in surface temperature and physical features correlated thermal and mechanical events. Video images recorded prior to ablation onset of tissue slabs clearly revealed two distinct phases: 1) progressive growth of a surface dehydration zone, and 2) surface deformation, implying subsurface bubble formation. Surface temperature recordings and video imaging revealed that the onset of CW ablation of soft biological media often initiated with a violent explosion, surface tearing, and tissue ejection. Histological inspection revealed intense coagulation in superficial layers near the irradiation site, whereas chiefly mechanical disruption was noted at the base of the crater. Ablation characteristics were consistent with theoretical calculations which indicate subsurface temperature peaks that increase in magnitude and surface proximity as energy deposition rates are increased. Results also suggested that mechanical properties of target media strongly influenced the extent of pressure built up, the nature of ablation onset, and the characteristics of the overall ablation pathway.

Acrylic Resins↗
PubMed1993

Real-time algorithm for retinal tracking.

Conventional retinal laser photocoagulation is presently performed by an ophthalmologist manually aiming a low-power laser beam at a desired site and firing a high-power laser for a preselected interval of time. To automate this process a retinal tracker must acquire a target, track small saccades, and identify loss of track during a large saccade. The authors successfully implemented a real-time algorithm that used a simple computer, video digitizing card, low light video camera, and fundus camera to perform rudimentary tracking on a photograph of a retina undergoing smooth circular motion. The algorithm tracked speeds up to 5 Hz, or 27 degrees/s, which equated to the retina moving in a 525 microns diameter circle.

Algorithms↗