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Radiography Testing with Flat Panel Type Image Detector Yasushi IKEDA, Yasutoshi MIZUTA, Japan Fine Ceramic Center, 2-4-1, Mutsuno, Atsuta-ku, Nagoya, 456-8587, Japan Yoshitaka KINOSHITA, Japan Nondestructive Testing Corporation, 4-4-3, Omorikita, Ota-ku, 143-0016, Tokyo, Japan Contact

ABSTRACT

The performances of a new type of imaging device, "Flat Panel Detector", has been studied. An amorphous Si imaging device with a wide active area and a thin thickness of the dimension has been prepared. The perfomance of the device are studied. The characteristics of phosphor scintillation screens used for the device have been tested. Various applications of the device for NDT are investigated. It has been cleared that the device has fairly high spatial resolution, high sensitivity and high dynamic range for usual x-ray testing. Especially, the image quality of the device has been very high. The devices are more sensitive than usual industrial x-ray films, and have the performance of nearly real-time radiography. By considering these aspects, the devices are more important in future radiography.

Keywords: a-Si Flat panel detector, Radiography, Welding, Pipes, Electronic bord

INTRODUCTION

Recently, new types of radioraphic imaging devices, Flat Panel Detector (FPD)", have been developed. These are using amorphous Si or Se films as radiation sensitive layers. Their active imaging areas are very wide, and the thicknesses of the devices are fairly thin to be comparable to the conventional film cassettes. The radiation sensitivities of the devices are fairly high, so that nearly real-time imaging can be possible. The spatial resolutions of the devices are in similar levels of conventional image intensifiers (I.I.), but the noise levels of the new devices are fairly smaller than those of I.I.-TV systems. Therefore, very high image qualities can be achievable with the new FPD systems. The FPDs have been developed so far mostly for the medical purposes^1-3), and very few papers have been reported for industrial applications of the FPDs still yet^4-5).
In this paper, we report some characteristic features of a new device using an amorphous Si detector, such as its principles, computer handling and basic performances for radiography testing⁶⁾. We also show some radiography examples obtained from various samples. The results obtained from small electronic parts to bigger concrete structure will be presented. In addition to radiography, other applications are considered.

PRINCIPLE AND BASIC STRUCTURE OF A NEW IMAGING DEVICE

Figure 1 shows the principle structure of an amorphous Si FPD. The radiations, such as x-rays, illuminate a scintillation screen according to the intensities. The following amorphous Si sensor is sensitive to the scintillation, so that the pixels of the sensor can detect the image of x-rays. The amorphous Si sensor has the matrix of pixels, which is composed of a photodiode area, a thin film transistor (TFT) switch, a scan line and a data line. The sensor is made from thin film deposit on a glass base-substrate.

Fig 1: Conceptual structure of an amorphous Si flat panel detector

Fig 2: Photomicrograph of a single pixel of the FPD

Figure 2 shows the photomicrograph of a single pixel of the FPD. The photodiodes produce an electrical charge that is proportional to the scintillation light that impinges on the area of photodiode. This charge is collected through the data line by a charge amplifier according to the gate pulse coming through the gate line and the function of the TFT.

Fig 3: shows the electronic schematic diagram of the FPD 2D-array and the outward appearance of FLASH SCAN FPD .

There are several types of the FPD. FLASH MRTRIS (Japanese version of FLASH SCAN: dpiX in USA) 30 has the 2,304 data lines and 3,200 gate lines, so that the total number of pixels is 7,372,800. The typical size of the pixel is 127mm´127mm for the FLASH MRTRIS series. The photodiode has a fill factor of 57% in the pixel. The overall active area is about 421mm´308mm.
Usually, the sensitivity of each pixel is different, which yields artifacts in radiographs. To avoid such background false images, gain and offset correction are needed.

EXAMPLES OF RADIOGRAPHY BY FLASH MRTRIS SEREIES

Various samples are imaged by the Flash MRTRIS. The x-ray images are shown in Fig. 4 (a) to (f). They are (a) a socket of piping with mantle, (b) Steel pipe and elbow, (c) CD player, (d) Surface erosion of steel pipe, (e) Steel pies with penetrameters, (f) a fish. Gammer-ray source and various x-ray energies are employed. The imaging time is ranging from several seconds to 30 seconds The image quality of these pictures are very fine, and the contrast resolution is much better than those of the conventional film method. Figure 5 shows the comparison between the resulting image with FPD and conventional film AGFA D7 processed with NIPS (a Film digitizer), original images and magnified ones. A wire typed penetrameter, S02 , is attached on the welding line of SUS steel (5 mm in thickness). In both FPD and AGFA images, the #3 wire can be seen. The sensitivity is not so different in those cases. Therefore, we can use such FPD for the radiography testing of welding .

(a) Corroded steel pipe with mantle Ir-192 340GBq Exposure 30sec FDD 500mm	(b) Steel pipe and elbow Ir-192 340GBq Exposure 30sec FDD 500mm	(c) CD player X-ray 90kV, 3mA posure 1.0sec FDD 800mm
(d) Surface erosion of steel pipe X-ray 150kV, 4mA Exposure 20sec FDD 600mm	(e) Steel pies with penetrameters X-ray 150kV, 3mA Exposure 20sec FDD 800mm	(f) Fish Micro-focus X-ray Exposure 20sec FDD 600mm
Fig 4: Examples of X-ray imaging by Flush MRTRIS 20

(a) Original image by Flash MRTRIS	(b) Film image by AGFA D7 ( NIPS)
(c) Magnified image by Flash MRTRIS	(d) Magnified film image (D7, NIPS)
Fig 5: Comparison of x-ray images by the Flash MRTRIS and Film (AGFA D7; NIPS) SUS: 5 mm, FDD:600 mm, 150 kVp, 3mA, Exposure: 2 sec (FPD) and 60 sec (D7)


Recently, real-tome imaging by such FPD has been possible like image intensifier TV system. However, the image is much clearer than those I.I. TV system, so that many application can be expected for such FPD system.

References

1. R.L.Weisfield et al.; SPIE Vol.3032, Medical Imaging, Physics of Medical Imaging, pp.14-21, 1997.
2. G.Fleiherr; Diagnostic Imaging, Vol.18, No.8, supplement, August 1996.
3. B.Allen; SPIE Vol.2163, Medical Imaging, Physics of Medical Imaging, pp.264-273, 1994.
4. E.E.Anderson et al.; SPIE Vol 3399, Medical Imaging, Physics of Medical Imaging, pp.180-186, 1998.
5. Y.Kinosita; JSNDI RT Report, No.10258, 1999; No.10271, 1999 (in Japanese)
6. Y.Ikeda, Y.Mizuta and Y. Kinoshita; Proc.of 5^th FENDT '99, Taiwan, pp.377-384,1999.

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