A computed tomographic imaging system for experimentalizing

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1 A computed tomographic imaging system for experimentalizing LU Yanping *1,2 WANG Jue 1,2 LIU Fenglin 1,2 YU Honglin 2 1. ICT Research Center, 2. Key Laboratory of Optoelectronic Technology and System of the Education Ministry of China, Chongqing University, Chongqing , P. R. China ABSTRACT Computed tomography (CT) is a non-invasive imaging technique, which is widely applied in medicine for diagnosis and surgical planning, and in industry for non-destructive testing (NDT) and non-destructive evaluation (NDE). So, it is significant for college students to understand the fundamental of CT. In this work, A CT imaging system named CD-50BG with 50mm field-of-view has been developed for experimental teaching at colleges. With the translate-rotate scanning mode, the system makes use of a Bq (20mCi) activity 137 Cs radioactive source which is held in a tungsten alloy to shield the radiation and guarantee no harm to human body, and a single plastic scintillator + photomultitude detector which is convenient for counting because of its short-time brightness and good single pulse. At same time, an image processing software with the functions of reconstruction, image processing and 3D visualization has also been developed to process the 16 bits acquired data. The reconstruction time for a image is less than 0.1 second. High quality images with 0.8mm spatial resolution and 2% contrast sensitivity can be obtained. So far in China, more than ten institutions of higher education, including Tsinghua University and Peking University, have already applied the system for elementary teaching. Keywords: computed tomography; imaging; experimentation; detector; data acquisition; image reconstruction 1. INTRODUCTION In the past thirty years, Computed Tomography (CT), a non-invasive imaging technique, has been extensively used not only in medicine for diagnosis and surgical planning, but also in non-destructive testing (NDT) and non-destructive evaluation (NDE) for many industrial applications such as defect inspecting for manufacture, assemblage validity examining for complex equipment, composite materials analyzing for production [1]. As well known, when γ ray or χ ray with intensity I 0 penetrates an object along path l, it will be attenuated to intensity I. If the attenuation coefficient is µ ( x, y), the relation of them can be described by Bill Law as I = I exp( µ ( x, y) dxdy ) 0 This law shows that the line integral of attenuation coefficient along path l can be obtained by the logarithm of I0 I. Radon has proved that the distributing of relative linear attenuation coefficient in an infinitely filmy slice is uniquely determined by all of its set of line integrals [2,3], that is 1 1 2π µ ( x, y) = lim m ( cosθ + sinθ +, θ θ 2π ) 2 l x y q d dq δ 0 δ q 0 Where ml (, l θ ) is the partial derivative of mlθ (, ) on l. CT just employs Bill Law and Radon Transform to * Project supported by the National Natural Science Foundation of China (No ) and the National 863 Planning of China (2006AA04Z104). luyp_cqu@126.com; Phone: ; Fax: International Symposium on Photoelectronic Detection and Imaging 2007: Related Technologies and Applications, edited by Liwei Zhou, Proc. of SPIE Vol. 6625, 66251D, (2008) X/08/$18 doi: / Proc. of SPIE Vol D-1

2 CoadDetectorioactiveSoDatmputeaAcquisitionurcereconstruct cross-section image with projections from many directions. The cross-section image represents the density distributing and conformation details. CT technique is the typical example that atomic physics applies in engineering and medical fields, so it is significant to the elementary teaching for college physics. In addition, with the wider and wider application of CT, it is necessary to help the engineering and medical students to know the theory and technique of CT. For those purpose, we researched and developed a series of CT teaching experimental system, CD-50BG is one among them. 2. DESCRIPTION OF SYSTEM 2.1 Framework CD-50BG CT teaching experimental system was designed for testing small dimensional objects. A mechanical rig contains a motorized translate-rotate-elevate table that holds the object, and two stationary stands that hold the radiation source and detector system. The motorized table has two DC motors that permit the object to be rotated and moved across a radiation beam, while a third DC motor is used to elevate the object to a position where the transect is to be tested. The scanning mechanism can assure a translate speed of up to 1~8mm/s and rotation speed of up to 1~6rpm. The scanning system is covered with a four-surface transparent glass cover that makes for observing the scanning process. Figure 1 shows a photograph of CD-50BG. However, besides of the scanning system, the whole CT system also includes a computer that controls the scanner and processes the acquired data. The framework is shown in Figure 2.RObj ect Image Out put Scanni ng Movement rcont r ol l er Figure 1. Photograph of CT scanning system Figure 2. Schematic framework of the CT system 2.2 Radioactive source and its shield The radioactive source of the system is 137 Cs with Bq (20mCi) radioactivity, which can generateγ-ray. The ray has the characters of strong penetrability, fine stability and single energy. But at the same time, it is harmful to human body for being radiated by too large doseγ-ray. While the system is working, operator will stand near by it for operating and observing. So it is necessary to shield the radiation. In this system, Tungsten alloy is taken to machining a radiation shield because the penetrability of γ-ray in the high-density metal is puny. Proc. of SPIE Vol D-2

