Aliasing on MRI, also known as wrap-around, is a frequently encountered MRI artifact that occurs when the field of view (FOV) is smaller than the body part being imaged. The part of the body that lies beyond the edge of the FOV is projected onto the other side of the image ⁵.

This can be corrected, if necessary, by oversampling the data. In the frequency direction, this is accomplished by sampling the signal twice as fast. In the phase direction, the number of phase-encoding steps must be increased with a longer study. However, if the FOV and matrix size (phase-encoding steps) are increased and simultaneously the number of excitations (or number of signal averages) reduced to half, the imaging time can be kept constant with correction of aliasing ⁵.

More detail

The basis of aliasing lies in "analogue-to-digital conversion", wherein the continuous MR signal picked by the receiver coil is converted into its digital counterpart for presentation as a grey-scale image. This ubiquitously involves the sampling of the continuous signal at pre-defined intervals. For greater fidelity in signal conversion, the sampling rate should be at least twice the highest frequency within the signal (Nyquist rate). At lower sampling rates, high-frequency signals become indistinguishable from lower frequency signals, i.e. they become aliases ⁵.

On MRI, spatial localisation within a single image depends on the frequency signature of the MR signal originating from that portion. Within a given bandwidth, higher frequency signals come from the periphery of the image and are aliased over the lower frequency (relatively) central portion of the image. Aliasing on MRI can occur in both phase and frequency axis.

Remedy

Aliasing on MRI can be compensated for by ⁵:

• enlarging the field of view (FOV)

• using pre-saturation bands on areas outside the FOV

• anti-aliasing software

• switching the phase and frequency directions

• use a surface coil to reduce the signal outside of the area of interest

See also

• MRI artifacts

• aliasing in ultrasound