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Proceedings ArticleDOI

3-D ultrasound imaging using forward viewing CMUT ring arrays for intravascular and intracardiac applications

18 Sep 2005-Vol. 2, pp 783-786

TL;DR: Full synthetic phased array volumetric ultrasound imaging is demonstrated using a forward-viewing CMUT ring array with 64 elements, in both the conventional (8 MHz) and collapse (19 MHz) regimes of operation.

AbstractForward-viewing ring arrays can enable new appli- cations in intravascular and intracardiac ultrasound. We have demonstrated full synthetic phased array volumetric ultrasound imaging using a forward-viewing CMUT ring array with 64 elements, in both the conventional (8 MHz) and collapse (19 MHz) regimes of operation. Measured SNR of an echo from a plane reflector at 5 mm is 29 dB for 8 MHz and 35 dB for 19 MHz. The 6-dB axial and lateral resolutions for the B-scan of the wire target is 189µm and 0.112 radians for 8 MHz, and 78µm and 0.051 radians for 19 MHz. Rendered 3-D images of a Palmaz-Schatz stent are also shown, demonstrating that the imaging quality is sufficient for clinical applications.

Topics: Phased array (51%), Ultrasonic sensor (50%)

Summary (2 min read)

Introduction

  • Forward-viewing intravascular ultrasound enables new procedures in medicine such as diagnosing severely occluded blood vessels or guiding the placement of stents.
  • A ringshaped forward-viewing transducer provides clearance for the guidewire in catheter-based applications.
  • Also, the forwardviewing ring array is capable of volumetric imaging, which is highly desirable because it reduces operator dependence in clinical ultrasound.
  • Previous efforts have found it challenging to design and fabricate ring arrays using piezoelectric transducers with sufficient performance in the forward-looking mode [1].
  • This paper presents the characterization of a CMUT ring array and its imaging capabilities.

II. CMUT RING ARRAYS

  • CMUTs offer several advantages over piezoelectric transducers for use in medical imaging [4].
  • The microlithography process used to make CMUTs churns out batches of transducers with the fine dimensions required for high-frequency ring arrays.
  • The wide bandwidth of CMUTs in immersion improves the resolution of ultrasound images.
  • Operating the CMUT in collapse affords particularly good characteristics for imaging, including higher echo signal levels and higher frequency [5].
  • The operator can choose conventional mode operation for lower frequency and better penetration for navigation, or collapse mode for higher frequency and resolution for diagnosis.

III. EXPERIMENTAL SETUP

  • Four integrated circuits, with 16 independent pulsers and amplifiers each, were wire bonded to the elements of the CMUT in a 209 pin grid array (PGA) electronics package, as shown in Fig.
  • Although this arrangement is capable of full phased array operation, full synthetic phased array imaging was performed to simplify the data collection and to acquire the most general data set for offline reconstruction with various beamforming schemes.
  • The CMUT was biased at 30 V for conventional operation and 100 V for collapse mode.
  • Elements were excited one at a time with a 25-V pulse, and A-scans were acquired from the entire array for each transmit element.
  • Each A-scan was collected at a sampling rate of 500 MS/s for both conventional and collapse mode operation, and using 16 averages.

A. A-Scan Results

  • For the conventional case, the pulse width was 60 ns; for collapse mode, the pulse width was 27 ns.
  • Fig. 3 shows the FFT of the pulse-echo signal after it has been filtered using a Gaussian bandpass filter with a 6-dB band from 5.5 to 13 MHz for conventional, and 10 to 27.5 MHz for collapse.
  • In conventional mode, the device operates at 8.3 MHz with a 6-dB fractional bandwidth of 70%, and in collapse, 19 MHz with a fractional bandwidth of 69%.
  • The experimental axial and lateral line spread functions (LSF) of the array are shown in Fig. 8 alongside the simulated results.
  • Finally, volume images of several targets were generated and are shown in Figs.

V. CONCLUSION

  • The authors have demonstrated volumetric ultrasound imaging with a forward-viewing CMUT ring array.
  • These results show that CMUT ring arrays with custom front-end integrated circuits can produce images with ample quality for clinical use.

