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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

(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

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 =

+ III

where

lip1 indicates the absolute value of the polarized component, Ip, and ID is the diffuse light

component. The polarized component is defined:

Ij.=O

= o

I =1=

1=+1±

Li if L=O

and the diffuse component is defined:

ifIIpl=O

'total

lIP'

10 if

lip1 =

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

J1s

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

0.159 cm

792 nm

0.194 cm

= 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

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

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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