Post on 09-Jan-2022
2015 International Nuclear Atlantic Conference - INAC 2015 São Paulo,SP, Brazil, October 4-9, 2015 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-06-9
RELATIONSHIP BETWEEN IMAGE PLATES PHYSICAL
STRUCTURE AND QUALITY OF DIGITAL RADIOGRAPHIC IMAGES IN WELD INSPECTIONS
Davi F. Oliveira1,2
, Aline S. S. Silva1, Alessandra S. Machado
1, Célio S. Gomes
1,
Joseilson R. Nascimento1 and Ricardo T. Lopes
1
1Laboratório de Instrumentação Nuclear/COPPE
Universidade Federal do Rio de Janeiro
68509 Rio de Janeiro, RJ
davi@lin.ufrj.br.br
aline@lin.ufrj.br
celio@lin.ufrj.br
alemachado@lin.ufrj.br
joseilson@lin.ufrj.br
ricardo@lin.ufrj.br
2Departamento de Física Aplicada - Instituto de Física
Universidade do Estado do Rio de Janeiro
Rua São Francisco Xavier, 524
20550-900 Rio de Janeiro, RJ
davi.oliveira@uerj.br
ABSTRACT
In the last decades a new type of detector which is based on photostimulable luminescence was
developed. There are currently many kinds of image plates (IPs) available on the market, originating from
different manufacturers. Each kind of plate distinguishes itself from the others by its peculiar physical
structure and composition, two factors which have a direct influence upon the quality of the digital
radiographic images obtained through them. For this study, several kinds of IPs were tested in order to
determine in which way such influence takes place. For this purpose, each kind of IP has been
characterized and correlated to its response in the final image. The aim of this work was to evaluate
procedures for employing Computed Radiography (CR) to welding inspections in laboratory conditions
using the Simple Wall Simple Image Technique (SWSI). Tests were performed in steel welded joins of
thickness 5.33, 12.70 and 25.40 mm, using CR scanner and IPs available on the market. It was used an X-
Ray equipment as radiation source. The image quality parameters Basic Spatial Resolution (BSR),
Normalized Signal-to-Noise Ratio (SNRN), contrast and detectability were evaluated. In order to
determine in which way the IPs’ properties are correlated to its response in the final image, the thickness
of the sensitive layer was determined and the grain size and the elemental composition of this layer were
evaluated. Based on the results drawn from this study, it is possible to conclude that the physical
characteristics of image plates are essential for determining the quality of the digital radiography images
acquired with them. Regarding the chemical composition of the plates, it was possible to determine that,
apart from the chemical elements that were expected to be found (Ba, I and Br), only two plates, with
high resolution, don’t have fluorine in their composition; the presence of Strontium was also detected in
the chemical composition of the plates supplied by a specific manufacturer. Regarding the grain size and
the thickness of the IPs’ sensitive layers, we could determine that the dimensions of such parameters were
smaller on the plates presenting higher BSR, SNRN and contrast. However, the image plates, which
produced images with the highest resolution, have also proven to be the least sensitive ones. All these
parameters have a direct influence in the detectability of the defects found in the welded joints, were it
was possible to observe that for images obtained with plates with small grain size and thinner sensitive
layer, the defects could be better visualized, including small cracks and pores.
INAC 2015, São Paulo, SP, Brazil.
1. INTRODUCTION
Computed radiography (CR) with phosphor plates (Image Plate – IP) has become the
most common commercially available device used to perform digital radiography [1].
Development of CR systems enabled it to become a viable alternative to the
conventional method in many aspects of industrial radiography applications, such as
weld inspection, corrosion detection and evaluation of deteriorated mechanisms in
pipelines and equipments [2,3].
IPs used for industrial radiography contain BaFBr:Eu+2
active layer in which, after
radiographic exposure, latent image is formed. The scanning IP in CR system with laser
beam enables photostimulated luminescence (PSL) effect and thus transforming latent
image into visible light [4,5]. The creation of latent images on IPs is based on the
energy levels of the electrons within a crystal lattice.