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3-D Ultrasound Imaging Using Forward Viewing
CMUT Ring Arrays for Intravascular and
Intracardiac Applications
David T. Yeh
,
¨
Omer Oralkan
, Ira O. Wygant
, Matthew O’Donnell
, and Butrus T. Khuri-Yakub
Edward L. Ginzton Laboratory
Stanford University, Stanford, CA 94305–4088
Email: dtyeh@stanford.edu
Biomedical Engineering Department
University of Michigan, Ann Arbor, MI 48019–2099
Abstract Forward-viewing ring arrays can enable new appli-
cations in intravascular and intracardiac ultrasound. We have
demonstrated full synthetic phased array volumetric ultrasound
imaging using a forward-viewing CMUT ring array with 64
elements, in both the conventional (8 MHz) and collapse (19
MHz) regimes of operation. Measured SNR of an echo from a
plane reflector at 5 mm is 29 dB for 8 MHz and 35 dB for 19
MHz. The 6-dB axial and lateral resolutions for the B-scan of
the wire target is 189 µm and 0.112 radians for 8 MHz, and
78 µm and 0.051 radians for 19 MHz. Rendered 3-D images of
a Palmaz-Schatz stent are also shown, demonstrating that the
imaging quality is sufficient for clinical applications.
I. INTRODUCTION
Forward-viewing intravascular ultrasound enables new pro-
cedures in medicine such as diagnosing severely occluded
blood vessels or guiding the placement of stents. A ring-
shaped forward-viewing transducer provides clearance for the
guidewire in catheter-based applications. Also, the forward-
viewing ring array is capable of volumetric imaging, which
is highly desirable because it reduces operator dependence in
clinical ultrasound.
Previous efforts have found it challenging to design and fab-
ricate ring arrays using piezoelectric transducers with sufficient
performance in the forward-looking mode [1]. Consequently,
there have been several efforts to make ring arrays using
Capacitive Micromachined Ultrasonic Transducers (CMUTs)
with the goal of volumetric intravascular ultrasound [2], [3].
This paper presents the characterization of a CMUT ring array
and its imaging capabilities.
II. CMUT R
ING ARRAYS
CMUTs offer several advantages over piezoelectric trans-
ducers for use in medical imaging [4]. The microlithography
process used to make CMUTs churns out batches of transduc-
ers with the fine dimensions required for high-frequency ring
arrays. The wide bandwidth of CMUTs in immersion improves
the resolution of ultrasound images.
Operating the CMUT in collapse affords particularly good
characteristics for imaging, including higher echo signal levels
and higher frequency [5]. In addition, the CMUT can be
Fig. 1. (a) Ring array wire bonded to electronics; (b) 16-channel trans-
mit/receive circuit; (c) 64-element CMUT ring array.
switched between its two operating modes during the imaging
procedure. The operator can choose conventional mode opera-
tion for lower frequency and better penetration for navigation,
or collapse mode for higher frequency and resolution for
diagnosis.
The parameters for the CMUT ring array presented are
as follows: ring diameter, 2 mm; number of elements, 64;
element pitch, 102 µm; element size, 100×100 µm; cells per
element, 9; cell membrane radius, 13 µm; electrode radius,
9 µm; membrane thickness, 0.4 µm; gap distance, 0.15 µm;
collapse voltage, 50 V. The very small element area required
for this application makes ultrasound imaging difficult, yet
the CMUT provides sufficient performance to produce clear
images.
III. E
XPERIMENTAL SETUP
Four integrated circuits, with 16 independent pulsers and
amplifiers each, were wire bonded to the elements of the
CMUT in a 209 pin grid array (PGA) electronics package, as
shown in Fig. 1. Although this arrangement is capable of full
phased array operation, full synthetic phased array imaging
was performed to simplify the data collection and to acquire
the most general data set for offline reconstruction with various
beamforming schemes. The CMUT was biased at 30 V for

