We present the feasibility of functional ultrasound-modulated optical tomography (UOT) in tissue phantoms with two optical wavelengths. By using intense acoustic bursts and a CCD camera-based speckle contrast detection technique, we observe variations of UOT signal at different optical absorptions. In addition, the results from Monte Carlo simulations highly correlate with the experimental outcomes. By irradiating the sample at two optical wavelengths, we quantitatively estimate the total concentration and the concentration ratio of double dyes in inclusions inside tissue phantoms. Therefore, UOT is potentially able to supply functional imaging of the total concentration and oxygen saturation of hemoglobin non-invasively in biological tissues

Kim, Chulhong

Wang, Lihong V.

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Toward functional ultrasound-
modulated optical tomography: a
phantom study
Chulhong  Kim, Lihong V. Wang
Chulhong  Kim, Lihong V. Wang, "Toward functional ultrasound-modulated
optical tomography: a phantom study," Proc. SPIE 6856, Photons Plus
Ultrasound: Imaging and Sensing 2008: The Ninth Conference on Biomedical
Thermoacoustics, Optoacoustics, and Acousto-optics, 68561U (28 February
2008); doi: 10.1117/12.761377
Event: SPIE BiOS, 2008, San Jose, California, United States
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Toward functional ultrasound-modulated optical tomography: a 
phantom study 
 
Chulhong Kim and Lihong V. Wang* 
 
Optical Imaging Laboratory, Department of Biomedical Engineering, Washington University in St. 
Louis, Campus Box 1097, One Brookings Dr. St. Louis, Missouri, 63130 
 
 
 
ABSTRACT 
 
We present the feasibility of functional ultrasound-modulated optical tomography (UOT) in tissue phantoms with two 
optical wavelengths. By using intense acoustic bursts and a CCD camera-based speckle contrast detection technique, we 
observe variations of UOT signal at different optical absorptions. In addition, the results from Monte Carlo simulations 
highly correlate with the experimental outcomes. By irradiating the sample at two optical wavelengths, we 
quantitatively estimate the total concentration and the concentration ratio of double dyes in inclusions inside tissue 
phantoms. Therefore, UOT is potentially able to supply functional imaging of the total concentration and oxygen 
saturation of hemoglobin non-invasively in biological tissues. 
 
Keywords: Ultrasound-modulated optical tomography, total hemoglobin concentration, oxygen saturation of 
hemoglobin, functional imaging. 
 
INTRODUCTION 
1. Motivation 
Imaging functional information of living subjects, such as the total hemoglobin concentration (HbT) and hemoglobin 
oxygen saturation (SO2), has become important in many medical fields, for examples, imaging normal and abnormal 
brain activation1 and studying tumor physiopathology2. Currently, a few non-ionizing-optical imaging techniques, such 
as diffuse optical tomography (DOT)3 and photoacoustic imaging4,5, can estimate the HbT and SO2 in living subjects 
non-invasively. Due to strong light scattering in biological tissue, however, pure optical imaging techniques like DOT 
suffer from poor spatial resolution which degrades with increasing imaging depth. Photoacoutic imaging overcomes 
such drawbacks of DOT because its spatial resolution depends on ultrasound parameters, whose scattering is 2–3 orders 
of magnitude weaker than light scattering in biological tissues. Similar to photoacoustic imaging, ultrasound-modulated 
optical tomography (UOT)6 is able to provide strong optical contrast and high ultrasonic spatial resolution. Therefore, 
thanks to the strong optical contrast, UOT has potential for functional imaging. 
The principles of UOT is that multiply scattered light passing through an ultrasonic focal volume is acoustically phase-
modulated by both ultrasound-induced particle displacement and changes in refractive index. By computing the ratio of 
the ultrasound-modulated light intensity to the unmodulated light intensity at each scanned ultrasonic focal volume, 
images represent the optical heterogeneities of the tissue. The mechanisms of UOT has been theoretically studied by 
Leutz et al.7, Wang8, and Sakadžic´ and Wang9,10. Because of the low modulation efficiency and uncorrelated speckle 
grains, low signal-to-noise ratio (SNR) is a major problem in UOT. A number of detection techniques have been 
pioneered to detect weak modulated signals efficiently such as parallel speckle detection using a CCD camera11, 
photorefractive crystal based detection12, fabry-perot interferometer based detection13. Rather than devoting efforts to 
the detection of weak signals, Kim et al. and Zemp et al.14,15 explored the use of intense acoustic bursts as a significant 
signal enhancement mechanism. Recently, Xu et al.16 and Bossy et al.17 reported the measurement of mechanical 
contrast of tissues using UOT in detail. In this Proceedings artile, we applied two optical wavelengths to UOT toward 
studying the feasibility of functional imaging, total hemoglobin concentration and hemoglobin saturation of oxygen, of 
tissue. By using intense acoustic bursts and using the speckle contrast detection technique14,15 at two optical 
                                                          
