Spatial Resolution Performance Tests of a Multiple-Wavelength
NIR Light Transmission Scanner
N.M. Uzunov1,4, M. Bello4, G. Baldazzi3, G. Moschini2,4, P. Rossi2,5, R. Pani6,7
1
Department of Natural Sciences, Shumen University, Shumen, Bulgaria. 2 Department of Physics, Padova University, Padova, Italy.
3
Department of Physics, Bologna University, Bologna, Italy. 4 INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy.
5
INFN Padova Section, Padova, Italy. 6 Department of Experimental Medicine, “La Sapienza” University, Roma, Italy.
7
INFN Roma Section, Roma, Italy.
INTRODUCTION
In our previous publication we have shown some
preliminary results and images obtained using a multiplewavelength NIR scanning device designed in the
Laboratory for Radiopharmaceuticals and Molecular
Imaging (LRIM) at the National Laboratories of Legnaro,
INFN [1]. The device described there has been designed
for simple straight-line scanning of the NIR light
transmitted through the scanned object.
In this article we present the test results from our
conceptually new NIR scanning device realized for smallobject and tissue imaging in XY plane.
EXPERIMENTAL SETUP
The NIR transmission scanner is realized on the basis of
256 pixels InGaAs array detector Hamamatsu G9203256D, placed in a socket of a NIR detector board. Unlike
the model used in [1] however, we have changed the mode
of scanning fixing the panel with NIR light emitting diodes
and NIR detector with the detector board to the scanner
compartment. The scanning is performed while the
scanned object, placed in a platform with a thin CaF2
window is moving in XY plane (figure 1).
A stepping motors mechanical system, based on bipolar
stepping motors model 4018L Liengineering with optical
encoder controlled by a stepper board system of Robot
Factory, has been constructed to realize movement in XY
plane. The scanner is able to scan the whole optical
window with dimensions of 50 mm x 80 mm in two
directions in steps as small as 10-3 mm.
A set of five NIR light-emitting diodes fixed to the
compartment containing the NIR detector and the NIR
detector board is placed at a distance of 25 cm above the
scanned object.
The stepping motors guidance, the NIR diodes emission
and the image acquisitions are fully controlled by a PCbased multifunction intelligent board National Instruments
FPGA 7831R. A graphical user interface based on the
developing environment LabView 8.5 was created to
control the scanner functions as well as to visualize and
process the acquired images.
Detector signals of the NIR light transmitted through the
object at five different wavelengths are collected at each
step by switching consecutively the NIR light-emitting
diodes. The set of NIR LED employs the following
wavelengths: 940 nm, 1070 nm, 1200 nm, 1300 nm and
1550 nm. The scanning procedure produces five images of
the NIR light transmitted through the biological objects
analyzed.
EXPERIMENTAL RESULTS
Fig. 1. Block-diagram of multiple-wavelength contact NIR
transmission scanner.
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Each of NIR diodes emits light in a cone with an
opening angle of 150. It is important to adjust the light
cones emitted by the diodes with their axis of symmetry
perpendicular to the scanned surface in order to have a
homogeneously illuminated scanned surface. The diode’s
orientation in the LED panel is adjusted according its
signal in the NIR detector created at a fixed value of the
diode current. The signal level read by the NIR detector
should be almost equal in all detector cells.
Measurements of the NIR diodes current causing a
certain light intensity and respectively a certain response in
NIR detector have been conducted. Coefficients of the
polynomial fit through the measured values have been used
to calculate the diodes’ currents to equalize the light
intensity produced in the NIR sensor by the five diodes.
The spatial resolution of the scanner has been
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determined along two perpendicular directions using
USAF 1951 resolution standard, placed at different
distances from the detector. For convenience the scan
direction perpendicular to the detector sensible array was
assigned X and the direction parallel to the detector array
was assigned Y. The dependence of the spatial resolution
in directions X and Y versus the distance to the scanned
object has been measured for each of the five NIR
wavelengths.
The overall spatial resolution dependence of the system
calculated from the measured five wavelengths of NIR
light versus the distance along X direction is shown in
figure 2 (detector without FOP). The corresponding overall
spatial resolution dependence of the system versus the
distance along Y direction is shown in figure 3 (detector
without FOP).
400
Spatial resolution ( μm)
350
300
250
200
150
100
Sensor without FOP
Sensor with FOP
50
0
0
5
10
15
20
25
30
35
40
Sensor-to-object distance ( mm)
Fig. 2. Overall spatial resolution along the direction X calculated
from the spatial resolution at five wavelengths of NIR light
versus the distance to the scanned object.
CONCLUSIONS
The analysis of the spatial resolutions of the NIR
detector measured at 5 NIR light wavelengths shows that
the partial spatial resolutions do not differ significantly.
As it is seen from figure 2 and figure 3, the spatial
resolution of the scanner along the two directions of the
scanned surface, provisionally assigned X and Y, are not
equal. This is due to the difference in the pixel dimensions
of Hamamatsu G9203-256D: the pixel size is 50 μm x
500 μm [4] oriented with its longer size along the X axis.
The use of FOP slightly deteriorates the spatial resolution
along both directions, resulting in a shift of about 50 μm in
both graphs. This is due to the fact that the FOP is
compounded by optical fibers with a diameter of 6 μm
each with optical isolation between the fibers. Nevertheless
the spatial resolution of the detector is satisfactory good for
imaging of thin layers of tissues in transmitted NIR light
and of a tissue bioluminescence as well.
ACKNOWLEDGEMENTS
One of the authors (N. Uzunov) would like to
acknowledge the support received from the ICTP, Trieste,
Italy and in particular the Program for Training and
Research in Italian Laboratories (TRIL).
350
300
Spatial resolution ( μm)
A fiber optical plate (FOP) model FOP(D35T20.5)
containing ordered optically isolated fibers with diameter
of 6 μm has been optically coupled to the NIR detector in
order to apply the so-called tilting-collimator technique
[2,3]. It has a cylindrical shape with diameter of 35 mm
and height of 20.5mm. The spatial resolution of the system
thus formed (NIR detector with FOP) has been measured
scanning the USAF 1951 resolution standard at several
distances from the FOP surface for the five NIR
wavelengths. The dependence of the overall calculated
spatial resolution along directions X and Y versus the
distance to the scanned object, is shown in figure 2 and
figure 3 respectively (detector with FOP).
250
200
150
100
Sensor without FOP
Sensor with FOP
50
0
0
5
10
15
20
25
30
35
40
Sensor-to-object distance (mm)
Fig. 3. Overall spatial resolution along the direction Y calculated
from the spatial resolution at five wavelengths of NIR light
versus the distance to the scanned object.
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[1] N. M. Uzunov et al., LNL Annual Report (2008) 141.
[2] N. M. Uzunov et al., Physics in Medicine and Biology, 51
(2006) N199.
[3] N. M. Uzunov et al., LNL Annual Report (2008) 139.
[4] http://jp.hamamatsu.com/products/sensorssd/pd101/pd105/pd109/ G9203-256D/index_en.html.
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