Computed Tomography Artifacts

In computed tomography (CT), the term artifact is applied to any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object. CT images are inherently more prone to artifacts than conventional radiographs because the image is reconstructed from something on the order of a million independent detector measurements. The reconstruction technique assumes that all these measurements are consistent, so any error of measurement will usually reflect itself as an error in the reconstructed image.

Streak artifact:

Streaks are often seen around materials that block most X-rays, such as metal or bone. These streaks can be caused by undersampling, photon starvation, motion, beam hardening, or scatter. This type of artifact commonly occurs in the posterior fossa of the brain, or if there are metal implants.The streaks can be reduced using newer reconstruction techniques.

The streak artifacts emanating from the implant.

Partial volume effect:

This appears as "blurring" over sharp edges. It is due to the scanner being unable to differentiate between a small amount of high-density material (e.g., bone) and a larger amount of lower density (e.g., cartilage). The processor tries to average out the two densities or structures, and information is lost. This can be partially overcome by scanning using thinner slices.

Diagram showing partial volume effect.

Motion artifact:

This is seen as blurring and/or streaking, which is caused by movement of the object being imaged. Motion blurring might be reduced using a new technique called incompressible flow tomography (IFT).

CT brain displaying a motion artifact.

Beam hardening:

This can give a "cupped appearance". It occurs when there is more attenuation in the center of the object than around the edge. This is easily corrected by filtration and software.

CT brain displaying a beam hardening artifact.

Windmill:

Streaking appearances can occur when the detectors intersect the reconstruction plane. This can be reduced with filters or a reduction in pitch.

CT image of a 12-mm-diameter acrylic sphere supported in air, obtained with 0.6-mm section acquisition and beam pitch of 1.75, shows windmill artifact.

Picture of a windmill.