Abstract

The computerized industrial tomography is one of the NDT method entirely based on the computer reconstruction technique that offers a very detailed 2D and 3D graphic images of the tested object. After a brief description of a simple laboratory tomograph, we present some important features of our software package, specially developed for tomographic analysis. In the last part, some relevant 2D and 3D tomograms made with our laboratory equipment and some consideration regarding the material identification from tomograms are showed. We use the Filtred Back Projection based algorithm for reconstructing the 2D tomograms and a direct representation of the 3D-matrix method for 3D tomograms.

Introduction

The computerized tomography technique is based on successive attenuation measurements of a narrow X-ray or -ray beam after it passes through a tested object in several directions. By means of an adequate mathematical tomographic algorithm, a map of attenuation coefficients for the scanned surface is reconstructed. In fact, this map of attenuation coefficients is sometimes called a "density map" of the materials from the slice.

Fig 1: General presentation of a simplest tomographic equipment

The laboratory tomograph we built is a 1st generation classical model (fig. 1): it has a gamma ray source and a detector, so that only one measurement is possible at a given time. With this model, the data acquisition time is relatively long, but the simplicity of its design and the low cost were the major selection criteria.

The tomograph operates in the following way. The tested object is placed on the mechanical unit enabling its translation, rotation and motion along Z-axis. The object is then exposed to the collimated beam. Two collimators are placed between the source and the detector. Accurate motion control and data acquisition is accomplished by a microcontroller equipped personal computer. A preamplifier, an amplifier and two-channel analyzers process the data collected from the detector. Data storage and processing, reconstruction algorithms and the final 2D and 3D tomograms display are carried out by specialized program packages running on the PC.

Reconstruction algorithm

Several classes of reconstruction algorithms are available, such as algebraic, iterative, analytical, etc. We have found that one of the faster and accurate algorithms is the Filtered Back Projection (FBP) algorithm, combined with convolution filtering method. Various basic algorithm description, including the FBP, could be found in [1] - [7].

A reconstructed slice, or a 2D tomogram, is usually 0.4 - 3 mm thick which is generally determined by the radiation beam width. The scanned slice is plotted by associating a given color to every "density" value, corresponding to a given distribution of the attenuation coefficients. Thus, a tomogram becomes a "false colors" representation of the scanned slice of the object.

The 3D tomograms or, more exactly, the representation of the successive slice tomograms, is based on the 3D direct matrix representation method, described in [8], [9] and [10].

We have to point some specific advantages of the tomographic reconstruction:

• The technique gives the absolute radiation absorption coefficient for each pixel from the tomogram, which leads to high accuracy detection of small variations (faults and flows) in the investigated object;
• The dimension of the flow is absolute (not projection or equivalent dimension), blurred only by scattering effects, and a 3D tomogram shows the real shape and dimension of object's constituents (absolute maximum length of a flaw, for example).

Software package

An NDT tomograph procedure includes motion control, data acquisition and storage, tomographic reconstruction, fault detection analysis of 2D and 3D tomograms and report generation. The program package providing all these facilities was developed in our laboratory [11], [12] and includes the following features:

• Displacement control and data acquisition, according to the programmed parameters, with facilities regarding the optimization of the scanned area;
• Reconstruction of the 2D tomogram using the FBP algorithm with various filters. Several reconstruction options are implemented, each of them being specific for same class of objects, thus giving good results regardless of the object nature.
• Display of a 2D and/or 3D tomogram and a histogram of the attenuation coefficient. Several operations are available, such as: dimensions and attenuation coefficients measurements, color representation, rotation, magnification, lighting, wire-frame plotting, 3D filtering, linear/spline interpolation, cutting, semi-transparency display, surface and volume measurements for internal objects, selection and removal of internal objects, animation, etc.;
• 2D and 3D fault detection analysis and as a result, height, area, volume, equivalent density measurement and report generation for those measurements;
• Determination of materials of which the investigated object is made, using available data in the database, as well as introducing data related to new materials into the data base;

