.
Significance:
Raman spectroscopy (RS) applied to surgical guidance is attracting attention among scientists in biomedical optics. Offering a computational platform for studying depth-resolved RS and probing molecular specificity of different tissue layers is of crucial importance to increase the precision of these techniques and facilitate their clinical adoption.
Aim:
The aim of this work was to present a rigorous analysis of inelastic scattering depth sampling and elucidate the relationship between sensing depth of the Raman effect and optical properties of the tissue under interrogation.
Approach:
A new Monte Carlo (MC) package was developed to simulate absorption, fluorescence, elastic, and inelastic scattering of light in tissue. The validity of the MC algorithm was demonstrated by comparison with experimental Raman spectra in phantoms of known optical properties using nylon and polydimethylsiloxane as Raman-active compounds. A series of MC simulations were performed to study the effects of optical properties on Raman sensing depth for an imaging geometry consistent with single-point detection using a handheld fiber optics probe system.
Results:
The MC code was used to estimate the Raman sensing depth of a handheld fiber optics system. For absorption and reduced scattering coefficients of 0.001 and
, the sensing depth varied from 105 to
for a range of Raman probabilities from
to
. Further, for a realistic Raman probability of
, the sensing depth ranged between 10 and
for the range of absorption coefficients 0.001 to
and reduced scattering coefficients of 0.5 to
.
Conclusions:
A spectroscopic MC light transport simulation platform was developed and validated against experimental measurements in tissue phantoms and used to predict depth sensing in tissue. It is hoped that the current package and reported results provide the research community with an effective simulating tool to improve the development of clinical applications of RS.
A methodology is presented for analytically solving simplified spherical harmonics equations (SP) in a finite homogeneous absorbing and scattering cylindrical medium. The SP equations are a reliable approximation to the radiative transfer equation for describing light propagation inside turbid media. The equations consist of a set of coupled partial differential equations (PDEs). The analytical solution developed here is for a steady-state isotropic point source located at an arbitrary point inside a cylindrical turbid medium. Partial-reflection boundary conditions are considered, as they realistically model the refractive index mismatch between a turbid medium and its surroundings (air), as occurs in practice in biomedical optics. The eigen method is used to decouple the set of SP PDEs. The methodology is applied to the SP, which has proved to be sufficiently accurate in practice, but it is readily generalizable to higher orders. The solution is compared with the analytical solution of the diffusion equation as well as to gold standard Monte Carlo simulations for validation, against which it shows good agreement. This work is important, as it provides an additional tool for validating numerical solutions of SP equations for curved geometries, namely, cylindrical shapes, which are often used in practice.
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