* Corresponding author: lhwang@biomed.wustl.edu 
Photons Plus Ultrasound: Imaging and Sensing 2008: The Ninth Conference on Biomedical
Thermoacoustics, Optoacoustics, and Acousto-optics, edited by Alexander A. Oraevsky, Lihong V. Wang,
Proc. of SPIE Vol. 6856, 68561U, (2008) · 1605-7422/08/$18 · doi: 10.1117/12.761377
Proc. of SPIE Vol. 6856  68561U-1
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generatorvx
 
 
wavelengths, we quantitatively estimated the relative total concentration of red and blue dye mixtures and the ratio of 
the red dye concentration to the total concentration in inclusions in tissue mimicking phantoms. In addition, the 
variation of UOT signals at various optical absorptions was modeled with Monte Carlo simulations and compared with 
the experimental results. 
 
 
EXPERIMENT 
 
The experimental schematic is shown in Fig. 1. Two lasers (JDS Uniphase, HeNe 1145; 633-nm wavelength and Melles 
Griot, 56ICS153/HS; 657-nm wavelength) illuminated a tissue mimicking phantom sequentially. An average laser 
power of ~30 mW/cm2 was delivered to the tissue phantom. A 1-MHz focused ultrasound transducer with 25 mm active 
aperture and 38 mm focal length (Ultran, VHP100-1-138) generated acoustic waves. The ultrasonic focal zone was ~2 
mm in diameter and ~20 mm in length. An ultrasound peak pressure of less than 1.9 MPa was applied to the focal point. 
Therefore, the mechanical index at this frequency was smaller than 1.9, which is within the typical safety limit for 
diagnostic ultrasound.18 We used a gelatin-cornstarch phantom (10% gelatin and 10% cornstarch by weight) whose 
optical reduced scattering coefficient was ~9 cm–1 at both wavelengths measured by the oblique-incidence diffuse 
reflectance spectroscopy.19 A red dye was made from the mixture of Fiesta Red and India Red inks, whereas a blue dye 
was made of Trypan Blue. The absorption coefficients of the red dye in the mother liquid at 633- and 657-nm 
wavelengths measured by the light transmission were ~2.2 and ~1.0 cm–1, respectively, whereas the ones of the blue dye 
in the mother liquid at the two wavelengths were ~11.9 and ~5.2 cm–1, respectively. The optically absorptive targets 
were made of the dye solutions and 10% gelatin. A CCD camera (Basler, A312f; 12-bit, 640×480 pixels) captured 
speckle patterns emerging from the sample. By placing a lens tube, which acted as an iris, in front of the CCD camera, 
we matched the average speckle size to a single CCD pixel size. Ultrasonic bursts were synthesized by a function 
generator (Agilent, 33250A) and amplified by an RF amplifier (Amplifier Research, 75A250), and subsequently drove 
the ultrasound transducer. We used a low 1 Hz duty cycle to prevent damage to the transducer. The function generator 
triggered a pulse-delay generator (Stanford Research, DG535) that produced two CCD trigger pulses for each burst. The 
burst synchronized speckle pattern was captured with ultrasound; then, another one was captured without ultrasound. 
The exposure time of the CCD camera was equivalent to the duration of an ultrasonic burst. The laser speckle contrast 
was computed with and without ultrasound modulation. The UOT signal was defined as the change in speckle contrast 
between ultrasound on and off (off value – on value). 
 