Laboratory Tomograph

The industrial tomography apparatus TOMORAY-01 (fig.2) was built in 1992. It utilizes the mono-detector geometry and a 30 Ci¹⁹²Ir radiation source. The following are its technical characteristics:

Fig 2: TOMORAY Tomograph

• Size: 1240x940x1480 mm
• Weight: 800 kg;
• Radiation source:¹⁹²Ir / 30 Ci;
• Maximum size of investigated object: 250x250x250 mm, Weight : 50 Kg;
• Maximum attenuation thickness (Fe equiv.): 100 mm;
• Average (2D) data acquisition and processing: 20 - 200 min;
• Maximum resolution: 0.4 mm;
• High radiological security.

2D Tomograms

In order to illustrate the industrial application of the computerized tomography (see the general presentation from [13] and [14]) we present in following a part of the 2D (fig. 3 to fig.6) and 3D tomograms (fig.7 and fig.8) made with the TOMORAY-1 apparatus. More details are presented in [15], [16] and [17].

Fig 3: Cross-section of a 90 mm diameter aluminum rod cage rotor from an asynchronous electric motor

Diameter of reconstruction: 110 mm; scanning step : 0.4 mm; number of projections : 60

Large faults consisting in air bubbles resulting form casting and some high-density material segregation possibly as a result of differences in cooling, may be seen .

Fig. 4: A cross section of an encapsulated varistor. Length of reconstruction area: 85 mm Scanning step : 0.4 mm Number of projections : 60 The size of the capped varistor and the bolt screw terminals may be clearly observed in the cross section.

Fig 5: A cross section of a high sensitivity Hall transducer. Length of reconstruction area: 98 mm Scanning step : 0.4 mm Number of projections : 60 Several coils and some ferromagnetic material showing very well shaped geometry, may be observed.

Fig 6: A cross section of a pine tree trunk. Diameter of reconstruction: 160 mm; Projections: 180; Scanning step: 0.4 mm. The annual rings of the tree and an internal crack can be clearly observed.

3D Tomograms

Fig 7: A roman statue from the National Museum of Romanian History. The statue, 12 cm high and made of bronze, was reconstructed using 167 successive longitudinal planes. The resolution was 0.4 mm Scanning step : 0.4 mm Number of projection : 60 3D-array size : 6.2 MB

Fig 8: 3D plot and analysis of a circular weld of a 44 mm diameter 4 mm thick steel pipe; Number of planes: 25; Scanning step : 0.4 mm; Number of projections : 90

Fig 9: A tomogram and histogram of a specimen containing different materials, such as: silicon, steel, aluminum, carbon, copper, ceramic, bronze, molybdenum and different mixtures

The most interesting and spectacular results of tomography reconstruction technique are the plots and analyses of 3D scanned objects. The method consists of successive slice scans and the reconstruction of the object by means of an original reconstruction algorithm, named 3D array direct plotting ([10]and [11]). This means that 3D scanning of an object results in an array (3D matrix) containing voxels (elementary volumes) that describe every internal or external detail of the object, so that any interesting surface or view of the object can be plotted subsequently. In order words, the object with its every internal or external detail is stored in a 3D array and may be fully displayed using any view angle or lighting source, may be partially or fully cut through. Some materials or any part of the object may be removed, magnified or represented in a semitransparent manner. Using this procedure, the object is 3D fully digitized and stored.

Reconstruction details are remarkable, so that internal and even external analysis may be directly carried out using computer's display. During analysis, various interesting internal details, obtained by adequate choice of plotted positions and surfaces, were pointed out. Cutting performed through the statue, in some interesting areas, showed hidden internal details. Its middle area was studied in order to check the hypothesis that the statue was made by joining or by soldering two pieces. The hypothesis was contradicted by 3D tomographic analysis as may be observed in the illustrated images. The 3D analysis of the statue also pointed to another interesting detail: the left leg was apparently broken and repaired by gluing the pieces together. Using the data processing program, the gluing material was removed, and an image of a perfectly circular hole was reconstructed. The hole was drilled with a 2 mm spiral drill, possibly with the intention of inserting a reinforcement bolt.