6.5 7 7.5 8 8.5
−2
0
2
Time (µs)
Voltage (mV)
(a)
6.5 7 7.5 8 8.5
−2
0
2
Time (µs)
Voltage (mV)
(b)
Fig. 2. Pulse-echo response: (a) Conventional; (b) Collapse mode;
0 5 10 15 20 25 30 35 40
−40
−30
−20
−10
0
Frequency (MHz)
Normalized Magnitude (dB)
(a)
0 5 10 15 20 25 30 35 40
−40
−30
−20
−10
0
Frequency (MHz)
Normalized Magnitude (dB)
(b)
Fig. 3. FFT of filtered pulse-echo response: (a) Conventional; (b) Collapse
mode.
conventional operation and 100 V for collapse mode. Elements
were excited one at a time with a 25-V pulse, and A-scans
were acquired from the entire array for each transmit element.
Each A-scan was collected at a sampling rate of 500 MS/s for
both conventional and collapse mode operation, and using 16
averages.
IV. R
ESULTS
A. A-Scan Results
Pulse-echo data of a plane reflector (the oil-air interface) at
5 mm was taken from a single element. For the conventional
case, the pulse width was 60 ns; for collapse mode, the pulse
width was 27 ns. The echo, shown in Fig. 2, demonstrates the
wide bandwidth of the CMUT. Fig. 3 shows the FFT of the
pulse-echo signal after it has been filtered using a Gaussian
bandpass filter with a 6-dB band from 5.5 to 13 MHz for
0 16 32 48 64
6
8
10
12
Array Element Index
f
o
(MHz)
−3 σ
+3 σ
µ
µ = 8.3 MHz
σ = 307 kHz
(a)
0 16 32 48 64
0
50
100
Array Element Index
Bandwidth (%)
−3 σ
+3 σ
µ
µ = 69.7%
σ = 9.4 %
(b)
Fig. 4. Uniformity across ring array, conventional mode: (a) Center
frequency; (b) Fractional bandwidth.
0 16 32 48 64
17
18
19
20
21
Array Element Index
f
o
(MHz)
−3 σ
+3 σ
µ
µ = 18.9 MHz
σ = 187 kHz
(a)
0 16 32 48 64
0
50
100
Array Element Index
Bandwidth (%)
−3 σ
+3 σ
µ
µ = 69.0%
σ = 1.57%
(b)
Fig. 5. Uniformity across ring array, collapse mode: (a) Center frequency;
(b) Fractional bandwidth.
conventional, and 10 to 27.5 MHz for collapse. In conventional
mode, the device operates at 8.3 MHz with a 6-dB fractional
bandwidth of 70%, and in collapse, 19 MHz with a fractional
bandwidth of 69%. The SNR of a plane reflector at 5 mm is
29 dB for conventional and 35 dB for collapse.
B. Imaging Results
A conical volume was reconstructed offline using the full
64×64 set of A-scans from an imaging target with weightings
for full-aperture resolution [6] and cosine apodization. All
images are shown with 40 dB of dynamic range. The imaging
phantom is shown in Fig. 6, and consists of three steel wires,
each 0.3 mm in diameter. Fig. 7 shows the Y-Z and X-Z
planes of the conical volume (depicted in Fig. 6). Because of
the higher frequency and reduced acoustic crosstalk, collapse
mode produces images with a narrower main lobe and fewer

