ARTIFACTS IN COMPUTED TOMOGRAPHY - 20241007_130237

ARTIFACTS IN COMPUTED TOMOGRAPHY

Definition of Artifact

  • An artifact is any discrepancy between the CT numbers in the image and the expected CT number based on the linear attenuation coefficient.

Appearance of Artifacts

  • Common appearances of artifacts in CT images include:

    • Streaks

    • Shading

    • Bands

    • Rings

Sources of Artifacts

  • Artifacts can originate from:

    1. The Patient

    2. Imaging Process

    3. Equipment

Patient Induced Artifacts

  • Types of patient-induced artifacts include:

    • Motion: Caused by voluntary or involuntary movements, leading to a "ghosting" effect where the CT image shows superimposed images.

    • Beam Hardening: Occurs when higher energy photons are less attenuated by tissue, resulting in artifacts, often described as "cupping."

    • Metal Artifact: Caused by metallic objects affecting photon absorption, leading to incomplete profiles in images.

    • Out-of-Field Artifact: When the patient's body obstructs detectors, leading to streaks and shading, especially if the patient is not entirely within the scanning field of view.

Motion Artifact

  • Causes:

    • Voluntary Motion: Patient movements that can be minimized by good communication and understanding of the procedure.

    • Involuntary Motion: Movement that happens without the patient's control, mitigated by shortening the scan time.

  • Effects:

    • Produces a "ghosting" effect in the CT images where outlines appear duplicated or blurred.

Beam Hardening

  • Defined as the increase in the average energy of the x-ray beam passing through tissue.

  • High energy photons pass through more easily and reach the detectors causing artifacts.

  • Artifacts are more pronounced in the center of the image, hence the term "cupping."

  • Common areas of manifestation include:

    • Skull (petrous-pyramids)

    • Upper chest and shoulders

    • Hips

  • Remedies for Beam Hardening:

    • Increase kVp

    • Decrease slice thickness

    • Increase filtration using bowtie filters

Metal Artifact

  • Appearance: Manifest as "star streaking" artifacts due to metallic objects inside or outside the patient.

  • Remedies:

    • Removal of external metallic objects

    • Use MAR (metal artifact reduction) software

    • Adjusting gantry angulation

Out of Field Artifact

  • Definition: Occurs when parts of the patient are outside the scanning field of view (SFOV).

  • Effects: Believed to result in streaks and shading in images.

  • Remedies:

    • Select a larger SFOV

    • Tape patient tissue out of the way

    • Position patient arms above their head when scanning the chest and abdomen.

Equipment and Imaging Process Artifacts

  • Other sources can include the following:

    • Partial Volume Effect: Arises when dense objects protrude into the x-ray beam affecting image quality.

      • Remedies:

        • Using thin slice selection during scanning.

    • Photon Starvation: Lead to noise in images where CT numbers fluctuate unrealistically in uniform materials.

      • Remedies:

        • Increase kVp, mAs, or slice thickness to reduce noise.

    • Ring Artifact: Caused by detector malfunction in third-generation scanners.

      • Remedies:

        • Calibration or replacement of detectors.

    • Tube Arcing: Results from tungsten vapor affecting electron collisions, producing a crackling sound.

      • Remedies:

        • Replacement and gas burn-off of the tube.

Summary of Artifact Appearance Origins

  • Artifacts can manifest as:

    • Streaks (Physics)

    • Shading (Beam Hardening)

    • Rings (Detector issues)

    • Partial Volume Effects (Scanning technique)

    • Motion (Patient movement)

    • Tube Arcing (Equipment malfunction)

    • Out of Field & Other Effects (Imaging conditions)