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.

WRAP AROUND OR ALIASING ARTEFACT

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.

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

CAUSE

Aliasing along the frequency encoding axis is known as

frequency wrap.

Caused by under sampling the frequencies that are present in

the echo regardless from where it originates.

If the Nyquist theorm 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.

FREQUENCY WRAP

Aliasing along the phase axis of the image is known as phase

wrap

Caused by under sampling along the phase axis

When a phase encoding is done it changes the phase from 0 to

3600 inside the FOV along the phase encoding direction

The same phase curve is repeated outside the FOV.

PHASE WRAP

Since the signal coming from outside the FOV is having the same

phase value which as inside, it is allocated a space were signal

from inside the FOV is kept.

PHASE WRAP

Enlarging the FOV so that all the anatomy creating signal is

within the FOV eliminates this artefact

Applying saturation bands will also null signal from this area.

REDUCTION IN GENERAL

Increasing the sampling frequency so that all the frequencies

are digitised sufficiently will eliminate the artifact

The above method increase the noise.

In order to avoid that a frequency filter is used to filter out

frequencies that occur outside the selected FOV.

Most of the systems apply this automatically.

FREQUENCY WRAP REDUCTION

This is termed as no phase wrap or anti-fold over.

It increases the number of phase encoding performed

(achieved by enlarging the FOV)

Increasing the FOV will avoid duplication of phase values so

that each and every point will have a different phase than

another.

PHASE WRAP REDUCTION

THE ABOVE METHOD DECREASES THE SPATIAL RESOLUTION

(phase encoding is increased to compensate this)

INCREASING THE PHASE ENCODING INCREASES SCAN TIME

(system automatically reduce the NEX in order to compensate for

this)

REDUCING NEX WILL INCREASE MOTION ARTEFACT

PHASE WRAP REDUCTION