High-resolution ultrasonic imaging using an etalon detector array
Sheng-Wen Huang,1,a?Yang Hou,2Shai Ashkenazi,1and Matthew O’Donnell3
1Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
2Department of Electrical Engineering and Computer Science, University of Michigan,
Ann Arbor, Michigan 48109, USA
3Department of Bioengineering, University of Washington, Seattle, Washington 98195, USA
?Received 15 June 2008; accepted 26 August 2008; published online 15 September 2008?
A photoacoustic imaging system was built and tested to demonstrate the feasibility of
high-resolution low-noise ultrasonic imaging based on parallel detection using polymer etalons. Its
capability of detecting ultrasound at different elements simultaneously in the optical end was
verified by imaging three 49 ?m beads. An average noise-equivalent pressure of 3.6 kPa over 50
MHz for 50 ?m diameter detection elements in a two-dimensional array with a diameter of 1.35
mm and a detection bandwidth of 75 MHz at –3 dB was measured. These results demonstrate the
potential of polymer etalons for high-frame-rate high-resolution three-dimensional photoacoustic
and ultrasound pulse-echo imaging. © 2008 American Institute of Physics.
High-frequency ?above 20 MHz? ultrasonic imaging,
including pulse-echo1,2and photoacoustic ?also called optoa-
coustic? imaging,3,4has been applied to applications demand-
ing high resolution. However, no two-dimensional ?2D?
piezoelectric array has been realized for real-time high-
resolution three-dimensional ?3D? ultrasonic imaging. In
fact, high-frequency piezoelectric 2D arrays would naturally
suffer from increased noise levels and wiring and fabrication
complexities due to large element count and small element
size and spacing. To avoid these issues, one way is to detect
and generate ultrasound optically.5
Three properties make resonant optical ultrasound
transducers ?ROUTs?5–17suitable for realizing dense large-
element-count high-frequency 2D detection arrays. First, the
signal-to-noise ratio ?SNR? or sensitivity of a ROUT does
not directly depend on its element size and can be improved
by increasing the probing light. Second, no complex wiring
and electromagnetic interference exist around a ROUT
array. Third, the space for optoelectrical transduction and
electronics is not limited by the array size or element spac-
ing. In short, a ROUT array’s performance and complexity
are irrelevant to its element size.
Polymer Fabry–Pérot etalons are a special type of
ROUT.5,10–14Their basic structure is a thin polymer layer
with semitransparent mirrors coated on both its sides. The
optical reflectivity of an etalon is a function of wavelength.
When the probing light experiences a round-trip phase shift
of 2m?, where m is an integer, in the etalon, resonance oc-
curs and the reflectivity drops. In the presence of ultrasound,
pressure modulates the etalon thickness and therefore the
phase shift. At a wavelength near resonance, the phase
modulation is transformed into reflectivity modulation with
high gain, and ultrasound detection can be performed by
measuring the reflected light power as a function of time.
An ultrasound detection element can be formed on an
etalon by focusing the probing light onto an area of interest.
Such optically defined elements can be as small as the order
of 10 ?m. To form an array, different areas can be probed in
sequence.10In this way, however, imaging frame rate is
greatly limited. Therefore, to achieve high frame rate, dis-
tributing the probing light at a fixed wavelength over a large
area to enable parallel detection is preferred.13,14Note that
although charge-coupled device ?CCD? arrays can be used
for parallel detection at a time instant,9,12they are not suit-
able for high-frame-rate imaging where low noise and
parallel detection at ?1000 instants ?i.e., 1000 consecutive
time samples? are required. Element-by-element wavelength
adjustment11is also infeasible in this case.
In this study, a photoacoustic imaging system was built
and tested to demonstrate the feasibility of high-resolution
low-noise ultrasonic imaging based on parallel detection us-
ing polymer etalons. As shown in Fig. 1?a?, the output light
from a continuous-wave tunable laser ?HP 8168F, Agilent
Technologies, Santa Clara, CA? was approximately colli-
mated by a lens ?L1? to illuminate an etalon. A unity-
magnification two-lens system comprised of L1 and L2
mapped the light reflected from the etalon onto a photode-
tector plane. The etalon was made on a glass substrate by
coating in sequence a 30 nm gold mirror, a 6 ?m SU-8
?MicroChem Corp., Newton, MA? polymer layer, another 30
nm gold mirror, and an additional 1.5 ?m SU-8 protection
As shown in Fig. 1?b?, one tip of a fiber with a 50 ?m
?in diameter? core was put on the photodetector plane to
deliver light from a 50 ?m element on the etalon surface to
a photoreceiver ?1811-FC, New Focus, San Jose, CA?. A
2D translation stage, driven by two motorized actuators
?T-LA60, Zaber Technologies Inc., Richmond, BC, Canada?,
scanned this tip to emulate a photodetector array. Detecting
a?Electronic mail: firstname.lastname@example.org.
FIG. 1. ?Color online? Setup for photoacoustic imaging. BS, PD, and L
stand for beamsplitter, photodetector, and lens, respectively.
APPLIED PHYSICS LETTERS 93, 113501 ?2008?
0003-6951/2008/93?11?/113501/3/$23.00© 2008 American Institute of Physics