The high resolution of the 3D plot is pointed out by weld scales and drops. A "software" cutting method may be used to observe internal caverns caused by the welding process, as well as one high-density material inclusion, which possibly came from the welding electrode. The investigation went on, by choosing the adequate plan to measure the maximum size of the gap occurring in the most significant section.

Materials identification

Another direction we work on was the development of an accurate algorithm for tomographic identification of constituent materials. Two methods were investigated. The first method involves the attenuation coefficient value correction [19], in order to have a linear response across the full range of the material attenuation coefficients. Using this compensation method we are able to identify materials, sufficiently represented in a tomogram, with less then 2% accuracy. An example is shown in fig. 9 where a test specimen composed of many materials, such as silicon, steel, aluminum, ceramic, carbon, copper, bronze, molybdenum and other composite materials, was reconstructed. The histogram (placed on the upper right side) shows the distribution of the materials of the tomogram and has a very clear separation between the corresponding peaks.

We developed also a more accurate method for materials identification for our equipment, similar to the dual energy method, which is based on the data correlation from two tomograms made at different energy. This algorithm, described in [19] and [20] is able to identify the density and Zeff (effective atomic number) values for the materials contained in tomograms with 3-5 % accuracy.

Conclusions

After many experiments in the tomography domain, despite of high equipment costs, it appears that for every class of objects to be studied by Computerized Tomography, there is an adequate configuration leading to optimum affordable cost/performance ratio.

The Computerized Tomography results obtained until present have been well received by the NDT specialists, who expressed their interest in testing this method in various industrial applications.

