Photoacoustic tomography of foreign
bodies in soft biological tissue
Lihong V. Wang
Journal of Biomedical Optics 16(4), 046017 (April 2011)
Photoacoustic tomography of foreign bodies in soft
Xin Cai, Chulhong Kim,∗Manojit Pramanik, and Lihong V. Wang
Washington University in St. Louis, Department of Biomedical Engineering, Optical Imaging Laboratory, St. Louis,
Abstract. In detecting small foreign bodies in soft biological tissue, ultrasound imaging suffers from poor sensitivity
advantage of strong optical absorption contrast and high ultrasonic resolution. A PA imaging system is employed to
to 800 nm and a 5-MHz spherically focused ultrasonic transducer. PA images were obtained from various targets
including glass, wood, cloth, plastic, and metal embedded more than 1 cm deep in chicken tissue. The locations
and sizes of the targets from the PA images agreed well with those of the actual samples. Spectroscopic PA imaging
was also performed on the objects. These results suggest that PA imaging can potentially be a useful intraoperative
imaging tool to identify foreign bodies.C ?2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3569613]
Keywords: photoacoustic tomography; foreign body; soft tissue imaging.
Paper 10536PR received Sep. 30, 2010; revised manuscript received Feb. 20, 2011; accepted for publication Mar. 2, 2011; published
online Apr. 18, 2011.
Wounds, especially those inflicted by weapons or other explo-
sions, may contain debris of various materials, such as glass,
wood, cloth, plastic, and metal. Retained foreign bodies may
cause inflammatory, allergic, and infectious complications.1
Furthermore, the wounded tissue is devitalized. Treatment is
more likely to work well when both foreign bodies and devital-
ized tissue are identified and removed early.2Current imaging
modalities employed for detecting foreign bodies include x-
ray computed tomography (CT), magnetic resonance imaging
(MRI), and ultrasound (US) imaging. X-ray CT is the mass
screening tool for detecting radiopaque foreign bodies. How-
ever, it has obvious drawbacks. Repeated x-ray exposure can be
harmful to the human body despite using low doses of radiation.
Most importantly, x-ray contrast is not appropriate for detecting
radiolucent substances, such as wood, cloth, and plastic. MRI is
perior soft-tissue contrast compared to other imaging methods.
However, MRI is not suitable for detecting metallic fragments
because it gives rise to strong interference artifacts.3It may also
present potential hazards for the patient due to magnetically in-
duced movement of the metallic object through the soft tissues.4
Other disadvantages of MRI include limited spatial resolution
and high cost. US imaging is widely used in clinics because
of its real-time display, zero ionizing radiation exposure, and
affordable price. Unfortunately, it suffers from poor sensitiv-
ity (52.6%) and specificity (47.2%) in detecting small foreign
bodies.5Moreover, distinguishing vital from nonvital tissues in
wounds is challenging. Photoacoustic (PA) imaging, however,
*Current address: University of Buffalo, State University of New York, 328 Bonner
Hall, Buffalo, New York 14260.
Address all correspondence to: Lihong V. Wang, Washington University in
St. Louis, Department of Biomedical Engineering, Optical Imaging Labora-
tory, St. Louis, Missouri 63130. Tel: 314-935-6152; Fax: 314-935-7448; E-mail:
can image blood vessels and functions without extrinsic con-
trast agents at high resolution and can image much deeper than
other optical technologies.6–8Imaging of the wound areas can
potentially be made real time. The wavelength tuning capability
helps identification of debris types.
A deep reflection mode PA imaging system was used.9Figure 1
shows the schematic of the system. Upon laser excitation, the
tissue generates ultrasonic waves, known as PA waves. The PA
waves were first received by an ultrasonic transducer, ampli-
fied by a pulser/receiver (5072PR, Panametrics-NDT, Watham,
Maryland), digitized by an oscilloscope (Tektronix TDS 5054,
Natham, Massachusetts) and stored in a computer, which also
controlled an XY-linear translation stage (XY-6060, Danaher
Motion, Washington, DC) for raster scanning. A photodiode
(SM05PD1A, Thorlabs, Newton, New Jersey) was used to com-
pensate for the energy instability of laser pulses.
