Simple method to determine the resolution and
sensitivity of systems for optoacoustic tomography
M
´
etodo sencillo para determinar la resoluci
´
on y sensibilidad de sistemas para tomograf
´
ıa
optoac
´
ustica
M. G. Gonz
´
alez
∗†1
, E. Acosta
∗
, G. Santiago
∗
∗
Universidad de Buenos Aires, Facultad de Ingenier
´
ıa,
Grupo de L
´
aser,
´
Optica de Materiales y Aplicaciones Electromagn
´
eticas (GLOMAE)
Paseo Col
´
on 850, C1063ACV, Buenos Aires, Argentina
†
Consejo Nacional de Investigaciones Cient
´
ıficas y T
´
ecnicas, (CONICET)
Godoy Cruz 2290, C1425FQB, Buenos Aires, Argentina
1
mggonza@fi.uba.ar
Abstract—In this paper we present a method to determine
the spatial resolution and sensitivity of systems for optoacoustic
tomography (OAT). The method consists of obtaining the
image of a sample based on a transparent film embedded in
agarose. The film has a certain pattern made with a laser
printer that allows to obtain the system spatial resolution.
Moreover, since the damage threshold of the ink pattern
is similar to that of living tissue, it is also possible to
determine if the system is sensitive enough to be applied
on biological samples. The method is straightforward,
fast and repeatable, and was tested in a OAT system for
obtaining two-dimensional images developed in our laboratory.
Keywords: optoacoustic tomography; spatial resolution;
sensitivity.
Resumen— En este trabajo se presenta un m
´
etodo
para determinar la resoluci
´
on espacial y la sensibilidad de
sistemas para tomograf
´
ıa optoac
´
ustica (TOA).
´
Este consiste
en la obtenci
´
on de la imagen de una muestra basada en una
l
´
amina transparente embebida en agarosa. La l
´
amina posee
un determinado patr
´
on realizado con una impresora l
´
aser
que permite determinar la resoluci
´
on espacial del sistema.
Por otro lado, como su umbral de da
˜
no es similar a del tejido
vivo, tambi
´
en es posible establecer si el sistema posee la
suficiente sensibilidad para ser usado en muestras biol
´
ogicas.
El m
´
etodo es directo, r
´
apido y repetible, y fue probado en
un sistema TOA para obtenci
´
on de im
´
agenes bidimensionales
desarrollado en nuestro laboratorio.
Palabras clave: tomograf
´
ıa optoac
´
ustica; resoluci
´
on espacial;
sensibilidad.
I. INTRODUCTION
The optoacoustic (OA) phenomenon is the generation
of acoustic waves due to thermoelastic expansion caused
by absorption of short optical pulses, combining optical
absorption contrast with high resolution of ultrasound. When
the OA technique is used to perform tomography (OAT),
the pressure profiles generated by the optical excitation are
captured with ultrasonic sensors that surround the area of
interest. This methods provides high resolution maps of
optical absorption [1].
The OAT main goal is to obtain images from OA signals.
This demands solving two inverse problems: one acoustic
and the other optical [2]. In both cases, the ultrasonic signals
are measured. In the acoustic inverse problem, the energy
deposited in the sample is mapped, while the objective
of the optical inverse problem is to obtain the image of
the absorption coefficient. The OA effect applied to obtain
images of living objects is the goal that presents the greatest
challenges in order to solve both inverse problems. Optically,
large variations in the dispersion and absorption coefficients
of living tissues lead to very complex, non-linear inverse
problems. On the other hand, acoustically, the geometry of
the detection system, as well as the heterogeneity and losses
usually present in the sample, lead to distortions and artifacts
in the obtained images [2]. There are a several techniques
for obtaining images in OAT systems. The approach that has
had the best experimental results is the backprojection tech-
nique, a time domain algorithm very simple to implement.
Algorithms in the time domain are based on projecting each
one of the one-dimensional OA time signals into the three-
dimensional space in a way that is consistent with the flight
time principle [2].
