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Biomedical subjects

S Rastegar

Publications and source records attributed to S Rastegar.

27 records · Page 2Linked to original sources
PubMed1994

Modeling of intraluminal heating of biological tissue: implications for treatment of benign prostatic hyperplasia.

A computer model for predicting the thermal response of a biological tissue to different intraluminal heating modalities is presented. A practical application of the model is to calculate the temperature distributions during thermal coagulation of prostate by contact heating and radiative heating. The model uses a two-dimensional axisymmetric diffusion approximation method to calculate the light distribution during radiative heating. The traditional Pennes' bio-heat equation is used to calculate the temperatures in the presence of blood flow. An implicit finite difference scheme with nonuniform grid spacings is used to solve the diffusion equation for light distribution and the bio-heat equation. Model results indicate that the radiative heating of prostate by Nd:YAG (1064 mm) and diode (810 mm) lasers can be a more effective and efficient means of coagulating a large volume of prostate, as compared to contact heating of the tissue. Blood perfusion is shown to provide a considerable heat sink as the laser exposure time is increased. Surface cooling by irrigation during the laser irradiation of tissue is shown to be an effective method for delaying tissue explosion and obtaining a large volume of coagulated tissue. The model also shows that the volume of the coagulated tissue is appreciably altered by a change in the rate of energy deposition.

Animals↗
PubMed1994

Dye-enhanced ablation of enamel by pulsed lasers.

Laser removal of dental hard tissue has been proposed as a replacement for or augmented approach to the dental handpiece. The main limitation for widespread usage of lasers in dentistry has been inefficient ablation of dental hard tissue, accompanied by potential laser-induced damage to the surrounding tissue. The research focuses on a novel approach for enhancement of tissue ablation and confinement of laser interaction to a small tissue volume by controlled placement of an exogenous dye on the enamel surface. Studies were done with both pulsed alexandrite and pulsed Nd:YAG lasers, with indocyanine green and India ink, respectively, used as photo-absorbers. These dye-enhanced laser processes demonstrated the feasibility of this technique for cavity preparation. While control studies produced little or no appreciable crater, average preparation depth for the dye-enhanced ablation was from 1 to 1.5 mm, with a diameter of approximately 0.6 mm. Knoop hardness measurements show that, surrounding the crater, there is small annular region slightly softened by the laser action. SEM studies of the interior structure of the tooth did not show significant damage to the surrounding tissue. Temperature measurement studies indicated that the pulsed nature of the laser, combined with the photo-absorbing dye, effectively prevented significant temperature rise at the pulp. The remarkable effectiveness of this technique in creating cavity preparations and the absence of any notable collateral damage to the surrounding tissue suggest that dye-enhanced pulsed-laser ablation could be used as an alternative to the dental handpiece in selected procedures.

Carbon↗
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↗
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↗
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↗
PubMed1988

Technique for measurement of one-dimensional instantaneous ablation velocity.

An oscillating mirror scanned a He-Ne laser beam along the length of a thin rod while the rod was ablated by an argon laser beam. The shadow of the scanning beam on an image plane provides a picture of the instantaneous ablation velocity of the rod. This configuration provides a unique method for investigating one-dimensional ablation.

Laser Therapy↗
PubMed1988

Laser ablation of discs of agar gel.

Discs of agar gel mixed with ink were used to study ablation effects with an argon laser as a light source. Varying amounts of ink were added resulting in a variation of the attenuation coefficient between 0.45 and 6.3 mm-1. For laser beam irradiation horizontally incident on a vertical sample, the average velocity of ablation was found to be approximately constant for thicknesses up to 1.7 mm. When the laser beam was directed vertically on a sample held horizontally, the vaporized debris present in the beam attenuated the incident laser energy to such a degree that the average ablation velocity decreased by a factor of approximately five. Horizontal beam experiments for various attenuation coefficients showed that an attenuation coefficient of about 1.7 mm-1 is optimal for fast penetration of discs thicker than 4 mm. Thus, based upon the optical properties of a given tissue, there may exist an optimum laser wavelength to maximise ablation velocity.

Agar↗
PubMedDate not supplied

Intervention in autoimmune diabetes by targeting the gut immune system.

BB rats and nonobese diabetic (NOD) mice spontaneously develop autoimmune insulin dependent diabetes and serve as models for human type I diabetes. During progression of the disease the cytokine pattern elaborated by islet infiltrating immune cells shifts from a Th2 or Th0 toward Th1 type. Only the latter is associated with "destructive" insulitis. We discuss here attempts to modulate disease progression by targeting the gut immune system with bacterial immunostimulants. Oral dosing of diabetes prone BB rats with lipopolysaccharide (LPS) or the Escherichia coli extract OM-89 lead to a Th2-shift of pancreatic mRNA expression. In vitro studies showed that repeated exposure toward LPS or OM-89 lead to downregulation of proinflammatory macrophage responses. In the NOD mouse, repeated oral dosing of OM-89 caused a Th2 shift in the gut cytokine gene expression, probably because of desensitization of macrophages and other antigen presenting cells. Concomitantly, diabetes prevention by oral insulin was improved. In conclusion, oral dosing with bacterial immunostimulants dampens Th1 type immune reactivities of the gut immune system and thereby promotes oral tolerance mechanisms. Downregulation of proinflammatory immune reactivities by repeated exposure to bacterial stimulants requires intact desensitization mechanisms in macrophages or other antigen presenting cells.

Adjuvants, Immunologic↗