Ultrasound-modulated optical tomography (UOT) detects ultrasonically modulated light to spatially localize multiply scattered photons in turbid media with the ultimate goal of imaging the optical properties in living subjects. A principal challenge of the technique is weak modulated signal strength. We discuss ways to push the limits of signal enhancement with intense acoustic bursts while conforming to optical and ultrasonic safety standards. A CCD-based speckle-contrast detection scheme is used to detect acoustically modulated light by measuring changes in speckle statistics between ultrasound-on and ultrasound-off states. The CCD image capture is synchronized with the ultrasound burst pulse sequence. Transient acoustic radiation force, a consequence of bursts, is seen to produce slight signal enhancement over pure ultrasonic-modulation mechanisms for bursts and CCD exposure times of the order of milliseconds. However, acoustic radiation-force-induced shear waves are launched away from the acoustic sample volume, which degrade UOT spatial resolution. By time gating the CCD camera to capture modulated light before radiation force has an opportunity to accumulate significant tissue displacement, we reduce the effects of shear-wave image degradation, while enabling very high signal-to-noise ratios. Additionally, we maintain high-resolution images representative of optical and not mechanical contrast. Signal-to-noise levels are sufficiently high so as to enable acquisition of 2D images of phantoms with one acoustic burst per pixel

Kim, Chulhong

Wang, Lihong V.

Zemp, Roger J.

We present a novel signal-enhancement method for ultrasound modulated optical tomography that can increase the amount of modulated light, compared to previous methods. By applying intense acoustic bursts, particle displacements of several microns are possible at a very local scale. A CCD camera captured the speckle pattern emerging from a laser-illuminated tissue phantom and differences in laser speckle contrast were measured between ultrasound on and off states. When CCD triggering was synchronized with burst initiation, ultrasound radiation force-induced displacements were detected with our technique, and resulting shear waves were shown to degrade image contrast and spatial resolution. Deleterious effects of shear waves were minimized by delaying CCD camera acquisition several ms until shear waves are adequately attenuated. Our system signal-to-noise ratio (SNR) is sufficiently high to perform UOT scanning without signal averaging. Because of substantially improved SNR compared to previous techniques, the use of intense acoustic burst is a new promising method for ultrasound modulated optical tomography

Kim, Chul hong

Caltech Authors - Main

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High-SNR ultrasound-modulated
optical tomography with intense
acoustic bursts
Chul hong  Kim, Roger J. Zemp, Lihong V. Wang
Chul hong  Kim, Roger J. Zemp, Lihong V. Wang, "High-SNR ultrasound-
modulated optical tomography with intense acoustic bursts," Proc. SPIE 6086,
Photons Plus Ultrasound: Imaging and Sensing 2006: The Seventh
Conference on Biomedical Thermoacoustics, Optoacoustics, and Acousto-
optics, 608617 (6 March 2006); doi: 10.1117/12.648533
Event: SPIE BiOS, 2006, San Jose, California, United States
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High-SNR Ultrasound-Modulated Optical Tomography with Intense 
Acoustic Bursts 
 
Chul hong Kim, Roger J. Zemp, and Lihong V. Wang∗ 
 
Optical Imaging Laboratory, Department of Biomedical Engineering, 3120 TAMU,  
Texas A&M University, College Station, Texas 77843-3120 
 
 
ABSTRACT 
 
We present a novel signal-enhancement method for ultrasound modulated optical tomography that can increase the 
amount of modulated light, compared to previous methods. By applying intense acoustic bursts, particle displacements 
of several microns are possible at a very local scale. A CCD camera captured the speckle pattern emerging from a laser-
illuminated tissue phantom and differences in laser speckle contrast were measured between ultrasound on and off 
states. When CCD triggering was synchronized with burst initiation, ultrasound radiation force-induced displacements 
were detected with our technique, and resulting shear waves were shown to degrade image contrast and spatial 
resolution. Deleterious effects of shear waves were minimized by delaying CCD camera acquisition several ms until 
shear waves are adequately attenuated. Our system signal-to-noise ratio (SNR) is sufficiently high to perform UOT 
scanning without signal averaging. Because of substantially improved SNR compared to previous techniques, the use of 
intense acoustic burst is a new promising method for ultrasound modulated optical tomography. 
 
Keywords: Ultrasound modulated optical tomography, speckle-contrast detection, acoustic radiation force, shear 
waves. 
 
