3.11 Artifact Recognition and Management

Artifact Recognition and Management

An artifact is any feature in an MR image that does not correspond to the actual anatomical structure. Recognizing, understanding the physical cause of, and mitigating artifacts is a primary role of the MRI technologist and a heavily weighted topic on the ARRT exam.

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Motion and Ghosting Artifacts

Motion is the most common artifact in clinical MRI. It appears as image blurring or as repeating, distinct "ghost" images of the moving structure.

• The Physics: Motion artifacts propagate exclusively along the phase encoding axis. This occurs because the time interval between phase encoding steps (TR, typically seconds or minutes) is much longer than the time required for frequency encoding (milliseconds). Any motion occurring between phase steps introduces phase errors, which the Fourier transform misinterprets as spatial location.
• Types of Motion:
  • Random Motion: Patient movement, swallowing, coughing, tremors.
  • Periodic Motion: Respiration, cardiac cycles, pulsatile blood flow.
• Management Strategies:
  1. Spatial Presaturation Pulses (Sat Bands): Placing an RF saturation band over moving tissue (e.g., anterior chest wall on spine scans) or inflowing blood vessels nulls their signal before they can emit signal in the imaging slice.
  2. Phase Swapping: Swapping the phase and frequency axes to redirect the ghosting away from the anatomy of interest (e.g., swapping axes on an axial brain scan so eye-blinking ghosts propagate laterally, leaving the brain stem clear).
  3. Respiratory Compensation (ROPE/Bellows): Reorders phase encoding lines to match the patient's breathing cycle, reducing respiratory ghosting.
  4. Gating: Synchronizing data collection with the cardiac cycle (ECG/pulse oximeter) or respiratory cycle, scanning only during periods of minimal motion.
  5. Propeller/Radial K-space Filling: Collecting k-space data in a rotating radial pattern (e.g., PROPELLER, BLADE, RADAR) allows for motion correction during reconstruction.

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Aliasing / Wrap-around

Aliasing occurs when anatomy outside the selected FOV is excited by the RF pulse but is mapped onto the opposite side of the image.

• The Physics: Aliasing occurs when spatial frequencies exceed the Nyquist limit for the selected sample rate (undersampling).
  • Frequency Aliasing: Automatically filtered out by hardware filters (bandpass filters in the receiver).
  • Phase Aliasing: The primary clinical concern. Tissues outside the phase FOV are mapped to the opposite side of the image.
• Management Strategies:
  1. Phase Oversampling (No Phase Wrap): The scanner increases the FOV along the phase axis (by acquiring extra phase lines) and then digitally crops the extra data so it is not displayed. This prevents wrap without changing spatial resolution or increasing scan time (since step size is maintained, but NEX is halved or step spacing is adjusted).
  2. Increase FOV: Encompassing the entire anatomical part within the FOV eliminates aliasing but reduces spatial resolution if the matrix size is constant.
  3. Swapping Phase/Frequency: Aligning the phase axis with the shorter dimension of the anatomy to ensure no tissue falls outside the phase FOV.

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Chemical Shift Artifact

Chemical shift arises because hydrogen protons in fat precess at a slightly lower frequency than hydrogen in water due to shielding by electron clouds.

• The Physics: The precessional frequency difference is 3.5 parts per million (ppm).
  • At $1.5\text{ T}$, this shift is $\approx 220\text{ Hz}$.
  • At $3.0\text{ T}$, this shift is $\approx 440\text{ Hz}$.
• Type I Chemical Shift (Readout/Frequency): Occurs along the frequency encoding axis. Because the scanner uses frequency to determine spatial location, the lower frequency fat signal is shifted toward the lower frequency side of the image. This results in a bright band of signal overlap on one side of a water-fat interface (e.g., the kidney or optic nerve) and a dark band of signal void on the opposite side.
• Type II Chemical Shift (Phase Cancellation / Out-of-Phase): Occurs in Gradient Echo (GRE) sequences within voxels that contain both fat and water (boundary voxels). Because they precess at different frequencies, fat and water protons alternate between being in-phase and out-of-phase. If the echo time (TE) is set when they are out-of-phase (e.g., $2.2\text{ ms}$ at $1.5\text{ T}$), their signals cancel, producing a dark border around organs ("india ink" artifact).
• Management Strategies:
  • Type I: Increase receiver bandwidth (spreads frequencies across fewer pixels, decreasing spatial shift), use fat suppression, or scan at a lower field strength.
  • Type II: Select TE values where fat and water are in-phase (multiples of $4.4\text{ ms}$ at $1.5\text{ T}$ or $2.2\text{ ms}$ at $3.0\text{ T}$).

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Magnetic Susceptibility Artifact

Magnetic susceptibility is the degree to which a substance magnetizes when exposed to an external magnetic field.

• The Physics: Large differences in susceptibility (e.g., between tissue and surgical metal, or tissue and air in the sinuses) cause severe local magnetic field distortions. This leads to rapid spin dephasing, resulting in signal voids (black holes) and extreme spatial distortion.
• Sequence Sensitivity: Worst on Gradient Echo (GRE) sequences due to the lack of a 180-degree refocusing RF pulse.
• Management Strategies:
  1. Use Spin Echo (SE) or Fast Spin Echo (FSE) sequences (the 180-degree pulse refocuses dephasing).
  2. Decrease echo time (TE) to reduce the time allowed for dephasing.
  3. Increase receiver bandwidth (reduces spatial distortion).
  4. Decrease voxel size (thinner slices, higher matrix) to limit intra-voxel dephasing.

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Gibbs Ringing (Truncation Artifact)

Gibbs ringing appears as a series of parallel lines of alternating high and low intensity near sharp, high-contrast boundaries (e.g., CSF and spinal cord, or skull and brain).

• The Physics: It is caused by the mathematical limitation of the Fourier transform when reconstructing a step function from a finite number of data points (truncation of high-frequency k-space data). In the spine, it can mimic a syringomyelia (cervical syrinx).
• Management Strategies:
  1. Increase the phase matrix (e.g., from 192 to 256 or 512). This reduces pixel size, compressing the ringing lines close to the boundary, making them invisible to the eye.
  2. Apply smoothing filters during image reconstruction.

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RF Zipper Artifact

An RF zipper artifact appears as a dense line of noise (alternating bright and dark dots) crossing the image perpendicular to the frequency encoding axis.

• The Physics: It is caused by stray radiofrequency signals entering the scan room and being picked up by the receiver coil. These signals leak through a tear in the Faraday shield (room lining) or enter if the scan room door is left slightly ajar.
• Management Strategies:
  1. Ensure the scan room door is completely closed during scanning.
  2. Check door contact fingers for damage or debris.
  3. Inspect and repair the Faraday cage shielding.
  4. Remove unauthorized electronic devices (e.g., cell phones, patient monitoring equipment) from the magnet room.

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Artifact Reference Table