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QUESTIONS
» Pulse Sequences

How do we change image contrast? What are the effects of parameter changes? This section explores image contrast and image acquisition methods.

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Crusher Gradients

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Name an instance of gradient spoiling. (You may read "crusher gradients" in some texts. They are the same thing.)

One instance of spoiler gradient use is at the end of a sequence to spoil Mxy prior to the next RF excitation pulse.

Another instance is either side of the slice select gradient which is applied during the application of the 180° refocusing pulse in a SE pulse sequence. They are applied because the 180° pulse may not be exactly 180°, and it may not refocus everything at time TE as we expect. The crusher gradient after the 180° pulse destroys the FID (free induction decay: the decaying signal), the one before the 180° pulse is to balance the one after. Zipper type artefacts may be seen in the absence of spoiler gradients, because the RF pulses used are not perfect.

Further reading on this topic:
Books: MRI From Picture to Proton p219, 238-239, MRI The Basics p156 Online: Medcyclopaedia

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