During irradiation the photon energy is absorbed in the active layer of the IP, directly by
generating pairs of electrons and holes. The electrons are stored in F-centers – energy
levels between the valence and conduction bands ̶ while the holes are captured by Eu2+
ions [6]. The latent image which is formed by the spatial variation of the generated
electron and hole storage centers can be subsequently recovered by scanning the image
plate point by point with a focused red laser beam (λ = 700 nm), which converts it into
visible light (PSL) [7]. Emitted visible light is collected by light guides, amplified in
photomultiplier tube and transformed into digital image by A/D converter, which can be
viewed and analyzed [8,9].
The physical characteristics of phosphor plates are of substantial importance in
determining the quality of the radiographic images acquired through them. Industrial
use of CR requires development of evaluation methods for certain image parameters in
order to achieve a higher quality as possible [10].
This paper presents a study assessing the influence of the parameters which characterize
the IP regarding the quality of digital radiographic images. The IPs’ characterization has
been made by determining the thickness of the sensitive layer, evaluating the grain size
and the elemental composition of this layer. In order to evaluate the quality of the
images acquired by the CR system, the following parameters have been analyzed: dose,
basic spatial resolution (BSR), normalized signal-to-noise ratio (SNRN) and
detectability of the defects.
2. MATERIALS AND METHODS
In order to analyze the quality parameters of digital radiographic images by comparing
the physical characteristics of different kinds of IPs, a two-step methodology was
applied: first, each IP has been duly characterized, and then an evaluation on the quality
of the images obtained through each plate has been performed.
In this study, 8 different kinds of phosphor plates supplied by 3 different manufacturers
(A, B and C) have been analyzed. Table 1 show the description of the Image Plates
analyzed. The Blue Plates are the ones with the highest resolution; then, we have the
high resolution plates (HR) and, finally, the standard resolution plates (ST).
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Table 1: Description of the ImagePlates analyzed
Phosphor Plate Manufacturer Plate Type
1 A
2 B1
3 C1
Blue
Plates
4 B2
5 C2
6 C3
HR
Plates
7 B3
8 C4
ST
Plates
2.1. IPs’ Characterization
The characterization of the plates has been made by using a micrometer to determine the
thickness of the sensitive layer of each IP and by evaluating the grain size and the
chemical composition of each IPs’ sensitive layer with the aid of a scanning electron
microscope (SEM).This study was performed using a Jeol 2000 FX scanning transmission
electron microscope operating at 200 kV, equipped with a Noran Energy Dispersive X-ray
(EDX) and an ASID module for compositional mapping.
2.2. Quality of the Digital Radiographic Images
The aim of this step was to evaluate procedures for employing Computed Radiography
(CR) to welding inspections in laboratory conditions using the Single Wall Single
Image Technique (SWSI).Tests were performed in steel welded joins of thickness of
5.33 mm (S1), 12.70 mm (S2) and 25.40 mm (S3), where defects were generated during
the welding process for detectability analysis. Figure 1 shows the samples used.
S1 S2 S3
Figure 1: Test Samples
Figure 2 shows the experimental setup used in this study. It was used an X-ray tube,
model XMB 225, manufacturing of Yxlon, placed at a distance of 600 mm from the IP
and a computed radiography system. The samples were placed directly on the cassette
containing the IP. The exposure parameters used in this experiment are presented in
Table 2.
The computed radiography system used was HD-CR 35 NDT (Durr). For analyzing and
processing the images was used the software Isee. This system has a bit depth of 16 bits
for codification and a 50 µm pixel size.
INAC 2015, São Paulo, SP, Brazil.
Figure 2:Experimental Setup
Table 2: Exposure parameters.
