CT IMAGE ARTIFACTS

• Artifacts are defined as anything appearing on the image that is not present in the object scanned.
• They can be broadly classified as physics-based (resulting from the physical processes associated with data acquisition), patient-based, or equipment-induced.
• Artifacts can seriously degrade the quality of CT images.
• Recognizing various artifacts and understanding why they occur and how they can be prevented or reduced is an important aspect of image quality assurance.

Ring Artifact

• They are caused by imperfect detector elements either faulty or simply out of calibration.
• The problem must be reported to a service engineer for repair.

Beam Hardening Artifact

• The appearance of dark bands or streaks between dense objects in the image.
• This occurs because the portion of the beam that passes through one of the objects at certain tube positions is hardened less than when it passes through both objects at other tube positions.
• Streak artifacts at the interface between high-density material surrounded by low-density material System unable to process high spatial frequency.

Partial Volume Artifact

• Partial volume artifacts arise when a voxel contains many types of tissue. It will produce a CT number as an average of all types of tissue.
• This is the source of the partial volume effect and will appear as bands and streaks.
• Using thinner slice and some computer algorithms can reduce partial volume artifacts

Edge Gradient Effect

• A common clinical example is artifacts that result when barium and air lie adjacent to each other in the stomach.
• Artifacts from the edge gradient effect are largely unavoidable but are somewhat reduced by thinner slices.
• Using a low HU-value oral contrast, such as water (a neutral HU contrast agent) in place of a barium suspension can eliminate the streak artifacts from the gastrointestinal tract

Metallic Artifacts

• Metal objects in the SFOV will create streak artifacts.
• The best way to reduce metallic artifacts is to minimize the metal present in the SFOV.
• Patients are asked to take off any removable metal objects such as jewelry before scanning begins

Out-of-Field Artifacts

• Out-of-field artifacts appear as streaks and shading on the image. And caused by anatomy that extends outside of the selected SFOV.
• If the patient arms are outside the scan field they will not be seen on the image, but their presence can lead to severe artifacts in the image. In addition, obese patients may obstruct the reference detectors, further contributing to image artifacts.
• Out-of-field artifacts can be avoided when an SFOV can be selected that is larger than the patient.
• Encouraging patients to raise their arms out of the way of the SFOV will also avoid artifact

MOTION ARTIFACT

• Artifacts from patient motion typically appear as shading, ghosting (objects appear to have a shadow), streaking or blurring.
• If a patient or structure moves as the gantry rotates the object will be detected as being in several positions and represented in the image as such artifact

Motion artifact can be caused by:

Patient swallowing, Breathing, Pulsatility of heart and vessels, Patient moving

Cardiac Motion artifact: Image reconstruction window - best phase selection

SOLUTIONS OF MOTION ARTIFACT

• Shorten scan time.
• ECG gating: this can be used prospectively to trigger image acquisition during a specific point on the ECG when heart motion is lowest, or retrospectively by reconstructing acquired data from a specific ECG phase.
• Breath hold.
• Ensure comfortable patient position.
• Tell the patient to stay still and give clear instructions.

RegardingArrhythmia artifact (Extrasystole)

Modern scanners allow limited ECG editing of these artifacts, e.g. by deleting the information of a single irregular beat, often with significant improvement in image quality

In conclusion

• Many types of artifacts exist in CT.
• More than one type of artifact may play a role in a specific image.
• All artifacts degrade the image quality.
• It is helpful to be able to identify the types and causes of artifacts.