Gradient echo pulse
sequences
Conventional gradient echo
Steady state Coherent gradient echo
Steady state Incoherent gradient echo
Steady state free precession
Ultra fast sequences
Echo planer imaging (EPI)
Gradient echo pulse sequences
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Conventional gradient echo
• Uses variable flip angles so that, TR and
therefore the scan time, can be reduced without
producing saturation.
• A gradient instead of 180 rephasing RF pulse is
used to rephase the FID.
• The frequency encoding gradient is used for this
purpose.
• A gradient is quicker to apply than a 180 pulse
• Therefore the minimum TE can be reduced.
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Frequency encoding gradient is initially
applied negatively to speed up the
dephasing of the FID.
Then its polarity is reversed producing
rephasing of the gradient echo.
Gradient does not compensate for
magnetic field inhomogenities
So the resultant echo displays great deal
of T2* information
Used to acquire T2*, T1, and proton
density weighting
Allow for reduction in scan time as the TR
is greatly reduced
Conventional gradient echo
TR
RF
RF
FID
Frequency
encode
TE
Echo
rephase
dephase
FID
Uses of gradient echo
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Used for single slice breath-hold
acquisitions in the abdomen
Used for dynamic contrast
enhancement
Used to produce angiographic type
images, because the flowing nuclei
which have been previously excited,
always give a signal as gradient
rephasing is not slice selective.
Manipulating Parameters
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Flip angle and TR determines the degree
of saturation and, therefore the T1
weighting.
For saturation flip angle should be large
and TR short so that full recovery cannot
occur.
To prevent saturation, flip angle should be
small and the TR long enough to permit
full recovery.
TE controls the amount of T2* dephasing
To minimize T2* Te should be short
To maximize T2* TE should be long.
Typical values
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T1 weighting
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Large flip angle 70 -110 degrees
Short TE 5-10 ms
Short TR less than 50 ms
Average scan time several seconds to minutes
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Small flip angle 5 -20 degrees
Long TE 15 -25 ms
Short TR enough for full recovery as flip angle is small
Scan time several seconds to minutes
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Small flip angle 5 -20 degrees
Short TE 5 -10 ms
Short TR for full recovery as flip angle is small
Scan time several seconds to minutes
T2* weighting
Proton density weighting
The steady state
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This is a stage where the TR is shorter than the
T1 and T2 times of the tissues.
There is no time for the transverse magnetization
to decay, before the pulse sequence is repeated
There is coexistence of both longitudinal and
transverse magnetization
The flip angle and TR maintain the steady state
which holds the longitudinal and transverse
components and the NMV steady during the data
acquisition
Flip angles of 300 to 450 and TR of 20 to 50 ms
achieves a steady state.
B0
Longitudinal
component
held steady
NMV held steady
Transverse component held steady
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If steady state is maintained, the transverse
component does not have time to decay
during pulse sequence.
This transverse magnetization, produced as
a result of previous excitations , is called the
residual transverse magnetization(RTM)
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The residual transverse magnetization
(RTM) affects the contrast as it results in
tissues with long T2 times , appearing
bright on the image
Most gradient echo sequences use the
steady state, as the shortest TR and scan
times are achieved.
Gradient echo sequences are classified
according to whether the residual
transverse magnetization is in phase
(coherent) or out of phase (incoherent).
Learning point
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The steady state involves
repeatedly applying RF pulses at TR
less than the T1 and T2 of all
tissues
This train of RF pulses generates
two signals
1. A FID signal which occurs as a result of
the withdrawal of the RF pulse and
contains T2* information
2. A spin echo whose peak occurs at the
same time as an RF pulse
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This happens because every RF pulse
contains individual radio waves that have
sufficient energy to rephase a previous
FID
These radiowaves rephase the RTM left
over from previous RF excitation pulses to
form a spin echo.
This ocurs at exactly the same time as the
next RF pulse as the RTM takes the same
time to rephase as it took to dephase.
Therfore when utilizing steady state , the
TR =TAU of the spin echo.
RF pulse 2
RF pulse 1
FID
Produces own
FID and
rephases FID
of pulse 1
FID
dephasing
RF pulse 3
Spin
echo
FID
rephasing
TR
A FID and a spin echo occur at each
RF pulse
Summary
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Any two RF pulses produce a spin echo.
The first RF pulse excites the nuclei
regardless of its net amplitude
The second RF pulse rephases the FID
resulting from the first.
The spin echoes produced are sometimes
called Hahn or stimulated echoes.
This concept applies to all pulse sequences
that use steady state.
