The document compares and contrasts spin echo (SE) and gradient echo (GE) magnetic resonance imaging sequences. For SE, a 90 degree excitation pulse flips magnetization into the X-Y plane, followed by a 180 degree rephasing pulse. For GE, the flip angle (FA) determines how much magnetization is flipped, and the polarity of the readout gradient is changed to rephase spins. TR and TE parameters determine contrast for SE, while FA and TE determine contrast for GE, with different combinations producing T1, PD, or T2 weighted images.

1. SPIN ECHO GRADIENT ECHO
A. It starts with a 90º excitation pulse. The A. Start with a FA α. Depending on α, more or less
magnetization is flipped into the X-Y-plane. magnetization is flipped into the X-Y plane.
B. Immediately the spins dephase… B. The spins are dephasing…
C. The spins dephase a bit more… then a 180 C. And dephase a bit more until the polarity of the GRO
rephasing pulse is given. is changed after which…
D. The spins are mirrored around the Y axis. D. The spins rephase…
E. The spins rephase until… E. And rephase a bit more until…
F. The spins are in phase again creating an “echo “ F. The spins are in-phase again creating an “echo”.

2. (1) switching on the Slice Select gradient (G ). Simultaneously a 90º
SS
RF-pulse (1) Slice selecting with GSS.
(2) is given to ‘flip’ the net-magnetization into the X-Y plane. Then (2) Send excitation pulse.
the Phase Encoding gradient (G ) (3) Phase encoding.
PE
(4) Switch on GRO. First negative polarity, and then change
(3) is switched on to do the first phase encoding. G
SS polarity to positive.
(4) is switched on again during the 180º rephasing pulse (5) Signal sampling during GRO.
(5) so the same protons which were excited with the 90º pulse (2) are
affected. Then the Frequency Encoding or Read Out gradient
(G )
RO
(6) is switched on during which
(7) the signal is sampled.
TR and TE in a SE sequence are FA and TE determine the image
related in terms of image contrast. contrast of GE sequences
A short TR and short TE gives T1 High FA and short TE gives T1
weighted contrast weighted contrast.
. A long TR and a short TE gives PD A medium FA and short TE gives
contrast. PD contrast.
A long TR and long TE gives T2 A low FA and long TE gives T2
weighted contrast. weighted contrast.

3. Created by Curtsy to
Arun A Evert J Blink
Dr Shaji’s MRI & Research Center Kerala Application Specialist MRI