Imaging Sequences part I

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Imaging Sequencespart I

• Gradient Echo
• Spin Echo
• Fast Spin Echo
• Inversion Recovery

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Goals of Imaging Sequences

• generate an RF signal perpendicular to ?0
• generate tissue contrast
• minimize artifacts

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Measuring the MR Signal
RF signal from precessing protons
RF antenna
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Gradient Echo

• simplest sequence
• alpha flip-gradient recalled echo
• 3 parameters
• TR
• TE
• flip angle
• reduced SAR
• artifact prone

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Gradient Echo
dephase
gradient
rephase
signal
FID
gradient recalled echo
? RF pulse
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Partial Flip
?0
ML
M
MXY
tt0
tt0
MXY M sin(?)
ML M cos(?)
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Dephasing in the xy-planeview from the top
Mxy??
dephase
Mxy
phase coherency
phase dispersion
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Rephasing in the xy-planeview from the top
rephase
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MR Signal During Rephasing
RF signal echo
RF antenna
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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

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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

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T2 and T2 Relaxation

• 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

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T2 and T2 Relaxation

• T2 relaxation influences contrast in gradient
  echo imaging
• T2 relaxation influences contrast in spin echo
  imaging

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Gradient Echopulse timing
??
RF
slice
phase
readout
echo
signal
TE
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Gradient Echoadvantages

• 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

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Gradient Echodisadvantages

• 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

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Gradient Echochanging TE
TE 9 FA 30
TE 30 FA 30
susceptibility effect
T2 weighting
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Gradient Echomagnetic susceptibility
post-surgical change blooming artifact
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Gradient Echoin-phase / opposed-phase
TE 13.42
TE 15.66
in-phase
opposed-phase
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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

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In-Phase / Opposed-Phase TE Times (msec)
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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

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Conventional GRTE 20, FA 15
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Gradient Echo

• Spoiled GR
• aka SPGR, RF-FAST
• spoiling destroys accumulated transverse
  coherence
• maximizes T1 contrast

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Gradient Echo

• Contrast enhanced GR
• aka SSFP, CE-FAST
• infrequently used because of poor S/N
• generates heavily T2 weighted images

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Gradient Echo

• other varieties
• MTC
• T2 - like weighting
• IR prepped
• 180 preparatory pulse
• DE (driven equilibrium) prepped
• 90-180-90 preparatory pulses
• T2 contrast

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MTC GRTE 13, FA 50
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Spin Echo

• widely used sequence
• 90-180-echo
• 2 parameters
• TR
• TE
• generates T1, PD, and T2 weighted images
• minimizes artifacts

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Spin Echo
gradient
readout
frequency encode
??? RF pulse
???? RF pulse
signal
FID
spin echo
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Gradient versus Spin Echo
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900 Flip
??0
After ML0 MXYM
Before MLM MXY0
tt0
tt0
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Dephasing in the xy-planeview from the top
Dephasing begins immediately after the 900 RF
pulse.
Mxy??
Mxy
phase coherency
phase dispersion
900 RF
t0
tTE/2
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Rephasing in the xy-planeview from the top
1800 RF
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1800 Flip
dephased
rephased
1800 RF
900 RF
tTE/2
tTE
t0
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Spin Echopulse timing
???
????
RF
slice
phase
readout
echo
signal
TE
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WNMR Race
900 RF
t0
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WNMR Race
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WNMR Race
1800 RF
tTE/2
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WNMR Race
tTE
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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

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Spin Echoadvantages

• high signal to noise
• least artifact prone sequence
• contrast mechanisms easier to understand

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Spin Echodisadvantages

• high SAR than gradient echo because of 900 and
  1800 RF pulses
• long TR times are incompatible with 3D
  acquisitions

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Spin Echo Contrast

• T1 weighted
• short TR (450-850)
• short TE (10-30)
• T2 weighted
• long TR (2000 )
• long TE (gt 60)
• PD weighted
• long TR, short TE

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Spin Echo Contrast

• 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

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Spin Echo Contrast
T1 weighted
T2 weighted
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Spin Echo Contrast
PD weighted
T2 weighted