Journal
Papers |
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Y.-H. Wang and P.-C. Li |
Ultrasonic
Imaging |
Vol. 33, pp.
189-196, July, 2011. |
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Ultrafast photoacoustic imaging
and its application to real–time
3d imaging with improved
focusing |
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The restricted temporal resolution of
photoacoustic imaging due to limited
frame rates often prohibits its
applications in areas such as real-time
3D imaging. This paper presents an
ultrasound/photoacoustic multimodality
imaging system that provides an
ultrafast frame rate and consists of an
ultrasound transducer array with plane
wave excitation and a laser with pulse
repetition frequency up to 2000 Hz. Its
application to real-time 3D
photoacoustic imaging is demonstrated
and a synthetic-aperture focusing
technique is applied to improve the
elevational focusing quality of the
mechanically-scanned 1D array. A 3D
frame rate of 12 Hz in a volume covering
a 19.2 mm x 19.2 mm scanning surface is
demonstrated.
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Y.-F. Li and P.-C. Li |
Ultrasonic
Imaging |
Vol. 33, pp. 109-118, April, 2011 |
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The data-transfer and computation requirements
are compared between software-based beamforming
using a phased array (PA) and a synthetic
transmit aperture (STA). The advantages of a
software-based architecture are reduced system
complexity and lower hardware cost. Although
this architecture can be implemented using
commercial CPUs or GPUs, the high computation
and data-transfer requirements limit its
real-time beamforming performance. In
particular, transferring the raw rf data from
the front-end subsystem to the software back-end
remains challenging with current
state-of-the-art electronics technologies, which
offset the cost advantage of the software back
end. This study investigated the tradeoff
between the data-transfer and computation
requirements. Two beamforming methods based on a
PA and STA, respectively, were used: the former
requires a higher data transfer rate and the
latter requires more memory operations. The
beamformers were implemented in an NVIDIA
GeForce GTX 260 GPU and an Intel core i7 920
CPU. The frame rate of PA beamforming was 42 fps
with a 128-element array transducer, with 2048
samples per firing and 189 beams per image (with
a 95 MB/frame data-transfer requirement). The
frame rate of STA beamforming was 40 fps with 16
firings per image (with an 8 MB/frame
data-transfer requirement). Both approaches
achieved real-time beamforming performance but
each had its own bottleneck. On the one hand,
the required data-transfer speed was
considerably reduced in STA beamforming, whereas
this required more memory operations, which
limited the overall computation time. The
advantages of the GPU approach over the CPU
approach were clearly demonstrated.
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S.-Y. Su and P.-C. Li |
Optics Express |
Vol. 19, No. 2, pp.1174-1182, January, 2011 |
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A Q-switched Nd:YAG laser providing nanosecond
pulse durations and millijoule pulse energies is
suitable for typical biomedical PA applications.
However, such lasers are both bulky and
expensive. An alternative method is to use a
diode laser, which can achieve a higher pulse
repetition frequency. Although the energy from a
diode laser is generally too low for effective
PA generation, this can be remedied by using
high-speed coded laser pulses, with the signal
intensity of the received signal being enhanced
by pulse compression. In this study we tested a
version of this method that employs coded
excitation. A 20-MHz PA transducer was used for
backward-mode PA detection. A frequency-coded PA
signal was generated by tuning the interval
between two adjacent laser pulses. The
experimental results showed that this
methodology improved the signal-to-noise ratio
of the decoded PA signal by up to 19.3 dB,
although high range side lobes were also
present. These side lobes could be reduced by
optimizing the compression filter. In contrast
to the Golay codes proposed in the literature,
the proposed coded excitation requires only a
single stimulus.
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B.-Y. Hsieh, S.-L. Chen, T. Ling, L. J. Guo, and
P.-C. Li |
Optics Letters |
Vol. 35, No. 17, pp. 2892–2894, September,
2010 |
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The combination of intravascular ultrasound
and intravascular photoacoustic imaging has
been proposed for improving the diagnosis of
arterial diseases. We describe a novel
scan-head design for implementing such
multimodality imaging. The proposed device
has the potential to achieve a
sufficiently small size for
clinical intravascular applications. The
design aims for efficient image data
acquisition for facilitating real-time
three-dimensional imaging and reducing the
required laser pulse repetition frequency.
The integrated scan head consists of a
single-element, ring-shaped transducer
for sideward ultrasound transmission, a
multimode fiber with a cone-shaped mirror
for optical illumination, and a single
polymer microring with mechanical scanning.
The phantom imaging and some experimental
results are presented. A microring array
can be realized in the future to achieve
high-frame-rate intravascular multimodality
imaging.
