Imaging Sequences part I Gradient Echo Spin Echo

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 FID gradient recalled echo

Partial Flip z z 0 ML M 0 RF y x t=t 0 y MXY x t=t 0+ MXY = M sin( ) ML = M cos( )

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

Rephasing in the xy-plane view from the top y y Mxy z phase dispersion rephase x z Mxy phase coherency minus t 2* decay x

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

T 2* 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

T 2* decay • T 2* decay is always faster than T 2 decay • gradient echo imaging cannot recover signal losses from – magnetic field inhomogeneity – magnetic susceptibility – water-fat incoherence

T 2 and T 2* Relaxation • T 2 is the spin-spin relaxation time • T 2 M is the contribution to relaxation induced by inhomogeneities of the main magnet (predominant factor) • T 2 MS is the contribution to relaxation induced by magnetic susceptibility in the object

T 2 and T 2* Relaxation • T 2* relaxation influences contrast in gradient echo imaging • T 2 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 3 D acquisitions

Gradient Echo disadvantages • difficult to generate good T 2 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 T 2* 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

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

Gradient Echo • image contrast depends on sequence • conventional GR scan – aka GRASS, FAST – decreased FA causes less T 1 weighting – increased TE causes more T 2* weighting

Conventional GR TE 20, FA 15

Gradient Echo • Spoiled GR – aka SPGR, RF-FAST – spoiling destroys accumulated transverse coherence – maximizes T 1 contrast

Gradient Echo • Contrast enhanced GR – aka SSFP, CE-FAST – infrequently used because of poor S/N – generates heavily T 2* weighted images

Gradient Echo • other varieties – MTC • T 2 - like weighting – IR prepped • 180 preparatory pulse – DE (driven equilibrium) prepped • 90 -180 -90 preparatory pulses • T 2 contrast

MTC GR TE 13, FA 50

Spin Echo • widely used sequence – 90 -180 -echo • 2 parameters – TR – TE • generates T 1, PD, and T 2 weighted images • minimizes artifacts

Spin Echo gradient frequency encode RF pulse readout RF pulse signal FID spin echo

Gradient versus Spin Echo

900 Flip z z 0 Before ML=M MXY=0 0 RF y x t=t 0+ 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. Mxy phase coherency 900 RF t=0 x 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 Mxy phase coherency minus t 2 decay t=TE x

1800 Flip z z dephased y z z x 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 T 2 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 3 D acquisitions

Spin Echo Contrast • T 1 weighted – short TR (450 -850) – short TE (10 -30) • T 2 weighted – long TR (2000 +) – long TE (> 60) • PD weighted – long TR, short TE

Spin Echo Contrast T 1 weighted - T 1 relaxation predominates • Short TE minimizes differences in T 2 relaxation • Short TR maximizes differences in T 1 relaxation T 2 weighted - T 2 relaxation predominates • Long TE maximizes differences in T 2 relaxation • Long TR minimizes differences in T 1 relaxation

Spin Echo Contrast T 1 weighted T 2 weighted

Spin Echo Contrast PD weighted T 2 weighted

Summary • Detection of the MR signal only occurs in the transverse plane • Gradient echo – Alpha degree pulse, dephase-rephase-echo – Contrast (T 1/T 2*) depends on sequence type • Spin echo – 90 degree pulse, dephase, 180 degree pulse, rephase-echo – T 1 weighted: short TR, short TE – PD weighted: long TR, short TE – T 2 weighted: long TR, long TE