GE-Coherent residual transverse
magnetization
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This pulse sequence uses a variable flip
angle excitation pulse followed by gradient
rephasing, to produce a gradient echo.
Steady state is maintained by selecting TR
shorter than T1 and T2
There is therefore RTM left over when the
next excitation pulse is applied.
The RTM is kept coherent by a process
known as rewinding.
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Rewinding is achieved by reversing
the slope of the phase encoding
gradient after readout.
This results in RTM rephasing, so
that it is in phase at the beginning of
the next repetition
This alows the RTM to build up so
that tissues with a long T2 time
produce a high signal.
Coherent gradient echo pulse
sequence
TR
Phase
encode
FID
readout
echo
Rewinder
gradient
Sample image
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T2* weighted image using a coherent gradient
echo. TE 15 ms, TR 40 ms, flip 350, breath
holding single slice obtained in 11s
Uses of coherent gradient echo
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This pulse sequences produce T2*
weighted images.
As fluid is bright they give an
angiographic, myelographic or
arthrographic effect.
Can be used to determine whether a
vessel is patent, or whether an area
contains fluid.
Can be acquired slice by slice, or in a 3D
volume acquisition
As the TR is short, a sliice can be acquired
in a single breath hold.
Parameters
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To maintain the steady state
• Flip angles 30 – 45
• TR 20-50 ms
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To maximize T2* long TE 15- 25 ms
Use gradient moment rephasing to
accentuate T2*
Average scan time
• Seconds for single slice
• 4-15 min for volumes
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(to minimize T2* to produce T1 or proton
density weighting TE should be the
shortest possible)
Advantages & Disadvantages
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Very fast scans,
breath holding
possible
Very sensitive to
flow so good for
angiography
Can be acquired in
a volume
acquisition
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Poor SNR in 2D
acquisitions
Magnetic
susceptibility
increases
Loud gradient
noise
Incoherent(spoiled) RTM
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Pulse sequence that use incoherent RTM
begin with a variable flip angle excitation
pulse, and use gradient rephasing to
produce agrdient echo.
The steady state is maintained so that
RTM is left over from the previuos RF.
The RTM is spoiled so that its effect on
image contrast is minimal.
There are two ways to achieve spoiling
• Digitized RF spoiling
• Gradient spoiling
RF spoiling
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RF spoiling is achived by controling the phase of
the digitised RF pulses that are transmitted.
The digitized RF is transmitted at a specific
frequency & phase
The resultant NMV and transverse component are
fillped to a certain position in the transvere plane.
The receiver coil can lock onto the phase of the
RF that has just being transmitted and receives
only signal at that phase.
Transverse magnetization at other phases or
positions in the transverse plane are not recived
by the coil
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Each RF is delivered at a different
phase and the receiver coil is locked
to recieve signal only at that phase
This process continues and the TRM
which is at a different phase is
ignored by the receiver coil.
So the effect of RTM on the image is
eleminated.
T2* is therefore cannot predominate
and T1 and proton density weighting
prevails.
Uses
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RF spoild GE pulse sequencs produce
T1 or proton density weited images,
although fluid may have a rather
high signal due to gradient rephasing
Can be used for 2D and volume
acquisition and as the TR is short the
2D acquisition can be used to acquire
T1 weighted breath-hold images.
Demonstrate good T1 anatomy
Example for RF spoiled GE 5.21
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T1/proton density weighted image using RF
spoiling. TE 6 ms, TR 35 ms, flip 35, part of
volume acqusition which took 7 minutes
Parameters
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To maintain steady state
• Flip angle 30-45
• TR 20-50 ms
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To maximise T1 ; short TE 5-10ms
Average scan time; several seconds for
single slice, 4-15 min for volumes
Advantages
• Can be acquired in a volume or 2D
• Breath holding possible
• Good SNR and anatomical detail in volumes
Gradient spoiling
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Gradient spoiling is the opposite of rewinding.
The slice select, phase encoding, and frequency
encoding gradients can be used to dephase the
RTM, so that it is incoherent at the beginning of
the next repetition.
T2* effects are reduced
Uses and parameters are similar to those in RF
spoiling.
Can be used to achieve T2* when the
parameteres are similar to those in conventional
GE.(because GS is less efficient than RF spoiling
and moreT2* information is present in the signal)
Steady state free prcession (SSFP)
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Can be used to get shortest possible TR
and scan time with steady state GE
Used to produce more T2 weighted images
than conventional gradient echo
sequences.
The pulse sequences used here help to
obtain images that have a sufficiently long
TE and less T2* when using steady state
than other gradient echo pulse sequences.
This is achieved in the manner described
below.
