Photoacoustic Imaging Using a Two-Dimensional
CMUT Array
I. O. Wygant, X. Zhuang, P. S. Kuo, D. T. Yeh, O. Oralkan, B. T. Khuri-Yakub
Edward L. Ginzton Laboratory
Stanford University, Stanford, CA 94305
Email: iwygant@stanford.edu
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
I. INTRODUCTION
In photoacoustic imaging, the optical absorption properties
of a material are imaged by detecting the ultrasound emitted
when the material is illuminated by a laser. The emitted
ultrasound is due to the brief thermal expansions that occur
when the laser’s energy is absorbed by the material. Those
regions that are more optically absorbent than others will
generate a stronger acoustic signal. Thus, with an ultrasound
transducer array or a mechanically scanned transducer, an
image of the optical absorption properties of the material can
be constructed. For medical imaging, photoacoustics is
interesting because materials in the body have different optical
absorption properties. A common motivation put forth for the
development of photoacoustic imaging is the desire to image
vascularization for the detection of cancerous tumors.
Laser pulse widths of around 10 ns and wavelengths
between 600 nm and 1000 nm are typically used for
photoacoustic imaging [1-4]. The wavelength is chosen to
provide sufficient penetration and good optical contrast
between the materials being imaged. The pulse length must be
brief enough such that the volume expansions are short and
efficiently generate ultrasound.
Photoacoustic imaging has been extensively studied.
Impressive results have been published for the imaging of
humans and small animals [1-4]. In this work, a capacitive
micromachined ultrasonic transducer (CMUT) array is used to
photoacoustically image a vessel-like phantom. Previous
photoacoustic imaging work has typically relied on a single
mechanically scanned piezoelectric transducer for detection of
the laser-generated ultrasound. Using a CMUT array in place
of a mechanically scanned element has a number of
advantages. Large, two-dimensional arrays can be reliably
fabricated using CMUT technology. A transducer array greatly
speeds up the data acquisition time for a given aperture size.
CMUT array geometries such as the ring array have also been
demonstrated [5]. A ring array has the practical benefit that the
laser light can come through the hole in the center of the array.
Finally, CMUTs typically have wider bandwidth than
comparable piezoelectric transducers. This is especially
important given the broadband nature of the laser-generated
ultrasound.
II. E
XPERIMENTAL SETUP
A diagram illustrating the setup is shown in Fig. 1. The
CMUT array is located at the bottom of a 5 cm x 5 cm x 3 cm
tank. Vegetable oil is used to couple ultrasound between the
array and phantom. Vegetable oil is used because it is
nonconducting and thus the array and electronics do not need to
be insulated. The phantom consists of three 1.3-mm
polyethylene tubes passing through a 2 cm x 2 cm x 3 cm block
of tissue mimicking material (ATS Laboratories, Bridgeport,
CT). The center tube is filled with black ink to provide optical
contrast for the photoacoustic imaging.
The laser illuminates the phantom from the side. Ideally
the laser should uniformly illuminate the material being
imaged. Thus the laser beam is defocused to a 1/e
2
diameter of
approximately 6 mm. A ground glass diffuser in front of the
tank further diffuses the laser light. The laser is a Q-switched,
Nd:YAG laser with a 1.064 µm wavelength and 12-ns FWHM
pulse duration. The energy of each laser pulse is 2.3 mJ. The
laser was fired at a rate of 10 Hz. A photograph of the
phantom and tank is shown in Fig. 2.
Figure 1. Vessel-like photoacoustic imaging phantom. Three 1.3-mm
diameter tubes are inside a block of tissue mimicking material. The center
tube is filled with ink to provide optical contrast. The phantom is illuminated
by a laser from the side.
Figure 2. Photograph of the phantom and tank. The transducer array is
located at the bottom of the tank. The diffuser at the left diffuses the laser
beam to provide more uniform illumination of the sample.
III. CMUT ARRAY AND INTEGRATED ELECTRONICS
The transducer array has 256 elements (16 × 16 elements).
Each element is 250 µm × 250 µm. Thus, the entire array size
is 4 mm × 4 mm. The transducers have a center frequency of
5 MHz. The CMUT array was fabricated using a sacrificial
silicon nitride process. A few of the key CMUT device
parameters are shown in Table 1. A picture of the packaged
device is shown in Fig. 3. The CMUT array and electronics are
shown in Fig. 4. A more thorough description of the design
and fabrication of the CMUT array is given in [6]. A
description of the CMUT array and integrated electronics is
given in [7].