Sample IP X-ray
Source
Focal Size
(mm)
Voltage
(kV)
Current
(mA)
Expossure
Time (s)
1 100
2 85
3 300
4 45
5 32
6 36
7 22
S1
8
110 8.0
17
1 87
2 78
3 274
4 41
5 31
6 31
7 18
S2
8
160 7.5
15
1 250
2 220
3 811
4 113
5 78
6 84
7 55
S3
8
Yxlon 225 3
225 4.5
37
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The digital radiographic images were evaluated in accordance with the current standards
[12-17] to the following parameters: radiographic sensitivity, through wire IQI, basic
spatial resolution (BSR), through duplex wire IQI and normalized signal to noise ratio
(SNRN), through the software ISee [11]. Table 3 shows the requirements of radiographic
quality for the images.
Table 3: Requirements of radiographic quality for the images
Sample Thickness
(mm)
Essential wire
ASTM (ISO)
BSR
(µm) SNRN
S1 5.33 6 (12) 100 >100
S2 12.70 8 (10) 100 >100
S3 25.40 10 (8) 100 >100
For the detectability analysis was developed a comparative study among the images
obtained from CR and Digital Detectors Array (DDA) - Flat Panel. The DDA system
used was DXR250V, manufactured by GEIT.
3. RESULTS
The experimental results described hereinabove revealed the elemental composition of each
IPs’ sensitive layer (results shown in Table 4), as well as their average grain size and the
thickness of their sensitive layers (results shown in Table 5).
Table 4: Chemical composition of each IPs’ sensitive layer
Element Concentration (%) Image Plate
Fluorine Bromine Strontium Iodine Barium
1 A 6.46 25.42 - 5.40 42.96
2 B1 5.69 24.31 - 5.28 43.94
3 C1 3.81 23.94 - 7.82 48.69
4 B2 - 30.89 - 12.36 17.93
5 C2 - 28.50 1.09 7.53 48.41
6 C3 8.77 24.63 1.85 6.03 41.46
7 B3 6.23 24.68 - 4.49 35.56
8 C4 7.28 25.15 1.09 6.02 36.77
Regarding the elemental composition of each IPs’ sensitive layer, it is interesting to point
out that, in addition to the presence of Barium, Bromine and Iodine ― which were expected
to be found ―, Fluorine and Strontium were detected in some of the plates. We could also
determine that only two plates, with high resolution, don’t have fluorine in their
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composition and the element Strontium is only to be found in the plates produced by one
specific manufacturer (C).
The data presented in Table 5 shows that blue plates have smaller grains and their
sensitive layers are thinner than the others, matching what had already been predicted by
theoretical studies. Figure 3 shows SEM images of each IPs’ sensitive layer obtained with a
1,000 x magnification rate.
Table 5: Average grain size and thickness of each IPs’ sensitive layer
Image Plate Thickness of the
Sensitive layer (µµµµm) Grain Size (µµµµm)
1 131 ± 50 3.11 ± 0.65
2 154 ± 50 4.05 ± 1.13
3 112 ± 50 4.62 ± 0.99
4 173 ± 50 4.16 ± 1.03
5 154 ± 50 5.28 ± 1.45
6 168 ± 50 5.05 ± 1.39
7 268 ± 50 7.55 ± 2.27
8 227 ± 50 10.89 ± 3.84
A
B1
C1
B2
C2
C3
B3 C4
Figure 3: SEM images of each IP model with a 1,000x magnification rate.
INAC 2015, São Paulo, SP, Brazil.
As stated hereinabove, in addition to determining the physical characteristics of each
kind of IP, this study also assessed the image quality obtained with each plate. Table 6
below shows the results drawn from this assessment, which has evaluated the Dose,
Contrast, BSR and SNRN. According to the specific standards, the values indicated in
red would be all reproved.
Contrast was approved in all images, both at the central area and at the extremities of
each welding. In general, the best values were found in the blue plates: IPs 1 and 2.
