Dual-modal photoacoustic/CT imaging system
Kaiye Xia (夏开业), Xiaohui Zhai (翟晓晖), Zhaoheng Xie (谢肇恒), Kun Zhou (周 坤),
Yutao Feng (冯豫韬), Guangjie Zhang (张广杰), and Changhui Li (李长辉)*
Department of Biomedical Engineering, College of Engineering, Peking University, Beijing 100871, China
*Corresponding author: chli@pku.edu.cn
Received August 12, 2018; accepted October 24, 2018; posted online November 26, 2018
Photoacoustic (PA) tomography (PAT) breaks the barrier for high-resolution optical imaging in a strong light-
scattering medium, having a great potential for both clinical implementation and small animal studies. However,
many organs and tissues lack enough PA contrast or even hinder the propagation of PA waves. Therefore, it is
challenging to interpret pure PAT images, especially three-dimensional (3D) PA images for deep tissues, without
enough structural information. To overcome this limitation, in this study, we integrated PAT with X-ray com-
puted tomography (CT) in a standalone system. PAT provides optical contrast and CT gives anatomical in-
formation. We performed agar, tissue phantom, and animal studies, and the results demonstrated that PAT/CT
imaging systems can provide accurate spatial registration of important complementary contrasts.
OCIS codes: 170.5120, 170.7440.
doi: 10.3788/COL201816.121701.
Small animals, especially mice and rats, are widely used in
biomedical research. Various imaging techniques have
been developed for small animal studies
[1]
. In comparison
with conventional imaging techniques like magnetic reso-
nance imaging (MRI), X-ray computed tomography (CT),
positron emission tomography (PET), and single-photon
emission computed tomography (SPECT), optical imaging
has superior characteristics, including non-radioactivity,
high sensitivity, and cost-effectiveness. Although tissues’
optical properties contain rich structural and functional
information, the strong light scattering by tissues makes
pure optical imaging methods always suffer low spatial res-
olution when imaging deep in live animals. Over the past
decade, the emerging photoacoustic (PA) tomography
(PAT) has become a very promising method for small an-
imal imaging, including structural
[2–6]
, functional
[7–9]
, and
molecular imaging
[10–15]
.
PAT is a hybrid imaging technique that combines op-
tical absorption contrast and ultrasonic detection
[16–19]
.In
PAT, short-pulsed light illuminates the tissue surface and
endogenous or exogenous optical absorbers in the tissue
absorb the diffused photons and become heated, then
the subsequent thermoelastic expansion generates acous-
tic waves (PA signals). After receiving PA signals at
multiple locations over the tissue surface, images of these
underneath absorbers can be reconstructed. Since the soft
tissue scatters ultrasound waves about two orders weaker
than light, PAT can achieve high ultrasonic resolution
based on optical absorption contrast. Although PAT
has demonstrated its power in label-free deep tissue imag-
ing for animal study, such as small-animal whole-body
imaging
[20–24]
, most of these results relied on high PA con-
trast from limited blood-rich tissues, such as the blood ves-
sels, liver, or kidney. It is challenging to interpret pure PA
3D imaging of deep tissues accurately with out other com-
plementary information. Ultrasound is a commonly used
method to integrate with PAT, but ultrasound also suffers
from low tissue contrast. Inspired by the successful PET-
CT, which integrates high-resolution anatomical informa-
tion by CT and functional information by PET, in this
study, we explored multimodal imaging that fused PAT
with CT. Several previous works have been done on inte-
grating PAT with X-ray imaging
[25]
or CT
[26–28]
. However,
in these previously reported works, PAT and X-ray imag-
ing or CT were at different individual imaging facilities,
i.e., the object (like a mouse) has to be relocated when per-
forming different imaging, which may lead to the change
of morphology.
In this study, we integrated PAT with an X-ray-based
CT to develop a standalone system. Our results demon-
strated that this dual-modality imaging system has the
advantages of both PAT and CT. The dual-modal imag-
ing system in this study contains a PAT and a micro-CT,
as shown in Fig.
1. PAT and CT were nearly orthogonally
aligned, and the imaging object locates in the center. Both
PAT and CT were fixed on an optical table, and the im-
aging object was mounted on a rotational stage by a
waterproof bearing and rotated during imaging.
The PAT system contains a 48-element customized US
linear array (by TomoWave. Inc., Houston, Texas, USA)
to detect the PA signal. Each element has a size of 1 mm ×
1 mm with a center frequency of 1 MHz. The element
pitch is 1.35 mm. The water tank (made of a transparent
acrylic material) has an inner diameter of 10 cm, and its
side wall has an opening slit that allows the array to re-
ceive PA signals. The generated PA signal is amplified and
acquired by a customized data acquisition (DAQ) system
(by TomoWave. Inc., Houston, Texas, USA). More detail
of the PAT probe and the DAQ was described in our pre-
vious work
[29]
. The illuminating source is a Q-switched
Nd:YAG laser (LS-2137/2, LOTIS TII, Minsk, Belarus)
that generates 1064 nm, 10 Hz laser pulses. A one-to-four
optical fiber bundle guides the light to the imaging target.
The four-branch fiber bundle terminals are rectangular
COL 16(12), 121701(2018) CHINESE OPTICS LETTERS December 10, 2018
1671-7694/2018/121701(4) 121701-1 © 2018 Chinese Optics Letters