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Physics, Techniques and Procedures

Zipper artefact

linear artefact observed in MR Fourier transform imaging. If these artefacts propagate in the phase encoding direction they are generally caused by injection of a narrow bandwidth RF signal into the receiver chain. For example, if there is feedthrough of the frequency synthesizer output into the received signal, this will appear as a signal component that is exactly on resonance and therefore will be superimposed on the signal from the centre of the object in the frequency encoding direction. If the phase of the feedthrough signal changes pseudo-randomly from shot to shot, the effect will be a propagation in the phase-encoding direction. Thus, the artefact appears as a random signal variation along a line through the centre of the object in the readout direction, propagating in the phase-encoded direction. This artefact is sometimes called a central artefact. Presence of this artefact indicates an imperfection in the transmitter/receiver electronics.

If the RF contamination is at a frequency other than that of the synthesizer but still within the bandwidth of the received signal, the zipper will be shifted in the frequency-encoding direction, and will be superimposed on those voxels whose frequency in the presence of the readout gradient is equal to the frequency of the unwanted contamination. If the contamination has a wider bandwidth the artefacts may have some width in the frequency-encoding direction. These types of artefacts can be avoided by proper RF shielding.

It is possible to obtain a similar linear artefact at the centre of the field of view in the phase-encoding direction and propagating in the readout direction. The fact that it appears in the centre in the phase-encoding direction indicates that the contamination is the same in all shots, i.e. it is not affected by the phase-encoding gradient. A typical cause for such an artefact is a free induction decay FID induced by a 180 pulse. This artefact can be avoided by proper sequence design.

NP