Using an obliquely incident laser beam to measure optical properties of turbid media

A simple and quick approach was invented to measure optical properties of tissue-like turbid media. A laser beam with oblique incidence to the medium causes the center of the diffuse reflectance that is several transport mean free paths away from the incident point to shift from the point of incedence. The amount of shift is used to compute the reduced scattering coefficient by a simple formula. This formula is a function of the refractive index of the turbid medium divided by that of the incident medium and the angle of incidence off the surface normal for a semi-infinite turbid medium having a much smaller absorption coefficient than the reduced scattering coefficient. For a turbid medium having a comparable absorption coefficient with the reduced scattering coefficient, a revision to the above formula was made. The slope of the diffuse reflectance can be used to compute the penetration depth. Both the computation of the reduced scattering coefficient and penetration depth are based on simple and quick algorithms. the validity condition of the algorithms for slabs of turbid media are studied. This technique has potential for noninvasive, in vivo, real-time diagnosis of disease or monitoring of treatments

Use of a laser beam with an oblique angle of incidence to measure the reduced scattering coefficient of a turbid medium

A simple and quick approach is used to measure the reduced scattering coefficient (μ_s’) of a semi-infinite turbid medium having a much smaller absorption coefficient than μ_s’. A laser beam with an oblique angle of incidence to the medium causes the center of the diffuse reflectance that is several transport mean-free paths away from the incident point to shift away from the point of incidence by an amount Δx. This amount is used to compute μ_s’ by μ_s’ = sin(α_i)/(nΔx), where n is the refractive index of the turbid medium divided by that of the incident medium and α_i is the angle of incidence measured from the surface normal. For a turbid medium having an absorption coefficient comparable with μ_s’, a revision to the above formula is made. This method is tested theoretically by Monte Carlo simulations and experimentally by a video reflectometer

Animated simulation of light transport in tissues

Time-resolved light transport in composite tissues is simulated using the Monte Carlo technique. Snapshots of spatial distributions of physical quantities, including light absorption rate, light fluence rate, and diffuse reflectance rate, are presented. Such multiple snapshots with a given time interval can be shown sequentially to achieve an animation effect. This animated simulation is a tool that aids in the general understanding of light transport in tissues. For example, the simulation of time-resolved spatial distribution of light fluence rate inside a tissue illustrates how fast light is dispersed inside tissues. The simulation of diffuse reflectance rate as a function of time of a short-pulsed laser incident upon a piece of tissue containing a buried object shows that early reflected light does not carry imaging information of the object. The imaging quality of the object can thus be improved by rejecting the early-arriving reflected light

Analysis of diffusion theory and similarity relations for light reflectance by turbid media

Both diffusion theory and similarity relations for light reflectance by semi-infinite turbid media have been analyzed by comparing their computational results with Monte Carlo simulation results. Since a large number of photon packets are traced, the variance of the Monte Carlo simulation results is small enough to reveal the detailed defects of diffusion theories and similarity relations. We have demonstrated that both diffusion theory and similarity relations provide very accurate results when the photon sources are isotropic and one transport mean free path below the turbid medium surface or deeper. This analysis has led to a hybrid model of Monte Carlo simulation and diffusion theory, which combines the accuracy advantage of Monte Carlo simulation and the speed advantage of diffusion theory. The similarity relations are used for the transition from the Monte Carlo simulation to the diffusion theory

Optimized radial and angular positions in Monte Carlo modeling

In Monte Carlo simulations of light transport in tissues, a grid system is set up to score physical quantities. This study of cylindrically symmetrical problems found that the optimized radial and angular positions for the averaged physical quantities in each grid element are off-center. The error of the extrapolated physical quantities at the light-incidence point using the centered radial positions is up to 14.3%

Hybrid model of Monte Carlo simulation and diffusion theory for light reflectance by turbid media

