Gradient echo pulse sequence and its application

Jayanti GyawaliB.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 angleTo exaggerate T1 : short TRTo diminish T2: short TE

To diminish T1: Small flip angleTo diminish T1: TR is fairly longTo exaggerate T2*: TE is long

To diminish T1: Small Flip angleTo diminish T1: TR is fairly LongTo diminish T2*: TE is Short

Long TR: 100ms+Short TR: less than 50msShort TE: 15-25msLow flip angles: 5-20 degreeLarge 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.ExampleFLASH: 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) SiemensLAVA 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)

SiemensSSFP: SS Free Precession(GE)T2 FFE: Fast Field Echo (Philips)

Both S+ and S- used for image formationAlso known as balanced SSFPHave 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 imagingT1w 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 GE3D 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.