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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. z.  0. y. x. Measuring the MR Signal. RF signal from precessing protons.

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imaging sequences part i
Imaging Sequencespart I
  • Gradient Echo
  • Spin Echo
  • Fast Spin Echo
  • Inversion Recovery
goals of imaging sequences
Goals of Imaging Sequences
  • generate an RF signal perpendicular to 0
  • generate tissue contrast
  • minimize artifacts
slide3

z

0

y

x

Measuring the MR Signal

RF signal from

precessing protons

RF antenna

gradient echo
Gradient Echo
  • simplest sequence
    • alpha flip gradient-recalled echo
  • 3 parameters
    • TR
    • TE
    • flip angle
  • reduced SAR
  • artifact prone
gradient echo1
Gradient Echo

dephase

gradient

rephase

signal

RF pulse

FID

gradient recalled

echo

partial flip

z

z

0 RF

y

x

y

x

Partial Flip

0

ML

M

MXY

t=t0

t=t0+

MXY =M sin()

ML=M cos()

dephasing in the xy plane view from the top

y

y

x

x

z

z

Dephasing in the xy-planeview from the top

Mxy

dephase

Mxy

phase coherency

phase dispersion

rephasing in the xy plane view from the top

y

y

Mxy

x

x

z

Mxy

z

phase coherency

minus t2* decay

phase dispersion

Rephasing in the xy-planeview from the top

rephase

slide9

z

0

y

x

MR Signal During Rephasing

RF signal

“echo”

RF antenna

t2 decay
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 decay1
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
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
t2 and t2 relaxation1
T2 and T2* Relaxation
  • T2* relaxation influences contrast in gradient echo imaging
  • T2 relaxation influences contrast in spin echo imaging
gradient echo pulse timing
Gradient Echopulse timing



RF

slice

phase

readout

echo

signal

TE

gradient echo advantages
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
gradient echo disadvantages
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
gradient echo changing te
Gradient Echochanging TE

TE 9

FA 30

TE 30

FA 30

susceptibility effect

T2* weighting

slide18

Gradient Echomagnetic susceptibility

post-surgical change

“blooming” artifact

slide19

Gradient Echoin-phase / opposed-phase

TE 13.42

TE 15.66

in-phase

opposed-phase

water fat dephasing
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
gradient echo2
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
gradient echo3
Gradient Echo
  • Spoiled GR
    • aka SPGR, RF-FAST
    • spoiling destroys accumulated transverse coherence
    • maximizes T1 contrast
gradient echo4
Gradient Echo
  • Contrast enhanced GR
    • aka SSFP, CE-FAST
    • infrequently used because of poor S/N
    • generates heavily T2* weighted images
gradient echo5
Gradient Echo
  • other varieties
    • MTC
      • T2 - like weighting
    • IR prepped
      • 180 preparatory pulse
    • DE (driven equilibrium) prepped
      • 90-180-90 preparatory pulses
      • T2 contrast
spin echo
Spin Echo
  • widely used sequence
    • 90-180-echo
  • 2 parameters
    • TR
    • TE
  • generates T1, PD, and T2 weighted images
  • minimizes artifacts
slide30

Spin Echo

gradient

frequency encode

readout

 RF pulse

 RF pulse

signal

FID

spin

echo

90 0 flip

z

z

0 RF

y

x

y

x

900 Flip

0

Before

ML=M

MXY=0

After

ML=0

MXY=M

t=t0

t=t0+

dephasing in the xy plane view from the top1

y

y

x

x

z

z

Dephasing in the xy-planeview from the top

Dephasing begins

immediately after

the 900RF pulse.

Mxy

Mxy

phase coherency

phase dispersion

900 RF

t=0

t=TE/2

rephasing in the xy plane view from the top1

y

y

Mxy

x

x

z

z

Mxy

phase coherency

minus t2 decay

phase dispersion

t=TE/2

t=TE

Rephasing in the xy-planeview from the top

1800 RF

180 0 flip

z

z

z

z

y

x

y

x

y

x

y

x

1800 Flip

dephased

rephased

1800 RF

900 RF

t=0

t=TE/2

t=TE

spin echo pulse timing
Spin Echopulse timing





RF

slice

phase

readout

echo

signal

TE

w nmr race
WNMR Race

900 RF

t=0

slide39

WNMR Race

1800 RF

t=TE/2

effects of the 180 0 pulse
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
Spin Echoadvantages
  • high signal to noise
  • least artifact prone sequence
  • contrast mechanisms easier to understand
spin echo disadvantages
Spin Echodisadvantages
  • higher SAR than gradient echo because of 900 and 1800 RF pulses
  • long TR times are incompatible with 3D acquisitions
spin echo contrast
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 contrast1
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
slide46

Spin Echo Contrast

T1 weighted

T2 weighted

spin echo contrast2
Spin Echo Contrast

PD weighted

T2 weighted

summary
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