Transcript An Introduction to MRI

Imaging Sequences
part I
•
•
•
•
Gradient Echo
Spin Echo
Fast Spin Echo
Inversion Recovery
Goals of Imaging Sequences
• generate an RF signal
perpendicular to 0
• generate tissue contrast
• minimize artifacts
Measuring the MR Signal
z
RF signal from
precessing protons
0
y
x
RF antenna
Gradient Echo
• simplest sequence
– alpha flip gradient-recalled echo
• 3 parameters
– TR
– TE
– flip angle
• reduced SAR
• artifact prone
Gradient Echo
dephase
gradient
rephase
signal

RF pulse
FI
D
gradient recalled
echo
Partial Flip
z
z
0
ML
M
0 RF
y
x
t=t0
y
MXY
x
t=t0+
MXY = M sin()
ML = M cos()
Dephasing in the xy-plane
view from the top
y
y
Mxy
dephase
z
x
Mxy
phase coherency
z
phase dispersion
x
Rephasing in the xy-plane
view from the top
y
y
Mxy
z
phase dispersion
rephase
x
z
x
Mxy
phase coherency
minus t2* decay
MR Signal During Rephasing
z
RF signal
“echo”
0
RF antenna
y
x
T2* decay
• occurs between the dephasing and the
rephasing gradients
• rephasing incompletely recovers the
signal
• signal loss is greater with longer TEs
• decay generates image contrast
T2* decay
• T2* decay is always faster than T2
decay
• gradient echo imaging cannot recover
signal losses from
– magnetic field inhomogeneity
– magnetic susceptibility
– water-fat incoherence
T2 and T2* Relaxation
1
1
1
1
=
+
+
T 2 * T 2 T 2M T 2MS
• T2 is the spin-spin relaxation time
• T2M is the contribution to relaxation induced
by inhomogeneities of the main magnet
(predominant factor)
• T2MS is the contribution to relaxation induced
by magnetic susceptibility in the object
T2 and T2* Relaxation
• T2* relaxation influences contrast in
gradient echo imaging
• T2 relaxation influences contrast in
spin echo imaging
Gradient Echo
pulse timing
RF

slice
phase
readout
echo
signal
TE
Gradient Echo
advantages
• faster imaging
– can use shorter TR and shorter TEs
than SE
• low flip angle deposits less energy
– more slices per TR than SE
– decreases SAR
• compatible with 3D acquisitions
Gradient Echo
disadvantages
• difficult to generate good T2
weighting
• magnetic field inhomogeneities
cause signal loss
– worse with increasing TE times
– susceptibility effects
– dephasing of water and fat protons
Gradient Echo
changing TE
TE 9
FA 30
susceptibility effect
TE 30
FA 30
T2* weighting
Gradient Echo
magnetic susceptibility
post-surgical change
“blooming” artifact
Gradient Echo
in-phase / opposed-phase
TE 13.42
in-phase
TE 15.66
opposed-phase
Water/Fat Dephasing
• MR signal is a composite of fat and
water in the imaging voxel
• water and fat resonate at slightly
different frequencies
• cyclic variation in relative phase of
fat and water resonance results in
signal variations dependent on TE
times
Phase Cancellation
3
2.5
2
1.5
SI
Fat
Water
1
Composite
0.5
0
-0.5
-1
TE
In-Phase / Opposed-Phase
TE Times (msec)
Field Strength
(T)
W-F Offset
(Hz)
in
out
in
out
in
out
in
out
in
out
in
out
in
0.5
1
1.5
75
0.00
6.71
13.42
20.13
26.84
33.55
40.26
46.97
53.68
60.39
67.10
73.81
80.52
150
0.00
3.36
6.71
10.07
13.42
16.78
20.13
23.49
26.94
30.33
33.74
37.14
40.55
225
0.00
2.24
4.47
6.71
8.95
11.18
13.42
15.67
17.89
20.13
22.37
24.60
26.84
Gradient Echo
• image contrast depends on
sequence
• conventional GR scan
– aka GRASS, FAST
– decreased FA causes less T1
weighting
– increased TE causes more T2*
weighting
Conventional GR
TE 20, FA 15
Gradient Echo
• Spoiled GR
– aka SPGR, RF-FAST
– spoiling destroys accumulated
transverse coherence
– maximizes T1 contrast
Gradient Echo
• Contrast enhanced GR
– aka SSFP, CE-FAST
– infrequently used because of poor
S/N
– generates heavily T2* weighted
images
Gradient Echo
• other varieties
– MTC
• T2 - like weighting
– IR prepped
• 180 preparatory pulse
– DE (driven equilibrium) prepped
• 90-180-90 preparatory pulses
• T2 contrast
MTC GR
TE 13, FA 50
Spin Echo
• widely used sequence
– 90-180-echo
• 2 parameters
– TR
– TE
• generates T1, PD, and T2 weighted
images
• minimizes artifacts
Spin Echo
gradient
frequency encode
 RF pulse
readout
 RF pulse
signal
FI
D
spin
echo
Gradient versus Spin Echo
Gradient Echo
Spin Echo
dephase
gradient
gradient
frequency encode
 RF pulse
rephase
readout
 RF pulse
signal
signal
FID
spin
echo

