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Imaging Sequences part 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
z y x 0 RF signal from precessing protons RF antenna 4
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Gradient Echo simplest sequence 3 parameters reduced SAR
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 RF pulse FID
gradient recalled echo
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Partial Flip t=t0 t=t0+ 0 ML M 0 RF MXY MXY = M sin() ML = M cos()
z y x z y x 0 RF 0 ML M MXY t=t0 t=t0+ MXY = M sin() ML = M cos() 7
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Dephasing in the xy-plane view from the top
z y x z Mxy dephase Mxy phase coherency phase dispersion 8
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Rephasing in the xy-plane view from the top
z Mxy phase coherency minus t2* decay y x z Mxy phase dispersion rephase 9
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MR Signal During Rephasing
z y x 0 RF signal “echo” RF antenna 10
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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 Echo pulse timing
RF slice phase readout echo signal TE
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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
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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
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Gradient Echo changing TE
FA 30 TE 30 FA 30 susceptibility effect T2* weighting
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Gradient Echo magnetic susceptibility
post-surgical change “blooming” artifact
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Gradient Echo 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 GR TE 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 IR prepped
T2 - like weighting IR prepped 180 preparatory pulse DE (driven equilibrium) prepped preparatory pulses T2 contrast
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MTC GR TE 13, FA 50
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Spin Echo widely used sequence 2 parameters
TR TE generates T1, PD, and T2 weighted images minimizes artifacts
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Spin Echo gradient frequency encode readout RF pulse RF pulse
signal FID spin echo
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Gradient versus Spin Echo
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900 Flip t=t0 t=t0+ 0 RF 0 Before ML=M MXY=0 After ML=0 MXY=M z y
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Dephasing in the xy-plane view from the top
z Dephasing begins immediately after the 900 RF pulse. y x z Mxy Mxy phase coherency phase dispersion 900 RF t=0 t=TE/2 34
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Rephasing in the xy-plane view from the top
z Mxy phase dispersion y x z Mxy phase coherency minus t2 decay 1800 RF t=TE/2 t=TE 35
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1800 Flip dephased rephased 900 RF 1800 RF t=0 t=TE/2 t=TE z y x z y x
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Spin Echo pulse timing RF slice phase readout echo signal TE
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WNMR Race 900 RF t=0
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WNMR Race
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WNMR Race 1800 RF t=TE/2
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WNMR Race t=TE
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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 Echo advantages high signal to noise
least artifact prone sequence contrast mechanisms easier to understand
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Spin Echo disadvantages
higher 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 T2 weighted PD weighted
short TR ( ) short TE (10-30) T2 weighted long TR (2000 +) long TE (> 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
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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
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