We report herein a novel three-dimensional photoacoustic computed tomography (PACT) system for small-animal whole-body imaging. The PACT system, based on a 512-element full-ring ultrasonic transducer array, was cylindrically focused and capable of forming a two-dimensional image in 1.6 seconds. The pulsed laser could either illuminate directly from the top or be reshaped to illuminate the sample from the side. Top illumination was mainly used for mouse brain and mouse embryo imaging. Side illumination provided in vivo anatomical images of an adult mouse. By translating the mouse along the elevational direction, the system provided serial cross-sectional images

Anastasio, Mark

Chatni, Muhammad

Guo, Zijian

Maslov, Konstantin

Wang, Lihong V.

Xia, Jun

Caltech Authors - Main

PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Ring-shaped confocal photoacoustic
computed tomography for small-
animal whole-body imaging
Jun  Xia, Muhammad  Chatni, Konstantin  Maslov, Zijian
Guo, Mark  Anastasio, et al.
Jun  Xia, Muhammad  Chatni, Konstantin  Maslov, Zijian  Guo, Mark
Anastasio, Lihong V. Wang, "Ring-shaped confocal photoacoustic computed
tomography for small-animal whole-body imaging," Proc. SPIE 8223, Photons
Plus Ultrasound: Imaging and Sensing 2012, 82230G (23 February 2012);
doi: 10.1117/12.906056
Event: SPIE BiOS, 2012, San Francisco, California, United States
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Ring-shaped confocal photoacoustic computed tomography for small-
animal whole-body imaging 
 
Jun Xiaa, Muhammad Chatnia, Konstantin Maslova, Zijian Guoa, Mark Anastasioa, and Lihong V. 
Wanga*.  
aDept. of Biomedical Engineering, Washington University in St. Louis, St. Louis, MO USA 63130            
*Correspondence should be addressed to LHWang@biomed.wustl.edu 
 
ABSTRACT   
We report herein a novel three-dimensional photoacoustic computed tomography (PACT) system for small-animal 
whole-body imaging. The PACT system, based on a 512-element full-ring ultrasonic transducer array, was cylindrically 
focused and capable of forming a two-dimensional image in 1.6 seconds. The pulsed laser could either illuminate 
directly from the top or be reshaped to illuminate the sample from the side. Top illumination was mainly used for mouse 
brain and mouse embryo imaging. Side illumination provided in vivo anatomical images of an adult mouse. By 
translating the mouse along the elevational direction, the system provided serial cross-sectional images. 
Keywords: Photoacoustic computed tomography, small-animal imaging, whole-body imaging, focal-line image 
reconstruction 
 
1. INTRODUCTION  
Due to the widespread use of animal models for human disease studies, small-animal imaging plays an increasingly 
important role in biomedical research. Previously, the majority of small-animal whole-body imaging systems were based 
on magnetic resonance imaging (MRI) or X-ray computed tomography (X-ray CT) for anatomic evaluation, and positron 
emission tomography (PET) for functional evaluation. However, these imaging techniques have their own limitations, 
for instance, X-ray CT and PET utilize ionizing radiation, which may be carcinogenic and will confound experimental 
results in oncology, and MRI requires a very high magnetic field and long imaging time (more than one hour) for small-
animal imaging.  
Recently, photoacoustic imaging emerged as a promising tool for small-animal imaging [1, 2]. By combining optical 
sensitivity and ultrasonic imaging depth scalability, this hybrid technology provides high-resolution images beyond the 
diffusion limit of optical microscopic imaging technologies. Over the past few years, several array-based photoacoustic 
whole-body imaging systems have been proposed, including 64-element arc array [2, 3], 64-element half-ring array [4], 
and 128-element hemisphere array [5]. However, these systems have either limited boundary coverage or a long data 
acquisition time, which limits their applications on pre-clinical research. In this report, we present a novel whole-body 
imaging system based on a 512-element full-ring ultrasonic transducer array.  
 
2. SYSTEM DESIGN  
Figure 1 shows the schematic diagram of the system. The key components include a 512-element full-ring ultrasonic 
transducer array, a 64-channel data acquisition (DAQ) system, and two laser systems.   
The 512-element full-ring transducer array (Imasonic, Inc.) has a central frequency of 5 MHz and a reception 
bandwidth greater than 80%. The piezocomposite elements were spaced with a lateral pitch of one wavelength (0.3 mm) 
and a kerf of 0.1 mm. The array element was mechanically shaped into an arc to produce an axial focal depth of 19 mm 
within the imaging plane. The combined foci of all elements generate a central imaging region of 20 mm diameter and 1 
mm thickness. The elevation focus and the small pitch enable each array element to be treated as a point detector in a 
two-dimensional (2D) image reconstruction. During the experiment, signals acquired from all 512 elements were first 
amplified and selected by a custom-designed connector board consisting of 512 preamplifiers, 64 8:1 multiplexers, and 
Photons Plus Ultrasound: Imaging and Sensing 2012, edited by Alexander A. Oraevsky, Lihong V. Wang,
Proc. of SPIE Vol. 8223, 82230G · © 2012 SPIE · CCC code: 1605-7422/12/$18 · doi: 10.1117/12.906056
Proc. of SPIE Vol. 8223  82230G-1
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64 post-amplifiers, and then digitalized by a 64-channel data acquisition system [6]. The two stage amplification 
achieves a total gain of 60 dB. Due to the 8:1 multiplexing, the current system can acquire one frame of data per 1.6 
seconds.  
The excitation source consists of two lasers. An optical parametric oscillator (OPO) laser, tunable from 400 to 680 
nm and 740 to 2000 nm, and a Ti:sapphire laser, tunable from 680 to 900 nm. The laser beam was first homogenized 
using a ground glass, and then passed through a conical lens to form a ring-shaped light. The ring-shaped light was then 
focused using an optical condenser made from acrylic to project a light band around the animal. The thickness of the 
light band was 5 mm, and the diameter was determined by the cross-sectional diameter of the animal. By removing the 
conical lens and the optical condenser, the system can also provide planar top illumination for brain [7] and embryo 
imaging [8]. 
 
