Photoacoustic tomography of small animal brain with a curved array transducer

Photoacoustic tomography of small animal brain with a

curved array transducer

Xinmai Yang

Washington University in St. Louis

Department of Biomedical Engineering

Optical Imaging Laboratory

St. Louis, Missouri 63130

Anastasios Maurudis

John Gamelin

Andres Aguirre

Quing Zhu

University of Connecticut

Department of Electrical Engineering

Storrs, Connecticut 06269

Lihong V. Wang

Washington University in St. Louis

Department of Biomedical Engineering

Optical Imaging Laboratory

St. Louis, Missouri 63130

Abstract.

We present the application of a curved array photoacoustic

tomographic imaging system that can provide rapid, high-resolution

photoacoustic imaging of small animal brains. The system is opti-

mized to produce a B-mode,

90-deg

field-of-view image at

sub-

200-

resolution at a frame rate of

1 frame/second

when a

10-Hz

pulse repetition rate laser is employed. By rotating samples, a

complete

360-deg

scan can be achieved within

15 s

. In previous

work, two-dimensional

2-D

ex vivo

mouse brain cortex imaging has

been reported. We report three-dimensional

3-D

small animal brain

imaging obtained with the curved array system. The results are pre-

sented as a series of 2-D cross-sectional images. Besides structural

imaging, the blood oxygen saturation of the animal brain cortex is

also measured

in vivo

. In addition, the system can measure the time-

resolved relative changes in blood oxygen saturation level in the small

animal brain cortex. Last, ultrasonic gel coupling, instead of the pre-

viously adopted water coupling, is conveniently used in near-real-

time 2-D imaging.

2009 Society of Photo-Optical Instrumentation Engineers.

DOI: 10.1117/1.3227035

Keywords: photoacoustics; imaging; imaging systems

Paper 08423R received Dec. 2, 2008; revised manuscript received Jun. 26, 2009;

accepted for publication Jul. 14, 2009; published online Sep. 16, 2009.

1 Introduction

In recent years, photoacoustic imaging

PAI, also referred to

as optoacoustic or thermoacoustic imaging

has emerged as a

promising novel biomedical imaging modality.

–

As a hybrid

imaging modality, PAI can provide ultrasound-resolution im-

ages with intrinsic optical contrast in regions up to

5cm

deep.

Besides structural information, PAI can also detect

functional changes and disorders

in vivo

since these changes

and disorders usually induce local optical contrast through

changing blood volume and oxygenation. Therefore, PAI rep-

resents a novel technology from the perspectives of both ul-

trasound and optical imaging: it adds new contrast and func-

tional information to ultrasound imaging, and it greatly

extends the depth of high-resolution optical imaging.

In the past decade, as small animal models have been es-

tablished for many human diseases,

in vivo

small animal im-

aging techniques have developed dramatically. PAI has at-

tracted much attention in this area due to the aforementioned

merits.

particular,

reconstruction-based

PAI—

photoacoustic tomography

PAT

—has been successfully ap-

plied to small animal brain imaging, and its applications in

structural, functional, and molecular imaging have been

demonstrated.

–

Frequently, PAT involves the scanning of a single ultra-

sound detector. The use of a single detector is inexpensive and

can provide a good signal-to-noise ratio

SNR

for an image;

however, the data acquisition duration is long, usually

20 min

per cross section. The prolonged measurement time

presents great challenges for the control of small animal

physiological parameters, especially when time-resolved

functional information is desired or multiple slices of cross-

section images are needed. Therefore, real-time or near real-

time PAT scans are necessary, which may be realized by using

array transducers.

The use of ultrasound array transducers for PAT has been

explored by several groups.

–

In these studies, linear or

curved array transducers were employed to reduce data acqui-

sition time and meet clinical needs. However, these systems

are neither designed nor optimized for small animal use. We

have developed a curved array photoacoustic system opti-

mized for high-resolution tomography of small animal

brains.

The array was custom fabricated by Imasonic, Inc.

Besançon, France

, using piezocomposite technology for

high sensitivity and SNR. The system employs 128-element

ultrasonic transducers operating at

5 MHz

with 80% band-

width for resolution of fine features such as brain vascula-

tures, while retaining high sensitivity for deep imaging. To

mimic the scanning of a single element transducer, all the

elements are arranged to form a quarter circle of

25 mm

ra-

dius; therefore, it takes three rotations to get the

360-deg

view data. Each individual element has an elevation height of

10 mm

with an azimuthal pitch of one wavelength

0.308 mm

and a kerf of

0.1 mm

. Furthermore, each ele-

ment is cylindrically focused with a focal distance of

19 mm

1083-3668/2009/14

Address all correspondence to: Lihong Wang, Washington University in St.