Bibliography

1. G.N. RAMACHANDRAN & A.V. LAKSHMINARAYANAN , "Three-dimensional Reconstruction from Radiographs and Electron Micrographs Application of Convolutions instead of Fourrier Transforms ", Proc.Nat.Acad.Sci.USA , 1971
2. R. GORDON , "A Tutorial on ART ", IEEE Transaction on Nuclear Science , 1974
3. L.A. SHEPP & B.P. LOGAN , "The Fourier Reconstruction of a Head Section ", IEEE Trans. on N. Sc. , 1974
4. T.F.BUDINGER & G.T.GULLBERG , "Three-dimensional Reconstruction in Nuclear Medicine Emission Imaging ", IEEE Transaction on Nuclear Science , 1974
5. Z.H. CHO , "General Views on 3-D Image Reconstruction and Computerized Transvers Axial Tomography ", IEEE Transaction on Nuclear Science , 1974
6. R.A. BROOKS & G. DI CHIRO , "Principles of Computer Assisted Tomography ( CAT ) in Radiographic and Radioisotopic Imaging ", Phys.Med.Biol. , 1976
7. Y.S. KWOH , I.S. REED & T.K. TRUONG , "Back Projection Speed Improvement for 3-D Reconstruction ", IEEE Transaction on Nuclear Science , 1977
8. M.IOVEA, A.MARINESCU, P.CHITESCU,T. SAVA - " Three - Dimensional Method Of Representation In Industrial Computerized Tomography"- The 6-th Joint PS-APS International Conference on Physics Computing, Lugano, Switzerland, august 22-26 1994:
9. M.IOVEA, A.MARINESCU, P.CHITESCU, T. SAVA - "SOME ASPECTS CONCERNING THE STAGE OF APPLYING THE COMPUTERIZED INDUSTRIAL TOMOGRAPHY IN ROMANIA", International Conference on Industrial Computerized Tomography, Berlin, 8-10 June 1994.
10. M.IOVEA, A.MARINESCU, P.CHITESCU, C.RIZESCU, GH.GEORGESCU " TRIDI MENSIONAL ANALYSIS BY COMPUTED TOMOGRAPHY" National Physics Conference, Baia Mare, Romania, Nov.-Dec. 30-02, 1995 [abs., pg. 54]
11. M.IOVEA, A.MARINESCU, P.CHITESCU, C.RIZESCU, GH.GEORGESCU - "GRAPHIC ENVIRONMENT FOR INDUSTRIAL COMPUTERIZED TOMOGRAPHY" - The Fourth International Conference in Central Europe on Computer Graphics and Visualization'96, University of West Bohemia, Plzen, Czech Republic, February 12-16, 1996 [pg. 267-268];
12. M.IOVEA, A.MARINESCU, C.RIZESCU, GH. GEORGESCU - "AN OBJECT ORIENTED MODELLING SYSTEM FOR INTERACTION SIMULATION IN 3D INDUSTRIAL TOMOGRAPHY" - Proceedings of the 19th International Conference on INFOR-MATION TECHNOLOGY INTERFACES, Pula,Croatia, 17-20 June 1997, [pg. 457-462]
13. P.REIMERS , J. GOEBBELS , H.P.WEISSE & K.WILDING , "SOME ASPECTS OF INDUSTRIAL NON-DISTRUCTIVE EVALUATION BY X-RAY AND GAMMA-RAY COMPUTED TOMOGRAPHY", Nuclear Instruments and Method in Physics Research , 1984
14. P.REIMERS , W.B. GILBOY and J.GOEBBELS , " RECENT DEVELEPMENTS IN THE INDUSTRIAL COMPUTERIZED TOMOGRAPHY WITH IONIZING RADIATION" NDT Intern. 1984
15. M.IOVEA, A.MARINESCU, C.RIZESCU, GH. GEORGESCU - "USING COMPUTER-IZED TOMOGRAPHY IN NON-DESTRUCTIVE TESTING OF THE SOLID MATERIALS" - The Annual Symposium of the Institute of Solid Mechanics, Romanian Academy, Bucharest, Romania, December 15-16, 1994 [pg. 357-364]
16. M.IOVEA, C.RIZESCU, A.MARINESCU, GH. GEORGESCU - "SOME ASPECTS OF COMPUTERIZED TECHNOLOGIES BY USING GAMMA-RAY COMPUTED TOMOGAPHY AND ULTRASONIC IMAGING IN NDT", The 3rd edition of the National Symposium of the Romanian Association of Non-Destructive Testing, Mamaia, Romania, May 29-31, 1996 [pg. 104-111]
17. M.IOVEA, C.RIZESCU, A.MARINESCU, GH. GEORGESCU - "NON DESTRUCTIVE TESTS OF PIPES USING INDUSTRIAL COMPUTERIZED TOMOGRAPHY", National Conference of Energy, Neptun-Olimp, 1-5 September 1996 [pg. 201-205]
18. M.IOVEA, C.RIZESCU, GH. GEORGESCU - "NON-DESTRUCTIVE EVALUATION OF ARCHAEOLOGICAL AND WORK OF ART OBJECTS BY USING COMPUTERIZED TOMOGRAPHY", International Restoration and Conservation Session", Satu-Mare , 19-23 October 1997 [pg. 26-32]
19. M.IOVEA, C.RIZESCU, A.MARINESCU, C. RIZESCU, GH. GEORGESCU. I.DUMITRESCU - "A DIRECT GAMMA RAY CALIBRATION METHOD FOR DENSITY MEASUREAMENT WITH APPLICATIONS IN INDUSTRIAL TOMOGRAPHY ", 3rd General Conference of the Balkan Physical Union, Cluj-Napoca , 2-5 September1997, [pg. 466]
20. M.IOVEA, C.RIZESCU, A.MARINESCU, GH. GEORGESCU - "ADVANCED NON-DESTRUCTIVE TECHNIQUES USING GAMMA RAY COMPUTERIZED TOMOGRAPHY - 3rd General Conference of the Balkan Physical Union, Cluj-Napoca , 2-5 September 1997, [pg. 476]