BSR was not approved only in IP 8 for all three inspected samples. As for the SNRN, it
was not approved in IPs 7 and 8 for all the samples, and neither was it approved in IP 4
for sample 3.
Table 6: Assessment of quality parameters for each IP.
Contrast IQI (ISO) Samples IP Dose (Gy)
Center Extremity BSR (µm) SNRN
1 0.266 15 14 50 234
2 0.227 15 14 50 180
3 0.866 15 14 50 139
4 0.127 14 13 80 103
5 0.097 14 14 80 160
6 0.089 14 13 80 136
7 0.055 13 12 100 64
S1
8 0.039 14 13 130 63
1 0.465 13 12 50 221
2 0.395 12 13 50 161
3 1.553 11 12 50 134
4 0.211 12 11 80 116
5 0.144 13 12 80 145
6 0.144 13 11 80 130
7 0.076 11 11 100 63
S2
8 0.058 11 11 130 63
1 1.230 11 10 80 155
2 1.166 10 10 80 101
3 4.163 9 9 65 108
4 0.517 10 9 100 83
5 0.389 10 9 100 109
6 0.392 10 9 100 109
7 0.254 9 9 100 66
S3
8 0.162 9 9 130 52
In order to better evaluate and compare the results drawn from the experiment described
above, comparative charts have been created. Figures 4-9 below indicate the correlation
between the quality parameters mentioned above and the grain size and thickness of
each IPs’ sensitive layer.
The curves shown in Figures 4, 5, and 6 indicate that, in most cases, the larger the grain
size on the IP sensitive layer, the higher the IPs’ sensitivity and the lower its BSR and
INAC 2015, São Paulo, SP, Brazil.
SNRN. This is due to the fact that large phosphor grains intensify the scattering effect,
which affects the quality of the image in a negative way.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8
Dose (Gy)
Grain Size (µµµµm)
Image Plate
Grain Size 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 4: Relationship between the grain size (µm) and the dose (Gy), for each
plate.
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8
BSR (µµµµm)
Grain Size (µµµµm)
Image Plate
Grain Size 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 5: Relationship between the grain size (µm) and the BSR (µm), for each
plate.
INAC 2015, São Paulo, SP, Brazil.
0.0
50.0
100.0
150.0
200.0
250.0
300.0
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8
SNRN
Grain Size (µµµµm)
Image Plate
Grain Size 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 6: Relationship between the grain size (µm) and SNRN, for each plate.
Moreover, since the grain size of each plate has been defined based on an average
assessment, such values are subject to a certain level of uncertainty. This uncertainty is
represented by the standard deviation of the average sizes of the grains measured for
this study. Therefore, the lower the uncertainty is, the more similar in size the grains
are. It was possible to observe that the level of uncertainty is higher for those plates
whose sensitive layer is composed by rather large grains; hence, the higher the
uncertainty level, the lower the BSR and SNRN.
By analyzing the curves shown on Figures 7, 8, and 9 below, it can be noticed that, the
thicker the sensitive layer of an IP is, the higher its sensitivity is and, consequently, the
lower its BSR and SNRN are. This is due to the fact that, on thicker sensitive layers, the
storage phosphors are more likely to be excited by the luminescence emitted by other
phosphor grains, which directly affects the quality of the image.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0
50
100
150
200
250
300
1 2 3 4 5 6 7 8
Dose (Gy)
Sensitive Thickness (µµµµm)
Image Plate
Sensitive Thickness 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 7: Relationship between the thickness of the sensitive layer (µm) and the dose
(Gy), for each plate.
INAC 2015, São Paulo, SP, Brazil.
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
0
50
100
150
200
250
300
1 2 3 4 5 6 7 8
BSR (µµµµm)
Sensitive Thickness (µµµµm)
Image Plate
Sensitive Thickness 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 8: Relationship between the thickness of the sensitive layer (µm) and the BSR
(µm), for each plate.
