Sohi Rastegar

Measurement and calculation of the two-dimensional backscattering Mueller matrix of a turbid medium

We present both experimental and Monte Carlo-based simulation results for the diffusely backscattered intensity patterns that arise from illumination of a turbid medium with a polarized laser beam. A numerical method that allows the calculation of all 16 elements of the two-dimensional Muller matrix is used; moreover, it is shown that only seven matrix elements are independent. To validate our method, we compared our simulations with experimental measurements, using a turbid medium consisting of 2.02-microm -diameter polystyrene spheres suspended in deionized water. By varying the incident polarization and the analyzer optics for the experimental measurements, we obtained the diffuse backscattering Mueller matrix elements. The experimental and the numerical results are in good agreement.

Light backscattering polarization patterns from turbid media: theory and experiment.

We present both experimental measurements and Monte-Carlo-based simulations of the diffusely backscattered intensity patterns that arise from illuminating a turbid medium with a polarized laser beam. It is rigorously shown that, because of axial symmetry of the system, only seven elements of the effective backscattering Mueller matrix are independent. A new numerical method that allows simultaneous calculation of all 16 elements of the two-dimensional Mueller matrix is used. To validate our method we compared calculations to measurements from a turbid medium that consisted of polystyrene spheres of different sizes and concentrations in deionized water. The experimental and numerical results are in excellent agreement.

Nonlinear finite-element analysis of the role of dynamic changes in blood perfusion and optical properties in laser coagulation of tissue

A nonlinear finite-element program was developed to simulate the dynamic evolution of coagulation in tissue considering temperature and damage dependence of both the optical properties and blood perfusion rate. These dynamic parameters were derived based on the Arrhenius rate process formulation of thermal damage and kinetics of vasodilation. Using this nonlinear model, we found that the region of increased blood flow that formed at the periphery of the coagulation region significantly reduces the heat penetration. Moreover, increased scattering in the near-surface region prevents light penetration into the deeper region. Therefore, if the dynamic parameters are ignored, a relatively significant overestimation of the temperature rise occurs in a deeper area resulting in an overestimation in predicted depth of coagulation. Mathematical modeling techniques that simulate laser coagulation may not provide reliable information unless they take into account these dynamic parameters.

Laser thermal ablation

Abstract— —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 μm 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.

Light and temperature distribution in laser irradiated tissue: the influence of anisotropic scattering and refractive index

The rigorous method of discrete ordinates was used to evaluate the effects of anisotropic scattering and optical discontinuity at the boundaries on light and temperature distribution in tissue. The influence of optical parameters of tissue on its thermal response was examined by using a finite element solution of the heat conduction equation. Calculations were performed for wide ranges of scattering albedo, the anisotropy factor, as well as interface reflectivities. This study shows that the presence of an optical discontinuity due to an air-tissue interface forces the maximum peak intensity to move from subsurface to the surface for tissue with high scattering albedo, which leads to a higher fluence rate in the near surface region. Temperature field calculations show a higher subsurface temperature for a highly scattering medium during tissue coagulation. Neglecting the anisotropic properties of tissue as well as the optical discontinuity at the boundaries would result in considerable error in the calculated temperature rises. Additionally the accuracy of the photon diffusion theory for predicting light and temperature distribution near the tissue surface is examined.

Monte Carlo modeling for implantable fluorescent analyte sensors

A Monte Carlo simulation of photon propagation through human skin and interaction with a subcutaneous fluorescent sensing layer is presented. The algorithm will facilitate design of an optical probe for an implantable fluorescent sensor, which holds potential for monitoring many parameters of biomedical interest. Results are analyzed with respect to output light intensity as a function of radial distance from source, angle of exit for escaping photons, and sensor fluorescence (SF) relative to tissue autofluorescence (AF). A sensitivity study was performed to elucidate the effects on the output due to changes in optical properties, thickness of tissue layers, thickness of the sensor layer, and both tissue and sensor quantum yields. The optical properties as well as the thickness of the stratum corneum, epidermis, (tissue layers through which photons must pass to reach the sensor) and the papillary dermis (tissue distal to sensor) are highly influential. The spatial emission profile of the SF is broad compared that of the tissue fluorescence and the ratio of sensor to tissue fluorescence increases with distance from the source. The angular distribution of escaping photons is more concentrated around the normal for SF than for tissue AF. The information gained from these simulations will he helpful in designing appropriate optics for collection of the signal of interest.