Light reflectance by semi-infinite turbid media is modeled by a hybrid of Monte Carlo simulation and diffusion theory, which combines the accuracy of Monte Carlo simulation near the source and the speed of diffusion theory distant from the source. For example, when the turbid medium has the following optical properties—absorption coefficient 1 cm^(-1), scattering coefficient 100 cm^(-1), anisotropy 0.9, and refractive-index-matched boundary—the hybrid simulation is 7 times faster than the pure Monte Carlo simulation (100,000 photon packets were traced), and the difference between the two simulations is within 2 standard deviations of the Monte Carlo simulation

Non-invasive detection of skin cancers by measuring optical properties of tissues

Skin cancer is the most frequently occurring cancer of all cancers. Each year over 500,000 new cases of skin cancer will be detected. A high percentage of skin cancers are diseases in which fatalities can be all but eliminated and morbidity reduced if detected early and treated properly. These skin lesions are distinguished generally by subjective visual inspection and their definitive diagnosis requires time-consuming expensive histopathological evaluation of excisional or incisional biopsies. In vivo experimental evidence published in the literature has shown that cancerous skin lesions have different total diffuse reflectance spectra than non- cancerous lesions or normal skin. Therefore, cancerous skin lesions may be differentiated from non-cancerous skin lesions by comparing the optical properties of the skin lesions with those of the surrounding normal skin sites, where the optical properties of the normal skin sites are used to account for different types of skin or different areas of skin. We have demonstrated that the effect of melanin concentration on the diffuse reflectance may be removed by extrapolating the reflectance at different wavelengths to an apparent pivot point. Because the concentration of melanin does not indicate malignancy, the removal of its effect is important to avoid false detection. The total diffuse reflectance depends on the albedo and anisotropy of tissues. Therefore, the total diffuse reflectance will remain the same as long as the anisotropy and the ratio between the absorption coefficient and the reduced scattering coefficient remain the same. Separating the absorption and scattering effects should enhance the detection sensitivity of skin cancers

Multiple-source optical diffusion approximation for a multilayer scattering medium

A method for improving the accuracy of the optical diffusion theory for a multilayer scattering medium is presented. An infinitesimally narrow incident light beam is replaced by multiple isotropic point sources of different strengths that are placed in the scattering medium along the incident beam. The multiple sources are then used to develop a multilayer diffusion theory. Diffuse reflectance is then computed using the multilayer diffusion theory and compared with accurate data computed by the Monte Carlo method. This multisource method is found to be significantly more accurate than the previous single-source method

Computation of the optical properties of tissues from light reflectance using a neural network

We have established a neural network to quickly deduce optical properties of tissue slabs from the diffuse reflectance distribution. Diffusion theory based on multiple image sources mirrored about the two extrapolated boundaries is used to prepare the training and testing sets for the neural network. The neural network is trained using backpropagation with the conjugate gradient method. Once the neural network is trained, it is able to deduce optical properties of tissues within on the order of a millisecond. The range of the tissue optical properties that is covered by our neural network is 0.01 - 2 cm^(-1) for absorption coefficient, 5 - 25 cm^(-1) for reduced scattering coefficient, and 0.001 - 1 cm for tissue thickness. A separate network is also trained for thick tissue slabs. A simple experimental setup applying the trained neural network is designed to measure tissue optical properties quickly

Ultrasound-modulated optical tomography for dense turbid media

Continuous-wave ultrasonic modulation of scattered laser light has been used to image objects in tissue-simulating turbid media for the first time. We hypothesized that the ultrasound wave focused into the turbid media modulates the laser light passing through the ultrasonic focal zone. The modulated laser light collected by a photomultiplier tube reflects the local mechanical and optical properties in the focal zone. Buried objects in 5-cm thick tissue phantoms (absorption coefficient µ_a = 0.1 cm^(-1), reduced scattering coefficient µ_s' = 10 cm^(-1)) were located with millimeter resolution by scanning and detecting alterations of the ultrasound-modulated optical signal