Computer Tomography Technique Lecture 3 Image Quality Prepared

Computer Tomography Technique Lecture 3: Image Quality Prepared by: Behzad Ommani Master of Medical Engineering Instructor Radiology Group September, 2012

Motion Artifacts Corrective Steps 1. Give clear breathing instructions to the patient and reinforce the importance of holding still. 2. Use positioning aids or immobilization devices. 3. Consider sedation, particularly for pediatric patients. 4. Use shortest scan time possible. 5. For cardiac protocols, consider βblockers. Manifestation Possible Cause Motion Voluntary or involuntary patient motion (shading, streaking, blurring, or ghosting)

Metallic Artifacts Corrective Steps 1. Whenever possible, remove metallic objects from SFOV. 2. Angle gantry. 3. Increase technique, particularly k. Vp. 4. Use thin slices Note : beam hardening, partial volume, edge gradient, and aliasing all contribute to the streaks that result from metal in the SFOV. Manifestation Possible Cause Metallic Objects present that are beyond the dynamic range of the scanner (streaks)

Metallic Artifacts Metal Artifact Reduction software (MAR)

Ring Artifacts Corrective Steps 1. Recalibrate 2. if rings persist, call service Manifestation Possible Cause Ring Detector problem (a single ring or concentric rings)

Out of Filed Artifacts Corrective Steps 1. An SFOV can be selected that is larger than the patient or Target region. 2. Drive out limbs or artifactors Manifestation Possible Cause Out of Filed Anatomy that extends outside of the selected SFOV or incorrect SFOV. (streaking, blurring)

Out of Filed Artifacts

Tube Arcing Artifacts Corrective Steps 1. Gas send out or Vacuum 2. Tube Replacement Manifestation Possible Cause Tube Arcing There is a large difference in electrical potential (streaking)

Aliasing Artifacts Corrective Steps 1. Slowing gantry rotation speed 2. Reducing the helical pitch 3. Increasing scan time Manifestation Possible Cause Aliasing Too few samples (streaking)

Beam Hardening Artifacts

Beam Hardening Artifacts Corrective Steps 1. Filtration 2. Calibration Correction 3. Beam Hardening Correction Software 4. Gantry Tilt ( Bone ) 5. Increase KVP Manifestation Possible Cause Beam Hardening X-ray beams are composed of different energies. (streaking or Cupping)

Calibration Correction Beam Hardening Correction Software

Partial Volume Artifacts Corrective Steps 1. Thinner slices Manifestation Possible Cause Partial Volume Partial volume effect (streaking)

Edge gradient effect Artifacts Corrective Steps 1. Largely unavoidable. 2. Somewhat reduced by thinner slices. 3. Use low or neutral HU-value oral contrast in place of barium. 4. Change the Patient Position. Manifestation Possible Cause Edge gradient effect Angle of x-ray beam varies between two similar views (straight line radiating from high-contrast areas, such as barium adjacent to air).

Spiral interpolation Artifacts Corrective Steps 1. Lower pitch Manifestation Possible Cause Spiral interpolation artifacts (subtle inaccuracies in CT number) Images are created from views that are not all in the same plane