Unwanted Coherences and their Removal

Question 1

How can there be residual M_xy left after an imaging sequence?

Answer 1

If TR<5*T2 not all of the signals will have decayed and so there will be finite M_xy which will react to the next RF pulse.

Question 2

How will these residual coherences effect the scan?

Answer 2

• The second RF pulse will “store” the effect of the PE gradient from step 1 along the -z axis.
• PE gradient in step 2 is very close to PE gradient in step 1 and almost reverses the effect, resulting in net partial rephasing.
• Rephased M_xy component is essentially unencoded in PE and this effect will occur for every pair of consecutive pulses.
• Every sampled FID contains the same unencoded coherence.
• The FT of a constant effect leads to a band across the image.

Question 3

What two options do we have for the removal of these artefacts?

Answer 3

Spoiling (gradient or RF) and refocusing.

Question 4

How does gradient spoiling eliminate residual coherences?

Answer 4

Spoiler or crusher gradient is applied after the acquisition, typically in the slice select axis, to cause slice-wise dephasing.
This cannot be refocused by the PE gradient.

Question 5

How is RF spoiling used to eliminate unwanted coherences?

Answer 5

Incremented or pseudorandom phase shifts applied to subsequent RF pulses.
This flips any magnetisation in different directions, preventing the build up of a coherent pathway.

Question 6

What problems arise with RF spoiling?

Answer 6

The Fourier Transform assumes that the only phase change is due to the effect of the gradient (read or PE), but the phase of the magnetisation in an RF spoiled sequence is altering due to the direction of the B1 excitation vector.
To make RF spoiling work we need to compensate for the phase shift in the receiver.

Question 7

How does refocussing reduce residual coherences?

Answer 7

Gradients are rephrased after each acquisition, now the transverse magnetisation always starts from an un-encoded state.

Question 8

How can FOV be reduced in the PE direction without causing aliasing?

Answer 8

Destroy the longitudinal magnetisation outside the FOV before image excitation.
Use a combination of gradients and a pre-sat pulse to dephase the magnetisation outside the FOV.

Question 9

How can flow-effects cause unwanted coherences over a balanced gradient with one positive and one negative lobe?

Answer 9

The position of blood following at a constant velocity, v, is given by:
x(t)=x_0+vt
Net phase accumulated is then given by:
d.phi = gamma.int[0-T/2]G(t)(x_0+vt).dt-gamma.int[T/2-T]G(t)(x_0+vt).dt
First lobe causes a phase shift of gamma.G[(xT/2) + (v{T^2})/8]
Second lobe causes a phase shift of -gamma.G[(xT/2)+(3v{T^2}/8)]
Net Phase shift is -gamma.G(v{T^2}/4)
We now have a velocityy dependent phase shift so the phase no longer maps linearly to a loaction in the PE direction, causing artefacts in the image.

Question 10

How can we remove the effects of flow from our image?

Answer 10

Using a 1:-2:1 gradient sequence.