The width of OA signals in the time domain scales with
the size of the OA source. The limiting factor for the
achievable resolution is the highest detectable frequency.
However, it also depends on other factors such as duration of
the laser pulse, detection geometry, size, shape and number
of the sensor elements and image formation technique [3].
To characterize the spatial resolution in a OAT system, it
is possible to distinguish between the axial and the lateral
resolution. The former is defined along the acoustic axis of
each sensor intersecting the center of rotation of the detec-
tion surface (or arc in bidimensional schemes). It depends
mainly on the bandwidth of the detection system. On the
other hand, the lateral resolution is defined perpendicularly
to the acoustic axis of the sensor. It is spatialy variant and
depends both on the bandwidth and the aperture of the
transducer element. The more the source approaches the
center of rotation of the sensor array, the more the lateral
resolution improves until it is only bandwidth limited [4].
The resolution of an imaging system can be assessed
by the point spread function (PSF) which describes how
the system represents an elementary volume. To determine
the PSF experimentally, black polyethylene microspheres
Revista elektron, Vol. 2, No. 2, pp. 63-66 (2018)
ISSN 2525-0159
63
Recibido: 09/08/18; Aceptado: 17/09/18
water
Nd:YAG Laser
with a second harmonic generator
beamsplitter
x
y
z
pyroelectric
sensor
amplifier
oscilloscope
thermocouple
XYZ stage
sample
step
motor
Computer
lens
amplifier
cuvette
oscilloscope
lens
beamsplitter
sample
sensor
step motor
Fig. 1. Scheme (left) and picture (right) of the experimental setup used in this work.
100
200
300
400
500500500
Fig. 2. Disc pattern used to determined the system spatial resolution. Each
disc has a diameter of 100 µm. Distances are expressed in micrometers.
(or any very small object with known shape and size) are
usually used to generate the OA signals [4]–[6]. The black
coloration ensures high absorption and thus a strong OA sig-
nal. In order to avoid acoustic reflections, due to impedance
mismatch, the phantom is made of a gel (e.g. agarose)
with similar acoustic properties to water and with scattering
properties that provide an homogeneous illumination. The
dimension of the PSF is estimated comparing the full width
at half maximum (FWHM) obtained from the reconstructed
image with the known size of the object.
The sensitivity of a OAT system is determined measuring
the OA signal generated by a phantom with similar charac-
teristics to those of living tissue and comparing its value with
the signal to noise ratio (SNR) of the device. In this case,
the SNR depends on various parameters: magnitude of the
OA signal arriving at the detector, the ultrasonic sensor sen-
sitivity, the detection aperture, the number of tomographic
projections used in the reconstruction, the number of signal
averages and, obviously, the noise floor of the system [7].
In this work we propose another method to determine the
resolution and sensitivity performing only one measurement.
It consists of obtaining the image of a sample based on a
transparent film embedded in agarose. The film has a certain
pattern made with a laser printer that allows to obtain the
system spatial resolution. Since the damage threshold of the
ink pattern is similar to that of living tissue, it is also possible
to determine if the system is sensitive enough to be applied
on biological samples. This last feature makes the method
simpler than others since with a single measurement the
values of two parameters are obtained. The method was
tested in a 2-D OAT system based on an integrating line
detector and a backprojection reconstruction algorithm.
