The intensity of two photon excited fluorescence (TPF) in highly scattering media generated by ultrashort laser pulses as a function of depth of focal point inside the media were measured for investigating the spatial location of TPF in scattering media. Scattering medium attenuates the intensity of incident beam. TPF signal decayed exponentially as the focal point was scanned into the medium. TPF was not confined in the focal region and had a wider distribution as the scattering strength of the medium increased resulting degradation of optical sectioning ability.
The light scattering is the main reason to degrade the imaging resolution of optical tomography. In order to improve the imaging resolution, polarization and scattering angle differential technique was developed to suppress the effect of scattering in turbid medium. An explicit theory for the technique was presented and a transillumination imaging system under the technique was set up. Using this system, the feasibility of this technique was demonstrated through a serial of experimental measurements in model phantom and higher resolution was obtained than that maintained by conventional method. The technique shows promise in getting the high resolution imaging in conjunction with other optical tomography methods.
An optical topography algorithm is presented for a two dimensional stratified slab. Under the approximation of phase function, the Green function technique is employed to solve the time resolved propagation equation of ultrafast laser pulses within the slab. The initial condition of the Green function can be expressed with an explicit form and is correlated with the optical parameters and the incident angles. This leads to a straightforward numerical inverse algorithm to reconstruct the optical parameters simultaneously. The reconstruction algorithm shows promise in noninvasive detection of the structure of turbid medium.
In the present paper, we consider that a planar laser pulse falls obliquely on the surface of a stratified tissue slab. The Green function approach is developed to reconstruct two optical parameters of the stratified tissue simultaneously. By analyzing the strucutre of the fundamental solution, the transport equation of the Green function and the explicit form of the initial condition are derived. The initial condition of Green function is found that correlates to the parameters of tissue and incident angle. By the characteristic method, the measurable boundary data are propagated to the initial value layer by layer for each incident angle. The scattering and absorption coefficient of tissue are then reconstructed simultaneously layer by layer using the explicit expression of the initial condition of Green fucntion. A detailed algorithm for the reconstruction is presented and an imaging picture as a numerical examples is given.
In this paper, we consider a laser pulse normally incident on the surface of a stratified tissue. The Green function technique in the time domain is used to solve the inverse problem of the transport equation that describes the behavior of energy transport in tissue. The novel algorithm is applied to reconstruct the optical parameters of tissue, which provides the possibility of noninvasive diagnosis and image. The structure of the fundamental solution is analyzed and the equation for the Green function is given together with the initial and boundary conditions. The initial value of the Green function is related to the optical parameters of tissue, and this condition is employed to reconstruct the scattering coefficients or absorption coefficients when the other parameters are assumed to be known. The reflection data are required as the input data in the reconstruction algorithm. It is shown that one-sided reflection data are sufficient to reconstruct one of the optical parameters. The numerical results are presented for both the clean and noisy data to check the stability of the algorithm. This method also provides an efficient way to calculate the internal light distribution field as well as to reconstruct the internal optical parameters distribution.
A reiteration method based on a modified discrete lattice model is presented to determine the absorption and transport scattering coefficients according to the fit of the calculated radial distribution curve of diffuse reflectance to the measured one. The modified discrete lattice model which includes the effects of specular reflection and the reduced incident intensity serves to exactly calculate the probability P(x,y,m) that a normally incident photon is emitted from the lattice point(x,y) at mth step on the surface of a semi-infinite medium with the index of refraction n before reiteration. In the reiteration procedure, various parameters to absorption and transport scattering coefficients as input data in an optimal sequences are used to calculate the radial distribution of diffuse reflectance R((rho) ) on the basis of P(x,y,m). The parameters correspondent to the radial distribution of diffuse reflectance which fits the measured curve are adopted to be the absorption and transport scattering coefficients of the medium. Steady state diffuse reflectance measurements are performed on Intralipid/ink phantoms with various concentrations. It costs only a few minutes to determine the absorption and transport scattering coefficients from the radial distribution of diffuse reflectance by this method in a personal microcomputer. The good agreement between the results of this method and those obtained from the transmission measurements of scattering and absorption demonstrates the potential of this technique in clinical applications.
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