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3D Tomographic Visualisation of an Aluminium Manifold Speciman on Experimental Gamma-Ray Tomographic SystemUmesh Kumar, S. S. Datta
Isotope Applications Division, Bhabha Atomic Research Centre
Mumbai - 400 085, INDIA. Fax: 91-22-5505151
Email : email@example.com
V. R. Ravindran
Rocket Propellant Plant, Vikram Sarabhai Space Centre,
Industrial Computed Tomography (ICT) is one of the latest methods of non-destructive testing and examination. Different prototypes of Computed Industrial Tomographic Imaging System (CITIS) have been developed and experimental data have been generated in Isotope Applications Division. The experimental gamma rays based tomographic imaging system comprises of beam generator containing approx. 220 GBq (6 Curies) of Cs-137, a single NaI(Tl) - PMT integral assembly in a thick shielding and associated electronics, stepper motor controlled mechanical manipulator, collimators and required software. CITIS data is normally acquired in one orientation of the sample. It may be sometimes required to view a tomographic plane in a different orientation. Also, 3D visualization may be required with the available 2D data set. All these can be achieved by processing the available data. We have customized some of the routines for this purpose provided in IDL package to suit our requirements.
The manifold is an explosive device (pyrodevice) used in rockets and satellites, which is used to generate many output from a single input. The sample taken up for this experiment was the hardware of a manifold. In this paper, we describe 2D and 3D tomographic visualisation of this typical aluminium manifold specimen. First multiple CT slices are generated and then 3D presentation of the structure is explored for experimentation.
Keywords: Gamma rays, tomography, visualization, imaging, ndt
Most conventional computed tomographic scanners fall into one of four generations. The first and second-generation systems are suited for many low speed critical industrial research applications. The strength of first generation systems to which CITIS belongs, are their low cost because of simple design and extremely simple geometry and data collection scheme. These systems use only a single detector and there is almost no variations or small mismatch within various regions of the entire data set. Besides these, operation of CITIS is computer controlled for scanning mechanism and data acquisition system. This has been achieved by separate computer software.
In its simplest form a tomographic scanner is designed to obtain sets of parallel ray sums from a single plane from many directions around the object. A translate rotate tomographic scanner technique was adopted in the present work. The system consists of collimated source of gamma radiation from a Cesium-137 source, and a collimated detector. Each measurement of attenuation of the beam inside the specimen represents one ray sum. One set of parallel ray sum obtained by translations and rotation of the object is called a projection. Projections from many directions around the object are obtained by rotating the specimen and repeating the translation procedure at each new angular orientation.
It is a procedure for solving a mathematical problem by a series of operations following a set procedure. In computed tomography the mathematical process is used to convert the digitized transmission measurements into cross sectional images. There are two commonly used computation methods for image reconstruction from projection data. The Fourier transform method or filtered back projection method (FBP) normally operates in spatial frequency domain whereas a simplified version called Convolution Back Projection method (CBP) operates in special domain and is quite easy to implement. In the experiment presented in this paper, CBP method is employed for the development of image reconstruction software. The reconstructed image is processed by separate image processing software.
Cs-137 radioisotope based tomographic imaging system comprises of mainly three major sub-systems :
Figure 1 is the actual photograph of the experimental system and figure 2 shows the nuclear data processing unit. Data acquisition and control software running on a personal computer is used for overall control of the mechanical system data acquisition job for a full scanning sequence. The entire software for mechanical manipulator, data acquisition, MS Windows based image reconstruction and interactive image display has been developed at Isotope Applications Division and they are:
|Fig 1:||Fig 2:|
In rockets and satellites many pyrodevices are used to initiate operations like separation of stages, deployment of solar panel, ejection of heat shield, ignition of control rockets etc. In some cases, from a single explosive signal multiple explosive inputs may have to be given to activate several systems simultaneously. The manifold is an explosive device used to generate many output from a single input. The input can be a single explosive chord and it ignites the main explosive charge in the manifold and in turn relays the ignition to the required target through the many output explosive chords provided. The given sample is the hardware of a manifold. It has got two inputs (for redundancy) and three outputs, all connected to the central hole. In the actual device the central hole will be filled with explosive charge and the explosive chords will be connected to the input and output ports. Figure 3 is the photograph of the specimen. Table 1 below shows typical parameters of the experimental setup used for scanning the specimen described above.
|Radiation Source:||Caesium -137|
|Activity:||Approx. 220 GBq (6 Curies)|
|Counting time per sampling point:||0.4 to 1.0 s|
|Resolution of the PC based counter:||16 bit binary mode|
|Circle of Reconstruction:||80 mm dia (typical)|
|Scanning step size:||0.5 mm (typical)|
|Number of samples per projection:||161 (typical)|
|Number of equi-spaced angular positions:||100|
|Detector resolution:||1.0x5.0 mm|
|Overall scanning time:||4-6 h depending upon counting time|
The specimen was placed on the scanning platform standing vertically and projection data were obtained for tomographic reconstruction. The specimen was given a vertical displacement of 5 mm for a new plane for scanning. Figure 4 through figure 26 show twenty three such tomographic images obtained for 3D visualisation. As mentioned above, the detector resolution along a normal to the tomographic plane was 5 mm which restricted finer details to be visualised. Major structural details inside the specimen are clearly brought out in these reconstructed images. Figure 27 shows a three dimensional visualization of the specimen which was obtained after processing 2D experimental CITIS data. Figure 28 depicts a typical cut view through the specimen showing major structural details and finally figure 29 was generated which shows an oblique tomographic plane through the object. Figures are not to scale and the grey back plane visible in figures 27, 28 and 29 are due to processing and are not part of the specimen.
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The experiment was carried out as part of our development activities in the field of industrial computed tomographic imaging systems to explore various possibilities of data analysis and processing for presentation of specific information which is inherent in tomographic imaging. By making use of readily available software packages for 2D and 3D data processing, interesting information can be visualized in industrial tomography which would be difficult in conventional radiography.
We are thankful to Shri Gursharan Singh, Head, Isotope Applications Division, Bhabha Atomic Research Centre for his continued support and encouragement. Shri Umesh Kumar is also thankful to Dr. S. Kailash, Nuclear Physics Division, Bhabha Atomic Research Centre for his guidance.
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