II. MATERIALS AND METHODS
The experimental setup for the OA imaging system is
shown in Fig. 1. The sensor and the sample were both
immersed in a large cilindrical cuvette (115 mm of diameter
and 90 mm of height) filled with deionized water. The size
of the cuvette was chosen to avoid interference with the
reflected waves from the walls of the cuvette, within the
measurement time frame. The water temperature was mea-
sured with a calibrated thermocouple. This makes possible
to determine the speed of sound in the water surrounding the
sensor. A Nd:YAG laser with a second harmonic generator
(Continuum Minilite I, 532 nm) with a pulse duration of
5 ns, and a repetition rate of 10 Hz, was used as the light
source. In all the measurements carried out in this work,
the laser pulse energy was less than 13 mJ. A diverging
lens adapts the diameter of the laser beam to the size
of the sample (12 mm of diameter), trying to achieve an
homogeneous illumination. An ultrasonic sensor was fixed
and pointed to the center of the rotating sample stage
using a XYZ translation stage. Phantoms were fixed on the
sample stage and rotated (Newport PR50CC) through 360
degrees over a circumference with a diameter of 8 mm. Full
view data (360 degrees) minimizes the effect of a limited
view detection [8]. The sensor output was amplified with
a transimpedance amplifier (EG&G Optoelectronics Judson
PA-400), digitized by an oscilloscope (Tektronix TDS 2024,
2 GS/s, 200 MHz) and processed on a personal computer.
OA signals were recorded every degree and, at each angle,
averaged 64 times. The laser Q-Switch pulse served as the
trigger pulse. The value of the angular sampling period does
not lead to aliasing artifacts such as streaks [4]. The main
laser beam was sampled with a beamsplitter to measure the
laser pulse energy with a pyroelectric detector (Coherent
LMP10).
The ultrasonic sensor used in this work consist of a PVDF
film (25 µm of thickness) attached to an acrylic substrate
with dimensions 30 mm x 30 mm x 10 mm. The active de-
tection area is approximately 0.7 mm x 24 mm. These values
allow to achieve a homogeneous resolution in the scanned
region [9]. The frequency response of the system (sensor +
amplifier) was measured using the method detailed in [10]
obtaining a frequency cut-off of 20 MHz (-3 dB). Further
details about the implementation and characterization can be
found in references [10] and [11].
The projections of the initial pressure distributions into the
xy plane were obtained using the backprojection algorithm
described in [12].
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Fig. 3. Transparent film phantom embedded in agarose gel. The pattern
(3 x 3 mm) on the film surface was made with a laser printer.
The method presented in this paper is based on the
measurement of a phantom. This consists of transparent
film embedded in agarose gel. On the film was drawn the
pattern shown in the Fig. 2 with a laser printer. The pattern
is a set of pairs of black disks (100 µm of diameter) at
different distances. The agarose gel was prepared with 2.5%
(w/v) agarose in distilled water. First, a cylindrical base of
the agarose gel with a diameter of 15 mm and a height
of approximately 25 mm was prepared. Then, the object
(transparent film) was placed in the middle of the cylinder
and fixed with a few drops of the gel. Finally, another
layer of gel with a thickness of 1 mm was formed on top
of the sample object. The production of the phantoms is
straightforward,fast and they are stable for 15 days if stored
at 4
◦
C. The acoustical properties of the solidified agarose gel
are comparable to those of water [13]. Scattering properties
similar to tissue can be achieved by adding intralipid to the
agarose solution before it solidifies [7].
It is important to remark that the damage threshold of
the ink pattern is very close to the maximal permissible
skin exposure value established by the American National
Standards Institute [14] which, for a nanosecond laser in
the visible range (400 - 700 nm) with a repetition rate up to
10 Hz, is 20 mJ/cm
2
. Therefore, using this characteristic
in combination with the noise equivalent pressure value
of the detection system, it is possible to determine if the
OAT system is sensitive enough to be applied on biological
samples.
III. EXPERIMENTAL RESULTS
First, we studied the general performance of the system.
In this way, we made a measurement using the sample
(phantom UBA) shown in Fig. 3. The phantom was made
with the same procedure explained in the section II. The
image obtained is shown in Fig. 4. In the imaging plane
of the system (6 x 6 mm) any effect related to limited view
detection, finite detector size or insufficient spatial sampling
could be observed. However, there is a lack of homogeneity
in the map of absorption in the letters UBA. This is not
due to an error of the system, but to the imperfect adherence
of the toner to the transparent film. This fact was visually
checked using a microscope.
