NDTnet 1998 Aug, Vol.3 No.8

Abstract

The aim of this development is to provide a system able to display in real time a reconstructed image using a tomosynthesis method [1]. This technique has been shown to be able to detect defects and delaminations less than 0.5 mm between the propellant and the peripheral metal enveloppe on 3 meters diameter rocket boosters [2]. The object is moved on a known trajectory between the high energy X-Ray source and a detector based on Gd₂O₂S screen and a SIT camera. The geometric parameters are estimated in a calibration task [3].
The implementation of the tomosynthesis algorithm described in [4] is based on a set of look up tables which give for each position of the object, the projection of a given point of the object on the detector. Several hundreds of frames may be combined to compute the tomosynthesis image. This requires a specific video signal digitizer, a large disk space and a great computation time on a workstation.

In order to speed up the tomosynthesis reconstruction and to reach the video frame rate, we integrated a specialized architecture in a PC host. The geometric calibration parameters and the look up tables are pre-computed. The on-line tomosynthesis calculation is carried out by a set of 3 DSP's working in parallel in order to generate an output video signal. The first processor manages the real time 512 x 512 images acquisition and the output display. The two other processors handle a part of the digitized signal to perform the reconstruction. To guarantee a significant signal/noise ratio improvement, we fix at 36 the number of correlated frames. The object is moved at less than 1 degree/sec this allows to correlate 6 frames by a simple frame averaging without fuzzy distortion. Then in approximately 200ms, 6 packs of cumulated images are correlated by the tomosynthesis process.
This work is a first step for a final system development and some improvements are expected mainly in the field of automatic defect detection and the use of a priori information.

REFERENCES
1. DG Grant,"Tomosynthesis: a three- dimensional imaging technique " IEEE trans. on biomed. engineering, vol BME-19, n°1,(January 1972), pp20-28.
2. Ph. Rizo, M. Antonakios, P. Lamarque, "Solid rocket motor nondestructive examination using tomosynthesis methods", 14'h WCNDT, New Delhi, India, 1996.
3. Ph. Rizo, P. Grangeat, R. Guillemaud, "Geometric Calibration method for multiple-head cone beam SPECT system. " IEEE Transaction on nuclear science, (Oct. 1994).
4. Ph. Rizo, M. Antonakios, P. Lamarque, "Tangential radiography detectability improvement by tomosynthesis technique", QNDE 1994, Snowmass Co., USA, 1994.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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