Dispersion dependence of the optical anisotropy and the degree of depolarization
of fibrous tissues
D. A. Zimnyakov, G. V. Simonenko,
a
and V. V. Tuchin
Saratov State University, Saratov
Submitted April 5, 2010
Opticheski Zhurnal 77, 69–74 September 2010
This paper presents the results of studies of the optical characteristics of fibrous tissues. It is
shown that the macroscopic anisotropy of the refractive indices in fibrous biological tissues has
an anomalous character of the frequency dependence far from the absorption bands, and an
explanation is proposed for this effect. Moreover, it is shown in this paper that the dispersion
dependence of the degree of depolarization of radiation transmitted through biological tissue is well
approximated by a power function with an exponent equal to -4. © 2010 Optical Society of
America.
INTRODUCTION
The probing of biological tissues with polarized radia-
tion in the visible and near-IR regions is a promising ap-
proach in clinical-laboratory diagnostics, making it possible
to efficiently detect pathological changes in the structure of
the tissues.
1–3
The scattering of polarized light by randomly
inhomogeneous media, which include the vast majority of
biological tissues, causes it to be partially or totally depolar-
ized depending on the structural features of the tissue and
the wavelength of the probe radiation. The degree of depo-
larization of radiation transmitted through biological tissue
makes it possible to distinguish the optical signal formed
predominantly by single-scattering events in the layers of the
tissue.
4
The main factors that affect the degree of depolariza-
tion of the radiation are
1. the thickness of the probed layer,
2. the scattering-anisotropy parameter of the probed
medium,
5–7
3. the absorption coefficient of the medium, determined by
the concentration of chromophores for example, hemo-
globin in it,
8,9
4. the macroscopic anisotropy of the refractive index of the
biological tissue.
10
The macroscopic anisotropy of the morphological and
accordingly the optical characteristics of certain biological
tissues for example, muscle tissue, tendons, derma, caused
by their fibrillar structure with partially oriented close-
packed fibers,
10
is specifically manifested when polarization
diagnosis is used and such objects are visualized.
11,12
The
spatial distributions of the polarization characteristics of the
detected scattered radiation in particular, the degree of de-
polarization also have pronounced anisotropy.
It should be pointed out that the structural anisotropy of
such multiple-scattering media must manifest itself not only
in different values of the transport characteristics of the me-
dium the transport scattering coefficient
s
', the scattering
coefficient
s
, and the scattering-anisotropy parameter g for
the principal directions in the medium being probed, deter-
mined by the predominant orientation of the structure-
forming fibers.
10
A characteristic property of fibrous tissues
with partially oriented fibers is also the birefringence that
manifests itself at macroscopic scales that substantially ex-
ceed the typical transport length for the medium being
probed. Test samples of biological tissues are often charac-
terized by small damping values of the optical signal; as a
rule, L
t
1, where L is the sample thickness, and
t
is the
extinction factor. In the absence of selective absorption of the
probe radiation by the chromophores that enter into the com-
position of the biological tissue,
t
s
, where
s
is the
scattering factor of the biological tissue at the wavelength of
the probe radiation. An analysis of the features of radiation
transport in optically thin layers of biological tissues probed
in transmitted light (see, for example, Ref. 13) makes it pos-
sible to conclude that the detected optical signal mainly rep-
resents the partial components with optical path lengths in
the probed medium of s L (unscattered, or ballistic; singly
scattered; doubly scattered; etc.). In this case, the contribu-
tion of the partial components to the detected optical signal
quickly dies out as the dimensionless parameter
s
s that
characterizes the scattering multiplicity of the given compo-
nent increases and becomes negligible for singly scattered
components with s 1 /
s
. Such scattering regimes of the
probe radiation can be defined as low-multiplicity scattering
(see, for example, Ref. 10). When unscattered or low-
multiplicity scattered components in the detected optical sig-
nal give a substantial contribution, two factors should be
kept in mind in analyzing the polarization characteristics of
the detected radiation:
1. the depolarization effect due to multiple scattering of the
plane-polarized probe light in the medium, and
2. the transformation of the type of polarization in general,
from plane-polarized to elliptically polarized for the un-
scattered and low-multiplicity scattered components.
These factors should be taken into account when opti-
cally thin layers of biological tissues are probed while de-
tecting forward-scattered radiation or when optical coherent
tomography is used to investigate the structure of the surface
layers of biological tissues; this gives additional possibilities
in investigating pathological changes of the fibrillar structure
of the tissues. An informative parameter that describes the
577 577 J. Opt. Technol. 77 9, September 2010 1070-9762/2010/090577-05$15.00 © 2010 Optical Society of America