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Ultrasonic modulation of diffuse light

in turbid media

Lihong V. Wang, Xuemei Zhao

Lihong V. Wang, Xuemei Zhao, "Ultrasonic modulation of diffuse light in turbid

media," Proc. SPIE 2979, Optical Tomography and Spectroscopy of Tissue:

Theory, Instrumentation, Model, and Human Studies II, (18 August 1997); doi:

10.1117/12.280273

Event: BiOS '97, Part of Photonics West, 1997, San Jose, CA, United States

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Ultrasonic Modulation of Diffuse Light in Turbid Media

Lihong Wang* and Xuemei Zhao

Bioengineering Program

Texas A&M University

College Station, Texas 77843-3 120

Abstract

Continuous-wave ultrasonic modulation of laser light has been used to image objects buried

in tissue-simulating turbid

The

ultrasonic wave focused into the turbid media modulated

the laser light passing through the ultrasonic field. The modulated laser light collected by a

photomultiplier tube reflected primarily the local mechanical and optical properties in the focal

zone. Modulated signal was estimated using diffusion theory. The dependence of the ultrasound-

modulated optical signal on the off-axis distance of the detector from the optic axis was studied.

The mechanisms of ultrasonic modulation of light were discussed.

Key Words

Ultrasonic modulation, acousto-optics, optical imaging, optical tomography, turbid media.

Introduction

Nonionizing optical tomography of biological tissues including breast tissues has been an

active research field as indicated by this volume and others.3 The contrast mechanism of optical

imaging is based on the difference in optical properties between diseased and surrounding normal

biological tissues. Several optical imaging techniques being investigated include time-gated or time-

resolved optical imaging, frequency-domain optical imaging, and optical coherence tomography.

The time-resolved and frequency-domain techniques are mathematically linked by Fourier

transform and have achieved comparable results, namely, several millimeter resolution for

approximately five-centimeter thick tissues or tissue phantoms. Optical coherence tomography has

achieved <10 .tm resolution in both axial and lateral dimensions but is limited to a penetration

depth of a couple of millimeters into biological tissues.

Because biological tissues are optically turbid media, light is quickly diffused inside tissues

as a result of strong scattering. Light transmitted through tissues is classified into three categories:

Corresponding

author.

Tel: (409) 847-9040

Email: LWang@tamu.edu

Fax: (409) 847-9005

URL: http://biomed.tamu.edu/P4w

SPIE Vol. 2979 • 0277-786X/971$1O.OO

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ballistic light, quasi-ballistic light, and diffuse light. Ballistic light experiences no scattering by

tissue and travels straight through the tissue and hence carries direct imaging information. Quasi-

ballistic light experiences minimal forward-directed scattering and carries some imaging

information. Diffuse light follows tortuous paths and carries little direct imaging information and

overshadows ballistic or quasi-ballistic light.

For breast tissue of clinically useful thickness (5

10 cm), scattered light must be used to

image breast cancers. We have shown that for a 5-cm-thick breast tissue with the assumed

absorption coefficient ji 0. 1 cm1, reduced scattering coefficient i' =

cm', the detector must

collect transmitted light that has experienced at least 1 100 scattering events in tissue to yield enough

signal.4 Therefore, ballistic light or even quasi-ballistic light does not exist for practical purposes.

However, if a 10-mW visible or near JR laser is incident on one side of the 5-cm thick breast

tissue, we have estimated using diffusion theory that the diffuse transmittance on the other side is

on the order of 10 nW/cm2 or 1O'° photons/(s•cm2), which is detectable using a photomultiplier

tube capable of single-photon counting. Similarly, the diffuse transmittance through a 10-cm thick

breast tissue would be on the order of 1 pW/cm2 or 106 photons/(s•cm2).

To utilize the diffuse light for its abundance and overcome its lack of direct imaging

information, ultrasound-modulated optical tomography of turbid media has been studied. Marks et

al have investigated tissue imaging using the combination of pulsed ultrasound and laser light and

have detected the ultrasound-modulated optical signal in a homogeneous turbid medium without

buried objects.5 Wang et a! developed ultrasound-modulated optical tomography that combined

continuous-wave ultrasound and laser irradiation and successfully imaged buried objects in tissue-

simulating turbid media."2 The major advantage of using continuous-wave ultrasonic modulation

over pulsed ultrasonic modulation is the significant increase in signal-to-noise ratio. Leutz and

Maret reported the observation of ultrasonic modulation of multiple light scattering speckles.6

Kempe et al investigated the modulation of the optical field transmitted through a turbid medium by

a quasi-continuous-wave ultrasound beam.7

We report in this paper the theoretical considerations and experimental results of our recent

studies. Modulated signal was calculated using diffusion theory. The dependence of the

ultrasound-modulated optical signal on the off-axis distance of the detector from the optic axis was

studied. The mechanisms of ultrasonic modulation of light were discussed.

Hypothesis

The design of the experiment on ultrasound-modulated optical tomography was based on

the following hypothesis (Fig. 1). Continuous-wave ultrasound wave was propagated through a

turbid medium and focused to a small spot inside the medium. The ultrasound wave modulated

light passing through the ultrasonic field.

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Several possible modulation

mechanisms are discussed

follows (Fig. 2).

Ultrasound

wave generated pressure variation

(approach

The

pressure

variation induced a density change

Focus

in the medium as a result of the

______

compressibility

of the medium. The i-

optical

absorption and scattering

coefficients were proportional to the

number density of absorbers and

scatterers, respectively. The index __________________________________________________

of refraction varied with the density

Fig. 1 .

Illustration

of the hypothesis of ultrasound-

as well. Therefore, the density

modulated optical tomography. The ultrasound-modulated

variation modulated the optical

light carries the information of the optical and mechanical

properties of the medium at the

properties at the ultrasonic focal spot, which provides a

ultrasound frequency. The variation

localization mechanism for optical imaging.

of optical properties modulated the

light passing through the ultrasonic field.

Ultrasound wave generated particle displacement (approach 2). The particle displacement

caused optical pathllength to change. Coherent laser light passing through turbid media generated

speckles. Because speckles depended on the optical pathlength, the speckles varied at the

ultrasonic frequency.

Ultrasound wave may be considered as phonons, whereas light may be considered as

photons (approach 3). The photon-phonon interaction caused Doppler shift8 of the optical

frequency by the ultrasonic frequency. An optical detector functioned as a heterodyning device

between the Doppler-shifted light and unshifted light and produced a signal of the ultrasonic

frequency.

The modulated light carried the information of the optical and mechanical properties near

the focal spot, where the modulation off focus was assumed less than that at the focus. The

ultrasound-modulated light signal can be separated from the unmodulated light signal by an

electronic filter. Scanning the imaging system relative to the turbid medium would generate an

image of the medium based on the distribution of optical and mechanical properties. The ultrasonic

modulation depended on any light, which was primarily diffuse light rather than ballistic or quasi-

ballistic light for dense turbid medium. The imaging resolution depended on the size of the

ultrasonic focus.

Ultrasonic

rransducer

Ballistic

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