INTRODUCTION 
1. Motivation 
Ultrasound modulated optical tomography1,2 (UOT) is a novel bio-photonic imaging technique which takes advantage 
of strong optical contrast and high ultrasonic spatial resolution. The principle of UOT is that a high coherent laser 
illuminates a light-scattering sample, while being insonified with ultrasound. Light passing through the ultrasonic 
sample volume is acoustically modulated from time-varying particle displacements and changes in refractive index such 
that each optical speckle spot has a time-varying modulation component due to the ultrasonic interaction.3-5 By 
observing the modulated component of light at each ultrasonic sample volume position, images may represent the 
optical properties of the tissue. A major challenge in UOT is the low signal-to-noise ratio because of both low 
modulation depth and uncorrelated speckle grains. In order to detect weak modulated signals effectively, a number of 
groups have devoted efforts to develop new detection methods, including parallel detection with CCD cameras6-8, 
Fabry-Perot interferometer9, and photo-refractive crystal techniques10, to overcome this problem. In this paper, using a 
speckle-contrast detection technique11, we explored the use of intense ultrasound bursts, instead of the use of 
continuous-wave or pulsed ultrasound.12 By reducing the duty cycle on the ultrasonic transducer, it is possible to use 
much higher pressures, thus improving the ultrasound-modulated light signal level. Additionally, during timescales on 
the order of a ms, intense acoustic bursts induce localized tissue displacement due to acoustic radiation force. We 
experimentally demonstrate slight but statistically significant radiation-force-enhancement of UOT signals beyond pure 
ultrasound mechanisms. This enhancement, however, comes with a cost. Shear waves due to ultrasound radiation force 
are shown to degrade UOT image contrast and spatial resolution, however, this transient effect is mitigated as transient 
shear-displacements propagate away from the pushing region and are attenuated. By triggering CCD camera image 
acquisition several ms after the acoustic burst generation, pure ultrasound mechanisms dominate over shear-wave 
transients. We compare signal levels with three mechanisms; the continuous-wave ultrasound mechanism with low 
pressure, the transient effect during the pushing phase of an ultrasonic burst, and the steady-state effect during the 
                                                          
∗ lwang@tamu.edu; phone +1 979 847 9040; fax +1 979 845 4450 
Photons Plus Ultrasound: Imaging and Sensing 2006: The 7th Conference on Biomedical
Thermoacoustics, Optoacoustics, and Acousto-optics, edited by A. A. Oraevsky, et. al
Proc. of SPIE Vol. 6086, 608617 (2006) • 1605-7422/06/$15 • doi: 10.1117/12.648533
Proc. of SPIE Vol. 6086  608617-1
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oscillating phase of an ultrasonic burst. To demonstrate the SNR advantage of our method, we show high contrast UOT 
images of phantoms without signal averaging, an important step towards moving UOT towards in vivo imaging studies. 
2. Theory 
Acoustic radiation force (ARF) is generated when ultrasound interacts with an interface of mismatched acoustic 
impedance, or interacts with ultrasonic absorbers.13 When this occurs there is an inherent momentum transfer which 
generates a force. Under an acoustic plane wave assumption, ultrasound radiation force has the form: 
cWF /= .            (1) 
where, for a focused ultrasonic wave, F is the ARF at the focal spot, W is the absorbed ultrasonic beam power at the 
focal spot, and c is the speed of sound in the medium. These effects have been used in various imaging techniques to 
elucidate mechanical properties of tissue. Nightingale et al.14 used transient radiation force to perform Acoustic 
Radiation Force Impulse Imaging, wherein acoustic radiation force displaced tissue and the resulting motion was 
tracked with speckle-tracking methods. Greenleaf et al.15 performed imaging by using the beat-frequency of two con-
focal narrow band acoustic fields to generate local radiation-force that induced tissue-dependent acoustic waves 
detected by sensitive hydrophones.  
 