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A.-H. Liao, C.-C. Shen and P.-C. Li |
IEEE Transactions on Ultrasonics, Ferroelectrics
and Frequency Control |
Vol. 57, No. 2, pp. 317-326, February, 2010 |
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Ultrasound nonlinear imaging using
microbubble-based contrast agents has been
widely investigated. Nonetheless, its
contrast is often reduced by the
nonlinearity of acoustic wave propagation in
tissue. In this paper, we
explore the use of empirical mode
decomposition (EMD) and ensemble empirical
mode decomposition (EEMD) in the
Hilbert-Huang transform (HHT) for possible
contrast improvement. The HHT is designed
for analyzing nonlinear and nonstationary
data, whereas EMD is a method associated
with the HHT that allows decomposition of
data into a finite number of intrinsic
modes. The hypothesis is that the nonlinear
signal from microbubbles and the tissue
nonlinear signal can be better
differentiated with EMD and EEMD, thus
making contrast improvement possible.
Specifically, we tested this method on
pulse-inversion nonlinear imaging, which is
generally regarded as one of the most
effective nonlinear imaging methods. The
results show that the contrast-to-tissue
ratios at the fundamental and
second-harmonic frequencies were improved by
10.2 and 4.3 dB, respectively, after EEMD.
Nonetheless, image artifacts also appeared,
and hence further investigation is needed
before EMD and EEMD can be applied in
practical applications of ultrasound
nonlinear imaging
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L.-C. Chen, C.-W. Wei, J. S. Souris, S.-H.
Cheng, C.-T. Chen, C.-S. Yang, P.-C. Li,*
and L.-W. Lo* |
Journal of Biomedical Optics |
Vol. 15, 016010, February, 2010 |
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Photoacoustic tomography (PAT) has garnered
much attention for its high contrast and
excellent spatial resolution of perfused
tissues. Gold nanorods (GNRs) have been
employed to further enhance the imaging
contrast of PAT. However, the photon
fluences typically needed for PA wave
induction often also result in GNR shape
changes that significantly reduce the
efficiency of acoustic wave generation. In
this work, we propose, synthesize, and
evaluate amorphous silica-coated gold
nanorods (GNR-Si) in an effort to improve
contrast agent stability and ameliorate
efficiency loss during photoacoustic (PA)
wave induction. TEM and optical absorption
spectra measurements of GNR and GNR-Si show
that encasing GNRs within amorphous silica
provides substantial protection of nanorod
conformation from thermal deformation. PA
signals generated by GNR-Si demonstrate
considerably greater resistance to
degradation of signal intensity with
repetitive pulsing than do uncoated GNRs,
thereby enabling much longer, high-contrast
imaging sessions than previously possible.
The prolongation of high-contrast imaging,
and biocompatibility and easy surface
functionalization for targeting ligands
afforded by amorphous silica, suggest GNR-Si
to be potentially significant for the
clinical translation of PAT
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Y.-H. Chuang, P.-W. Cheng, S.-C. Chen, J.-L.
Ruan and P.-C. Li |
Ultrasonic Imaging |
Vol. 32, pp. 33-47, January, 2010 |
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Cavitation induced by ultrasound enhances
enzymatic fibrinolysis by increasing the
transport of reactants. However, the effects
of cavitation need to be fully understood
before sonothrombolysis can be applied
clinically. In order to understand the
underlying mechanisms, we examined the
effects of combining ultrasound,
microbubbles and thrombolytic enzymes on
thrombolysis. First, we evaluated the
relations between inertial cavitation and
the reduction in the weight of a blood clot.
Inertial cavitation was varied by changing
the amplitude and duration of the
transmitted acoustic wave as well as the
concentration of microbubbles used to induce
cavitation. Second, we studied the combined
effects of streptokinase and inertial
cavitation on thrombolysis. The results show
that inertial cavitation increases the
weight reduction of a blood clot by up to
33.9%. With linear regression fitting, the
measured differential inertial cavitation
dose and the weight reduction had a
correlation coefficient of 0.66.
Microscopically, enzymatic thrombolysis
effects manifest as multiple large cavities
within the clot that are uniformly
distributed on the side exposed to
ultrasound. This suggests that inertial
cavitation plays an important role in
producing cavities, while microjetting of
the microbubbles induces pits on the clot
surface. These observations preliminarily
demonstrate the clinical potential of
sonothrombolysis. The use of the
differential inertial cavitation dose as an
indicator of blood clot weight loss for
controlled sonothrombolysis is also possible
and will be further explored.
