RF Pulse

1
RF Pulse generates a B1 field that realigns the
precessing spins in the low energy state
In the case of a 90o pulse the alignment is
perpendicular to Bo the B1 field is
sufficiently strong to effect this realignment
2
The NMR/MRI Signal and FT
Signal acquired by the NMR/MRI unit
Fourier Transform (FT)
Line width ? 1/T2
3
Linewidth is a f(T2)
4
Signal appearance in a Spin-echo Pulse Sequence
5
Graphical Representation of T1
6
T1 Measurement Inversion Recovery
7
T2 Measurement by Spin Echoes
8
Exponential Recovery T1
9
T1W Image contrast
10
T1W Brain Image
11
T2W Brain Image
12
T2W Human brain image
13
Pulses, T1 and T2

• Pulses have nothing to do with either T1 or T2
• Relaxation mechanisms/processes pertain to the a)
  recovery of magnetization along the Bo or z-axis
  and/or b) the dephasing of the NMR/MRI signal in
  the x-y plane

14
The Importance of Relaxation Phenomena for fMRI
and Clinical MRI

• Small differences in T1 and T2 can provide
  clear-cut diagnostic interpretations
• Small changes in signal intensity from the
  decrease of deoxyhemoglobin and the increase of
  oxyhemoglobin due to capillary dilation with
  brain activation are the basis for BOLD fMRI
• MRI contrast agents all rely on changing T1 or
  T2 of nearby water protons to give better disease
  changes contrast