FMRI David Norris

15 important questions on FMRI David Norris

How is T1 characterized?

It's spin-lattice relaxation time. The magnetisation recovers back to the longitudinal axis.

How is T2 characterized?

Spin-spin relaxation. Lifetime of transverse magnetisation in a perfectly homogeneous main field. Over time, the coherence between spins is gradually lost and they fall out of pase.

Which two causes are there for transverse relaxation?

  1. intrinsic cause from spin-spin interaction. This is T2
  2. Extrinsic cause from the external magnetic field.


1 and 2 combined is T2* decay
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By which quantity is the signal intensity measured?

How much magnetisation is in the x-y plane at a given time ater excitation. This is determined by the transverse relaxation rate.

What will any process that causes T1 relaxation also cause?

Transverse relaxation.

How is transverse relaxation independent o T1 relaxation caused?

By loss of coherence.

What is inversion recovery?

Method for measuring T1. The magnetisation is initially inverted by a 180degrees pulse, after an inversion time (TI) it's interrogated by an escitation pulse (90 degrees).

How is T2* measured?

With the Fourier transform of the free induction decay (FID), which is directly acquired after excitation.
T2* is measured by examining the full width at half the maximum height. The faster the FID decays (shorter T2*) the broader the line will be.

What will happen if TR is short compared to T1?

Tissue with shorter T1 will relax faster during TR and give a higher signal.

What TR and TE will we use for T1 contrast?

Short TR and short TE;
there's an optimal TR value that maximally differentiates between two tissues that
differ in T1; 
Short TE to minimize T2 contrast.

What contrast do you get with a long TR and a long TE?

T2 or T2* depending on whether the acquisition is spin- or gradient echo.

What contrast do you get with a long TR and a short TE?

Proton density contrast.

Do you get a good contrast with a short TR and a long TE?

No; Short TR leads to stronger signal for tissue with a short T1.
A long TE leads to strong signal for tissue with a long T2.

The two effect tend to cancel.

Which contrast is usually used for diagnosis in medicine?

T1 and T2

Which contrast is used and what are the blakc dots?

T2*-weighted image. The black dots are veins.

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