relationship between image plates physical … between image plates physical structure and quality...
TRANSCRIPT
19th World Conference on Non-Destructive Testing 2016
1 License: http://creativecommons.org/licenses/by/3.0/
Relationship between Image Plates Physical Structure and Quality of Digital Radiographic Images in Weld
Inspections
Davi FERREIRA DE OLIVEIRA 1, Aline S. S. SILVA 1, Alessandra S. MACHADO 1, Célio S. GOMES 1, Joseilson R. NASCIMENTO 1, Ricardo T. LOPES 1
1 Federal University of Rio de Janeiro, Rio de Janeiro, Brazil
Contact e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]
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 evaluated procedures for employing Computed Radiography (CR) to welding inspections in laboratory conditions using the Simple Wall Simple Image Technique (SWSI). CR tests were performed in steel welded joins of thickness 5.33, 12.70 and 25.40 mm. 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 IPs 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. 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.
More info about this article: http://ndt.net/?id=19707
2
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 ( = 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. 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.
2. Materials and Methodology
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).
3
Table 1: Description of the Image Plates analyzed Phosphor Plate Manufacturer Plate Type
1 A Blue Plates 2 B1
3 C1 4 B2
HR Plates
5 C2 6 C3 7 B3 ST
Plates 8 C4 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
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 m pixel size.
4
Table 2. Exposure parameters.
Sample IP X-ray Source
Focal Size (mm)
Voltage (kV)
Current (mA)
Expossure Time (s)
S1
1
Yxlon 225 3
110 8.0
100 2 85 3 300 4 45 5 32 6 36 7 22 8 17
S2
1
160 7.5
87 2 78 3 274 4 41 5 31 6 31 7 18 8 15
S3
1
225 4.5
250 2 220 3 811 4 113 5 78 6 84 7 55 8 37
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.
5
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
Image Plate Element Concentration (%)
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 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 2 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 (m) Grain Size (m)
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
6
A
B1
C1
B2
C2
C3
B3 C4
Figure 2. SEM images of each IP model with a 1,000x magnification rate. 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.
7
Table 6. Assessment of quality parameters for each IP.
Samples IP Dose (Gy) Contrast IQI (ISO)
BSR (m) SNRN Center Extremity
S1
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 8 0.039 14 13 130 63
S2
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 8 0.058 11 11 130 63
S3
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 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 3, 4, and 5 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 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.
8
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 (G
y)G
rain
Siz
e (m
)
Image Plate
Grain Size 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 3. 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
(m
)G
rain
Siz
e (m
)
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 BSR (µm), for each plate.
9
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
SNR
NG
rain
Siz
e (m
)
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 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 6, 7, and 8 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 (G
y)Se
nsit
ive
Thi
ckne
ss (m
)
Image Plate
Sensitive Thickness 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 6. Relationship between the thickness of the sensitive layer (µm) and the dose (Gy), for each plate.
10
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
(m
)Se
nsit
ive
Thi
ckne
ss (m
)
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 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
SNR
NSe
nsit
ive
Thi
ckne
ss (m
)
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 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. 3.1 Detectability Digital images generated by DDA system were used as reference images for detectability evaluation. Figure 9 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.
11
S1
S2
S3
Figure 9. Defects map of the samples S1, S2 and S3.
Table 7. Detectability results.
Samples IP Detectability
(%) Samples IP
Detectability (%)
Samples IP Detectability
(%)
S1
1 100
S2
1 90.2
S3
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 8 95.9 8 71.1 8 70.2
Figures 10 and 11 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.
12
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 (%
)G
rain
Siz
e (m
)
Image Plate
Grain Size 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 10. 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 (%
)Se
nsit
ive
Thi
ckne
ss (m
)
Image Plate
Sensitive Thickness 5.33 mm 12.70 mm 25.40 mm
Blue Plates HR Plates ST Plates
Figure 11. Relationship between the thickness of the sensitive layer (µm) and the detectability, for each plate.
4. Conclusions
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 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
13
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.
Acknowledgements
The authors wish to thank to 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).
References
[1] United Nations Committee of the Effects of Anatomic Radiation. Sources and effects of ionizing radiation, vol. 1. Sources, annex D. Medical radiation exposure. New York: United Nations, p. 355. (2000) [2] U. Ewert, U. Zscherpel, C. Heyne, M.Jechow, “Strategies for Film Replacement”, VII Hungarian NDT-Conference, Eger, Hungary, April 12-14 (2011). [3] U. Zscherpel, “New concepts for corrosion Inspection of Pipelines by Digital Industrial Radiology (DIR)”, WCNDT 2000 - Industrial Plants and Structures, Rome, Italy, November (2000). [4] M. Rakvin, D. Markučič, B. Hižman, “Evaluation of Pipe Wall Thickness Based on Contrast Measurement using Computed Radiography (CR)”, Procedia EngineeringVol.69, pp.1216 – 1224 (2014). [5] K. Takahashi, “Progress in Science and Technology on Photostimulable BaFX:Eu2+ (X = Cl, Br, I) and Imaging Plates”, Journal of Luminescence, Vol.100, pp.307-315 (2002).
[6] M. Thoms, H. von Segger, A. Winnacker, “Spatial correlation and photostimulability of defect centers in the x-ray-storage phosphor BaFBr:Eu2+”,Physics Review B, Vol.44, pp.9240-9247 (1991). [7] M. Thorns, “The quantum efficiency of with image radiographic imaging plates”, Nuclear Instruments and Methods in Physics Research A, Vol.378, pp.598-611 (1996). [8] S. A. Mango, “How to Evaluate the Radiographic Performance Envelope of a Computed Radiography System”, Materials Evaluation, Vol.64, nº3, pp.297-302 (2006). [9] S. A. Mango, “Transitioning to Digital Radiography – When does it make sense?”, 16th WCNDT –World Conference on NDT, Montreal, Canada, August-September (2004). [10] A. S. S. Silva, D. F. Oliveira, A. S. Machado, J. R. Nascimento, R. T.Lopes, “An Evaluation of Imaging Plate Characteristics that Determine Image Quality in Computed Radiography”, Materials Evaluation, Vol.72, pp.392-397 (2014).
[11] BAM, User Manual for the Measuring Program ISee!,Verson 10.2. Disponível em: HTUhttp://www.kb.bam.de/~alex/ic.html (2007).
[12] ASTM E 2445, Standard Practice for Qualification and Long-Term Stability of Computed Radiology Systems (2005).
14
[13] ASTM E 2446, Standard Practice for Classification of Computed Radiology Systems (2005).
[14] EN 14784-1, Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates - Part 1: Classification of systems (2005).
[15] EN 14784-2, Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates - Part 2: General principles for testing of metallic materials using X-rays and gamma rays (2005).
[16] ISO 19232-1, Non-destructive testing - Image quality of radiographs - Part 1: Image quality indicators (wire type) - Determination of image quality value (2004).
[17] ISO 19232-5, Non-destructive testing - Image quality of radiographs - Part 5: Image quality indicators (duplex wire type) - Determination of image unsharpness value (2004).