Diffuse optical tomography: image reconstruction and verification

In this study, we intend to use diffuse optical Tomography [DOT] as a noninvasive, safe and low cost technique that can be considered as a functional imaging method and mention the importance of image reconstruction in accuracy and procession of image. One of the most important and fastest methods in image reconstruction is the boundary element method [BEM]. This method is introduced and employed in our works. Generally, to image a biological tissue we must obtain its optical properties. In order to reach this goal we benefit from diffusion equation because tissue is highly scattering medium. Diffusion equation is solved by boundary element equation [BEM] in our research. First, we assume a double layer phantom with different scattering and absorption coefficients to simulate and verify precession and accuracy of image reconstruction by BEM. Light absorption can be affected by volume fraction of blood in skin. For a specific skin species the volume fraction is calculated and then the results are compared with the reconstructed values obtained by BEM. Since the depth of tissue is important in light absorption a two layer phantom with known values is made and the depths of layers are reconstructed by BEM then they are compared with the expected values. A homogenous phantom with known scattering and absorption coefficients was made and then these coefficients were reconstructed by BEM. Finally, an inhomogeneous phantom [phantom with defect] whose defect was in a known position was made and the absorption and scattering coefficients were reconstructed and compared with real values. Comparison between real or simulated values and reconstructed values of scattering and absorption coefficients, volume fraction of blood and thickness of phantom layers by BEM shows maximum errors of 24%, 7% and 35%, respectively. Comparison between BEM data and real or simulated values shows an acceptable agreement. Consequently, we can rely on BEM as a beneficial method in diffuse optical tomography image reconstruction

Mechanisms of laser-tissue interaction: II. tissue thermal properties

Laser-tissue interaction is of great interest due to its significant application in biomedical optics in both diagnostic and treatment purposes. Major aspects of the laser-tissue interaction which has to be considered in biomedical studies are the thermal properties of the tissue and the thermal changes caused by the interaction of light and tissue. In this review paper the effects of light on the tissue at different temperatures are discussed. Then, due to the noticeable importance of studying the heat transfer quantitatively, the equations governing this phenomenon are presented. Finally a method of medical diagnosis called thermography and some of its applications are explained

low-cost method for optical tomography

In this study, arrangement of a low-cost optical tomography device compared to other methods such as frequency domain diffuse tomography or time domain diffuse tomography is reported. This low-cost diffuse optical imaging technique is based on the detection of light after propagation in tissue. These detected signals are applied to predict the location of in-homogeneities inside phantoms. The device is assessed for phantoms representing homogenous healthy breast tissues as well as those representing healthy breast tissues with a lesion inside. A diode laser at 780nm and 50 mW is used as the light source. The scattered light is then collected from the outer surface of the phantom by a detector. Both laser and detector are fiber coupled. The detector fiber may turn around the phantom to collect light scattered at different angles. Phantoms made of intralipid as the scattering medium and ink as the absorbing medium are used as samples. Light is collected after propagation in the phantoms and the capability of the device in collecting data and detecting lesions inside the phantoms is assessed. The fact that the detection fiber orbits around the sample and detects light from various angles has eliminated the need to use several detectors and optical fibers. The results obtained from experiments are compared with the results obtained from a finite element method [FEM] solution of diffusion equation in cylindrical geometry written in FORTRAN. The graphs obtained experimentally and numerically are in good accordance with each other. The device has been able to detect lesions up to 13 mm inside the biological phantom. The data achieved by the optical tomography device is compared with the data achieved via a FEM code written in FORTRAN. The results indicate that the presented device is capable of providing the correct pattern of diffusely backscattered and transmitted light. The data achieved from the device is in excellent correlation with the numerical solution of the diffusion equation. Therefore, results indicate the applicability of the reported device. This device may be used as a base for an optical imaging. It is also capable of detecting lesions inside the phantoms.

propagation of laser light in skin by Monte Carlo- diffusion method: a fast and accurate method to simulate photon migration in biological tissues

Due to the importance of laser light penetration and propagation in biological tissues, many researchers have proposed several numerical methods such as Monte Carlo, finite element and green function methods. Among them, the Monte Carlo method is an accurate method which can be applied for different tissues. However, because of its statistical nature, Monte Carlo simulation requires a large number of photon pockets to be traced, so it is computationally expensive and time consuming. Although other numerical methods based on the diffusion method are fast, they have two important limitations: first, they are not valid near the bounder of sample and source, and second, their accuracy is less than Monte Carlo method. In this study, we combine the accuracy of Monte Carlo method and speed of the diffusion method. This hybrid method is faster than Monte Carlo Method and its accuracy is higher than the diffusion method. We first evaluate this hybrid model and the reflectance of a biological phantom is calculated by Monte Carlo method and this hybrid model. Then the propagation of laser light in the skin tissue has been studied. In this study, a combined method based on the Monte Carlo method and the diffuse equation is introduced. This hybrid method is five times faster than Monte Carlo Method, and its accuracy is higher than the diffusion method. The propagation of laser light in skin has also been studied by this hybrid method and its accuracy shows that it can be applied for laser penetration in biological tissues. It seems that this method is good for photo dynamic therapy [PDT] and optical imaging

Mechanisms of laser-tissue interaction: I. optical properties of tissue

Today, lasers are widely used in biology and medicine, and the majority of health centers and hospitals utilize modern laser systems for diagnosis and therapy applications. Researchers have introduced different medical applications for different lasers used in surgeries and other medical treatments. Medical lasers can be categorized in both diagnosis and therapy branches. Main difference between diagnosis and therapy applications is the type of laser-tissue interactions. In diagnosis, one tries to arrange a noninvasive method to study the normal behavior of tissue without any damage or clear effect on tissue. But in therapy, such as surgery, a surgeon uses laser as a knife or for affecting a specific region. So, the medical laser applications are defined by the interaction type between laser light and tissues. The knowledge of laser-tissue interaction can help doctors or surgeons to select the optimal laser systems and modify the type of their therapy. Therefore, we seek to review the mechanisms of laser- tissue interaction. In this paper, the optical properties of biological tissue such as absorption, scattering, penetration and fluorescence are reviewed. Also, the effects of these properties on laser penetration in tissue have been explained