3 2.3 Scanning The parallel beam, which is the earliest scanning model, is called as the first generation type CT technique [4], that is, rotate-translate (RT) model. This system employs this model. Its basal structure character is that only one detector acquires the intensity of a ray beam with little section each time. Obviously, the efficiency of the scanning model is very low. However, this model has the following advantages, just right explaining the fundamental of CT, no scattering and cross-talk affection to the acquired signal. It needs a data set composed with N N attenuation coefficient integral to reconstruct a N N image. So the detector acquires the signal from N directions, and N signals are acquired in each direction. The scanning procedure of this model includes two movements. One is in each direction, the source and the detector are synchronously translated N 1 times corresponding to testing object in equal step and acquiring time, thus, N testing data can be obtained in this direction. Another is that the source and the detector rotate around an axes of testing object in equal angle in the cross-section, N times rotation in range of 180 o. Through the two movements, a data set built by N N projections is obtained. The RT scanning model is just shown in Figure Detector and data acquisition The detector is a discrete pixel detector in which pixel data are measured from independent detector elements. Obviously, the elements are more, the scanning time is less in the same other conditions. As mentioned above, for clearly explaining the fundamental of CT, the system employs the first generation model, that is, RT model with one source and one detector. As shown in Figure 4, the detector element consists of a plastic scintillator and photomultitude, which is convenient for counting due to its short-time brightness and good single pulse. The scintillator size is approximately Φ10mm 60mm, while the photomultitude size is about Φ14mm 70mm. The dynamic range of the A/D converter is 16 bits. The sampling time is 0.1~0.8s/time. The detector has an anti-scatter collimator with a 1mm 4mm aperture before the scintillator. The Φ30mm 45mm collimator is made of tungsten alloy. The collimator blades are adjusted parallel to the rotation axis, and the pitch of the blade is identical to the detector element pitch. Proc. of SPIE Vol D-3

4 Tr ansl at e Rot at e Tr ansl at e Tr ansl at e Radi at i on r esour ce Det ect or Tr ansl at e Figure 3. The parallel beam scanning model: Translate-rotate Figure 4. Photograph of detector The data acquiring circuit is composed with high voltage dividing circuit, pre-amplifying circuit and signal processing circuit. The high voltage dividing circuit provides high voltage to photomultitude, enabling the faint optical signal to be converted and amplified to a strong electric signal. But the electric signal is so low that the electronic component can t discriminate it. So a preamplifier is taken to amplify the electric signal. The signal processing circuit is divided into secondary amplifying unit, discriminating unit, ECL-TTL level converting unit and counting unit. The pulse signal after the preamplifier can be amplified five times through the secondary amplifier. The discriminator only holds the effective pulse signal by filtering the dark current pulse signal from the photomultitude. The ECL-TTL level converter transforms the ECL level to TTL level to drive the counting circuit. The FPGA counting unit counts the pulse signal and converts it to a stronger digital signal. The structure of the circuit for detecting and data acquiring is just shown in Figure 5. Pl ast i c Scintillator Phot omul t i t ude Pr e- Ampl i f i er Mai n Amplifier Di scr i mi nat or Hi gh Vol t age Di vi der Count er ECL- TTL Level Conver t or Figure 5. The structure of the circuit for detecting and data acquiring 2.5 Image reconstruction and processing software Image processing software package consists of the programs of cross-section image reconstruction, image processing and 3D reconstruction. The cross-section image is reconstructed by Filter Back Projection (FBP) algorithm [5,6], which is denoted as followed Proc. of SPIE Vol D-4

5 π ar ˆ(, θ ) = pr [cos( θ φ), φ]*[cos( hr θ φ)] dφ 0 Where (, r θ ) is the given point, r is its polar distance, while θ is the polar angle. φ is the the viewing angle. p is the function of projection, h is the filtering function. ˆ( arθ, ) is the value of given point. In the reconstructing process, the selection of interpolation method and filtering function can affect the reconstruction results. Figure 6(a) shows shepp-logan phantom s sinogram, and (b), (c), (d) and (e) respectively show the reconstruction results by cubic interpolation, with Cosine, Bandlimit, Hamming and Hann filtering windows. While Figure 6(f), (g) and (h) respectively reveal the reconstruction results by Cosine filtering functions, with cubic, linear and nearest interpolation methods. Because the dimension of the testing object is small, the scanning imaging pixel matrix is small. It can generate and images. The typical reconstruction time is less than 0.1s/slice. The software also provides the image processing including data filtering, uniformity correction and so on. The processed serial slices can be used to reconstruct 3D image with surface rendering and volume rendering. The 3D image intuitionisticly shows the vivid configuration of the object. (a) Shepp-logan phantom s sinogram (b) Cosine and Cubic (c) Bandlimit and Cubic (d) Hamming and Cubic (e) Hann and Cubic (f) Cosine and Cubic (g) Cosine and Linear (h) Cosine and Nearest Figure 6. The reconstruction results by different interpolation methods and filtering function 3. EXPERIMENTS AND RESULTS 3.1 Performance testing The quality of an CT system may be stated as the ability to identify the presence of relatively small changes in the linear attenuation properties of the material within the interior of given objects [7]. Usually, two key criterias, spatial resolution and contrast sensitivity, are taken to measure the quality of a CT imaging system. Proc. of SPIE Vol D-5

