![]() ![]() ![]() Thus it is evident that the beam hardening artifact in image (B) was not confined strictly to the edge of the PVC casing, but was a continuous feature within the saprolite as well. Note that although the centers of images (B) and (D) are similar, the edges of the saprolite are brighter in image (B). The bright rim on the left was caused by imperfect centering of the column the image of the saprolite itself, however, has only very minor ring artifacts and no beam hardening. ![]() The scan shown in (D) was done using a self-wedge calibration through a relatively homogeneous portion of the column. Beam-hardening and ring artifacts have been reduced markedly but not totally, and image noise has increased considerably. Image (C) shows the result of pre-filtering the X-ray beam by passing it through 6.35 mm of brass. However, in this case some fractures close to the center have been obscured or altered. If the grayscale fluctuations caused by the rings are smaller than for the features of interest, this approach can be very successful. Image (B) is the result of a software correction of the ring artifacts in (A). The latter is visible most obviously as the bright ring around the outer part of the PVC. Scan (A) shows both ring and beam-hardening artifacts. The scans all represent 1-mm-thick slices collected with the X-ray source at 420 kV and acquisition times of 3 minutes. In objects with roughly circular cross sections this process can cause the edge to appear brighter than the interior, but in irregular objects it is commonly difficult to differentiate between beam hardening artifacts and actual material variations.įigure 5: Scans through a 6-inch-diameter column of saprolite encased in PVC pipe, showing scanning artifacts and the results of various strategies for remedying them. In X-ray CT images of sufficiently attenuating material, this process generally manifests itself as an artificial darkening at the center of long ray paths, and a corresponding brightening near the edges. This also means that, as the beam passes through an object, the effective attenuation coefficient of any material diminishes, thus making short ray paths proportionally more attenuating than long ray paths. The end result is a beam that, though diminished in overall intensity, has a higher average energy than the incident beam (Fig. Because lower-energy X-rays are attenuated more readily than higher-energy X-rays, a polychromatic beam passing through an object preferentially loses the lower-energy parts of its spectrum. The artifact derives its name from its underlying cause: the increase in mean X-ray energy, or “hardening” of the X-ray beam as it passes through the scanned object. The most commonly encountered artifact in CT scanning is beam hardening, which causes the edges of an object to appear brighter than the center, even if the material is the same throughout (Fig. In this section we discuss commonly encountered problems, and some approaches for solving them. Partial-volume effects, if not properly accounted for, can lead to erroneous determinations of feature dimensions and component volume fractions. Scanning artifacts can obscure details of interest, or cause the CT value of a single material to change in different parts of an image. Although the output of computed tomography is visual in nature and thus lends itself to straightforward interpretation, subtle complications can render the data more problematic for quantitative use. ![]()
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