Pulse sequences
Categorization
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Spin echo
• Conventional spin echo
• Fast spin echo
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Inversion recovery
Gradient echo
• Coherent
• Incoherent
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Steady state free precession
Ultra-fast imaging
Conventional spin echo
Illustration
 The situation before the patient is placed inside
the magnet
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The patient is now inside the bore of the
magnet. The magnetization of the
patient’s protons M0 is aligned along the Z
axis
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The 900 pulse is now applied. M0 is now
completely removed from the Z axis and
lies along the Y axis

Relaxation is now taking place. Spin-lattice (T1)relaxation
caused the magnetization to re-grow along the Z axis. Spinspin relaxation causes the magnetization vectors to dephase
(move apart) while still in the X-Y plane

1800 pulse is now given. All the vectors now point
in the opposite direction. The magnetization
vectors rephase (come together) in the X-Y
plane. As they come together the echo is being
formed

The magnetization is now completely rephased in
the X-Y plane and points along the Y axis. This
causes the full height of the echo. The actual MRI
signal is taken here.

Relaxation continues to take place. The
magnetic vectors again dephase in the X-Y
plane while the regrowth along the Z axis
continues

Complete relaxation has taken place. There is no net vector
in the X-Y plane, and the magnetization is full grown along
the Z axis. This is identical to the situation before the 900
pulse was applied

The coordinate system in relation to
the magnet
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Short TE and short TR gives T1
weighted image
Use two RF rephasing pulses
generating two spin echoes to
produce T2 and proton density
weighting
First echo has short TE and long TR –
produce proton density weighting
Second echo has a long TE and a
long TR – produce T2 weighting
uses
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Gold standard for most imaging
May be used for every examintion
T1 images useful for demonstrating
anatomy – because high SNR
With contrast enhancement T1 images
show pathology
T2 images also demonstrate pathology
Diseased tissues are generally more
oedematous and/or vascular. They have
increased water content and, have a high
signal on T2 images
Parameters

T1 weighting
• Short TE 10-20 ms
• Short TR 300 – 600 ms
• Typical scan time 4-6 min
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Proton density/T2
• Short TE 20 ms/long TE 80 ms+
• Long TR 2000 ms+
• Typical scan time 7-15 min
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Advantages
• Good image quality
• Very versatile
• True T2 weighting sensitive to pathology
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Disadvantages
• Scan times relatively long
Fast spin echo
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In contrast to conventional spin echo, fast
spin echo applies a train of 1800 pulses per
TR and different phase encoding steps are
used.
Each 1800 pulse produce an echo (proton
density & T2)
This drastically reduce the scan time
The number of 1800 pulses in the train
called the turbo factor or train length

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E.g. if in conventional SE 256 phase
matrix and 1 NEX is used, the scan
time is 256TR
In FSE if the turbo factor is 16, the
scan time is 256TR/16 =16TR
Uses
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Useful in most clinical applications
Central nervous system, pelvis,
musculoskeletal regions
Note
• Fat remains bright on T2
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Unless fat saturation techniques are used
• Muscles appear darker in FSE images
• Artefacts from metal implants is
significantly reduced
Parameters
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T1 weighting
•
•
•
•
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Short effective TE less than 20ms
Short TR 300 – 600 ms
Turbo factor 2-6
Typical scan time 30s to 1 min
T2 weighting
•
•
•
•
Long effective TE 100 ms
Long TR 4000 ms+
Turbo factor 8-20
Typical scan time 2 min
Advantages & Disadvantages
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Reduced scan time
High resolution
matrices and multiple
NEX can be used
Image quality
improved
Increase T2
information
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Some flow and motion
effects increased
Incompatible with
some imaging options
Fat bright on T2
Reduces magnetic
susceptibility effect,
so should not be used
when haemorrhage is
suspected
Inversion Recovery
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Starts with a 1800 inversion pulse.
This inverts NMV through 1800 into
full saturation.
When inverting pulse removed NMV
begins to relax back to B0
A 900 excitation pulse is then applied
at a time TI (Time from Inversion)
from the 1800 inversion pulse
TI
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TI determines the weighting &
contrast
Short TI gives T1 contrast
Long TI gives proto density contrast
After the 900 excitation pulse 1800
rephasing pulse is applied at a time
TE
This produces the spin echo
TR is the time between each 1800
inverting pulse
Uses
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conventionally used to produce
heavily T1 weighted images to
demonstrate anatomy & in contrast
enhanced imaging
Now more widely used in conjunction
with fast spin echo to produce T2
weighted images
Parameters
STIR (short TI inversion recovery)
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Uses TI that corresponds to the time it
takes fat to recover from full inversion to
the transverse plane so that there is no
longitudinal magnetization corresponding
to fat.
As a result the signal from fat is nulled.
Used to achieve suppression of fat in T1
weighted images.
TI 150 – 175 ms
FLAIR (Fluid attenuated inversion
Recovery)
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The signal from CSF is nulled by
selecting a TI corresponding to the
time of recovery of CSF from 180 to
the transverse plane and there is no
longitudinal magnetization present.
Used to suppress signal from CSF in
T2 weighted images
TI - 1700 -2200 ms