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Photoacoustic endoscopic imaging of
the rabbit mediastinum
Joon-Mo Yang, Christopher Favazza, Ruimin Chen,
Junjie Yao, Xin Cai, et al.
Joon-Mo Yang, Christopher Favazza, Ruimin Chen, Junjie Yao, Xin Cai,
Chiye Li, Konstantin Maslov, Qifa Zhou, K. Kirk Shung, Lihong V. Wang,
"Photoacoustic endoscopic imaging of the rabbit mediastinum," Proc. SPIE
8581, Photons Plus Ultrasound: Imaging and Sensing 2013, 85813V (4 March
2013); doi: 10.1117/12.2004980
Event: SPIE BiOS, 2013, San Francisco, California, United States
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Photoacoustic endoscopic imaging
of the rabbit mediastinum
Joon-Mo Yang
1
, Christopher Favazza
1
, Ruimin Chen
2
, Junjie Yao
1
, Xin Cai
1
, Chiye Li
1
, Konstantin
Maslov
1
, Qifa Zhou
2
, K. Kirk Shung
2
, and Lihong V. Wang
1
*
1
Optical Imaging Laboratory, Depart
ment of Biomedical Engineeri
ng, Washington University in St.
Louis, One Brookings Drive, Campus
Box 1097, St. Louis, Missouri, 63130, USA
2
Ultrasonic Transducer Resource Ce
nter, Department of Biomedical
Engineering, University of
Southern California, 1042 Downey Way, Univer
sity Park, DRB 130, Los Angeles, CA 90089, USA
ABSTRACT
Like ultrasound endoscopy, photoacoustic endoscopy (PAE) co
uld become a valuable addition to clinical practice due
to its deep imaging capabilit
y. Results from our recent
in vivo
transesophageal endoscopic imaging study on rabbits
demonstrate the technique’s capability to image major organs
in the mediastinal region, such as the lung, trachea, and
cardiovascular systems. Here, we present various features
from photoacoustic images from the mediastinal region of
several rabbits and discuss possible clinical contributions
of this technique and directions of future technology
development.
Keywords
: Photoacoustic endoscopy, endoscopic ultrasound, upper
gastrointestinal tract, transesophageal imaging,
rabbit mediastinum.
1. INTRODUCTION
Photoacoustic endoscopy (PAE)
1-7
is a promising tomographic endoscopy modality that provides a unique combination
of functional optical contrast and high spatial resolution at
clinically relevant depths, far exceeding the penetration
depths of conventional high-resolution optical imaging modalities
8-17
. Moreover, it can provide unprecedented
physiological and functional information of the target tissue with the aid of endogenous or exogenous contrast agents
18-
27
. With these attributes, PAE’
s potential clinical contributions could riva
l or exceed those of
current ultrasound
endoscopy, also called endoscopic ultrasound (EUS)
28,29
.
Results from our recent transesophageal imaging study on
rabbits demonstrate the technique’s ability to image major
organs in the mediastinal region, such as th
e lung, trachea, and cardiovascular systems (
Nature Medicine
, 2012)
3
. Also,
the simultaneous, multi-wavelength spectral imaging capability of our system enables the provision of a wealth of
functional information, such as the total hemoglobin concentration, the oxygen saturation of hemoglobin, and the
dynamics of the lymphatic system. In this paper, we present features from photoacoustic (PA) and ultrasonic (US)
images from the mediastinal re
gions of two rabbits acquired
in vivo
and discuss the possible c
linical contributions of
this technique and directions of future technology development.
2.
MATERIALS AND METHODS
2.1.
3.8-mm diameter integrated photoacoustic and ultrasonic endoscopic probe and system
For this study, we utilized the 3.8-mm diameter probe based endoscopic system reported in
Nature Medicine
3
.
Figure 1
shows the endoscope and its peripheral systems, composed of a micromotor driver circuit, a delay generator, a laser
system, a US pulser-receiver including an amplifier, a da
ta acquisition (DAQ) card, an
d a computer for recording
signals and displaying images.
*
Corresponding author:
lhwang@biomed.wustl.edu
Photons Plus Ultrasound: Imaging and Sensing 2013, edited by Alexander A. Oraevsky, Lihong V. Wang,
Proc. of SPIE Vol. 8581, 85813V · © 2013 SPIE · CCC code: 1605-7422/13/$18 · doi: 10.1117/12.2004980
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Computer
Input electric Output
PA,
pulses
US signals
US pulser-
receiver
A -A
+--
Input laser
pulses
11
Delay i
generator
Micromotor
driver circuit
The endoscope
Motorized
pullback stage
_.i
I
Figure 1.
Block diagram showing the 3.8-mm di
ameter endoscopic probe and its
connection to peripheral systems.
