5. Relaxation and Imaging
โข FID (free induction decay) is
the relaxation behavior
following a single RF pulse
โข most imaging done with
repetitive RF energy deposition
โข the interval between the RF
energy pulses is called the TR
interval (time to repetition)
7. Equilibrium
โข after 5 or so
repetitions, the
system reaches
equilibrium
โข similar to water
flowing into a
leaky bucket
relaxation
RF in
equilibrium
8. Differential Relaxation
โข short TR
โข lower absolute ML
โข marked difference
in relative signal
โข long TR
โข higher absolute ML
โข minimal difference
in relative signal
fat protons
water protons
20. T2* decay
โข occurs between the dephasing and
the rephasing gradients
โข rephasing incompletely recovers
the signal
โข signal loss is greater with longer
TEs
โข decay generates image contrast
21. T2* decay
โข T2* decay is always faster than
T2 decay
โข gradient echo imaging cannot
recover signal losses from
โmagnetic field inhomogeneity
โmagnetic susceptibility
โwater-fat incoherence
22. T2 and T2* Relaxation
โข T2* relaxation influences
contrast in gradient echo
imaging
โข T2 relaxation influences
contrast in spin echo imaging
24. Gradient Echo
advantages
โข faster imaging
โcan use shorter TR and shorter
TEs than SE
โข low flip angle deposits less
energy
โmore slices per TR than SE
โdecreases SAR
โข compatible with 3D acquisitions
25. Gradient Echo
disadvantages
โข difficult to generate good T2
weighting
โข magnetic field inhomogeneities
cause signal loss
โworse with increasing TE times
โsusceptibility effects
โdephasing of water and fat
protons
28. Gradient Echo
โข image contrast depends on
sequence
โข conventional GR scan
โaka GRASS, FAST
โdecreased FA causes less T1
weighting
โincreased TE causes more T2*
weighting
38. Dephasing in the xy-plane
view from the top
y
x
z Mxy
y
x
z
Mxyโ0
phase coherency phase dispersion
Dephasing begins
immediately after
the 900
RF pulse.
t=0 t=TE/2
900
RF
39. y
x
z Mxy
phase coherency
minus t2 decay
Rephasing in the xy-plane
view from the top
y
x
z
Mxyโ0
phase dispersion
t=TE/2 t=TE
1800
RF
40. z
y x
z
y x
z
y x
z
y x
t=TE/2 t=TE
1800
RF
t=0
900
RF
dephased
rephased
1800
Flip
46. Effects of the 1800
Pulse
โข eliminates signal loss due to
field inhomogeneities
โข eliminates signal loss due to
susceptibility effects
โข eliminates signal loss due to
water/fat dephasing
โข all signal decay is caused by
T2 relaxation only
47. Spin Echo
advantages
โข high signal to noise
โข least artifact prone sequence
โข contrast mechanisms easier to
understand
48. Spin Echo
disadvantages
โข high SAR than gradient echo
because of 900
and 1800
RF
pulses
โข long TR times are incompatible
with 3D acquisitions
49. Spin Echo Contrast
โข T1 weighted
โshort TR (450-850)
โshort TE (10-30)
โข T2 weighted
โlong TR (2000 +)
โlong TE (> 60)
โข PD weighted
โlong TR, short TE
50. Spin Echo Contrast
T2 Relaxation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500
msec
Mxy
long T2
short T2
T1 Relaxation
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1000 2000 3000 4000 5000
msec
ML
long T1
short T1
T1 weighted - T1 relaxation predominates
โขShort TE minimizes differences in T2 relaxation
โขShort TR maximizes differences in T1 relaxation
T2 weighted - T2 relaxation predominates
โขLong TE maximizes differences in T2 relaxation
โขLong TR minimizes differences in T1 relaxation