Ti-sapphire laser
512 pre-
amplifier
8:1 
multiplexing
64-channel 
amplifier & DAQ
3-axis 
motor
Computer 
Water
Conical 
lens
Mirror
Full-ring 
transducer 
array
Optical 
condenser
Full-ring
light 
Ground 
glass
Sa
m
pl
e
 
Figure 1. Schematic diagram of the photoacoustic computed tomography system for small-animal whole-body 
imaging. 
 
3. IMAGE RECONSTRUCTION  
Due the elevation focus of the transducer element, the detector cannot be treated as a point detector in a three-
dimensional (3D) image reconstruction. Instead, we can treat the focal line of the detector as an auxiliary line for 
applying appropriate delays in the 3D image reconstruction [8]. As shown in figure 2, to compute the delay of a point A 
in the 3D space, we first project A to the x-y plane (B). Point B is then connected to the center point of the transducer 
(C), crossing the focal line at point D. We then connect A and D, and extend the line to reach the transducer at E. The 
line AE is always perpendicular to the transducer surface (CD=DE), and is used for calculating the delay time between 
the imaging point A and the transducer. In the 3D image reconstruction, the original time-domain photoacoustic signals 
from each transducer are back-projected into the 3D imaging space based on the calculated delay time, and are then 
summed to form the reconstructed image [8, 9]. 
 
 
 
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iI` y _ n Q' IV
,X L ö B
E
Fnral Tina1 vv41 I I I Iv
 
 
 
Figure 2. Schematic of the 2D reconstruction, 3D direct reconstruction, and 3D focal-line reconstruction concepts. 
A: point of reconstruction. A': point A projected in 2D reconstruction. A'C: 2D reconstruction delay. AC: 3D 
direction reconstruction delay. AE: 3D focal-line reconstruction delay. x-y: 2D reconstruction plane. The focal line 
is perpendicular to the transducer plane (x-z). 
 
4. RESUTLS  
Figure 3a is an in vivo cortical vasculature image of an adult mouse with intact skin and skull. Figure 3b is an ex vivo 
image of a 15-day-old mouse embryo fixed in a gelatin gel. Figures 3a and 3b were acquired over a depth of 6 mm and 9 
mm, respectively, and maximum-amplitude-projected along the elevation direction. The 3D focal-line reconstruction 
algorithm was utilized to generate these images. Figure 3c is an in vivo cross sectional image of an adult mouse. Figures 
3a and 3b were acquired using the top illumination scheme, and figure 3c was acquired using the side illumination 
scheme. 
a
Ph
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e
Max
Min
 
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f-,
 
 
b
EY
LB
RB Ph
ot
oa
co
us
ti
c a
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ud
e
Max
Min
 
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C
GI
KN
SC
KN
SP
Ph
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co
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Figure 3. In vivo and ex vivo photoacoustic computed tomography images of mice. a. Maximum-amplitude-
projection image of the cortical vasculature in a living adult mouse. b. Maximum-amplitude-projection image of a 
15-day-old mouse embryo fixed in a gelatin gel. c. Cross-sectional image of a living adult mouse. EY, eye; GI, GI 
tract; KN, kidney; LB, limbs; RB, ribs; SC, spinal cord; SP, spleen. Scale bar, 5 mm. 
 
5. CONCLUSION  
A new small-animal whole-body imaging system has been developed based on the full-ring ultrasonic transducer array. 
The new system has an in-plane resolution of 0.1 mm and a frame rate of one frame per 1.6 seconds. Images of the 
mouse brain, embryo, and kidneys were successfully acquired using the system. 
 
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6. ACKNOWLEDGEMENTS  
This work was sponsored in part by National Institutes of Health grants R01 EB000712, R01 EB008085, R01 
CA134539, U54 CA136398, R01 EB010049, R01 CA157277, R01 CA159959, and 5P60 DK02057933. L.W. has 
financial interests in Microphotoacoustics, Inc. and Endra, Inc., which, however, did not support this work. 
 
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Ring-shaped confocal photoacoustic computed tomography for small-animal whole-body imaging

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Ring-shaped confocal photoacoustic computed tomography for small-animal whole-body imaging

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