Magnetic Resonance Imaging III

Question 1

What is the 3D perspective of Excitation/Relaxation?

Answer 1

Excitation drives Mz asymmetry into precessional plane
Asymmetry in precessional plane relaxes to equilibrium
Two relaxation mechanisms
Transverse
Longitudinal

Question 2

What is longitudinal relaxation (T1)?

Answer 2

T1 - slow
Typically 100’s or 1000’s of msecs
Spin-lattice relaxation
Energy loss mechanism
Return of z-magnetization to equilibrium

Question 3

What is transverse relaxation (T2)?

Answer 3

T2 - fast
Typically 10’s or 100’s of msecs
Spin-spin dephasing (ie. loss of coherence)
Each proton feels influence of fluctuating field from neighbouring spins: w = yB, dw= y.dB
Dephasing: Fluctuations in B yield fluctuations in w 

Question 4

What is the saturation recovery sequence?

Answer 4

90 degree pulse interrogates current state of Mz

Assume uniform field (no T2* effects)

Question 5

What is the inversion recovery sequence?

Answer 5

90 degree pulse interrogates current state of Mz

Assume uniform field (no T2* effects)

Question 6

What is the additional tranverse relaxation (T2*)?

Answer 6

Anything that causes a fluctuation in w will accelerate dephasing
B-field inhomogeneity
- So far assumed uniform Bo throughout sample
Bo is only slightly non-uniform (dB/B ~ 10^-6)
dw = y.dB
Accelerates dephasing by an order of magnitude. Completely swamps T2!
T2* < T2