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Ultrasound-modulated optical
tomography for dense turbid media
Lihong V. Wang, Xuemei Zhao, Steven L. Jacques
Lihong V. Wang, Xuemei Zhao, Steven L. Jacques, "Ultrasound-modulated
optical tomography for dense turbid media," Proc. SPIE 2676, Biomedical
Sensing, Imaging, and Tracking Technologies I, (24 April 1996); doi:
10.1117/12.238787
Event: Photonics West '96, 1996, San Jose, CA, United States
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Ultrasound-Modulated Optical Tomography of Dense Turbid Media
Lihong Wang, Ph. D.'
Xuemei Zhao, M. S.2
Steven L. Jacques, Ph. D.2
(1)
Bioengineering Program
234C Zachry Engineering Center
Texas A&M University, College Station, Texas 77843, USA
Email: lwang@aggie.tamu.edu
(2)
Laser Biology Research Laboratory
University of Texas M. D. Anderson Cancer Center
1515 Holcombe Boulevard, Houston, Texas 77030, USA
Abstract
Continuous-wave ultrasonic modulation of scattered laser light has been used to image objects in
tissue-simulating turbid media for the first time.' We hypothesized that the ultrasound wave
focused into the turbid media modulates the laser light passing through the ultrasonic focal zone.
The modulated laser light collected by a photomultiplier tube reflects the local mechanical and
optical properties in the focal zone. Buried objects in 5-cm thick tissue phantoms (absorption
coefficient 'a
= 0.1
cm', reduced scattering coefficient =
10
cm') were located with millimeter
resolution by scanning and detecting alterations of the ultrasound-modulated optical signal.
Key Words
Ultrasound-modulation, optical imaging, optical tomography, turbid media, scattering media,
biological tissue phantoms.
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SPIE Vol. 2676 191
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Introduction
Non-Optical Imaging
Breast cancer is the most common malignant neopasm and the leading cause of cancer
deaths in women in the United States. A means for prevention of breast cancer has not been found,
and early detection and treatment is the best solution to improve cure rate. At present,
mammography and ultrasonography are clinically used for breast cancer detection. Mammography
is currently the only reliable means of detecting nonpalpable breast cancers. As a supplementary
tool, ultrasound is used to evaluate the internal matrix of circumscribed masses found at
mammography or of palpable masses that are obscured by radiographically dense parenchyma at
mammography.26 However, x-ray mammography is ionizing radiation, and imaging of
radiographically dense breasts is difficult. Ultrasonography cannot detect many of the nonpalpable
cancers that are not visible on mammograms of good quality.3
Several other techniques are under investigation for breast cancer imaging. Magnetic
resonance imaging (MRJ) offers great promise for imaging of the radiographically dense breast.7'8
Breast MRI is superior to mammography in differentiating solid from cystic lesions and is
equivalent to manmiography in providing information regarding different parenchymal patterns.
Injection of intravenous contrast material with MRI increases cancer detectability in spite of the fact
that breast cancer and glandular tissues have similar magnetic resonance tissue characteristics.
However, breast MRI is expensive, has inferior spatial resolution to mammography, and cannot
image microcalcifications, which are often the sole indicators of breast cancer.3'
Breast computed tomography (CT) has been investigated for the differentiation of benign
from malignant solid masses. Breast CT involves the use of intravenous injection of iodinated
contrast material and has limited spatial resolution and high cost; hence, it is not suited for routine
breast cancer
ni3
Optical Imaging
Nonionizing laser light detection ofbreast cancer is a new and active field.1034 The optical
properties of normal and diseased breast tissues are usually different;35'36 therefore, it is possible to
detect breast cancers based on the measurement of optical properties. For example, the scattering
coefficient of fibrocystic tissue '-6OO cnf1) is approximated 50% higher than that of normal
glandular breast tissue (4OO cnf ) or 100% higher than that of normal breast adipose tissue (-.'300
cm') in the wavelength range of 500-1 100 am; the absorption coefficient of ductal carcinoma ('-4
cnf') is about 100% higher than the coefficients of normal glandular or adipose breast tissues (.2
cnf1) around 550 nm wavelength. The optical difference is not surprising because cancerous
tissues manifest significant architectural changes at the cellular and sub-cellular levels, and the
cellular components that cause elastic scattering have dimensions typically on the order of visible to
near-IR wavelengths. Some tumors are associated with vascularization, where blood causes
increased light absorption. The use of optical contrast agents can also be exploited to enhance the
optical contrast between normal and abnormal tissues.'4
Because tissues are optically turbid media, light is quickly diffused inside tissues as a result
of scattering. Light transmitted through tissues is classified into three categories: ballistic light,
quasi-ballistic light, and diffuse light. Ballistic light experiences no scattering by tissue and thus
travels straight through the tissue. Quasi-ballistic light carries some imaging information. Diffuse
light carries little direct imaging information and overshadows ballistic or quasi-ballistic light.
One of the techniques is called "early-photon imaging".2"5'24 If diffuse light is rejected,
and ballistic or quasi-ballistic light is collected, buried objects can be detected. This technique uses
a short-pulse laser (<1 ps pulse width) to illuminate the breast tissue. Only the initial portion of
transmitted light is allowed to pass to a light detector, and the late-arriving light is gated off by a
fast optical gate. If only ballistic light is detected, the imaging is called ballistic imaging. It has been
shown that ballistic imain is possible only for breast tissue thickness less than 0.14 cm or 42
mean free paths (mfp).
Most
ballistic imaging techniques reported in the literature have
92 ISPIE Vol. 2676
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