x
y
z
y-y
z
-2 0 2
2
4
6
8
10
(mm)
(mm)
d = 0.3 mm
Fig. 6. Phantom of three wires.
Fig. 9. Photograph of spring, 3-D rendered ultrasound of spring, cross
sections with 40 dB dynamic range.
artifacts than conventional mode imaging. The image SNR in
conventional is 50 dB compared to 22 dB for the SNR of the
wire A-scan. In collapse mode, the image SNR is 48 dB and
the A-scan SNR, 24 dB.
The experimental axial and lateral line spread functions
(LSF) of the array are shown in Fig. 8 alongside the simulated
results. The 6-dB axial and lateral resolutions for for the B-
scan of the wire target is 189 µm and 0.112 radians for 8 MHz,
and 78 µm and 0.051 radians for 19 MHz.
Finally, volume images of several targets were generated
and are shown in Figs. 9,10,11 to exhibit the quality of images
produced by this ring array.
Fig. 10. Photograph of Palmaz-Schatz stent, undeployed, 3-D rendered
ultrasound of stent, cross sections with 40 dB dynamic range.
Fig. 11. Photograph of Palmaz-Schatz stent, deployed, 3-D rendered
ultrasound of stent, cross sections with 40 dB dynamic range.
V. C ONCLUSION
We have demonstrated volumetric ultrasound imaging with
a forward-viewing CMUT ring array. Future work involves
developing a fully integrated system with flip-chip bonded
electronics for incorporation into a catheter probe. These
results show that CMUT ring arrays with custom front-end
integrated circuits can produce images with ample quality for
clinical use.
A
CKNOWLEDGMENT
This work was supported by the National Institutes of
Health. Thanks to Bill Broach and the Portable Power group
at National Semiconductor Corporation for supporting us with
assistance in circuit design and for providing the custom inte-
grated circuits. Thanks to Volcano Therapeutics for providing
the stents. David Yeh is supported by a National Defense
Science and Engineering Graduate Fellowship.
R
EFERENCES
[1] Y. Wang, D. Stephens, and M. O’Donnell, “Initial results from a forward-
viewing ring-annular ultrasound array for intravascular imaging, in Proc.
IEEE Ultrason. Symp., vol. 1, Oct. 2003, pp. 212–215.
[2] U. Demirci, A. S. Ergun,
¨
O. Oralkan, M. Karaman, and B. T. Khuri-
Yakub, “Forward-viewing CMUT arrays for medical imaging. IEEE
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July 2004.
[3] F. L. Degertekin, R. O. Guldiken, and M. Karaman, “Micromachined
capacitive transducer arrays for intravascular ultrasound, in Proc. SPIE
MOEMS Display and Imaging Systems III, vol. 5721, no. 1, San Jose,
CA, 2005, pp. 104–114.
[4]
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O. Oralkan, A. S. Ergun, J. A. Johnson, U. Demirci, M. Karaman,
K. Kaviani, T. H. Lee, and B. T. Khuri-Yakub, “Capacitive micromachined
ultrasonic transducers: Next-generation arrays for acoustic imaging?”
IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 49, no. 11, pp.
1596–1610, Nov. 2002.
[5] B. Bayram, E. Hæggstr
¨
om, G. G. Yaralioglu, and B. T. Khuri-Yakub, A
new regime for operating capacitive micromachined ultrasonic transduc-
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[6] S. J. Norton, Annular array imaging with full-aperture resolution, J.
Acoust. Soc. Am., vol. 92, no. 6, pp. 3202–3206, Dec. 1992.

