FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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11.3 Computer Management of Image Information-* »^
Radiographie images provide information beyond the human range of perception.
The digital values assigned to each pixel in terms of location and intensity
are combined and stored in matrix array as one complete image.
Enhancement is never a substitute for good x-ray technique. Maximizing the
signal (primary radiation) and minimizing noise (scattered radiation) are
necessary; a high signal to noise ratio (S/N ratio) is desirable. Therefore,
an unvarying technique that keeps scatter and geometrical unsharpness to a
minimum is critical to obtaining a qualified radiographie image. In butt
weld radiography of pipelines, the technique is held constant throughout the
job.
Once the raw digitized image is stored in the computer memory, the computer
can begin to enhance it. The S/N ratio of the raw image depends not only or,
the technique, but also depends on the efficiency of the detector.
Brightness transfer functions can be applied to images so that different
results occur. For example, if the intensity variation (film density
variation) across an image falls within some limited narrow range, then that
range can be expanded to encompass the human range of perception. The
result is an image with broader intensity (density) range. Even though real
time radiography provides positive images, the field can be reversed to give
negative images which the film interpreter is accustomed to seeing. By
relating intensity values to quantity of penetrating energies, the computer
can calculate the thickness properties of the inspected piece.
Sometimes the density variation across an image is variable. In the case of
radiographing a pipe section using a film set flat against the pipe rather
than rounded about the pipe, the edges of the developed film appear much
lighter than the center. This is referred to as blowout. And, though image
detectors are analogous to flat film, the computer can assign a function to
the variation of this intensity across the image and, with enhancement,
provide a consistent intensity across the image. This process is called
gradient removal. A shortcoming of gradient removal might be that more of
the image appears to be qualified because of the consistent density.
Histograms relating the thickness of the weld to the distance along a line
drawn across the weld can be drawn by the computer. Any discontinuities can
be measured for length and width. By changing slightly the orientation of
the source on the weld, it is also possible to locate and determine
discontinuities in the third dimension. With such capabilities, three
dimensional topographical radiographie images of a weldment can result.
Computers excel at organizing volumes of information, so records management
is greatly facilitated with real time radiography. The computer can record
information such as operator identification and operating functions
performed. The traceability of welds is made easy by the ability of these
systems to record joint identification, location, and discontinuity data
directly on the imaging media. A variety of media can be used to store
digitized data. These include videotape, magnetic discs, and laser discs.