For PA imaging, laser pulses (584 nm, ~10 ns pulse width, ~0.3 mJ/pulse) from a tunable dye laser (Cobra HRR, Sirah),
pumped by a solid-state, diode-pumped Nd:YLF laser (INNOSLAB IS811-E, EdgeWave), are guided by a multimode
optical fiber (BFL22-365, Thorlabs) and emitted through the
central hole of a single element focused US transducer
(LiNbO
3
, ~36 MHz, 65% fractional bandwidth), which is coaxially
aligned with the optical fiber. After exiting the fiber,
the laser beams are further directed to
the target tissue by a scanning mirror, and finally generate PA waves once
absorbed by the target tissue. The PA wa
ves that propagate to the scanning mirror are reflected by the same mirror, sent
to the US transducer, converted into electrical signals, am
plified by the US pulser-recei
ver (5072PR, Panametrics), and
digitally recorded by the DAQ card (NI PCI-5124, National Instruments). For US imaging, we utilized the same US
pulser-receiver which provided sharp elect
ric pulses to the US transducer to ge
nerate acoustic pulses for US imaging
and also amplified the US an
d PA signals detected by the transducer
. With the endoscopic system, we acquired
volumetric PA and US images with a B-scan frame rate of
~4 Hz. More information on the endoscope’s structure is
available in our previous reports
3
.
With the laser fluences used in these experiments, the endoscope’s maximal radial imaging depth was ~7 mm from the
endoscope’s surface, and the angular fiel
d-of-view was limited to approximately
270°, due to partial blocking by the
probe housing. Experimentally measured highest PA and US resolutions in the focal zone of the transducer were
respectively ~55 μm and ~30 μm in the axial direction, and
~80 μm and ~60 μm in the transverse direction, but the
transverse resolution varied with target distance.
2.2.
Animal experiment
We transesophageally imaged the mediastinum of two adu
lt New Zealand white rabbits (~4 kg, Myrtles Rabbitry). The
animals were fasted, beginning ~12 hr before the experime
nts, to reduce the likelihood of ingesta in the imaged
gastrointestinal tracts. Before starting
the endoscopic imaging experiments, we fi
rst anesthetized the rabbits with 35–50
mg/kg
of ketamine and 5–10 mg/kg
of xylazine (IM). While anesthetized, the rabbit was intubated and supplied with
maintenance gas for anesthesia (1.5–3.0% isoflurane). An
endotracheal tube cuff was inserted into the trachea and
inflated to prevent aspiration of water
into the lung. The rabbit wa
s placed on an inclined
stage (~10°) in supine
position. Just prior to probe insertion, water was introduced
into the esophagus and stomach, using an enteral feeding
syringe connected to a rubber feeding tube (8–12 F). The water provided the necessary acoustic coupling and functioned
as a lubricant during the imaging procedure. After the stom
ach and esophagus were filled with water, the endoscopic
probe was inserted through the mouth and advanced ~30 cm into the esophagus. Simultaneous PA and US imaging were
immediately initiated. While acquiring images, the probe was slowly and mechanically pulled out of the esophagus
using a motorized translation stage and with a constant pullback speed of ~160 μm/s. Each imaging session required a
scanning time of ~10 min. Abou
t 2,800 B-scan slices at ~40 μm intervals
were recorded for each volumetric set of PA
imaging at 584 nm and US imaging. Throughout the experiment, the rabbit’s anesthesia level and vital signs were
continuously monitored. After the experiment, the rabbit was euthanized by an overdose of sodium pentobarbital (150
mg/kg) injected in the marginal ear vein.
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(a)
-60°
MV
+60°
+120°
MD
Rabbit 1
(b)
MV
+60°
+120°
-120°
Rabbit 2
(d)
MV
+60°
;
+120°
MD
-120°
o.
o.
o
All procedures in the animal experiments followed the prot
ocol approved by the Institutional Animal Care and Use
Committee at Washington University in St. Louis.
3.
RESULTS
In
Figure 2
, we present two sets of coregistered radial-maximum amplitude projection (RMAP) images showing the PA
(
a–b
) and US (
c–d
) image features of the mediastinal
regions of the two rabbits acquired
in vivo
. Each image was
processed from a C-scan data set covering a cylindrical volu
me ~11 cm long with a ~18 mm diameter, excluding signals
generated from the esophagi (wall thickness
~1 mm) to show anatomical structures
in the mediastinal regions only. The
PA structural images (
a–b
) were created from the PA data acquired at a 584 nm laser wavelength, in which the PA
signal is only sensitive to the total hemoglobin concentration. The US images (
c–d
) represent the echogenicity
distribution of the imaged structures like the conventional pul
se-echo imaging that detects acoustic waves reflected from
target tissue.
Figure 2.