Y (mm)
Z (mm)
−6 −4 −2 0 2 4 6
2
3
4
5
6
7
8
9
(a)
Y (mm)
Z (mm)
−6 −4 −2 0 2 4 6
2
3
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7
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9
(b)
X (mm)
Z (mm)
−6 −4 −2 0 2 4 6
2
3
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7
8
9
(c)
X (mm)
Z (mm)
−6 −4 −2 0 2 4 6
2
3
4
5
6
7
8
9
(d)
Fig. 7. Slices of 3-D volume, 40 dB range: (a) Y-Z plane, conventional; (b) Y-Z plane, collapse mode; (c) X-Z plane, conventional; (d) X-Z plane, collapse
mode.
3 3.5 4 4.5
-40
-20
0
Axial Distance (mm)
N
o
r
m
a
l
i
z
e
d
M
a
g
n
i
t
u
d
e
(
d
B
)
Experiment
Simulation
(a)
3 3.5 4 4.5
-40
-20
0
Axial Distance (mm)
N
o
r
m
a
l
i
z
e
d
M
a
g
n
i
t
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d
e
(
d
B
)
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Simulation
(b)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
-40
-20
0
sin(
θ
)
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o
r
m
a
l
i
z
e
d
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a
g
n
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e
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)
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Simulation
(c)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
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-20
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sin(
θ
)
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r
m
a
l
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z
e
d
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a
g
n
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t
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d
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(
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B
)
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Simulation
(d)
Fig. 8. Experimental versus simulated line spread functions: (a) Axial LSF, conventional; (b) Axial LSF, collapse mode; (c) Lateral LSF, conventional; (d)
Lateral LSF, collapse mode.
Citations
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Proceedings ArticleDOI
18 Sep 2005
Abstract: Real-time catheter-based ultrasound imaging tools are needed for diagnosis and image-guided procedures. The continued development of these tools is partially limited by the difficulty of fabricating two-dimensional array geometries of piezoelectric transducers. Using capacitive micromachined ultrasonic transducer (CMUT) technology, transducer arrays with widely varying geometries, high frequencies, and wide bandwidths can be fabricated. A volumetric ultrasound imaging system based on a two-dimensional, 16×16-element, CMUT array is presented. Transducer arrays with operating frequencies ranging from 3 MHz to 7.5 MHz were fabricated for this system. The transducer array including DC bias pads measures 4 mm by 4.7 mm. The transducer elements are connected to flip-chip bond pads on the array back side with 400-µm long through-wafer interconnects. The array is flip-chip bonded to a custom- designed integrated circuit (IC) that comprises the front-end electronics. Integrating the front-end electronics with the transducer array reduces the effects of cable capacitance on the transducer's performance and provides a compact means of connecting to the transducer elements. The front-end IC provides a 27-V pulser and 10-MHz bandwidth amplifier for each element of the array. An FPGA-based data acquisition system is used for control and data acquisition. Output pressure of 230 kPa was measured for the integrated device. A receive sensitivity of 125 mV/kPa was measured at the output of the amplifier. Amplifier output noise at 5 Mhz is 112 nV/√Hz. Volumetric images of a wire phantom and vessel phantom are presented. Volumetric data for a wire phantom was acquired in real-time at 30 frames per second. Keywords-ultrasound imaging, catheter, capacitive micromachined ultrasonic transducer, CMUT, integrated electronics, volumetric, real-time

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Proceedings ArticleDOI
18 Sep 2005
Abstract: Photoacoustic imaging is a promising complement to pulse-echo ultrasound imaging because it provides contrast between areas with different light absorption characteristics. Specifically, regions with higher blood concentration can be identified, which is useful for imaging vascularization and the early detection of cancer. Here we present volumetric photoacoustic images of a vessel-like phantom. The phantom consists of three 1.3-mm diameter tubes inside a tissue mimicking material. The center tube is filled with ink to provide optical contrast. A two-dimensional capacitive micromachined ultrasonic transducer (CMUT) array is used for acoustic detection. The use of a two-dimensional transducer array eliminates the drawbacks of a mechanically scanned system and enables volumetric imaging. CMUT technology enables new types of transducer arrays that would benefit photoacoustic imaging. Fully populated two-dimensional arrays, annular ring- arrays, and high-frequency arrays have all been demonstrated using CMUT technology and have advantages for photoacoustic imaging systems. Other advantages of CMUT technology for photoacoustic imaging include a wider bandwidth than comparable piezoelectric devices and ease of integration with electronics. Keywords-photoacoustics, capacitive micromachined ultrasonic transducer, CMUT, three-dimensional, integrated electronics

29 citations


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  • ...CMUT array geometries such as the ring array have also been demonstrated [5]....