Composition of RF pulse
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RF pulse contains radio waves of
differing amplitudes. The magnetude
of RF pulse is an average of these
amlitudes. E.g.
• 10 waves of amplitude of 100
• 2 waves of amplitude of 300
• 15 waves of amplitude of 600
• 5 waves of amplitude of 1800
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The average amplitude = 19600/32
= 61.250
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Therfore every RF pulse contains waves
that on their own have sufficient
magnitude to move magnetic moments
within the NMV through 1800.
These radio waves are therefore able to
rephase a FID.
In SSFP, the steady state can be
maintained by using a flip angle between
300 and 450 with a TR of 20-50ms.
Every TR an excitation pulse is applied.
When the RF is switched off a FID is
produced.
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After the TR another excitation pulse is applied
which also produces its own FID.
The radiowaves within it that have an amplitude of
180 rephase the FID from the previous pulse, and
a spin echo is produced.
Each RF pulse therefore not only produces its own
FID, but also rephases the FID produced from the
previous excitation.
As nuclei take as long to rephase as they took to
dephase, the echo from the first excitation pulse
occurs at the same time as the third excitation
pulse.
However this cannot be sampled, as RF cannot be
transmitted and received at the same time.
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To receive the spin echo, a rewinder
gradient is used to speed up the rephasing
process after the RF rephasing has begun.
The rewinding moves the echo so that it
occurs before the next excitation pulse,
rather than during it.
This way the resultant ehco can be
received.
It demonstrate more true T2 weighting
than conventional gradient echo
sequences.
Because
• The effective TE is now longer than the TR.
• The rephasing is initiated by an RF pulse rather
than a gradient so that more T2 and less T2*
information is present.
1
2
3
FID 1
Echo of FID 1
FID 3
FID 2
1
TE
TR
2
3
Rewinder
gradient
FID 1
Effective TE
Echo of FID 1
Effective TE & actual TE
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Actual TE is the time between the
echo and the next excitation pulse
Effective TE is the time from the
echo to the excitation pulse that
created its FID
• Effective TE = (2xTR) – actual TE
• If TR = 50 ms, actual TE= 10 ms
Then effective TE = 90 ms
Uses of SSFP
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Used to acquire
images that
demonstrate true
T2 weighting.
Especially useful
in the brain and
joints and on most
systems can be
used with both 2D
and 3D volume
acquisitions.
Effective TE 71 ms, TE 9ms, TR
40 ms, flip angle 350, volume
scan time 9 minutes.
Parameters
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To maintain steady state; flip angle 30-45,
TR 20-50 ms
Actual TE affects the effective TE unless
the system uses a fixed TE.
Average scan time 4-15 min volume
acquisition
Some manufacturers suggest decreasing
the effective TE to reduce magnetic
susceptibility, and increasing the flip angle
to create more transverse magnetisation
which results in higher SNR
Comparison between Coherent,
incoherent
&
SSFP
RF
pulse
RF pulse
FID + spin echo
Spin
echo
FID
RF pulse
FID
Gradient
echo
Gradient
echo
TE
Coherent
TE
Incoherent
FID
TE
Spin echo
moved away
from RF
pulse by
rewinding
gradient
SSFP
Ultra-fast sequences
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Advances have been made in developing
very fast pulse sequences.
Usually employ the coherent or incoherent
gradient echo sequences.
The TE is significantly reduced by:
• Aplying only a portion of the RF exciation
pulse.
• Reading only a portion of the echo
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TE kept to a minimum (2.5 – 3.0 ms)
TR and therefore the scan time is reduced.
TR as low as 10 ms is achieved and about
16 slices can be achieved in a single
breath hold.
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Many ultra-fast sequences use extra
pulses applied before the pulse sequences
begins, to pre-magnetise the tissue.
This way certain contrast can be obtained.
Pre-magnetisation is achieved in the
following manner.
• Applying a 1800 pulse before the pulse
sequence and a specified delay time similar to
inversion recovery.
• Applying a combination of 900/1800/900 pulses
before the pulse sequence begins.
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(first 90 pulse produces transverse
magnetisation. 180 pulse rephases this,
and at a specific time later second 90
pulse is applied. This drives the coherent
transverse magnetisation into the
longitudinal plane. It is available to be
fliped when the pulse sequence begins.
This is used to produce T2 contrast and is
sometimes known as driven equilibrium)
Echo planer imaging(EPI)
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Fills all the lines of k-space during one TR
Uses a single echo train
Multiple Echos are generated and each is
phase encoded by a different slope of
gradient to fill all the required lines of k
space.
Echoes are generated either by 180
rephasing pulses or by gradients.
Gradients are much faster