RMAP images of the mediastinal regi
ons of the two rabbits (views from th
e inside of the esophagus). The left-
and right-hand sides of these RMAP images corre
spond to the lower and upper esophagus, respectively. (
a
,
b
) Normalized
PA-RMAP image showing the total hem
oglobin distribution, with the esophag
eal signals excluded during the RMAP
construction. AL, accessory lobe; LL, left
lobe; RL, right lobe of the lung; AO, ao
rta; CVC, caudal vena
cava; CA, carina;
TC, trachea. (
c
,
d
) Coregistered US-RMAP images showing the echogeni
city distribution for the organs presented in
a
and
b
, respectively. In each image, the vertical
φ
-axis corresponds to the angular range of 270°, and the horizontal
z
-axis
corresponds to the pullback length of 11 cm. The approximate mid-ventral (MV) position and angular measures from the
MV are marked along the vertical
φ
-axis, where the positive and negative values correspond to the right and left sides of
the animal, and MD denotes the mid-dorsa
l position. The scale
bars represent 1 cm for the horizontal direction only.
The two PA images (
a–b
) commonly show the structures of major organs, such as the left lobe (LL), the right lobe (RL)
of the lung, the carina (CA), and the tr
achea (TC), in the mediastinal regions.
However, the PA image acquired from
Rabbit 2 shows the accessory lobe (AL), the aorta (AO), and ca
udal vena cava (CVC) clearly, which were also observed
in the previous PA image reported in our recent paper (
Nature Medicine
, 2012)
3
. These major structures were surgically
validated after the endoscopic imaging procedures.
To understand the three-dimensional config
uration of the imaged structures in
Figure 2
, we produced two volumetric
images and present them in
Figure 3
. In these images, the contrast differences
between PA and US imaging are clearly
shown.
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(a)
Rabbit 1
(b)
Rabbit 2
PA amplitude
0
US amplitude
MME
1
0
1
Figure 3.
Three-dimensionally-rendered coregist
ered PA-US mediastinum images of
the two rabbits, processed from
the data presented in
Figure 2
. Each image covers a ~11 cm
range with a ~18 mm diameter. The red color corresponds
to the PA signals and the green color to
the US signals. The left-hand side (–
z
) of each image corresponds to the lower
esophagus of the animal. The horizont
al and vertical scale
bars represent 1 cm and 5 mm, respectively.
4.
DISCUSSION
The presented results show the integrated endoscopic sy
stem’s complementary contrast production ability and the
importance of the dual-mode imaging for the better understanding of morphologic structures of the target tissue.
Although traditional EUS imaging technique is used for imagi
ng the human mediastinum, as we presented in this paper,
PAE could provide unprecedented image information in clini
cal applications, with superior image contrast. Here, we
summarize several unique features of PAE over the traditiona
l EUS imaging technique, including the provisions of (1)
the optical-absorption-based contrast over a wide spectral ra
nge with high spatial resolution at super depths, exceeding
the limit of conventional optical microscopy; (2) high reso
lution (vascular) imaging with intrinsic contrast; (3)
functional imaging through spectral unmixing, such as total hemoglobin concentration and oxygen concentration; (4)
greater flexibility in employing existing and novel contrast ag
ents compared to EUS, with the potential for molecular
imaging; (5) high contrast lymphatic system imaging with the aid of exogenous contrast agents; (6) simultaneous dual-
modality imaging with complementary contrast (PAE-EUS).
So far, we have focused on developing endoscopic probes th
at can be used for imaging th
e gastrointestinal tract, for
which several-millimeter diam
eter probe sizes are acceptable. However, to
apply this technique for intravascular
imaging or use in the instrument channel of a video endoscope, related catheters should be further miniaturized,
typically ~1 mm for intravascular imaging and ~2.5 mm to use in the instrument channel. For clinical applications,
achieving a real-time image frame rate (typically ~30 Hz) is essential to acquire volumetric images minimizing motion
artifacts. Also, implementing a flexible-shaft based endoscope would promote broadening the applications of this
technique. Development of array transducer-based electronic scanning endoscopic systems would be a promising
direction because such systems could provide cross-sectional images with a single shot of a laser pulse and also would
enable achieving a high image frame ra
te. In our recent endoscopic studies
3,4
, we demonstrated acoustic resolution PA
endoscopes with image resolution determ
ined by the acoustic parameters. However, the lateral resolution could be
greatly improved if an optical focusing method were applied, as commonly used in optical-resolution photoacoustic
microscopy
21,30-34
.
ACKNOWLEDGEMENT
We thank Seema Dahlheimer for her attentive reading of the manuscript. This work was sponsored in part by National
Institutes of Health grants R01 CA157277, R01 NS46214
(BRP), R01 EB000712, R01 EB008085, and U54 CA136398
(Network for Translational Research). L.W.
has a financial interest in Microphotoacoustics, Inc. and Endra, Inc., which,
however, did not support this work.
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