The transducer array is flip-chip bonded to a custom-
designed integrated circuit (IC) that comprises the front-end
circuitry. The result is that each element is connected to its
own amplifier via a 400-µm long interconnect. This means of
integration mitigates the effect of parasitic cable capacitance
and simplifies connecting the transducer array to an external
system. The IC allows for the selection of a single element at a
time. Thus 256 laser firings are required to acquire a single
image with no averaging.
TABLE I. CMUT DEVICE PARAMETERS
Cell diameter, µm
36
Element pitch, µm
250
Number of cells per element
24
Membrane thickness, µm
0.6
Cavity thickness, µm
0.1
Insulating layer thickness, µm
0.15
Silicon substrate thickness, µm
400
Flip-chip bond pad diameter, µm
50
Through-wafer interconnect diameter, µm
20
Figure 3. Package containing the transducer array and electronics.
Figure 4. CMUT array flip-chip bonded to an integrated circuit that
comprises the front-end circuitry for the array.
CMUT array
and electronics
Oil
1.3-mm diameter
polyethylene
tubes
Ink-filled
tube
Laser
Diffuser
4 mm
Tissue mimicking
material
3 cm
3.5 cm
IV. RESULTS
Conventional pulse-echo imaging data and photoacoustic
imaging data were acquired for the phantom. Photoacoustic
data was acquired by recording an element’s output after the
laser excitation. The pulse-echo data was averaged 16 times to
improve the signal-to-noise ratio. The photoacoustic data was
averaged 4 times. Example pulse-echo and photoacoustic data
(i)
(ii)
(iii)
(a)
(i)
(ii)
(iii)
(b)
Figure 5. (a) Pulse-echo images of the phantom shown with 30-dB dynamic range. (b) Photoacoustic images shown with 20-dB dynamic range. The
volume rendered image is shown in (i). XZ and YZ cross sections are shown in (ii) and (iii) respectively.
acquisitions are shown in Fig. 6 and Fig. 7 respectively. The
signal from the ink-filled tube can be identified in both figures.
The individual element acquisitions are bandpass filtered and
then used for image reconstruction. Both the photoacoustic
image and pulse-echo image are constructed using a standard
delay and sum image reconstruction algorithm.
0102030
-2
0
2
Amplitude (mV)
Time (µs)
Signal from
ink-filled tube
Figure 6. Example pulse-echo A-scan. Reflections from the three tubes can be
identified.
0 5 10 15 20 25 30 35
-2
0
2
Amplitude (mV)
Time (µs)
Signal from
ink-filled tube
Figure 7. Photoacoustic data acquired for a single element. The signal from
the ink-filled tube can be clearly seen. The signals seen in the first 5-µs are
due to electronic noise of the laser and laser light incident on the transducer
array.
Volume rendered and cross-sectional views of the pulse-
echo and photoacoustic images are shown in Fig. 5. The three
tubes are clearly seen in the pulse-echo image. The ink-filled
tube is substantially brighter than the other tubes in the
photoacoustic image.
V.
CONCLUSION
Photoacoustic images obtained with a CMUT transducer
array and integrated electronics are presented. These results
demonstrate some of the advantages of CMUT technology for
photoacoustic imaging. A transducer array such as the one
used in this work has clear acquisition time advantages over a
mechanically scanned system. By increasing the laser
repetition rate, real-time images could be obtained with the
system described here. Image resolution could be improved by
using a larger aperture size. CMUT arrays as large as
128 × 128 elements have been fabricated [8]. The use of such
large transducer arrays for photoacoustic imaging would
provide both outstanding image quality and fast acquisition
times.
A
CKNOWLEDGMENTS
Funding was provided by the National Institutes of Health.
The authors would like to thank National Semiconductor for
the fabrication of the integrated circuits. Bill Broach and the
members of the portable power group at National
Semiconductor provided valuable circuit and process
discussions. Ed Binkley of Promex Industries, Santa Clara, CA
provided packaging and flip-chip bonding support. David Yeh
is supported by a National Defense Science and Engineering
Graduate Fellowship. Xuefeng Zhuang is supported by a
Weiland Family Stanford Graduate Fellowship.
R
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