To achieve deep penetration of light, a tunable near-infrared
Ti:sapphire laser (LT-2211A, LOTIS TII, Minsk, Belarus)
pumped by a Q-switched Nd:YAG laser (LS-2137/2, LOTIS
TII, Minsk, Belarus) was used for PA excitation. The laser pulse
duration was <15 ns, and the repetition rate was 10 Hz. The
incident laser beam on the tissue surface was controlled to be
less than the ANSI standard for maximum permissible exposure
To receive deep PA signals with minimal ultrasonic attenu-
ation, a 5-MHz central frequency ultrasonic transducer (V308,
Panametrics-NDT, Watham, Maryland) was used. This trans-
ducer had a spherical focus with a 2.54-cm focal length, a 1.91-
cm active diameter, and a 72% nominal bandwidth based on the
full width at half maximum amplitudes. The spatial resolutions
Materials and Methods
Photoacoustic Imaging System
1083-3668/2011/16(4)/046017/4/$25.00 C ?2011 SPIE
Journal of Biomedical Optics April 2011rVol. 16(4) 046017-1
Cai et al.: Photoacoustic tomography of foreign bodies in soft biological tissue
Fig. 1 Schematic of the deep-penetrating reflection-mode photoa-
coustic imaging system. A Cartesian coordinate is shown.
were 144 μm in the axial direction and 560 μm in the transverse
direction. The scanning time depends on the laser pulse repeti-
tion rate, the scanning step size, and the field of view. Typical
values are a 0.1-mm scanning step size for a 1-D scan and a
10-Hz laser pulse repetition rate. The acquisition time is ∼6 s
for a B-scan. Note that the signal was not averaged for any of
the images. The transducer was located inside a water container
with an opening of 5×5 cm at the bottom, sealed with a thin,
clear polyethylene membrane. The object was placed under the
membrane, and ultrasonic gel was used for US coupling. To
reduce the surface PA wave generation, a concave lens, a spher-
ical conical lens, and an optical condenser were used to form
the dark-field illumination.11
With some slight modifications to the PA imaging system, we
can get coregistered pulse-echo US images with the PA images.
A function generator (33250A, Agilent, Santa Clara, Califor-
nia) instead of the laser was used as the trigger source. The
pulser/receiver (5072PR, Panametrics-NDT, Watham, Mary-
land) was set to transmit/receive mode for pulse echo trans-
mission and detection. For this study, another clinical US imag-
ing system (iU22; Philips Healthcare, Andover, Massachusetts)
with a linear-array (L8-4, 128 elements, 4–8 MHz) was used for
comparison to PA images as well.
Ultrasound Imaging System
icking the human soft tissue.12Pieces of foreign bodies with
arbitrary shapes were embedded in the chicken breast tissue at
depths ranging from 3 to 10 mm. A metal blade, a piece of plas-
tic, a piece of wood, a piece of cloth, and a piece of glass were
used to test the ability of PA imaging to detect foreign bodies.
Also, two pieces of wood, two pieces of cloth, and a piece of
plastic were used for a comparison between PA imaging and US
imaging. Then, a piece of wood and a piece of cloth were used
for spectroscopic PA imaging.
To show the feasibility of PA imaging of foreign bodies, we
Photoacoustic Ability to Detect Foreign Bodies
Fig. 2 PA images and digital photographs for various foreign bodies.
Objects were embedded in chicken breast tissue: (a) PA MAP image
without objects embedded in the tissue, (b–d,f) PA MAP images, (e,i)
digital photographs, (g) PA B-scan image without objects embedded in
the tissue, and (h) PA B-scan images. TS: tissue surface.
an optical wavelength of 766 nm. We could clearly see the
objects in the PA images. Figures 2(a) and 2(g) show the PA
maximum amplitude projection (MAP) and B-scan without any
objects embedded in the tissue, respectively. We do not see
anything. Figures 2(b)–2(d) show the PA MAP images of a
piece of wood, a piece of glass, and a piece of cloth. Figure 2(f)
shows the PA MAP images of a piece of metal and a piece of
plastic. MAP was performed by projecting the maximum signal
amplitude from each A-line onto the XY plane. The shapes of
the objects agree well with those of the actual samples shown
in Figs. 2(e) and 2(i). Figure 2(h) shows the B-scan image of
the piece of metal and piece of plastic. Imaging depth can reach
>1 cm. The results indicate that PA is excellent for exactly
locating the foreign bodies of various debris materials.
US is not sensitive to tiny objects (a few millimeters) because
of their low acoustic contrasts or because of operator variability,
especially in the presence of ubiquitous US speckles.13By con-
trast, PA imaging provides excellent optical absorption contrast
and US spatial resolution, enabling detection of tiny objects. To
show this advantage of PA imaging, PA imaging and US imag-
ing were compared. A piece of wood and a piece of cloth were
embedded in chicken breast tissue and were imaged by both the
PA system and the clinical US system. Then, they were cut to
Figures 3(a) and 3(f) show digital photographs for the samples.
