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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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Respiratory Compensation

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Explain the role of respiratory compensation, to reduce motion artefact, in terms of shallow and steep phase encoding gradient slopes, the centre and periphery of k-space, the motion of the chest wall and SNR.

Most systems have a method known as respiratory compensation which greatly reduces ghosting from respiration. This method employs a set of bellows from which a signal may be obtained containing information about the time of maximum and minimum motion of the chest / abdominal wall. The central lines of k-space are filled when using shallow phase encoding gradient slopes, providing low spatial frequencies which dominate signal and contrast, whereas the outer lines are filled when using steep phase encoding gradient slopes which provide high spatial frequencies which dominate spatial resolution. Anatomy that moves along a steep phase encoding gradient slope produces maximum ghosting as there is a large phase shift between two points on the gradient. Anatomy that moves along a shallow phase encoding gradient however, produces less ghosting as there is a smaller phase shift between the two points. The system can perform the shallow phase encoding gradient slopes when the chest or abdominal wall movement is at a minimum, so that most of the data which provides image signal and contrast is acquired when the chest wall motion is low. It can reserve the steep phase encoding slopes for when the chest wall motion is at a maximum. In this way, the ghosting artefact is reduced.

Further reading on this topic:
Books: MRI From Picture to Proton p79-81, MRI The Basics p183-185, Q&A in MRI p142-144
Online: Basics of MRI, MRItutor, St Paul's 1, 2, 3, 4, 5, 6, 7, 8, Chickscope

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