Louis, Department of Biomedical Engineering, Optical Imaging Laboratory, St.

Louis, MO 63130. Tel:

314

935-6152; Fax:

314

935-7448; E-mail:

lhwang@biomed.wustl.edu

Journal of Biomedical Optics 14

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In the case of a

360-deg

scan, this focusing effect results in a

uniform central imaging region of approximate

16 mm

in di-

ameter. A 16-channel data-acquisition module and dedicated

channel detection electronics allow capture of a

90-deg

field-

of-view image in less than one second when a

10-Hz

pulse

repetition rate laser is employed, and a complete

360-deg

scan can be achieved through sample rotation within

15 s

The technical design and systematic evaluation of this sys-

tem has been reported in detail previously.

The main pur-

pose of current paper is to demonstrate that this curved array

is capable of

in vivo

small animal brain imaging, especially

for functional imaging. Since the curved array was designed

for small animal functional brain imaging, to test and demon-

strate its capability is the first step for more complex brain

functional detection in the future. We report the applications

of the system on small animal brain imaging, including 3-D

cross-sectional brain imaging,

in vivo

brain cortex imaging,

and near real-time hemodynamic detection on a rat brain cor-

tex. The results suggest that the current system can perform

PAT of small animal brains to provide both structural and

functional information

in vivo

2 Methods and Materials

The schematic for the noninvasive photoacoustic tomography

of rat brains is shown in Fig.

. A Q-switched Nd:YAG laser

LS-2137/2, LOTIS TII

–pumped tunable Ti:sapphire laser

LT-2211A, LOTIS TII

was employed to provide laser pulses

with a

FWHM

15 ns

, a pulse repetition rate of

10 Hz

, and

a wavelength range of

750 to 820 nm

. The incident energy

density of the laser beam was controlled to be less than

15 mJ

/

on the surface of the animal head, which is well

below the ANSI limit.

The beam was diverged with a con-

cave lens and homogenized by a circular diffuser to produce a

uniform illumination of approximately

20 mm

in diameter at

the sample. The laser light was positioned at the center of

curvature of the transducer and illuminated the sample or-

thogonal to the imaging plane of the transducer for maximum

uniformity.

In the animal experiments, Sprague Dawley rats

100 g

body weight

or Swiss Webster mice

25 g

body weight;

from Harlan Sprague Dawley, Inc., Indianapolis

were used.

For

in vivo

tests, a small animal was initially anesthetized by

the intramuscular injection of a mixture of

87 mg

/

ket-

amine plus

13 mg

/

xylazine. Subsequent anesthesia was

achieved by the inhalation of a mixture of

and isoflurane.

Before experiments, the hair on the head of the small animal

was depilated using hair removal lotion. The mouth and nose

of the animal were covered with a breathing mask to deliver

oxygen and anesthesia gas. A custom-designed animal holder

was used to fix the head of the animal with ear-pins and a

tooth-pin. The animal was placed in sitting position, and the

body of the animal was secured with surgical tapes to provide

support to the animal. During experiments, the small animal

and the animal holder were mounted on a rotary stage posi-

tioned at the center of the curved array transducer. The rotary

stage could be turned in

90-deg

increments to simulate the

angular view of a full ring array. The head of the small animal

was adjusted so that the brain cortex surface was parallel with

the imaging plane. After the data acquisition for PAT, the ani-

mal was sacrificed by the intraperitoneal injection of highly

concentrated pentobarbital.

In the measurements of blood oxygenation, we assumed

that deoxyhemoglobin

and oxyhemoglobin

HbO

were

the dominant absorbing compounds in blood at two wave-

lengths

and

. Relative total hemoglobin concentration

rHbT

and blood oxygen saturation

could then be cal-

culated using the detected optical absorptions at the two ap-

plied wavelengths:

–

rHbT =

HbO

−

HbO

−

where

is the absorption coefficient;

and

are the

known molar extinction coefficients of Hb and

HbO

, respec-

tively;

−

; and

and

HbO

are the con-

centrations of the two forms of hemoglobin, respectively. By

irradiating an animal head with light at two different wave-

lengths,

and

, independently, we could get two photoa-

coustic images that represent the distributions of the optical

energy deposition in the cerebral cortex corresponding to the

two wavelengths. The optical energy deposition is dependent

on the optical absorption and the light fluence at specific lo-

cation. Considering that the skin and skull covering the brain

are relatively homogeneous, the light fluence in the brain ce-

rebral cortex is similarly homogeneously distributed in the

horizontal plane at the two wavelengths. We can then calcu-

late the images of an absolute estimation of

based on

Eq.

