Artifacts

Circular ring artifacts

Circular ring artifacts are image interferences caused by detector inhomogeneities that appear in the reconstruction as circular structures. By means of a good light/dark adjustment of the detector these effects can be reduced. This correction offers an optimum possibility to eliminate circular ring artifacts completely.

Readout artifacts

Readout artifacts are originated by the readout electronics of TFT detectors. They already manifest themselves in the projections by erroneous gray value distributions, partially with gray value skips. The resulting error becomes critical in case of strong contrasts caused by highly absorbing materials resp. very large radiation longitudes. This results in an erroneous reconstruction of the absorption coefficients. After the characterization of the readout behavior of the detector this effect can be corrected from the tomography data sets. The parameterization of the artifact correction can be provided for a multitude of X-ray detectors without additional expense.

Double contours

A wrong assumption regarding the center of rotation during the image data acquisition entails artifacts in terms of double contours in the reconstruction. A mechanical calibration of the center of rotation often is not precise enough in case of high-resolution measurements. The center of rotation can be determined subsequently on the basis of the projection data. Thus double contours in the reconstructed volume are avoided.

Noise

Electronic noise or direct detector hits cause spontaneously blinking pixels (so-called impulse noise). In the best case those entail circular ring artifacts in the reconstruction, in the worst case they lead to a complete information loss of individual layers. The correction preserves structures and additionally offers the possibility to correct noise appearing in small clusters and lines.

Image retention artifacts

Retained images are luminescence artifacts of preceding radiographic images in the scintillator layer of indirectly converting detectors. Structures of the preceding projections are visible in the actual image and deteriorate the reconstruction result. Especially in the throughput-optimized production quality control, in which it is necessary to reduce measurement times, this effect can entail high pseudo-fault rates and unnecessarily high false rejects. The parameterization of the correction is realized by means of a characterization of the luminescence behavior of the detector.

Detector scattering

An unwanted signal spread is caused by detector-inherent scattering in the sensor material and in the electronics. Moreover backscattering takes place due to elements behind the detection layer which additionally broadens the signal. After the characterization of this internal scattering in dependence of the spectrum this artifact can be removed from the projection data sets. The corrected data show a higher image definition.

Object scattering

Besides system-specific disturbance impacts various physical effects also reduce the image quality of CT reconstructions. The reason for this is not only object scattering, but also mainly the beam hardening effect. This effect is caused by the polychromatic nature of the applied X-radiation. The artifacts are manifested by blurriness, loss of contrast and distortions.

Focal spot expansion

X-ray sources exhibit a certain expansion that leads to blurriness in the image depending on the geometrical magnification. This results in blurred structures especially in strongly magnifying images. Assumptions about the shape of the focal spot and the distribution of its intensity permit the correction of the geometrical blurriness.

Moreover, structures and edges can be enhanced and extracted for image evaluation by means of a tool especially developed for this purpose. Also segmentations and threshold binarizations can be done. In addition our algorithms and software packages include logical image combinations and offer morphological operations.