 
Figure 1: The UOT experimental Setup. 
 
By irradiating the sample at two optical wavelengths, λ1 and λ2, independently, we obtained two UOT images that 
represent the distributions of light fluences at the two wavelengths. The dependence of the amplitude of the UOT signal 
on the local optical absorption was approximated to ( )daµ−exp , where aµ  is the relative-local-optical absorption 
coefficient and d is the diameter of an buried object. By using this dependence, we calculated the concentrations of 
both blue and red dyes. Therefore, the total dye concentration, [ ] [ ]BR + , and the ratio of the red dye concentration to 
the total dye concentration, [ ] [ ] [ ]( )BRR + , can be calculated as follows: 
,
)()()()(
)()()()(][][
1221
1221
λελελελε
λελµλελµ
RBRB
aaBR −
∆−∆=+
 (1) 
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0.0
0.8 — ExperimentalSimulated
.
D) 0.7
ci)
0.6
cci
0
Z 0.5 R2=98%
0.' •0 2 4 6 8 10 12 14
Absorption coefficient [cm]
 
 
,
)()()()(
)()()()(
][][
][
1221
2112
λελµλελµ
λελµλελµ
∆−∆
−=+ aa
BaBa
BR
R
 (2) 
where [ ]R  and [ ]B  are the concentrations of red and blue dyes, respectively, relative to their mother liquids; aµ  is 
the measured relative optical absorption; Rε  and Bε  are the relative absorption coefficients of pure red and blue 
mother liquids, respectively, measured with our UOT system; and BR εεε −=∆ . 
 
RESULTS AND DISCUSSIONS 
 
The ratio of the ultrasound modulated light intensity to the unmodulated light intensity was computed using Monte 
Carlo simulations at different optical absorptions, and the simulated results were compared to the experimental results 
(Fig. 2). The Monte Carlo simulations are based on the temporal correlation transfer equation for acoustically 
modulated multiply scattered light.10 This numerical method can map the distribution of ultrasound-modulated light 
intensity in isotropically optical scattering (scattering anisotropy: 0) medium containing heterogeneous optical 
properties and a nonuniform ultrasonic field.10 The simulation parameters included a cylindrical-ultrasonic-focal zone of 
2 mm in diameter and 20 mm in length,  an ultrasonic pressure of 1.5 MPa, an optical wavelength of 633 nm, a 20 
mm-thick isotropically scattering medium with a reduced scattering coefficient of ~9 cm–1, and a cylindrical-optical-
absorbing object of 2.2 mm in diameter and 11 mm in length. The optical absorption coefficient of the embedded object 
in the medium varied from 1 to 12 cm–1. As the absorption coefficient increases, the UOT signal decreases as shown in 
Fig. 2. The decaying profile is close to an exponential fit instead of a linear fit. To validate our simulation results, we 
scanned a 20 mm-thick phantom containing five Trypan-Blue dyed objects (2.2 × 2.2 × 11 mm along the X, Y, and Z 
axes, respectively). The experimental parameters were the same as the above simulation parameters. Although they 
have different absolute values, the experimental and simulated results are highly correlated (R2 value: 98%) as shown in 
Fig. 2.  
 
 
Figure 2: Comparison of simulated modulation depths and experimentally measured changes in speckle contrast at various values of 
optical absorption coefficient. 
 