0.0
50.0
100.0
150.0
200.0
250.0
300.0
0
50
100
150
200
250
300
1 2 3 4 5 6 7 8
SNRN
Sensitive Thickness (µµµµm)
Image Plate
Sensitive Thickness 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 9: Relationship between the thickness of the sensitive layer (µm) and the
SNRN, for each plate.
The data presented above shows that the physical characteristics of an image plate wield
a considerable influence on the image quality. This explains why the response of an IP
cannot be predicted by a general principle. Hence, even if the phosphor grains of a
certain IP are relatively small in size, the BSR can be low in case its sensitive layer is
rather thick.
INAC 2015, São Paulo, SP, Brazil.
2.1. Detectability
Digital images generated by DDA system were used as reference images for
detectability evaluation. Figure 10 shows the defects map obtained from this technique
for the three samples. In this evaluation, was verified the power of detection and
identification of discontinuities in the CR images. The results obtained for detectability
of the defects are shown in table 7.
S1
S2
S3
Figure 10: Defects map of the samples S1, S2 and S3.
Table 7: Detectability results.
Samples IP Detectability
(%) Samples IP
Detectability
(%) Samples IP
Detectability
(%)
1 100 1 90.2 1 90.4
2 98.9 2 90.9 2 91.5
3 99.8 3 92.2 3 75.2
4 97.1 4 73.9 4 58.5
5 98.9 5 93.7 5 88.1
6 98.5 6 91.5 6 79.1
7 95.3 7 65.3 7 76.6
S1
8 95.9
S2
8 71.1
S3
8 70.2
Figures 11 and 12 indicate the correlation between the detectability of the defects found
and the grain size and thickness of each IPs’ sensitive layer. In these curves was
possible to observe that for images obtained using plates with small grain size and
thinner sensitive layer, the defects could be better visualized.
INAC 2015, São Paulo, SP, Brazil.
0.0
20.0
40.0
60.0
80.0
100.0
120.0
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8
Detectability (%)
Grain Size (µµµµm)
Image Plate
Grain Size 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 11: Relationship between the grain size (µm) of the sensitive layer (µm) and
the detectability, for each plate.
0.0
20.0
40.0
60.0
80.0
100.0
120.0
0
50
100
150
200
250
300
1 2 3 4 5 6 7 8
Detectability (%)
Sensitive Thickness (µµµµm)
Image Plate
Sensitive Thickness 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 12: Relationship between the thickness of the sensitive layer (µm) and the
detectability, for each plate.
4. CONCLUSION
Based on the results drawn from this study, it is possible to conclude that the physical
characteristics of phosphor plates are essential for determining the quality of the digital
radiography images acquired with them. Regarding the grain size and the thickness of
the IPs’ sensitive layers, we could determine that the dimensions of such parameters
were smaller on the plates presenting higher BSR, SNRN and contrast. However, the
image plates which produced images with the highest resolution have also proven to be
the least sensitive ones. In this way, it is possible to conclude that the efficiency of an IP
can be improved by increasing the sensitive thickness of the plate; but, on the other
hand, such an increase shall lead to a decrease in resolution. Moreover, it is also
INAC 2015, São Paulo, SP, Brazil.
possible to conclude that high-resolution phosphor plates are more efficient in absorbing
X-ray photons.
Regarding the chemical composition of the plates, it was possible to determine that,
apart from the chemical elements that were expected to be found (Ba, I and Br), only
two plates, with high resolution, don’t have fluorine in their composition; the presence
of Strontium was also detected in the chemical composition of the plates supplied by a
specific manufacturer.
All these parameters have a direct influence in the detectability of the defects found in
the welded joints, were it was possible to observe that for images obtained with plates
with small grain size and thinner sensitive layer, the defects could be better visualized,
including small cracks and pores.
ACKNOWLEDGMENTS
The authors wish to thank Carla Marinho and Marcos Aiub (Petrobras), Conselho
Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo
à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior (CAPES).
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