Aliasing or wrap around artifacts

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Post on 31-May-2015

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  • 1. WRAP AROUND OR ALIASING ARTEFACT
    Wrap or aliasing produces an image where anatomy that exists outside the FOV is folded on to the top of the anatomy inside the FOV.

2. CAUSE
Aliasing is cause when anatomy that exists outside the FOV is mismapped inside the FOV.
Anatomy outside the FOV still produces a signal if it is in close proximity to the receiver coil.
3. CAUSE
Data from this signals must be encoded, i.e. allocated a pixel position. If the data are under sampled, the signal is mismapped into pixels within the FOV rather than outside.
This can occur in both frequency encoding and phase encoding axis
4. FREQUENCY WRAP

  • Aliasing along the frequency encoding axis is known as frequency wrap.

5. Caused by under sampling the frequenciesthat are present in the echo regardless from where it originates. 6. If the Nyquisttheorm is not obeyed and frequencies are not sampled enough, the signal from the anatomy outside the FOV in the frequency encoding direction is mismapped into the FOV.

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