To investigate the potential of measuring HbT and SO2 using UOT, we scanned a phantom at two optical wavelengths 
(633 nm and 657 nm). An acoustic peak pressure of 1 MPa was applied to the sample. The phantom was 20 mm thick 
and contained seven objects (Fig. 3a). The sizes of all seven targets were identical, 2.2 × 2.2 × 11 mm. Two dye 
solutions were mixed at six concentration ratios ( [ ]R  : [ ]B  = 0:100, 30:70, 45:55, 75:25, 90:10, and 100:0 [%]) to 
mimic different levels of SO2 (Fig. 3a.). Figure 3b shows 1D images obtained at the two optical wavelengths. By 
approximating the local amplitude of UOT signal (Fig. 3c.) to ( )daµ−exp , we calculated each relative-local-optical 
absorption coefficient of six targets (Fig. 3d.). By substituting these relative optical absorption coefficients measured 
from the 1D images into Eqs. 1 and 2, we estimated the values of [ ] [ ]BR +  (Fig. 3e.) and [ ] [ ] [ ]( )BRR +  (Fig. 
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—633nmz •'
0.65
I'
—657nm
(b)
0 10 20 30 40 50 60 70
Relative position (mm]
(c)
&633nm
0.9 9657nm
CO
DC
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-o 0.8
0)N
CO
E
0.7
0.c0 10 20 30 40 50 60 70
Relative position (mm]
(d)
C.)
a
C.)
a
1.5
0
a.01
Cl)
.0
Cl)
a
>
. 0.5
a
a::
&633nm
—B— 657nm
0 10 20 30 40 50 60 70
Relative position (mm]
130
m
+
? 120 if4
(e)
e
a
su
re
m
e
n
t 
o
f 
I 
0 
I 
E100I-0 TT1
0 10 20 30 40 50 60 70
Relative position (mm]
]l([
R]
+(B
]) 
(%
] 
co
 
0 
0 
0 
is
ur
em
en
t o
f (F
 
.
 
0) 
D
 
0 
0 
I
E /b 0 (f)
0 20 40 60 80 100
Preset concentration of (R]/((R]+(B]) (%J
 
 
3f.). The experimentally estimated total concentration [ ] [ ]BR +  in each of the six objects is close to the preset value 
of 100% with an error up to ~30%. The estimated [ ] [ ] [ ]( )BRR +  values match well with the actual preset 
concentrations with an R2 value of 99%.  
  
      
 
 
 
Figure 3: (a) Photograph of the tissue mimicking phantom containing six objects dyed with different ratios of red dye concentration, [ ]R , to the total dye (red and blue inks) concentration, [ ] [ ]BR + . (b) 1D images of the phantom at wavelengths of 633 nm and 
657 nm. (c) Normalized change in speckle contrast at the position of each object, and (d) relative-local-optical absorption coefficient 
Proc. of SPIE Vol. 6856  68561U-4
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of each object. (e) UOT measurements of the total dye concentration, [ ] [ ]BR + , and (f) the fraction of the red dye concentration in 
the total dye concentration, [ ] [ ] [ ]( )BRR + .  
 
CONCLUSIONS 
 
In conclusion, this research has demonstrated the feasibility of functional ultrasound-modulated optical tomography in 
tissue phantoms.  The estimated total dye concentration and the ratio of the red dye concentration to the total dye 
concentration match the actual preset values well. The technique can potentially be extended to monitoring the total 
hemoglobin concentration and measuring the absolute hemoglobin oxygen saturation in vivo. 
 
ACKNOWLEDGEMENT 
 
We thank Hao F. Zhang and Roger J. Zemp for fruitful scientific discussions. This research was supported by NIH 
Grant Nos. R33 CA 094267 and R01 CA106728. 
 
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Toward functional ultrasound-modulated optical tomography: a phantom study

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Toward functional ultrasound-modulated optical tomography: a phantom study

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