10 Photon-Tissue Interaction Modelled by Monte Carlo Method …
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on oxygenation state. So the oxygen saturation can be determined by measuring at
two different wavelengths (pulse oxymetry). In “biological window” from 700 nm to
950 nm, the absorption coefficient is distinctly lower than the scattering coefficient,
which results in high penetration depth of the light. The reason for the light scattering
on living cells hasn’t been completely explained yet. For liver cells, it is shown that
the mitochondrion is the main reason for up to 80% of the light scattering. In order to
design optical sensors for photoplethysmography (PPG), optical tomography (OCT),
and other medical instrumentation systems, there are a number of computer-assisted
methods. Their basic principles will be described briefly here.
Scattering measurement techniques
As a precondition to calculate light distribution in biological tissues, the anisotropy
factor, scattering and absorption coefficient of the investigated tissue must be known.
Because their measured values are strongly dependent on the measurement technique,
there is an intense discussion on optical tissue parameters in the few past decades.
The relationship between pathogenous state and the optical tissue parameters is
particularly important for optical imaging techniques. The determination of the tissue
parameters can be made by in vivo and in vitro measurement. On one hand, the in vivo
methods provide the average parameters of the total illuminated area, so the intended
different tissue should be investigated together with in vitro methods. On the other
hand, the tissues which is taken from the living body and fixed under circumstances
show optical characters different from the live tissue. So in individual cases, it must
be decided, which method is suitable. Today, the single- or double-integrating sphere
methods [2, 4] are most often used for tissue optics studies (Fig. 10.4).
Diffusion model
The diffusion model can describe the place and time-resolved light distribution in
scattering tissues, if the scattering is far more than the absorption (μ s μ a ) and the
time range and the distance can be considered so that the light will be multiply scattered. Because the analytical solution can only be obtained in special case, numerical
solutions by finite element are often used so that the inhomogeneous structure and
complicated boundary conditions can be taken into consideration. At present, the
diffusion theory is used in optical tomography of the thick tissue layers (photon
density waves, forward-calculation in iterative image reconstruction).
Monte-Carlo simulation and ray tracing strategy
Monte-Carlo method is well-known in atom physics and other scientific disciplines.
It is successfully used in simulation of the light distribution in scattering tissues.
It is a kind of statistical method, which considers the light as particles (photons).
Before being scattered, an incident photon in a scattering tissue will fly through a
free path constrained by a random process. The scattering angle from the random
process of the same distribution can be calculated from the anisotropy factor g of the
tissue. The photon will fly in a new direction until it is scattered again. The track of
a photon is made until some stopping criteria (maximum path length, reach of the
detector) are satisfied. In order to get a statistically reliable result about the photon
167
on oxygenation state. So the oxygen saturation can be determined by measuring at
two different wavelengths (pulse oxymetry). In “biological window” from 700 nm to
950 nm, the absorption coefficient is distinctly lower than the scattering coefficient,
which results in high penetration depth of the light. The reason for the light scattering
on living cells hasn’t been completely explained yet. For liver cells, it is shown that
the mitochondrion is the main reason for up to 80% of the light scattering. In order to
design optical sensors for photoplethysmography (PPG), optical tomography (OCT),
and other medical instrumentation systems, there are a number of computer-assisted
methods. Their basic principles will be described briefly here.
Scattering measurement techniques
As a precondition to calculate light distribution in biological tissues, the anisotropy
factor, scattering and absorption coefficient of the investigated tissue must be known.
Because their measured values are strongly dependent on the measurement technique,
there is an intense discussion on optical tissue parameters in the few past decades.
The relationship between pathogenous state and the optical tissue parameters is
particularly important for optical imaging techniques. The determination of the tissue
parameters can be made by in vivo and in vitro measurement. On one hand, the in vivo
methods provide the average parameters of the total illuminated area, so the intended
different tissue should be investigated together with in vitro methods. On the other
hand, the tissues which is taken from the living body and fixed under circumstances
show optical characters different from the live tissue. So in individual cases, it must
be decided, which method is suitable. Today, the single- or double-integrating sphere
methods [2, 4] are most often used for tissue optics studies (Fig. 10.4).
Diffusion model
The diffusion model can describe the place and time-resolved light distribution in
scattering tissues, if the scattering is far more than the absorption (μ s μ a ) and the
time range and the distance can be considered so that the light will be multiply scattered. Because the analytical solution can only be obtained in special case, numerical
solutions by finite element are often used so that the inhomogeneous structure and
complicated boundary conditions can be taken into consideration. At present, the
diffusion theory is used in optical tomography of the thick tissue layers (photon
density waves, forward-calculation in iterative image reconstruction).
Monte-Carlo simulation and ray tracing strategy
Monte-Carlo method is well-known in atom physics and other scientific disciplines.
It is successfully used in simulation of the light distribution in scattering tissues.
It is a kind of statistical method, which considers the light as particles (photons).
Before being scattered, an incident photon in a scattering tissue will fly through a
free path constrained by a random process. The scattering angle from the random
process of the same distribution can be calculated from the anisotropy factor g of the
tissue. The photon will fly in a new direction until it is scattered again. The track of
a photon is made until some stopping criteria (maximum path length, reach of the
detector) are satisfied. In order to get a statistically reliable result about the photon