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» 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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Susceptibility Effects

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Which pulse sequence, out of SE and GE, has the greater sensitivity to susceptibility effects, and why?

The lack of a 180° pulse in gradient echo techniques means that T2* effects are not reversed during the sequence, and a greater dephasing of spins than in spin echo techniques results. For this reason, GE techniques possess a greater sensitivity to magnetic susceptibility effects. These are most noticeable at air-tissue interfaces, in the presence of metallic implants or in the presence of haemorrhage.

Magnetic susceptibility refers to the extent to which a substance will become magnetised when placed in a magnetic field. Think of it as how much a substance pulls the magnetic field lines of B0 about. Between two areas with different suceptibility, a small magnetic field gradient will exist. These gradients accelerate the dephasing between the protons on either side of the boundary. This causes signal loss.

susc_no_spinecho susc_yes_gradecho
Spin echo and gradient echo images. Susceptibility effects reduce the T2* in the vertebrae, causing signal loss in the GE image.

Further reading on this topic:
Books: MRI From Picture to Proton p100-101, Q&A in MRI p102-103, MRI The Basics p229-230

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