The next step was to obtain the image of the phantom
with the pattern detailed in Fig. 2. The results are shown in
Fig. 5 where the contour (black solid line) of the discs were
superimposed on the reconstructed image. From the figure,
with the naked eye and taking into account the separation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
x (mm)
y (mm)
Normalized amplitude
−1.5 −1 −0.5 0 0.5 1 1.5
−1.5
−1
−0.5
0
0.5
1
1.5
Fig. 4. Image obtained with the TOA system. The phantom reads UBA.
−1500 −1000 −500 0 500 1000 1500
1500
1000
500
0
−500
−1000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
x ( m)
y ( m)
μ
Normalized amplitude
μ
Fig. 5. Imaging obtained with the OAT system used the phantom with
the black discs pattern showed in Fig. 2.
−500 −400 −300 −200 −100 0 100 200 300
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
x ( m)
μ
Normalized amplitude
Fig. 6. The line profile of the reconstructed image (Fig. 5 at different y
values: -0.50 mm (dashed line), 0.06 mm (solid line), 0.65 mm (dash-dotted
line) and 1.2 mm (dotted line).
Revista elektron, Vol. 2, No. 2, pp. 63-66 (2018)
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between the discs, the resolution of the system is 200 µm.
In order to obtain a more precise value, in Fig. 6 the line
profile of the image of Fig. 5 for different values of y are
presented. The width of all the discs in the profiles (FWHM)
is approximately equal in all directions and its value is 190
± 10 µm. Considering the finite size of the discs, this can
be regarded as an upper limit of the resolution.
It is interesting to compare the previous value with the
theoretical maximal achievable resolution, R
bw
, using the
values of the sound speed (v
s
= 1480 m/s) and the cut-off
frequency of the detection system (f
co
= 20 MHz), R
bw
≈
0.8v
s
/f
co
= 60 µm [15]. The large difference is mainly due
to that this expression assumes an idealized scenario, i.e. full
view detection, point detector, continuous spatial sampling
and constant sound speed.
In order to obtain the image of Fig. 5, a minimum fluence
of 10 mJ/cm
2
was necessary. With this laser radiation value,
the minimum detected OA signal had a mean value of 60
Pa. Taking into account that the measured noise equivalent
pressure (NEP) of the detection system is 20 Pa [11], the
minimum signal to noise ratio (SNR) was greater than 3 dB.
Therefore, this OAT system is sensitive enough to be used in
biological applications such as in-vivo tissue measurements.
IV. CONCLUSIONS
In this paper we presented a method to determine the
spatial resolution and sensitivity of systems for OAT in a
single measurement. It is based on obtaining the OA image
of a transparent film with a printed pattern and embedded in
agarose. The method is straightforward, fast and repeatable,
and allows to determine if the system is sensitive enough to
be used on biological applications.
In order to test the method, we measured the resolution
and sensitivity of a homemade 2-D OAT system based on an
integrating line detector and a backprojection reconstruction
algorithm. The method determined that the spatial resolution
is 190 µm. Moreover, using laser pulse energies lower than
the sample damage threshold, an adequate SNR value has
been achieved, which allows obtaining high quality images.
In future work, we will carry out measurements using
phantoms with optical scattering properties similar to living
tissue adding intralipid to the agarose solution. Moreover,
we will probe ink patterns with different optical absorption
properties in order to study the dynamic range of the
detection system.
ACKNOWLEDGMENT
This work was supported by the Universidad de
Buenos Aires (UBACyT Grants 20020160100052BA and
20020170200232BA) and by the Agencia Nacional de Pro-
moci
´
on Cient
´
ıfica y Tecnol
´
ogica (PICT Grant 2016-2204).
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ISSN 2525-0159
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Enlaces de Referencia
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Copyright (c) 2018 Martin Gonzalez, Eduardo Omar Acosta, Guillermo Santiago
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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