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Polarized light transmission through
skin using video reflectometry:
toward optical tomography of
superficial tissue layers
Steven L. Jacques, Martin R. Ostermeyer, Lihong V.
Wang, Dawn V. Stephens
Steven L. Jacques, Martin R. Ostermeyer, Lihong V. Wang, Dawn V.
Stephens, "Polarized light transmission through skin using video
reflectometry: toward optical tomography of superficial tissue layers," Proc.
SPIE 2671, Lasers in Surgery: Advanced Characterization, Therapeutics, and
Systems VI, (17 May 1996); doi: 10.1117/12.240009
Event: Photonics West '96, 1996, San Jose, CA, United States
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Polarized light transmission through skin using video reflectometry:
toward optical tomography of superficial tissue layers.
Steven L. Jacques1'2, Martin Ostermeyer1, Lihong Wang1, Dawn Stephens2
1Laser Biology Research Lab., Univ. of Texas M. D. Anderson Cancer Center, Houston, TX 77030
2Rice University, Houston, TX 77030
ABSTRACT
The movement of polarized light through the superficial layers of the skin was visualized using
a video camera with a polarizing filter. This study constitutes a description of the impulse response to
a point source of incident collimated linearly polarized light. Polarization images reject unwanted
diffusely backscattered light from deeper in the tissue and the specular reflectance from the air/tissue
interface.
Two experiments were conducted:
(1) Video polarization reflectometry used a polarized HeNe laser (633 nm) pointing
perpedicularly down onto a phantom medium (0.900.-j.tm dia. polystyrene spheres in water). The video
camera was oriented 10° off the vertical axis and viewed the irradiation site where the laser beam met
the phantom. Video images were acquired through a polarizing filter that was either parallel or
perpendicular with the reference plane defined by the source, camera, and irradiation site on the
phantom medium's surface. The source polarization was parallel to the reference plane. The two
images (parallel and perpendicular) were used to calculate a polarization image which indicated the
attenuation of polarization as a function of distance between the source and point of photon escape
from the phantom. Results indicated a strong polarization pattern within -0.35 cm (-2.2 mfp') from
source. (mfp' =
1/(pa +
Jls').)
(2) Optical fiber reflectometry using a polarized diode laser (792 nm) coupled to a
polarization-maintaining single-mode fiber, and a multi-mode fiber collector to collect regardless of
polarization. Reflectance as a function of fiber separation was measured for the source fiber oriented
parallel and perpendicular with the reference plane. Results indicated that the strongest polarization
propagated within —0.43 cm (2.2 mfp') from source.
The polarization survived -'2.2 mfp', which for skin at 630-800 nm (mfp' —
0.066
cm)
corresponds to 1 .5
mm
(or 6.4 ps of travel at the speed of light). Using 6.4 ps as a maximum time of
survival, classical paths of photon transport (Feynman paths) were calculated to illustrate the expected
depth of interrogation by polarized imaging. The expected mean depth of photons is about 0.36 mm
at these longer wavelengths. Shorter wavelengths would result in a shorter mfp' and therefore more
superficial imaging of the skin.
Polarization images offer an inexpensive approach toward 2-D acquisition of time-gated
images based on the early light escaping the tissue. Polarization imaging is an opportunity for a new
form of optical image especially useful for dermatology.
O-8194-2045-X/96/$6.OO
SPIE Vol. 2671 / 199
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1. INTRODUCTION
Video images of the skin
or
other tissues are usually dominated by diffusely scattered light and
by specular light from the air/tissue surface. This paper considers an approach toward rejecting both
these components and acquiring images based on those few photons which only interrogate the
superficial layer of the tissue. The technique involves the use of polarized light delivered as a point
source on the tissue. The photons spread within the tissue and escape at the tissue surface for
observation by a video camera. However, the incident photons are polarized and the video camera
observes through a polarizing filter. Hence, the video images are sensitive to the polarized component
of the total escaping light.
This paper presents some preliminary work on the concept of video imaging using polarized
light. The feasibility of such imaging is demonstrated in liquid phantoms.
Aim
of this study:
To establish the behavior of polarized
light transport in the reflectance mode
in order to
specify the opportunity for video
polarization images
for imaging the superficial
epidermis and papillary dermis.