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S.-L. Wang and P.-C. Li |
Journal of Engineering in Medicine |
Vol. 224 (H2), pp. 143–154, 2010 (invited
paper) |
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This paper provides a review of advanced
algorithms for ultrasound image formation
and signal processing that are based on
aperture-domain data (i.e. the data recorded
by individual channels prior to beam
summation). First aperture-domain data are
defined and their properties described, then
two specific examples of phase-aberration
correction and vector velocity
estimation are presented. For
phase-aberration correction, sidelobe-reduction
techniques based on the coherence of the
received aperture-domain data were tested
with clinical breast data; the mean
improvements in the contrast and
contrast-to-noise ratios were 6.9 dB and
23.2 per cent, respectively. For flow
estimation, a conventional scanner can only
estimate the flow velocity parallel to the
beam axis. The proposed flow estimation
technique uses aperture-domain data for
two-dimensional flow-velocity estimation.
The experimental results demonstrate that
the estimation errors for the proposed
technique are 2.18 per cent and 18.11 per
cent in the axial and lateral velocity
components, respectively. Other applications
in which aperture-domain data can be used
are also discussed.
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S.-C. Chen, J.-L. Ruan, P.-W. Cheng, Y.-H.
Chuang, and P.-C. Li |
Ultrasonic Imaging |
Vol. 31, pp. 235-246, October, 2009 |
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The thrombus-targeted ultrasound contrast
agent with tirofiban, a glycoprotein IIb/IIIa
antagonist that can specifically bind to
activated platelets in the thrombus, was
designed to enhance both the image contrast
and the cavitation effect. The targeting
ability to thrombi was confirmed by
microphotography and high frequency
ultrasound (40 MHz) images (3.34plusmn0.30
dB enhancement). Our inertial cavitation
induction system showed that the enhancement
of percent weight loss was about 10%, and it
was further examined by using a microscope.
We conclude that the targeted microbubbles
are applicable not only for molecular
imaging of thrombus but also
improving effectiveness of
ultrasound-assisted thrombolysis
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S.-L. Wang and P.-C. Li |
IEEE Transactions on Ultrasonics, Ferroelectrics
and Frequency Control |
Vol. 56, No.10, pp. 2097-2110, October, 2009 |
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Some success has been demonstrated in the
extensive studies of adaptive imaging, but
these approaches are generally not suitable
for high-frame-rate (HFR) imaging where
broad transmit beams are required. In this
study, we propose an effective adaptive
imaging method suitable for HFR imaging
based on coherence-factor (CF)
weighting and the minimum-variance-distortionless-response
(MVDR) method. The CF is an index of
focusing quality estimated from
receive-channel data in which the amplitude
of each image pixel is weighted by the
corresponding CF so as to reduce the
unwanted sidelobes. Direct implementation of
CF weighting in HFR imaging does not provide
satisfactory results because the broad
transmit beams required for HFR imaging
reduce the accuracy of CF calculations. In
this study, we alleviated this problem by
applying the MVDR method. We test the
proposed method with the synthetic transmit
aperture method where only 8 firings are
required to form an image. Both simulations
and clinical breast imaging data were used,
and the proposed method enhanced the mean
contrast by around 4.6 dB and the mean
contrast-to-noise ratio by around 20%. The
results demonstrate that the proposed method
is effective at improving the image quality.
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L.-W. Chang, K.-H. Hsu and P.-C. Li |
IEEE Transactions on Ultrasonics,
Ferroelectrics and Frequency Control |
Vol. 56, No.9, pp. 1856-1869, September,
2009 |
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Color Doppler ultrasound is a routinely used
diagnostic tool for assessing blood flow
information in real time. The required
signal processing is computationally
intensive, involving autocorrelation, linear
filtering, median filtering, and
thresholding. Because of the large amount of
data and high computational requirement,
color Doppler signal processing
has been mainly implemented on
custom-designed hardware, with
software-based implementation—particularly
on a general-purpose CPU—not being
successful. In this paper, we describe the
use of a graphics processing unit for
implementing signal-processing algorithms
for color Doppler ultrasound that achieves a
frame rate of 160 fps for frames comprising
500 scan lines × 128 range samples, with
each scan line being obtained from an
ensemble size of 8 with an 8-tap FIR
clutter filter.
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Y.-L. Sheu and P.-C. Li |
IEEE Transactions on Ultrasonics, Ferroelectrics
and Frequency Control |
Vol. 56, No.5, pp. 1104-1112, May, 2009 |
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Most physical models used to evaluate
thermally induced photoacoustic waves in
biomedical applications are approximations
based on assumptions necessary to obtain
analytical results, such as thermal and
stress confinements. In contrast, using
numerical methods to solve the general
photoacoustic wave equations gives detailed
information on the wave phenomenon without
requiring as many assumptions to be made.
