FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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normally measured with a shim type penetrameter. Radiographie penetrameter
contrast refers to an integrated contrast along the line between source and
film, taking into account all the material along this line, whereas o^ is the
precision with which the n of a specific pixel anywhere within the interior of
the object can be determined. If the difference between the reconstructed ^'s
of two pixels is greater than 2a„ then there is a 95% chance that these two
pixels represent different M-'s in the object. Since in tomographic images any
two pixels or groups of pixels can be compared in this way, a^/y, is equivalent
to a much smaller radiographie contrast, typically two orders of magnitude
smaller.
5. CONCLUSIONS AND DISCUSSION
The tomographic scanning and reconstruction method has been used to detect and
locate a subsurface shrink cavity in the crotch area of a sectioned 5.08 cm
(2") valve casting.
The main body of the cavity has been located in three dimensions with a precision
in each dimension of approximately ±1 mm. The volume of the main body of
the cavity has been estimated to be about 40 mm 3 . The precision with which the
cavity can be located would be of considerable assistance in upgrading the
valve.
The ability to accurately reconstruct linear attenuation coefficients has been
demonstrated. The standard deviation in a given reconstructed linear attenuation
coefficient was determined to be approximately 0.011 cm"* or 3% of iron.
This is typical of CT images and represents an improvement of one to two orders
of magnitude over the contrast obtained with film radiography.
Although the demonstration scans in this study each required approximately 14
hours for data acquisition, the scan time can be greatly reduced by employing
arrays of detectors rather than a single detector. Scan times of 4 seconds or
less are typical of current generation medical CAT scanners. There is no fundamental
reason why CAT scanners intended for industrial purposes cannot achieve
similar scan times.
6. REFERENCES
1. L.M. Zaty, "Basic Principles of Computed Tomography Scanning",
Radiology of the Skull and Brain, Vol. V: Technical Aspects of
Computed Tomography, 1981.
2. D.A. Chesler, R.J. Stephen and N.J. Pele, "Noise Due to Photon Counting
Statistics in computed X-Ray Tomography", Journal of Computer
Assisted Tomography, 1(1), 1977.
3. R.A. Brooks and G.D. Di Chiro, "Statistical Limitation in X-ray
Reconstructive Tomography", Medical Physics, Vol. 3, No. 4, 1976.
4. P. Reimers and J. Goebbels, "New Possibilities of Nondestructive
Evaluation by X-Ray Computed Tomography", Materials Evaluation, 41,
1983.