Figures 3(b) and 3(c) show the PA B-scan images of the larger
wood and larger cloth, respectively. Figures 3(d) and 3(e) show
the US B-scan images of the larger wood and larger cloth, re-
spectively. Figures 3(g) and 3(h) show the PA B-scan images
of the smaller wood and smaller cloth, respectively. Figures 3(i)
smaller cloth, respectively. Although both modalities success-
fully imaged the objects, PA images had much higher contrasts
than US images. The contrast is defined as (I – Ib)/Ibwith I and
Ibrepresenting the average signal intensity of the foreign body
Comparison to Ultrasound Imaging
Journal of Biomedical OpticsApril 2011rVol. 16(4)046017-2
Cai et al.: Photoacoustic tomography of foreign bodies in soft biological tissue
Fig. 3 PA and US B-scan images of foreign bodies of two sizes. Objects were embedded in chicken breast tissue: (a,f) Digital photographs, (b,c) PA
B-scan images of the larger foreign bodies, (d,e) US B-scan images of the larger foreign bodies, (g,h) PA B-scan images of the smaller foreign bodies
and (i,j) US B-scan images of the smaller foreign bodies. TS: tissue surface; ST: surrounding tissue.
and that of the background (chicken breast tissue), respectively.
The PA contrasts of the larger wood and larger cloth are ∼11
and ∼13, respectively, whereas the corresponding US contrasts
the same, but the corresponding US contrasts were reduced to
only ∼0.6 and ∼0.7, respectively. The poor US contrasts made
it difficult to identify the smaller foreign bodies in US images.
US specificity is limited by the overlapping acoustic charac-
teristics of foreign bodies and the surrounding tissue.14If the
acoustic contrast, they may show up in US images but not in
PA images. Therefore, PA imaging and US imaging are comple-
mentary and will not supplant each other. However, PA images
and US images can be acquired from the same cross sections
of the sample.15–18The two types of images can be coregistered
naturally to combine the advantages of PA and US imaging. As
shown in Fig. 4, a piece of dark soft polyethylene plastic (thick-
ness, ∼150 μm) buried in chicken breast tissue was detected by
coregistered images. In the PA images shown in Figs. 4(a) and
It is not showing up in the coregistered US MAP and B-scan
images [Figs. 4(b) and 4(e)] because the piece of plastic has a
similar acoustic property with the tissue. As shown in Fig. 4(f),
the clinical US system cannot detect the piece of plastic either.
For spectroscopic PA study, we used the same Ti:sapphire laser
with the wavelength ranging from 750 to 800 nm, and the other
parameters are the same as those used in the previous PA ex-
periments. Figure 5 shows the PA spectra of a piece of green
cloth and a piece of brown wood. To validate the results, the
optical absorbance of the corresponding color ink was acquired
by a spectrophotometer (Cary 50 Bio, Varian, Walnut Creek,
California). Each curve was normalized by its own minimum
Spectroscopic PA Imaging
value. The correlation coefficients between the PA curve and
the absorbance curve in Figs. 5(a) and 5(b) are 0.82 and 0.92,
respectively. Spectral analysis would be helpful in choosing the
optimal light wavelengths for imaging specific objects to pro-
duce high-contrast images. In addition, viable and nonviable
tissue can potentially be distinguished spectrally.19
We have demonstrated that PA imaging can be used as a tool
to detect foreign bodies in biological tissue. PA imaging is sen-
sitive to intrinsic and extrinsic optical contrasts. In addition, it
is relatively inexpensive and portable. Therefore, photoacoustic
Discusions and Conclusion
Fig. 4 Coregistered PA and US images for a piece of plastic. The object
was embedded in chicken breast tissue: (a) Photoacoustic MAP image
(signals from the tissue surface were removed), (b) coregistered US
MAP image (signals from the tissue surface and from the polyethylene
membrane were removed), (c) digital photograph, (d) B-scan image of
(a) at the dashed line, (e) B-scan image of (b) at the dash line, and
(f) B-scan image acquired by the clinical US system. TS: tissue surface;
PM: polyethylene membrane.
Journal of Biomedical OpticsApril 2011rVol. 16(4)046017-3
Cai et al.: Photoacoustic tomography of foreign bodies in soft biological tissue Download full-text
Fig. 5 Optical absorption spectra of foreign bodies measured by PA
imaging. (a) PA amplitude of the brown wood and optical absorbance
of brown ink versus the optical wavelength. (b) PA amplitude of the
green cloth and optical absorbance of green ink versus the optical
imaging can potentially be useful in intraoperative procedures
for debris detection and removal.
US imaging is widely used in the clinic because of its real-
time display, nonionizing radiation exposure, and low cost. US
elastography can potentially improve the visibility of foreign
objects. Of course, PA imaging also has its limitations. It is
difficult to detect transparent or low optical absorption foreign
bodies, which may show up as negative contrasts in PA images.
Overall, PA and US images have complementary contrasts and
can be naturally coregistered. Coregistered images of the sam-
ple can potentially increase the sensitivity and specificity for
detecting foreign bodies.
We are grateful to Todd N. Erpelding for experimental assis-
tance. This work was sponsored by National Institutes of Health
Grant Nos. R01 EB000712, R01 EB008085, R01 CA134539,
and U54 CA136398 (Network for Translational Research).
L.V.W. has a financial interest in Microphotoacoustics, Inc. and
Endra, Inc., which, however, did not support this work.
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