Gradient echo pulse sequence and its application

Jayanti Gyawali
B.Sc.MIT 3rd
Year
Roll No: 1
Gradient Echo Pulse Sequence and It’s Application

Gradients are generated by coils of wire situated within the bore of the magnet.
This magnetic field interacts with the main magnetic field so that the magnetic
field strength along the axis of gradient coil is altered in a linear way.
The middle of the axis of the gradient remains at the field strength of main
magnetic field.
This is called magnetic isocentre.

There are three gradient coils situated within the bore of the magnet
These are named according to the axis along which they act when they are
switched on.

The Z gradient alters the magnetic field strength along the Z-(long) axis of the
magnet.
The Y gradient along Y-(vertical) axis of the magnet.
The X gradient along X-(horizontal) axis of the magnet.
The magnetic isocentre is the centre point of all axis which remains unaltered even
when the gradients are applied.

Slice selection Phase encoding Frequency encoding
Sagittal X Y Z
Axial(body) Z Y X
Axial(head) Z X Y
Coronal Y X Z

Apart from localization, gradients are also useful in the gradient echo sequences.
Used for spoiling or rewinding TM.
Rephasing of TM is done by gradients thus they eliminate 180 degree pulse and
make GRE sequences much faster.

There is no 180 degree pulse in GRE.
The flip angle in GRE is smaller, usually less than 90 degrees. Hence less scan
time.
T2 relaxation in GRE is called as T2* relaxation.

The TR, TE and flip angle affect image weighting and contrast.
T1 weighting
 To exaggerate T1:large flip angle
To exaggerate T1 : short TR
To diminish T2: short TE

To diminish T1: Small flip angle
To diminish T1: TR is fairly long
To exaggerate T2*: TE is long

To diminish T1: Small Flip angle
To diminish T1: TR is fairly Long
To diminish T2*: TE is Short

Long TR: 100ms+
Short TR: less than 50ms
Short TE: 15-25ms
Low flip angles: 5-20 degree
Large flip angles: 70-110 degree

It can be divided into two types depending on what is done with the residual
transverse magnetization.
If the residual TM is destroyed by RF pulse or gradient such that it will not
interfere with next TR- Spoiled or incoherent GRE sequence.
If the residual TM is not destroyed and is refocused such that after a few TRs
steady magnitude of LM and TM is reached.- Steady state or coherent GRE
sequences.

Gradient spoiling - dephase the residual transverse magnetization

These sequences usually provide T1 weighted GRE images.
Example
FLASH: Fast Imaging Using Low Angle Shot (Siemens)
SPGR: Spoiled GRASS(Gradient recalled acquisition in steady state) (GE)
T1 FFE: Fast Field Echo (Phillips)
 Can be acquired with echo times when water and fat protons are in phase and out of
phase.
Example: In and out of phase imaging.

Spoiled GRE sequences are modified to have time of flight MRA Sequences.
The 3D versions of these sequences can be used for dynamic multiphase post
contrast T1 weighted imaging.
Examples:
3D FLASH and VIBE(Volumetric interpolated breath-hold examination) Siemens
LAVA and FAME (GE)
THRIVE (Phillips).

When residual TM is refocused keeping TR shorter than T2 of tissues, a steady
state is reached, two signals are produced in each TR: FID(S+) and spin-echo (S-).
Depending on what signal is used to form the images, SS sequences are divided
into 3 types:

Only FID(S+) is used for image formation.
Since, S+ is formed after RF excitation it is called post excitation refocused.
Example:
FISP: Fast Imaging with Steady state Precession(Siemens)
GRASS: Gradient Recall Acquisition using Steady State(GE)
FFE: Fast Filed Echo(Phillips)

Only S- is used for image formation.
S- is formed just before next excitation hence named Pre-excitation refocused.
Example:
PSIF: Reversed FISP(Fast Imaging with SS free precession) Siemens
SSFP: SS Free Precession(GE)
T2 FFE: Fast Field Echo (Philips)

Both S+ and S- used for image formation
Also known as balanced SSFP
Have very short TR and TE, highest possible SNR, T2 more bright but lack
internal spatial resolution.
Example:
True FISP (Siemens)
FIESTA: Fast Imaging Employing SS Acquisition (GE)
b-FFE: Balanced Fast Filed Echo (Philips)

In and Out of Phase imaging
T1w 2D spoiled GRE for liver imaging
T1w 3D GRE(VIBE/3D FLASH/LAVA/THRIVE)
These are faster among T1w sequence and can be acquired in breath hold time.
Multiphase or dynamic studies.
MPRAGE: Magnetization Prepared Rapid Acquisition GE
3D inversion recovery that starts with 180 IR and followed by SS. TI-700ms, FA-30,
TR-12s.
Good grey-white differentiation. Used in epilepsy protocol.

Balanced SSFP(TRUFISP/FIESTA/BALANCED TFE)
Very fast sequences acquired in breath-hold time and have highest possible SNR among
all sequence.
Used in Cardiac, bowel, fetal imaging.
But lack internal spatial resolution.
Dual Echo at SS(DESS)
It combines two images formed separately from FISP and PSIF signals. It has both T1 and
T2 contrast hence anatomy as well as fluid is seen very well.
Joint imaging where articular cartilage, bone and synovial fluid seen very well.

Constructive Interference at SS(CISS)/FIESTA-C
3D version of TRUFISP in which two true FISP are combined.
This can provide thin slice high resolution images of posterior cranial fossa showing
cranial nerve dark against the background of bright CSF.
Used for hearing loss, vertigo, acoustic neuroma.

Gradient echo pulse sequence and its application

Gradient echo pulse sequence and its application

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Gradient echo pulse sequence and its application