Finite element analysis of temperature controlled coagulation in laser irradiated tissue

The Theoretical study of thermal damage processes in laser irradiated tissue provides further insight into the design of optimal coagulation procedures. Controlled laser coagulation of tissue was studied theoretically using a finite element method with a modulating laser heat source to simulate feedback controlled laser delivery with a constant surface temperature. The effects of uncertainty in scattering and absorption properties of the tissue, thermal denaturation induced changes in optical properties, and surface convection were analyzed. Compared to a single pulse CW irradiation in which a doctor would presumably stop CW laser delivery after noticing some effect such as vaporization or carbonization, the constant surface temperature scenario provided a better overall control over the coagulation process. In particular, prediction of coagulative damage in a constant temperature scenario was less sensitive to uncertainties in optical properties and their dynamic changes during the course of coagulation. Also, subsurface overheating under surface convective conditions could be compensated for under constant temperature irradiation by lowering the surface temperature.

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 thermodynamical processes in biological tissue are studied from a theoretical perspective. Light distribution is modeled by a discrete ordinate method, and heat transfer and ablation are modeled by an immobilized finite-element method. The effect of parametric variation of absorption, scattering, and scattering anisotropicity on the dynamics of the ablation process is examined. The manifestation of temperature 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.<<ETX>>

Theoretical analysis of equivalency of high-power diode laser (810 nm) and Nd:YAG laser (1064 nm) for coagulation of tissue: predictions for prostate coagulation

Profiles of light, temperature, and thermal damage distributions in tissue based on measured optical properties are examined theoretically for high power diode laser (810 nm) and Nd:YAG laser 1064 nm). Generally higher absorption and effectively lower optical penetration has been experimentally observed at the wavelength of diode laser as compared to that of Nd:YAG laser. Results of this study indicate that similar thermal damage volumes are expected to be obtained by the two lasers, in general. However, for same irradiation conditions a larger volume of damage and more charring near the surface is predicted when using the diode laser on prostate tissue, and similarly for myocardial tissue. Role of blood presence throughout tissue, in terms of its optical interaction, as well as the role of a small layer of blood between the laser and the tissue is also investigated for both wavelengths.

Heat shock improves recovery and provides protection against global ischemia after hypothermic storage

BACKGROUND Improved methods of donor heart preparation before preservation could allow for prolonged storage and permit remote procurement of these organs. Previous studies have shown that overexpression of heat-shock protein 72 provides protection against ischemic cardiac damage. We sought to determine whether rats subjected to heat stress with only 6-hour recovery could acquire protection to a subsequent heart storage for 12 hours at 4 degrees C. METHODS Three groups of animals (n = 10 each) were studied: control, sham-treated, and heat-shocked rats (whole-body hyperthermia 42 degrees C for 15 minutes). After 12-hour cold ischemia hearts were reperfused on a Langendorff column. To confirm any differences in functional recovery, hearts were then subjected to an additional 15-minute period of warm global ischemia after which function and lactate dehydrogenase enzyme leakage were measured. RESULTS Heat-shocked animals showed marked improvements compared with controls in left ventricular developed pressure (63+/-4 mm Hg versus 44+/-4 mm Hg, p<0.05) heart rate x developed pressure (13,883+/-1,174 beats per minute x mm Hg versus 8,492+/-1,564 beats per minute x mm Hg, p<0.05), rate of ventricular pressure increase (1,912+/-112 mm Hg/second versus 1,215+/-162 mm Hg/second, p<0.005), rate of ventricular pressure decrease (1,258+/-89 mm Hg/second versus 774+/-106 mm Hg/second, p<0.005). Diastolic compliance and lactate dehydrogenase release were improved in heatshocked animals compared with controls and sham-treated animals. Differences between heat-shocked animals and control or sham-treated animals were further increased after the additional 15-minute period of warm ischemia. Western blot experiments confirmed increased heat-shock protein 72 levels in heat-shocked animals (>threefold) compared with sham-treated animals and controls. CONCLUSIONS Heat shock 6 hours before heart removal resulted in marked expression of heat-shock protein 72 and protected isolated rat hearts by increased functional recovery and decreased cellular necrosis after 12-hour cold ischemia in a protocol mimicking that of heart preservation for transplantation. Protection was further confirmed after an additional 15-minute period of warm ischemia.