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 y x 0 RF signal from precessing protons RF antenna 4

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

Gradient Echo dephase gradient rephase signal  RF pulse FID gradient recalled echo

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

Dephasing in the xy-plane view from the top z y x z Mxy dephase Mxy phase coherency phase dispersion 8

Rephasing in the xy-plane view from the top z Mxy phase coherency minus t2* decay y x z Mxy phase dispersion rephase 9

MR Signal During Rephasing z y x 0 RF signal “echo” RF antenna 10

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 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 FA 30 TE 30 FA 30 susceptibility effect T2* weighting

Gradient Echo magnetic susceptibility post-surgical change “blooming” artifact

Gradient Echo in-phase / 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

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

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 IR prepped 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 2 parameters TR TE generates T1, PD, and T2 weighted images minimizes artifacts

Spin Echo gradient frequency encode readout  RF pulse  RF pulse signal FID spin echo

Gradient versus Spin Echo

900 Flip t=t0 t=t0+ 0 RF 0 Before ML=M MXY=0 After ML=0 MXY=M z y 33

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

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

1800 Flip dephased rephased 900 RF 1800 RF t=0 t=TE/2 t=TE z y x z y x

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 T2 weighted PD 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 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