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