6 The spatial resolution is the ability to resolve details and relates to the pixel size. According to the Nyquist theorem, the pixel size should be less than half the detector aperture size. By reducing the aperture size on the detector s shield and the pixel width, one can increase the spatial resolution. The spatial resolution is usually presented by either the modulation transfer function (MTF) of the CT system or by the maximum number of line pairs (lp) or the minimum diameter of hole distinguished in the image. Figure 7 presents cross-section images of a Line Pairs Phantom and a Hole Phantom obtained with the scanner. It shows that the modulation vanishes at more than lp/mm or less than 0.8mm. V "::,4j1) (a) Photograph of Hole Phantom (b) Image of Hole Phantom (c) Photograph of Line Pairs Phantom C C (d) Image of Line Pairs Phantom (e) Gray curve along a line in the image of Line Pairs Phantom Figure 7. Hole and Line Pairs Phantom for spatial resolution testing The contrast sensitivity, also called density resolution, is a measure of the ability to reveal small differences in the linear attenuation coefficients in the object, which are directly related to the differences in density of a homogeneous object. It depends on the ability to distinguish signal from noise in the image. The contrast sensitivity was determined to be less than 2% for a region of 1 cm 2 for the scanner, as shown in Figure 8. Bake1ite(1.g PVC(I.42 g/cm3) (a) Photograph of Density Phantom (b) Image of Density Phantom Proc. of SPIE Vol D-6

7 (c) Gray curve along a line in the image of Density Phantom Figure 8. Density Phantom 3.2 Application study The system also can be used in industrial application. But limited by the frame, the system merely has the capability of testing a object which is not beyond the size of Φ50mm 60mm. For there is only one detector, the typical scanning time for a slice is long to 30 minutes. It was used to scan a turbine engine lamina to measure its interior configuration. As shown in figure 9(a), the arc edges are clearly represented. In Figure 9(b), it shows a cross-section image of high-speed steel with crack. While Figure 9(c) shows the 3D image of another turbine engine lamina reconstructed by our developed software package with serial cross-section images. zp (a) 2D image of turbine engine lamina; (b) 2D image of high-speed steel; (c) 3D image of another turbine engine lamina Figure 9. Testing images in application 4. DISCUSSION AND CONCLUSION This paper has introduced CD-50BG, a CT experimental imaging system for college teaching. The instruments have been used in laboratories at many universities such as Tsinghua University, Peking University and so on. The system has the safe characteristics because it employs a radioactive source whose radioactivity is 20mCi and a reliable shielding vessel. Also it is economical for taking 137 Cs as source but not X-tube and for only taking one detector. But CD-50BG is only one member of the series of experimental system for college teaching. Some more systems also have been developed based on CD-50BG. They are: CD-50BG-A which is an improved system with 137 Cs(200mCi) and 4 detectors, CD-50BG-B which is an improved system with 60 Co (400mCi) and 4 detectors and CD-50BG-C which is an improved system with 60 Co (800mCi) and 16 detectors. The improved systems are approximate to the basic system, however, the scanning model no long is the first generation model but the third generation model, that is, rotate-only (RO) model because there are more than one detector used. The future research will be on the development of more mini-ct imaging systems for wider application, and also Proc. of SPIE Vol D-7

8 will devote us to farther improve the performance of portable industrial CT. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No ) and the National 863 Planning of China (2006AA04Z104). Industrial Computed Tomography Research Center (ICTRC) of Chongqing University released the ICT data set, which generated on the CD-50BG CT systems. The authors are especially grateful to their colleagues at the ICTRC for their assistance during this research. REFERENCE [1] Runzhen Huang, Kwan-Liu Ma and et al, Visualizing industrial CT volume data for nondestructive testing applications, Proceedings of the IEEE Visualization Conference, , [2] Zhu Jiehua, Lee Seung Wook, Ye Yangbo, Zhao Shiying and Wang Ge. X-ray transform and 3D Radon transform for ellipsoids and tetrahedral, Journal of X-Ray Science and Technology, Vol 12, No 4, , [3] Mitra Abhishek and Banerjee Swapna, A new interpolation free method for X-ray CT image reconstruction, Proceedings of 17th IEEE Symposium on Computer-Based Medical Systems, Vol 17, 54-59, [4] Asha Tripathi, P K Khatri, G L Baheti and K C Songara, Interpolation technique in computed tomography image visualization, Defence Science Journal, Vol 52, No 3, , July [5] U Van Stevendaal, J P Schlomka, M Grass, Filtered back-projection reconstruction technique for coherent-scatter computed tomography, Proceedings of SPIE - Medical Imaging 2003: Image Processing, Vol 5032 III, , [6] Horbelt Stefan, Liebling Michael, Unser Michael, Filter design for filtered back-projection guided by the interpolation model, Proceedings of SPIE - Medical Imaging 2002: Image Processing, Vol 4684 II, , [7] M S Rapaport, A Gayer, E Iszak, C Goresnic, A Baran and E Polak, A dual-mode industrial CT, Nuclear Instruments and Methods in Physics Research A, , Proc. of SPIE Vol D-8