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QUESTIONS
» 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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Motion Artefact

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Why is motion artefact only seen in the phase encoding direction?

Although the encoding methods in each direction are mathematically equivalent, a frequency-encode step takes much less time (of the order of milliseconds) than a phase encoding step (of the order of seconds). In the frequency-encode direction, all the samples of a signal are acquired in the time of a single echo, whereas in the phase-encode direction all the lines of k-space must be collected to obtain a complete data set for Fourier reconstruction—each line is separated by the time interval TR. This means that most motions that occur during clinical MRI are much slower than the rapid sampling process along the frequency encoding axis. For this reason although motion artefacts along the frequency encoding axis may occur, they are usually insignificant and the most effect they have is a slight blurring.

In the following images, the motion artefact (blurring in the phase encoding direction—up-down—across the whole image) is present in spite of the direction of the motion which occured during the scan.

motion_pe
Motion in the phase encoding direction (up-down). The strong contrast of the bottle-phantom simply represents where the phantom was when the central k-space lines were acquired (high up; some aliasing has occured). The edge information is in the centre of the image because this is where the phantom was when the peripheral k-space lines were acquired. The blurring of signal across the whole image in the phase encoding direction (up-down) is motion artefact.

motion_fe
Motion in the frequency encoding direction (left-right). The strong contrast of the bottle-phantom simply represents where the phantom was when the central k-space lines were acquired (to the right). The edge information is in the centre of the image because this is where the phantom was when the peripheral k-space lines were acquired. The blurring of signal across the whole image in the phase encoding direction (up-down) is motion artefact.

Motion artefacts are also considered in the K-Space Tool.

Further reading on this topic:
Books: MRI From Picture to Proton p79-82, MRI The Basics p183-185, 139, Q&A in MRI p142-144
Online: Basics of MRI, MRItutor, Chickscope, St Paul's 1, 2, 3, 4, 5, 6, 7, 8

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