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The capability of X-ray computed tomography (CT) to extract multiple slices from an object and then develop a 3-D-reconstructed volumetric image has proven to be a powerful tool in quality control. The technique can be used in different branches of industry where conventional radiography fails to reveal all the hidden flaws because of the inherent complications of X-ray beam alignment.
Along with the nondestructive testing (NDT) applications of CT techniques, the new trend of reverse engineering is another promising horizon for X-ray CT scanning. While the accurate 3-D-reconstructed images can be used to create point-clouds for design engineering, the ability of CT to reveal interior details of a complicated engineering part also can be valuable for design engineers.
CT techniques have seen tremendous advances in recent years. The reasons include:
Even with all the progress in CT techniques, creating high-quality CT images can be a challenging task in some cases, requiring training and experience for operators to fully acknowledge the effects of all the parameters involved.
One of the most important factors to understand in performing CT is the presence of scattered radiation. Presence of scattered radiation in any type of X-ray image tremendously degrades the quality of the final output and acts as noise in any receiver. It can produce uneven contrast and, by reducing the resolution of the final image, can consequently compromise the detection of some fine details. The problem of scattered radiation in computed tomography is more severe compared to other types of radiographic imaging because they are random in nature and can affect the reconstruction process by creating confusing artifacts.
The best practice to produce high-quality radiographic images is to eliminate-or at least reduce-the amount of scattered or indirect radiation. Scattered radiation is built up from the bouncing of X-ray beams through the surrounding walls of an X-ray cabinet, mechanical manipulator and, more importantly, inside the object. An object having compositions with high physical densities can produce more scattered radiation and it is challenging to eliminate them. There are several practical ways to achieve this goal.
X-ray MagnificationUsing a micro-focus X-ray tube as a source of radiation not only shows better resolution compared to conventional ones, it also has the advantage of reducing scattered radiation by applying high-projection magnification. Increasing the detector-part distance can help prevent the build-up of scattered radiation inside the part. It has been demonstrated that applying the projection magnification more than 4X in most cases can be helpful for this purpose. A larger detector-part distance is more effective in eliminating some of the scattered radiation because it is weaker compared to the direct beam.
Using the Right FiltersUsing higher kilo-voltages in radiography often can reduce the amount of scattering due to the higher penetration power of X-ray beams and the dependency of the attenuation coefficient (µ) on the energy of the X-rays. Using appropriate filters also can filter those parts of X-ray beams that have longer wavelengths, which show a higher tendency to produce scattered radiation. Filtering will shift the spectrum of the X-ray toward the shorter wavelengths and consequently increase the penetration power. This technique, called beam hardening, can be useful in performing CT scanning. Materials such as aluminum, copper and brass are ideal filters. The thickness of filters can be selected based on the thickness and physical density of the part.
A practical way to make an easy filter is by simply cutting small pieces of aluminum cans. If necessary, the piece can then be folded to create different thicknesses and can be attached on the tube window by a piece of tape. A CT technician can be creative in combining pre-filtration and post-filtration options in order to produce clearer images.
Adequate ShieldingShielding different parts of the X-ray cabinet, metallic fixtures or mechanical manipulator with layers of lead can be useful during CT scanning. For example, layers of lead have been used to cover the metallic parts of a mechanical manipulator and its rotation table. Making a simple opening window with lead around the area of interest also can absorb some of the scattered radiation during CT scanning.
In the case of very strong scattered radiation, attaching a thin foil of lead (around 0.001 inch) on the detector can help absorb some of the indirect radiation. This is because, as mentioned previously, the intensity of scattered radiation is much weaker than the direct X-ray beams and can be easily absorbed by that layer of lead. These sorts of lead foils are called intensifying screens and can be found in the packaging of some types of industrial radiographic films, which are considered wastes after use of the films.
Reduce Scattering RadiationThe technique of frame averaging during real-time radiography and particularly CT scanning is an effective method to eliminate some of the random noises and the traces of scattered radiations. The selection of the framing average depends on the frame rate of a detector, system memory and speed of the processor. It should be selected based on the scanning system. Selecting too many frames for averaging can reduce the sampling rate during the scanning and also compromise the final result by risking the elimination of some useful information.
Some algorithms for CT reconstruction have proven to be more effective than others in reducing noises and artifacts, but with the tradeoff of a slower processing time. For example, the high-quality cone-beam geometry reconstruction algorithm in most situations produces much cleaner results compared to Fast Fourier Transfer (FFT) parallel or fan reconstruction ones, but it naturally takes more time to process the data.
It seems that the creation of high-quality and accurate 3-D CT reconstructed images depends on careful attention to many parameters, but the final result is a rewarding and powerful NDT tool. The quality of reconstructed volumetric images allows operators to detect extremely fine details such as a fine number inside the part, which can be missed by an inadequate inspection technique. NDT