Evaluation of artifact in computed tomography

In CT image, the final image is formed only after the rather long, tortuous, and subtle process of reconstruction has reconstruction has taken place. The reconstruction algorithm makes use of all x-ray measurements in reconstruction between the patient's anatomy and machine. Therefore, CT image artifacts will be classified as to following. 1. Geometric artifacts are errors due to situations in which not all ray measurements occurred along lines with the proper orientation in space. Aliasing is an inherent problem due to an insufficient number of ray samples per beam with in the x-ray plane. In T/R and R/S scanner, it can be greatly diminished by increasing the sample rate, which may necessitate a slower scanning speed. It is only secondarily affected by the use of a smoothed reconstruction filter. Insufficient angular sampling can be reduced or eliminated by use of a smoothed reconstruction. Edge gradient streaks artifacts are an inherent problem due to the finite width of the scanning beam. This can be reduced by the use of a harder (higher energy) photon spectrum. Geometric misalignment artifacts are due to some kind of mechanical misalignment in the detectors or x-ray source. This can be corrected by either mechanical alignment or software correction. Partial volume effect can be mitigrated by overscanning or reduction of slice width. Motion artifacts can be greatly reduced by overscanning, short scan time, anesthesia and window setting control. 2. Algorithm distortions and artifacts are those artifacts in the image caused by the form of reconstruction algorithm used. Point spread effect can be reduced only by improving spatial resolution, and the intelligent manipulation of the window level and width viewer controls. 3. Attenuation measurement artifacts are caused by any malfunction of the radiation detectors that cause an inaccurate reading of the x-ray attenuation passing through the patient. Nonlinear attenuation errors can be partially overcome by use of a linearizing function in the software. 4. Photon spectrum effect artifacts are errors caused by alterations that occur in the x-ray energy spectrum as the beam passes through the patient. This effect is due to certain fundamental law of x-ray physics rather than any imperfection of equipment manufacture. Beam-hardening artifacts are caused by the inherent change induced in the x-ray energy spectrum. Harding in water is easily correctable in software. Hardening in bone can be eliminated with post reconstruction correction algorithms