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Proceedings ArticleDOI
Abstract: In photoacoustic (optoacoustic) medical imaging, short laser pulses irradiate absorbing structures found in tissue, such as blood vessels, causing brief thermal expansions that in turn generate ultrasound waves. These ultrasound waves which correspond to the optical absorption distribution were imaged using a two dimensional array of capacitive micromachined ultrasonic transducers (CMUTs). Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large two dimensional arrays, arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. In this study, a phantom consisting of three 0.86-mm inner diameter polyethylene tubes inside a tissue mimicking material was imaged using a 16 x 16 element CMUT array. The center tube was filled with India-ink to provide optical contrast. Traditional pulse-echo data as well as photoacoustic image data were taken. 2D cross-sectional slices and 3D volume rendered images are shown. Simple array tiling was attempted, whereby a 48 x 48 element array was simulated, to illustrate the advantages of larger arrays. Finally, the sensitivity of the photoacoustics setup to the concentration of ink in the tube was also explored. For the sensitivity experiment a different phantom consisting of only one 1.14-mm inner diameter polyethylene tube inside a tissue mimicking material was used. The concentration of the ink inside the tube was varied and images were taken.

23 citations


Proceedings ArticleDOI
31 Oct 2005
Abstract: Capacitive micromachined ultrasonic transducers (CMUTs) overcome many limitations of existing ultrasound transducer technologies enabling new applications of ultrasound, especially for medical imaging and treatment. Some of the most important of these advancements are the ability to fabricate transducer arrays with two dimensional geometries and high operating frequencies. Over the past decade, extensive research has been conducted on the fabrication, characterization, and modelling of CMUTs. Current research efforts focus on the integration of CMUTs in systems for new medical imaging tools. This paper briefly reviews CMUT technology and presents imaging results from two CMUT-based imaging systems. The first system is designed for use within a 5-mm endoscopic channel and is based on a two dimensional, 16-element times 16-element, 5-MHz CMUT array. To provide a means of integrating the CMUT array with electronics, each element of the array is connected to a flip-chip bond pad on the back side of the array via a through-wafer interconnect. The array is flip-chip bonded to a custom-designed integrated circuit (IC) that comprises the frontend circuitry for the transducer elements. The array and IC are connected to an FPGA-based data acquisition system that can acquire volumetric imaging data in real time. Volumetric pulse-echo and photoacoustic images obtained with this system are presented. The second system is based on a 64-element, 20-MHz, 2-mm diameter CMUT ring array. This array is also designed for use in catheter-based imaging applications. Ring arrays have the advantage of providing space in the center for a guidewire or other catheter-based instrument. Volumetric images obtained with the ring-array system are also presented

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Proceedings ArticleDOI
17 May 2006
TL;DR: A dual‐mode transducer can provide focused, noncontact ultrasound suitable for therapy and can be used to provide high quality real‐time images for navigation and monitoring of the procedure.
Abstract: In recent years, medical procedures have become increasingly non‐invasive. These include endoscopic procedures and intracardiac interventions (e.g., pulmonary vein isolation for treatment of atrial fibrillation and plaque ablation for treatment of arteriosclerosis). However, current tools suffer from poor visualization and difficult coordination of multiple therapeutic and imaging devices. Dual‐mode (imaging and therapeutic) ultrasound arrays provide a solution to these challenges. A dual‐mode transducer can provide focused, noncontact ultrasound suitable for therapy and can be used to provide high quality real‐time images for navigation and monitoring of the procedure. In the last decade, capacitive micromachined ultrasonic transducers (CMUTs), have become an attractive option for ultrasonic imaging systems due to their fabrication flexibility, improved bandwidth, and integration with electronics. The CMUT’s potential in therapeutic applications has also been demonstrated by surface output pressures as high as 1MPa peak to peak and continuous wave (CW) operation. This paper reviews existing interventional CMUT arrays, demonstrates the feasibility of CMUTs for high intensity focused ultrasound (HIFU), and presents a design for the next‐generation CMUTs for integrated imaging and HIFU endoscopic catheters.

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References
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Journal ArticleDOI
TL;DR: The first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array is presented and preliminary investigations on the effects of crosstalk among array elements on the image quality are performed.
Abstract: Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

468 citations


Journal ArticleDOI
TL;DR: The finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency higher than a cWU operating in the conventional regime below its collapse voltage.
Abstract: We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k/sub T//sup 2/) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes.

