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» Image Artefacts

These questions are concerned with MRI image features which do not represent the object in the field-of-view.

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Truncation vs. Motion

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Some motion artefacts can look a bit like truncation artefacts. How can you tell them apart?

Both types of artefact are propagated in the phase encoding direction. An important differential feature of (random) motion artefacts is that they propagate across the entire image. Truncation artefact diminishes quickly in amplitude with distance from a high contrast boundary. The size of a truncation artefact is related to the resolution of the image.

truncation artefact
Truncation artefact.

An excellent Fourier approximation tool may be found online (select a Rectangular pulse, click Calculate, and then play with the number of Fourier terms used).

Periodic motion (e.g. pulsation of blood in vessels) can also cause multiple ghosts which decrease in strength with distance from the originating structure. However, these ghosts will look like the originating structure (e.g. the aorta) and may be placed far from the originating structure. Trucation artefacts fade quickly and run parallel to a high contrast boundary.

Further reading on this topic:
Books: MRI The Basics p184, Q&A in MRI p130-132
Online: Basics of MRI, MRItutor, St Paul's, Chickscope

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