Artifact

X-ray CT scan is one of the methods of NDI (Non Destructive Inspection). CT scan can inspect the internal structure that can not be seen from outside without the need to destroy the sample object.

However, there are some problem with CT scan and artifact is one of the typical problem with CT scan. Artifact is a phenomenon that actually does not exist on the sample and can be seen on the result of CT image reconstruction. Therefor it highly interferes the observation.

Artifact is appeared with the reason of the structures and materials of hardware that when it operates, and also the ability of CT image reconstruction software. It has various types and strength.

The types of artifact are:

Ring artifact

Ring artifact is a type of artifact as well as a noise in CT reconstruction images that has circle shadows.

Ring artifact occur when the sensitivity correction of X-ray detection element has insufficient variation. Especially, ring artifacts occur when a weak X-ray is used. It is needed to pay attentions when the material of the sample can not be accepted strong energy of X-ray during the CT scan.

As a countermeasure, in the CT scan condition, it is effective to use the X-ray of which the energy is little stronger. It is better if you do the CT scan of the samples with ring structures or patterns like a tree rings, place it at a shifted position from the center of the rotating table, so that it can be easily identified the difference between actual patterns of the sample and ring artifacts.

Using CT image reconstruction software algorithm to reduce the ring artifact is also the solution of solving this problem. Particularly, the strong ring artifact must use software solutions. In this case, you need to use CT image reconstruction software with high capabilities of artifact reduction tools.

Our CT reconstruction software: TomoShop® has the tools that will strongly reduce ring artifacts.

☞ Go to “Ring artifact reduction“ page.

Vertical artifact

Vertical artifact will occur in the same cause of ring artifact on the theory. Vertical artifacts can be seen in the Z direction (XZ, YZ) of the image of slice images that CT image reconstruction has been completed.

Especially, Vertical artifact occur when a weak X-ray is used. It need paying attention when the material of the sample can not be accepted strong energy of X-ray during the CT scan.

As a countermeasure, in the CT scan condition, it is effective to use the X-ray of which the energy is little stronger.

Using CT image reconstruction software algorithm to reduce the vertical artifact is also the solution of solving this problem. Particularly, the strong ring artifact must use software solutions. In this case, you need to use CT image reconstruction program with high capabilities of artifact reduction tools.

Our CT reconstruction software: TomoShop® has the functions that will strongly reduce the vertical artifact.

☞ Go to “Vertical artifact reduction” page.

Metal artifact

Metal artifact occur if metals exist in the sample object of CT scan.

As a cause, the materials such as metals have a characteristic of absorbing high dose of X-ray, that occur beam-hardening phenomenon. If X-ray is absorbed and does not transmitted through the parts of sample objects, it could not obtain the shape information of those parts, and it creates shadows and whiter shapes like lights scatter radically around parts of objects on the slice images.

As a countermeasure, it needs to do beam-hardening correction to reduce and control beam-hardening phenomenon by using CT reconstruction software.

As note of the case of performing the CT scan, use high powered X-ray. There is also a method of performing a CT scan using a filter of some metal filter such as copper, aluminum., etc.

Our CT reconstruction software: TomoShop has strong beam-hardening correction functions that reduce strongly metal artifacts.

Cupping artifact (Cupping effect, Beam-hardening)

Cupping artifact (beam-hardening, cupping effect) is a phenomenon in output CT images caused by beam hardening problem. It is a phenomenon that the CT value in the center part falls from the outer side when scanning an object showing a uniform CT value originally like a water phantom. It is a troublesome problem which is said to be the fate of CT scan.

The cause is the beam hardening phenomenon caused by linear hardening of continuous X-rays (X-rays with continuous wavelength) and polychromaticity which is the characteristic of X-rays.

The following problems can be raised as a problem caused by the cupping artifact (beam-hardening) phenomenon.

• Detailed observation of the shape / nest / foreign matter of the white part is impossible. Also dark shadow areas similarly cause problems of observational uncertainty.
• Shape problems after reconstruction of slice images.
• Three dimensional analysis · Measurement and other problems arise due to the uncertainty of slice image shape. An accurate measured value can not be obtained.
• Other

Motion artifact

Motion artifacts caused for some reasons which are:

1. The correction of the rotation trajectory error of the stage (horizontal direction, vertical direction, camera mounting) is not sufficient.
2. The mechanical accuracy and adjustment of the rotating trajectory of the stage is not sufficient.
3. There is an unexpected movement of the sample that occurs during CT imaging.
4. Unexpected vibrations (horizontal and vertical) that occur during CT imaging occur randomly.

In particular, “4. Unexpected vibrations (horizontal and vertical) that occur during CT imaging occur randomly.” is a serious problem that bothers CT users.

☞ Click here to see the solution using CT software: TomoShop®.

Streak artifact

Streak artifact is considered that image noise generated due to lack of information gathering comes out in a certain direction.

Streak artifact is often seen in the case when the X-ray exporsure dose is considerably low level. In this case, you should increase the X-ray irreciation ammount.

On the other hand, it may be caused by beakdown or inadvertent of the camera (FPD, detector). In correction of some image data occurs in the image data collected by the failure and streak artifact occurs. In this case, inspection, repair and replacement of the camera (FPD, detector), DAS are required.

Even when the number of CT scan images is significantly small, streak artifacts are generated in  the image after reconstruction has made. In this case, you should increase the number of shots of CT scan. (Lower slice thickness)

There are also measures to reduce and correct streak artifact by using the noise reduction functions of CT imaging software.

 

TomoShop® (CT reconstruction software) has strong tools to reduce and correct cupping artifact in CT images.

☞　Please see the page of artifact reduction.

and so on.