112 citations


Journal ArticleDOI
TL;DR: The designed, forward-viewing annular CMUT array is suitable for mounting on the front surface of a cylindrical catheter probe and can provide Doppler information for measurement of blood flow and guiding information for navigation through blood vessels in intravascular ultrasound imaging.
Abstract: This paper reports the design and testing of forward-viewing annular arrays fabricated using capacitive micromachined ultrasonic transducer (CMUT) technology. Recent research studies have shown that CMUTs have broad frequency bandwidth and high-transduction efficiency. One- and two-dimensional CMUT arrays of various sizes already have been fabricated, and their viability for medical imaging applications has been demonstrated. We fabricated 64-element, forward-viewing annular arrays using the standard CMUT fabrication process and carried out experiments to measure the operating frequency, bandwidth, and transmit/receive efficiency of the array elements. The annular array elements, designed for imaging applications in the 20 MHz range, had a resonance frequency of 13.5 MHz in air. The immersion pulse-echo data collected from a plane reflector showed that the devices operate in the 5-26 MHz range with a fractional bandwidth of 135%. The output pressure at the surface of the transducer was measured to be 24 kPa/V. These values translate into a dynamic range of 131.5 dB for 1-V excitation in 1-Hz bandwidth with a commercial low noise receiving circuitry. The designed, forward-viewing annular CMUT array is suitable for mounting on the front surface of a cylindrical catheter probe and can provide Doppler information for measurement of blood flow and guiding information for navigation through blood vessels in intravascular ultrasound imaging.

96 citations


"3-D ultrasound imaging using forwar..." refers background in this paper

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Abstract: The problem of imaging with an annular array of transducers by employing all pairs of transducer elements around the circumference of the annulus as transmitters and receivers is considered. If θt and θr are, respectively, the angular locations of a pair of transmitting and receiving elements, then weighting the received signal with the positive weight 2‖sin(θt−θr)‖ before coherent summation results in an image point spread function of the form J1(R)/R. This corresponds to the point spread function of a full circular (area) aperture. Moreover, it is shown that the diameter of this synthetic aperture is twice that of the annulus. A more general weighting function is also derived that results in a point spread function of the form Jn(R)/Rn, n=1,2,..., which is shown to correspond to an apodized circular aperture of diameter twice that of the annulus.

50 citations


"3-D ultrasound imaging using forwar..." refers methods in this paper

  • ...Imaging Results A conical volume was reconstructed offline using the full 64×64 set of A-scans from an imaging target with weightings for full-aperture resolution [6] and cosine apodization....

    [...]


01 Jan 2002
Abstract: In this paper, we present some initial results from our proposed forward viewing ring-annular array for intravascular ultrasound imaging. We have investigated array design and image synthesis allowing simultaneous sideward imaging at 20 MHz and forward imaging at 10 MHz from the same array. This design is appropriate for over-the-wire delivery but limits the forward-looking aperture to a ring-annular array. In particular, a 1200 /spl mu/m diameter ring-annular array with 64 elements has been constructed. The measured forward- viewing radiation pattern of an individual element indicates an acceptance angle range of over 90/spl deg/. Currently, there are significant sensitivity and center frequency variations among the elements. Nevertheless, images of point targets are obtained as a proof-of-concept. Future improvements of the array will be discussed.

17 citations


"3-D ultrasound imaging using forwar..." refers background in this paper

  • ...Previous efforts have found it challenging to design and fabricate ring arrays using piezoelectric transducers with sufficient performance in the forward-looking mode [1]....

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Frequently Asked Questions (2)
Q1. What are the contributions mentioned in the paper "3-d ultrasound imaging using forward viewing cmut ring arrays for intravascular and intracardiac applications" ?

The authors have demonstrated full synthetic phased array volumetric ultrasound imaging using a forward-viewing CMUT ring array with 64 elements, in both the conventional ( 8 MHz ) and collapse ( 19 MHz ) regimes of operation. 

Future work involves developing a fully integrated system with flip-chip bonded electronics for incorporation into a catheter probe.