MRI artifacts

Aliasing / Wraparound Artifact

Cause: The sampled anatomical object is physically larger than the designated Field of View (FOV), resulting in data under-sampling.

Appearance: The anatomy outside the FOV folds over and projects onto the exact opposite side of the generated image.

Correction: Reorient phase and frequency directions, increase the FOV, activate "No Phase/Frequency Wrap", or use surface coils to limit regional signal sensitivity.

Chemical Shift Artifact

Cause: Driven by the inherent differences in the resonant precessional frequency of protons due to variations in their local chemical environments (specifically fat vs. water).

Appearance: Produces a dark band on one side of a tissue interface and a bright band on the opposite side.

Correction: Change the scan direction (or flip the phase/frequency axis), align the axis perfectly parallel to the fat-water interface, increase gradient strength (use steeper gradients), or use a fat-suppressed sequence like STIR.

Truncation / Gibbs Phenomenon

Cause: Occurs due to a lack of high spatial frequencies at sharp interfaces, creating a failure to perfectly map abrupt transitions between high and low-signal intensities.

Appearance: Multiple rings or ripples of regular periodicity parallel to high-contrast boundaries, false widening of tissue edges, and localized edge distortion.n matrix size (e.g., upgrade from 128 to 256 matrix), decrease pixel size, or apply raw data filters to strip out high-frequency truncation ripples.

Partial Volume Averaging

Cause: Selected slice thickness is too thick and/or the pixel matrix size is too large.

Appearance: Multiple overlapping anatomical structures become crowded and averaged together within the exact same pixel space, causing severe image blurring.

Correction: Decrease the FOV, program thinner slice dimensions, and maintain a smaller overall pixel size.

Motion / Ghosting Artifacts

Cause: Voluntary or involuntary patient movement (respiration, cardiac cycle, swallowing, or physical shifting) during data acquisition.

Appearance: Blurry "ghost images" that propagate strictly along the phase-encoding direction, accompanied by a general lack of crisp detail.

Correction: Patient sedation, physical positioning sponges, Cardiac (ECG) gating, Respiratory gating, breath-hold commands, swapping phase/frequency directions, or using fast pulse sequences (e.g., FGRE, FSE).

Magic Angle Artifact

Occurs when highly structured collagen fibers (like tendons or cartilage) lie at an angle of exactly $54.74^\circ$ relative to the main magnetic field (B0), artificially lengthening T2 relaxation time.

Appearance: A false, hyperintense signal spike within normal tendons (e.g., PCL, peroneal, or supraspinatus tendons) or joint cartilage, mimicking a tear.

Correction: This artifact is only visible on short TE sequences (T1, PD, GRE). To resolve it, switch to long TE sequences (like T2WI or Fast Spin Echo T2) where the false signal disappears. It is also inherently minimized on higher-field (3.0T) scanners.

Crosstalk / Slice Overlap Artifact

Cause: Multi-angle, multi-slice structural scan planning causes RF excitation pulses from adjacent slices to physically intersect and overlap.

Appearance: Localized, band-like signal loss (shadowing/darkness) across the image, most commonly seen in multi-angle scans of the lumbar spine.

Correction: Decrease the slice thickness by 30% or apply an interleaved scanning collection order (acquiring alternating slices to let adjacent tissues recover).

Free Induction Decay (FID) / Zipper / Star Artifact

Cause: Generated by internal system noise or an unshielded residual free induction decay signal leaking into the receiver.

RF Noise / External Interference Artifact

Cause: Stray electromagnetic waves from unshielded or malfunctioning medical equipment near the magnet room leaking into the scanner room.

Criss-Cross / Herringbone Artifact

Cause: Discrete RF spikes, abrupt gradient power supply surges, Analog-to-Digital (A/D) converter malfunctions, or severe electrical current fluctuations.

Quadrature / Amplifier Artifact

Cause: An unbalanced electronic gain or physical malfunction in the two independent signal channels of a quadrature receiver coil.

Moire Fringes / Zebra Artifact

Cause: An interference pattern caused by field non-uniformity or when a receiver coil accidentally picks up a stimulated echo, most commonly encountered during Gradient Echo (GRE) sequences.

Magnetic Susceptibility / Metal Artifact

Cause: Extreme distortion of the local magnetic field caused by materials with highly differing magnetic properties (Ferromagnetic, Paramagnetic, Diamagnetic) placed within the bore.

Eddy Currents & Nonlinearity (Cause, Appearance, and Correction)

Cause: Eddy Currents are small electric loops induced in the scanner's metal housing when gradients rapidly pulse on/off. Gradient Nonlinearity is a physical deviation where gradient fields fail to stay perfectly linear.