EXPERIMENTS 
 
The experimental setup is shown in Figure 1 with a Cartesian coordinate system as a reference. The light source 
(Coherent, Verdi; 532 nm wavelength) illuminated a tissue mimicking sample. The average laser power delivered to the 
phantom during CCD exposure period was ~12mW/cm2, a value below present safety limits. A focused ultrasound 
transducer (Ultran, VHP100-1-138; 1 MHz central frequency, 25.4 mm lens diameter, 38 mm focal length) generated 
acoustic waves. The focal zone of an ultrasonic wave was ~2 mm in width and ~20 mm in length. The ultrasound peak 
pressure at the focal spot for image experiments was applied up to 1.5 MPa, so the Mechanical Index (MI) at this 
frequency was ~1.5, which is below 1.9, the present maximum permissible MI for diagnostic ultrasound systems.16 We 
used a gelatin-cornstarch (10%-wt gelatin and 10%-wt cornstarch) phantom with reduced scattering coefficient of ~9.2 
cm-1 as measured by an Oblique-Incidence Reflectometer.17 The light transmitted through the sample yielded a speckle 
pattern which was detected by a digital CCD camera (Basler, A312f; 12-bit, 640×480 pixels). A function generator 
(Agilent, 33250A) synthesized 2-20 ms bursts that were subsequently amplified by an RF amplifier (ENI, Inc., 325LA) 
to drive the ultrasound transducer. Burst initiation triggered a pulse-delay generator (Stanford Research, DG535) that 
produced two CCD trigger pulses for each burst. A low 1-Hz duty cycle was used to prevent damage to the transducer. 
One burst-synchronized frame was captured on the CCD camera, after which an image was taken with no ultrasound. 
We measured laser speckle contrast on the CCD surface with and without the ultrasound modulation every second. 
CCD image capture can either be done at the initiation of an 8 ms burst (to see transient acoustic radiation force (TARF) 
effect) or near the end of a long burst, where transient effects have adequately attenuated (we refer to this as quasi 
continuous wave (quasi-CW) ultrasound). The exposure time of the CCD camera was set to 2 ms. We define our signal 
as the difference between ultrasound ON and OFF in speckle contrast measurements.  
L CCD
RF Amp
S
T
Z
Y
X
LTR
PDGFG
C
 
 
Figure 1: Experimental Setup: L- laser; CCD- CCD camera; RF amp- RF amplifier; FG- function generator; PDG- pulse-delay 
generator; T- ultrasound transducer; S- sample; LT- lens tube; R- rubber; C-personal computer. 
Proc. of SPIE Vol. 6086  608617-2
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1 2 3 4 5
Ultrasonic Pressure (MPa]
0.24
TARF(0
=0
C-,00
00.
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00,
=(0
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0.08
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Quasi-CW
- cw
0 4 8 12 16
CCD Trigger Delay (ms]
156
0.152
(0
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=(0
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(b)
148
0.144
Quasi-CW/
 
 
 
RESULTS AND DISCUSSIONS 
1. Signal enhancement due to intense acoustic bursts and shear wave effects 
To show signal enhancement using intense acoustic bursts, we measured the signal with increasing ultrasonic pressure 
(Figure 2a). As the pressure increases, the signal becomes larger and the difference between the TARF and quasi-CW 
regime becomes larger. The signal starts saturating after the ultrasonic pressure goes higher than ~2MPa. The SNR, 
defined as the mean of change in speckle contrast divided by the root-mean-square standard deviations of ultrasound on 
and off, with 1.5MPa quasi-CW was 260 compared to 287 with 1.5MPa TARF and 32 with 0.3MPa CW (Figure 2a). 
The SNR enhancement of quasi-CW mechanism over CW mechanism is about 8 times larger, but the SNR 
enhancement of TARF mechanism over quasi-CW mechanism is only 10% larger. Quasi-CW method can provide the 
comparable SNR to TARF method but is not affected by shear wave displacement and propagation. In addition, 
comparison of signals with different CCD trigger delay times can distinguish the effects of TARF and quasi-CW 
(Figure 2b). To show this, we applied a 20 ms, ultrasonic burst of ~2 MPa peak pressure, and triggered the CCD camera 
at 0, 2, 4, ···, 18 ms delays following the burst initiation. The largest change in speckle contrast is generated, when the 
burst and CCD trigger are synchronized (with zero delay). After this point, the TARF effect attenuates and the signal 
decreases as it approaches a steady-state condition, where quasi-CW ultrasound mechanisms dominate TARF 
mechanisms (Figure 2b). Although TARF offers signal enhancement over quasi-CW methods for the same acoustic 
pressure, the relative enhancement is only about 10% at the ultrasonic pressure of 2 MPa.  
 
