C T ARTIFACTS

     C T  ARTIFACTS

ARTIFACTS

With contemporary CT units, artifacts due to equipment defects are rare. Short scan times have diminished but not eliminated motion artifacts. Artifacts at the edge of high contrast objects may also be seen (we will call these "streak artifacts"). Beamhardening effects are minimized by reconstruction programs, but we need to understand the concept. The one equipment defect that may be seen is a ring artifact caused by failure of a detector

Motion Artifacts

Patient motion has a devastating effect on image quality. This is the primary reason for developing faster scan units, including the 50-f,Lsec unit used to image cardiovascular structures. When the patient moves during scan acquisition, the reconstruction program has no ability to make appropriate corrections because motion is random and unpredictable. The reconstructed image will display an object in motion as a streak in the direction of motion.

Streak Artifacts 

One of the basic assumptions in CT scanning is that each detector, at every position, will observe some transmitted radiation. If a high density material severely reduces the transmission, the detector may record no transmission. This violates the basic assumption, and the reconstruction program will not account for such a violation. Streaks will appear in the image. Even for high density materials that do transmit some x rays) the filter in the reconstruction program is not designed to perfectly reproduce the high density to average density interface. Some CT units expand the CT numbers well above those numbers associated with dense bone in order to assign a linear absorption coefficient to materials that are denser than bone. A 1989 survey indicates that almost all units now have CT numbers that range from at least - 1000 to + 3000, and some have a greater range. In these cases the CT numbers + 1000 to + 3000 are for materials whose densities are greater than dense bone, whose density is about 2 gm cm-3

Beam-Hardening Artifacts

 Unfortunately, the x-ray beam producing the CT image is not a monochromatic beam. The beam contains x rays with many energies, although we speak of the average energy of a CT x-ray beam as being about 70 keV. As a heterogeneous x-ray beam passes through the patient, the low energy protons are rapidly absorbed. This means the x-ray beam exiting the patient contains a lower percentage of energy photons than the beam had when it entered the patient. This effect is called "beam hardening."

Ring Artifact

Ring artifacts are the result of miscalibration of one detector in a rotate-rotate geometry scanner. (Of course, detector failure would also produce a ring artifact.) If a detector is miscalibrated, it will record incorrect data in every projection. This misinformation is reconstructed as a ring in the image, with the radius of the ring determined by the position of the faulty detector in the detector array. Faulty detectors in rotate-fixed units also record false information. However, this information is not visible in the reconstructed image because the faulty detector collects data from many angles (rings may appear in rotate-fixed geometry if the x-ray tube is not aligned correctly). Ring artifacts have virtually disappeared in contemporary CT units.