The photoacoustic wave generated by thermal
expansion is characterized by the heat
conduction theorem and the state,
continuity, and Navier-Stokes equations.
This study developed a numerical solution in
axis-symmetric cylindrical coordinates using
a pseudospectral time-domain scheme. The
method is efficient for large-scale
simulations since it requires only 2 grid
points per minimum wavelength, in contrast
to conventional methods such as the
finite-difference time-domain method
requiring at least 10-20 grid points. The
numerical techniques included Berenger's
perfectly matched layers for free wave
simulations, and a linear-perturbation
analytical solution was used to validate the
simulation results. The numerical results
obtained using 4 grid points per minimum
wavelength in the simulation domain
agreed with the theoretical estimates to
within an absolute difference error of
3.87 x 10(-2) for a detection distance of
3.1 mm
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S. Emelianov, P.-C. Li and M. O’DDonnell |
Physics
Today |
pp. 34-39, May 2009.(invited paper) |
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Sound waves generated by light are the basis
of a sensitive medical imaging technique
with applications to cancer diagnosis and
treatment.
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X. Cheng, J. Chen, C. Li, J.-H. Liu, I-M. Shen
and P.-C. Li |
IEEE Sensors Journal |
Vol. 9, No. 5, pp. 569-577, May, 2009 |
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This paper describes the development of a
miniature capacitive micromachined
ultrasonic transducer (CMUT) array suitable
for minimally invasive medical imaging and
diagnosis. In contrast to conventional
laboratory-scale CMUT platforms, which are
generally integrated on a silicon substrate
thicker than 550 mum, this imager array is
integrated on a probe shaped silicon
substrate with a typical shank dimension of
60 mum(width) times 40 mum(thickness) times
4-10 mm(length) for 1-D arrays, and 0.4-2.3
mm(width) times 100 mum(thickness) times
6-12 mm(length) for 2-D arrays. Such
miniature CMUT arrays are suitable for
implantation into tissue through a fine
incision or by being placed inside an organ
for close-range imaging. In a close-range
diagnosis made possible by using such
miniature CMUT arrays, ultrasound of a
higher frequency can be used and the
conflict associated with the penetration
depth and image resolution can be resolved.
This imager array was fabricated using a
two-layer polysilicon surface micromachining
process followed by a double-sided deep
silicon etching for substrate shaping. The
total mask count was eight. The central
frequency of ultrasound transmitted by a
circular 46 mum-diameter transducer was 3.8
MHz, while its fractional bandwidth was 116%
in water. A simple transducer-fluid model
was used to predict the acoustic
characteristics of this device in
water. Preliminary B-mode imaging using a
21-element 1-D array was demonstrated.
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J.-H. Liu, S.-Y. Chen and P.-C. Li |
IEEE Transactions on Ultrasonics, Ferroelectrics
and Frequency Control |
Vol. 56, No.2, pp. 379-386, February, 2009 |
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The design, fabrication, and evaluation of a
high-frequency single-element transducer are
described. The transducer has an annular
geometry, with the thickness of the
piezoelectric material increasing from the
center to the outside. This single-element
annular transducer (SEAT) can provide a
broader frequency range than a conventional
single-element transducer with a uniform
thickness (single-element uniform
transducer, or SEUT). We compared the
characteristics of a SEAT and a SEUT. Both
transducers used 36°-rotated, Y-cut lithium
niobate (LiNbO3) material. The
SEAT had a diameter of 6 mm and comprised 6
subelements of equal area (electrically
connected by a single electrode on each
side) whose thickness ranged from 60 μm
(center) to 110 μm (outside), which resulted
in the center frequency of the subelements
varying from 59.8 MHz to 25 MHz. The overall
center frequency was 42.4 MHz. The annular
pattern was constructed using an ultrasonic
sculpturing machine that reduced the
root-mean-square value of the surface
roughness to 454.47 nm. The bandwidth of the
SEAT was 19% larger than that of the SEUT.
However, compared with the SEUT, the 2-way
insertion loss of the SEAT was increased by
3.1 dB. The acoustic beam pattern of the
SEAT was also evaluated numerically by
finite-element simulations and
experimentally by an ultrasound beam
analyzer. At the focus (10.5 mm from the
transducer surface), the -6 dB beam width
was 108 μm. There was reasonable agreement
between the data from simulations and
experiments. The SEAT can be used for
imaging applications that require a wider
transducer bandwidth, such as harmonic
imaging, and can be manufactured using the
same techniques used to produce transducers
with multiple frequency bands.
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