 
 
Figure 2: (a) Comparison of signals applying TARF and quasi-CW at different ultrasonic pressures. (b) Comparison of signals in 
TARF and quasi-CW regimes varying CCD trigger delay times with the ultrasonic pressure of 2 MPa. 
2. 1D images 
To investigate the potential of using intense acoustic bursts for optical imaging, we scanned phantoms and compared 
image contrast and spatial resolution of images acquired during the TARF and quasi-CW regimes to images formed 
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0.08
00.
'0 0.06
=
0.04
=
2 0.02
C-,
0 4 8 12 16 20
V (mm]
I
—I.5MPaQuasi-CW
(0 0.800
NJ
0.2
=
C0
00 4 8 12
V (mm]
16 20
 
 
using lower pressure CW ultrasound. We imaged a gelatin-cornstarch phantom with two Trypan-Blue dyed objects (2.1 
mm × 2.1 mm × 10 mm and 2.1 mm × 1.8 mm × 10 mm) separated by 8 mm from center-to-center. As the ultrasonic 
pressure increases, the signal likewise increases. In addition, two absorbing objects are clearly seen in Figure 3. At the 
same pressure, the signal with TARF mechanism is only 10% larger than the signal with quasi-CW. This increase in 
signal comes with higher absolute image contrast and without significant loss in spatial resolution. The absolute image 
contrasts with TARF and quasi-CW are five times greater than the one with CW. Spatial resolution defined as the one-
way distance between the 25% and 75% of the maximum is approximately 2.2 mm in all of three mechanisms, which is 
comparable to the ultrasonic beam waist. 
 
 
Figure 3: Comparison of 1D images of two Trypan-blue dyed objects in a 1.9-cm thick tissue mimicking phantom with varying 
ultrasonic pressures. 
3. Image degradation due to shear waves 
To investigate shear wave effects on UOT images, we applied higher pressures up to 1.9 MPa using both TARF and 
quasi-CW mechanisms. Higher pressures generate larger force and larger shear displacement. In addition, TARF 
mechanism is affected by shear wave propagation. Increasing pressure increases the amount of non-local shear-wave 
induced modulation (Figure 4a). Increasing CCD exposure time also degrades image quality because shear wave 
propagation endangers non-locality of light modulation. 1D images using 2, 4, and 5 ms CCD exposure time CCD 
exposure times are shown in the Figure 4b. It is clear that the two objects, represented as dips in the signal, are most 
distinct for the shortest CCD exposure time. 
 
 
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I0.95
(0
=
.9' 0.9
C000
NJ
(0
0
=
0.75
4 8 12
V (mm]
16 20
01 . I IMax
V (mm]
-l
20
4
6
8
 
 
 
Figure 4: Image degradation as a function of (a) increasing ultrasonic pressure and (b) extending CCD exposure time.  
4. 2D images 
4.1. 2D imaging using the TARF mechanism 
Figure 5 shows a 2D image obtained using 1.5 MPa, and synchronizing the 2 ms burst and CCD triggering to allow 
sensitivity to the transient acoustic radiation force mechanism. 8 measurements were averaged. The gelatin-cornstarch 
sample with 19 mm thickness contained two Trypan-Blue dyed objects (separated by 11 mm, from center to center). 
The sizes of the two objects were approximately 2.1 mm × 2.4 mm × 13 mm and 1.4 mm × 2.5 mm × 13 mm along the 
X, Y, and Z axes. In the image, the objects are clearly seen with a relative image contrast of 36% and a resolution of ~2.3 
mm.  
 
Figure 5: Two-dimensional image of two Trypan-Blue dyed objects embedded in a 1.9 cm-thick tissue mimicking phantom acquired 
using TARF.  
4.2. 2D images using quasi-CW mechanism 
Like the TARF mechanism, the quasi-CW method can also provide substantial SNR for 2D imaging. As in section 4.1 
we scanned the 1.9 mm thick phantom possessing two Trypan-blue dyed targets. The sizes of the two objects were 
approximately 2.0 mm × 2.0 mm × 12.3 mm and 2.0 mm × 1.3 mm × 12.3 mm along the X, Y, and Z axes. An ultrasonic 
pressure of 1.5 MPa was applied. The CCD exposure time was set to 2 ms. Only 1 pair of on-off measurements were 
taken for each image pixel. We performed no averaging. In Figure 6, the objects are clearly seen with a relative image 
contrast of 31% and a resolution of ~2.0 mm.    
 
 
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High-SNR ultrasound-modulated optical tomography with intense acoustic bursts

Ultrasound-modulated optical tomography with intense acoustic bursts

https://authors.library.caltech.edu/71693/1/ao-46-10-1615.pdf

Ultrasound-modulated optical tomography with intense acoustic bursts

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