3 Results and Discussions

Three-dimensional

3-D

PAT brain images of a mouse were

obtained

in situ

noninvasively as a series of 2-D cross sec-

tions. The animal was sacrificed before the experiment,

mounted to the rotary stage, and then immersed in the water

tank. The laser wavelength used was

797 nm

. Figure

shows

the PAT cross sections at different brain horizontal planes.

Figure

shows the cortex surface. Figures

–

show

the interior brain structures underneath the superficial cortex

with a

2-mm

increment along the depth. To illustrate the fea-

tures shown in these PAT images, we have labeled some char-

Fig. 1

Schematic of the curved array photoacoustic tomography

system.

Yang et al.: Photoacoustic tomography of small animal brain with a curved array transducer

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acteristic tissue structures. An MRI image at a similar depth

as Fig.

is presented in Fig.

and shows good similari-

ties. In Fig.

, the ventricular system is clearly visible on

the image.

By rotating the sample, better quality images can be ob-

tained; however, the data acquisition speed is reduced. To get

near real-time imaging, we avoided rotation. With the current

quarter-ring array, we can have a

90-deg

view angle if no

rotation is employed. Although the image quality is degraded

and some information may be missing because of the incom-

plete data captured with no rotation, there are distinct advan-

tages. First, near real-time PAT images can be acquired, which

allow us to monitor the time-resolved change in a small ani-

mal. Second, instead of using water as the coupling material

between the transducer array and a sample, ultrasound gel can

be used as the coupling medium without worrying about cou-

pling problems caused by rotation. In PAT experiments re-

ported in the literature, water tanks have been used to hold

water for coupling while the detection transducer was

scanned. The use of ultrasound gel is much more convenient.

Furthermore, although with a

90-deg

view angle, the infor-

mation about the image is incomplete, we can focus on the

structures of interest one at a time without monitoring the

whole image.

Figure

shows an

in vivo

rat cortical cortex image ob-

tained with the curved array without rotating the transducer.

This image was obtained by using ultrasound gel as coupling

material; therefore, a water tank is not needed for this case. To

get complete structural information, a

360-deg

view angle is

required.

Because only

90-deg

view angle data was ac-

quired in the current situation, some structures are missing

from the image, including, notably, the sagittal sinus, which is

labeled in the open-skull photograph. However, Fig.

still

shows most of the blood vessels on the cortical surface, which

are oriented perpendicular or nearly perpendicular to the sag-

ittal sinus, and exhibits a good match with the open-skull

photograph shown in Fig.

. Most important, this PAT im-

age was acquired within

, which is nearly real-time

monitoring. Therefore, with the current curved array, we can

provide fast-frame images at

1 frame

/

second

. The fast

frame rate allows us to monitor the changes in these blood

vessels in Fig.

as a function of time.

Figure

shows the

in vivo

measured photoacoustic signal

strength as a function of time as the blood oxygenation level

on the rat brain cortex changes. The signal strength was ob-

tained from the single blood vessel indicated as A in Fig.

The animal experienced two physiological statuses: normoxia

and hyperoxia. Figure

shows the hemodynamic changes

on the rat brain cortex when the physiological status changed

from normoxia to hyperoxia, and Figure

shows the he-

modynamic result when it changed from hyperoxia to nor-

moxia. The laser wavelength used was

771 nm

. At this wave-

length, deoxyhemoglobin has a greater extinction than

Fig. 2

In situ

3-D mouse brain images obtained by the curved array

photoacoustic tomography system. The images show horizontal cross

sections from the dorsal to the ventral part of the brain, where the

imaging depth is

0 mm,

2.0 mm,

4 mm,

6 mm, and

8.0 mm from the top surface of the mouse’s brain, with an interval of

2 mm. Major tissue structures are indicated on the images. An MRI

image at a similar depth as

is shown in

. The color bar shows the

relative magnitude of optical absorption. CB: cerebellum; RH: right

hemisphere; LH: left hemisphere; LV: lateral ventricle; TV: third ven-

tricle.

Color online only.

Fig. 3

PAT image from the curved array system without any rotation,

i.e., only a 90-deg view angle.

PAT image;

open-skull

photograph.

Fig. 4

Rat brain cortex hemodynamics with the curved array trans-

ducer.

The photoacoustic signal strength change on the rat brain

cortex when the physiological status changed from normoxia to hy-

peroxia;

the photoacoustic signal strength change when the physi-

ological status changed from hyperoxia to normoxia. The laser wave-

length was 771 nm. The error bars are the standard errors at each

measurement point.

Yang et al.: Photoacoustic tomography of small animal brain with a curved array transducer

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