2. BACKGROUND
The total light, 'total, observed escaping the tissue may be divided into two components, a
diffuse component and a polarized component:
'total =
ID
+ III
where lip1 indicates the absolute value of the polarized component, Ip, and ID is the diffuse light
component. The polarized component is defined:
11
if
Ij.=O
=
L
= o
if
I =1=
1=+1±
Li if L=O
and the diffuse component is defined:
Ii
ifIIpl=O
ID
=
'total
-
lIP'
=
10 if
lip1 =
1
200 / SPIE Vol. 2671
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3. METHODS
Phantom
A phantom was prepared using water, polystyrene spheres as scatterers, and trypan blue as
absorber. The optical properties were measured by video reflectometry and utilizing diffusion theory
for analysis. The optical properties at the 633 nm and 792 nm were determined which corresponded
to the He Ne and diode lasers used in the study.
Table 1: Optical properties of phantom
(O.9OO-im-diameter polystyrene spheres in water as scatterer with Trypan blue as absorber)
wavelength
nwater
nspheres
Qsca
P J1s
g
i'
Jia
[cm] [cm1]
[]
[cm1] [cmi]
experimental
633 nm 6.07
0.20
792 nm 46.8
calculated (mie theory)
633 nm
1.3316
1.5721 1.9763 5.6x109 70.3
0.9136 6.07
0.20
792 nm
1.3290
1.5645
1.2999
5.6x109 46.3
0.8891 5.13
0.02
transport mean free path (mfp' =
1/(.t
+
t'))
633
nm
0.159 cm
792 nm
0.194 cm
1s
= i'/(1-g);
density of spheres, p =
Jls/Qsca/A
geometrical cross-sectional area of spheres, A =
icr2
=
ic(0.45x104
cm)2 =
0.636x108
cm2
For comparison, the optical properties of bloodless dermis in the range of 630-800 nm are
approximately
15
cm1, J.ta
0.25 cm4, which yields a mfp' of 0.066 cm. This mfp' is 2.4-fold
and 3.0-fold shorter than the phantom at 633 and 792 nm, respectively.
Optical
Properties of phantom:
(polystyrene spheres + trypan blue + water)
633nm 792nm
absorption
p 0.20
0.02 cm-1
scattering
70.3 46.3 cm'
anisotropy
g
0.914 0.889
red. scattering
t' 6.073
5.14 cm-1
equivalent to
skin scaled larger by 2.4-fold to 3-fold
(2.4 to 3 mm of phantom 1 mm of skin)
SPIE
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Experimental setup for polarized video imaging
The experimental setup is shown below. A polarized He-Ne laser beam was directed
perpendicularly down onto the liquid phantom. The camera acquired light which had entered the
medium then backscattered, passing through a polarizing filter in front of the camera. The images
were acquired by a frame grabber using a Macintosh computer and the IP Lab software.
Two images were acquired. First, the polarizing filter was aligned with the reference plane to
achieve a parallel image called 1=. Second, the polarizer was set at 900
with
respect to the reference
plane to achieve a perpendicular image called I. The reference plane was specified by the camera,
laser, and irradiation site on the phantom's surface.
A combined image called the polarization image, Ip, was prepared by calculating pixel by pixel
using the definition of Ip defined in section 2.
Experimental
setup: video camera
Optical fiber measurements
laser's polarization
filter parallel to
reference plane
reference plane
specified by
camera, laser, and
irradiation site on
phantom surface
To provide an alternative test of the movement of polarized light in a scattering medium, a
linearly polarized diode laser (792 nm) delivered via a polarization-maintaining single-mode fiber was
used as a source and a second multi-mode fiber was used to collect light. The setup is shown in the
figure below. The single-mode fiber preserved the polarization of the diode laser source as light was
transported to the phantom. The collecting fiber collected light with equal efficiency regardless of its
polarization and routed the light to an avalanche photodiode. The diode laser source was modulated
at 1 MHz by a network analyzer and the diode detector output was returned to the analyzer. The
modulation of the 1 MHz signal constituted the recorded data. Hence the influence of background
lighting was eliminated.
202 /SPIE Vol. 2671
polarizing filters
633 nm
HeNe
laser
16-bit
video
camera's
camera
polarization
filter is oriented
either
parallel or
perpendicular to
reference plane
'iantom
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