RF pulse
FID
gradient recalled
echo
900 Flip
z
z
0
Before
ML=M
MXY=0
0 RF
y
x
t=t0
y
x
t=t0+
After
ML=0
MXY=M
Dephasing in the xy-plane
view from the top
y
z
y
Dephasing begins
immediately after
the 900 RF pulse.
x
Mxy
phase coherency
900 RF
t=0
Mxy
x
z
phase dispersion
t=TE/2
Rephasing in the xy-plane
view from the top
y
y
Mxy
z
phase dispersion
1800 RF
t=TE/2
x
z
x
Mxy
phase coherency
minus t2 decay
t=TE
1800 Flip
z
z
dephased
y
z
x
z
y
x
rephased
y
900 RF
t=0
x
y
x
1800 RF
t=TE/2
t=TE
Spin Echo
pulse timing
RF


slice
phase
readout
echo
signal
TE
WNMR Race
900 RF
t=0
WNMR Race
WNMR Race
1800 RF
t=TE/2
WNMR Race
t=TE
Effects of the 1800 Pulse
• eliminates signal loss due to field
inhomogeneities
• eliminates signal loss due to
susceptibility effects
• eliminates signal loss due to
water/fat dephasing
• all signal decay is caused by T2
relaxation only
Spin Echo
advantages
• high signal to noise
• least artifact prone sequence
• contrast mechanisms easier to
understand
Spin Echo
disadvantages
• higher SAR than gradient echo
because of 900 and 1800 RF pulses
• long TR times are incompatible with
3D acquisitions
Spin Echo Contrast
• T1 weighted
– short TR (450-850)
– short TE (10-30)
• T2 weighted
– long TR (2000 +)
– long TE (> 60)
• PD weighted
– long TR, short TE
Spin Echo Contrast
T2 Relaxation
T1 Relaxation
1
1
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
long T1
M L 0.5
short T1
long T2
M xy 0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
short T2
0.1
0
0
1000
2000
3000
m sec
4000
5000
0
0
100
200
300
400
500
msec
T1 weighted - T1 relaxation predominates
•Short TE minimizes differences in T2 relaxation
•Short TR maximizes differences in T1 relaxation
T2 weighted - T2 relaxation predominates
•Long TE maximizes differences in T2 relaxation
•Long TR minimizes differences in T1 relaxation
Spin Echo Contrast
T1 weighted
T2 weighted
Spin Echo Contrast
PD weighted
T2 weighted
Summary
• Detection of the MR signal only occurs in the
transverse plane
• Gradient echo
– Alpha degree pulse, dephase-rephase-echo
– Contrast (T1/T2/T2*) depends on sequence type
• Spin echo
– 90 degree pulse, dephase, 180 degree pulse,
rephase-echo
– T1 weighted: short TR, short TE
– PD weighted: long TR, short TE
– T2 weighted: long TR, long TE