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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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SE / GE Differences

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What are the differences between spin echo (SE) and gradient echo (GE) pulse sequences?

The spin echo pulse sequence has a 90° excitation pulse (GE has a small excitation pulse), and SE refocuses some of the dephasing which occurs during the echo time using a 180° refocusing RF pulse. A GE does not have the 180°.

A spin echo pulse sequence and a gradient echo pulse sequence:

transverse plane  gradientechopsd (8K)

This difference has a number of consequences. One of the most significant is speed. Because only one RF pulse is applied in GE, the echo can be recorded more quickly, resulting in a shorter echo time. If low flip angles are used, TR can also be shorter. Shorter TR and TE times mean that GE is preferred for rapid imaging techniques. Another consequence is that in GE image contrast is dictated by T2*, unlike in SE where image contrast is dictated by T2. This is because the gradient reversal in GE refocuses only those spins that have been dephased by the action of the gradient itself; dephasing of spins due to magnetic field inhomogeneities is not reversed. In SE, they are reversed by the 180° pulse, and thus in SE the signal-to-noise ratio is higher.

Changing SE into GE

Consider a SE pulse sequence (phase encoding and slice selction gradients are not shown):

echosize_se
Spin echo pulse sequence.

Now we shall remove the 180° RF refocusing pulse. Notice the reduction in the echo size (which may reduce to nothing if T2* is short enough).

echosize_se_no180
Spin echo pulse sequence, without the 180° RF refocusing pulse.

Since we have no 180° RF refocusing pulse, we can measure the signal earlier (shorter TE); this will increase the echo size. To make MRI signal measurements faster (shorten the TR), we need to also reduce the RF excitation pulse so that there is some Mz ready for the next RF excitation pulse which will follow on shortly. This will reduce the echo size again.

echosize_ge
Gradient echo pulse sequence.

Further reading on this topic:
Books: MRI From Picture to Proton p219, MRI The Basics p87, 147, 165
Online: Basics of MRI 1, 2, MRItutor, e-MRI 1, 2

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