1. Magnetic Resonance Imaging 4
Gradient echo
V.G.Wimalasena
Principal
School of Radiography

2. The gradient echo pulse sequence
This refers to the use of a variable RF excitation pulse which flips the NMV through any angle (usually less than 900).
The transverse component of magnetization is less than that in spin echo, because only part of the longitudinal magnetization is converted to transverse magnetization.
300
900
Small transverse component
Full transverse component

3. After the RF pulse is withdrawn, the FID signal is immediately produced due to inhomogeneities in the magnetic field and T2* dephasing occurs.
The magnetic moments within the transverse component of magnetization dephase.
They are then rephased by a gradient.
The gradient causes a change in the magnetic field strength within the magnet.
The gradient rephases the magnetic moments so that a signal can be received by the coil.
The signal contains T1 and T2 information.
This signal is called a gradient echo.

4. Gradients Magnetic field Gradients perform many tasks.
They are generated by passing currents through coils of wire situated within the bore of the magnet.
The gradient field interacts with the main static field, so that the magnetic field strength along the axis of the gradient coil is altered in a linear way.
The middle of the axis of the gradient remains at the field strength of the main magnetic field.
This is called the isocentre.
High
A gradient magnetic field
Low

5. Gradients are used to either dephase or rephase the magnetic moments
The magnetic field strength increases relative to isocentre in one direction, and decreases in the other direction of the gradient axis.
Gradients are used to either dephase or rephase the magnetic moments
Bore of magnet
Isocentre
Magnetic field strength decreases (B0 - b)
Magnetic field strength increases (B0 + b)
Magnetic field strength remains constant (B0)
Nuclei slow down
Nuclei speed up

6. How gradient dephase & rephase
Dephasing
Rephasing
In phase
Out of phase
Out of phase F F S S S F
Rephasing gradient
Nuclei speed up
Nuclei speed up
Nuclei slow down
Dephasing gradient
Nuclei slow down
In phase FS

7. Timing parameters & weighting in gradient echo
RF pulse
Rephasing gradient
Gradient echo
TAU
TAU TE TR
The TR, TE and flip angle affect image weighting and contrast

8. T1 weighting in gradient echo
Flip angle is large, and
TR is short
to avoid full recovery of longitudinal magnetization and maintain saturation and to maximize T1 differences.
TE is short to minimize T2* differences

9. T2* weighting in gradient echo
Small flip angle, to minimize T1 recovery, and
Long TR
to allow full recovery of fat and water vectors
Long TE, so that fat & water to decay sufficiently to show the differences.
(in practice because of smaller flip angle, TR can be kept relatively short)

10. Proton density weighting in gradient echo
Small flip angle to minimize T1 recovery
Long TR to full recovery of longitudinal magnetization
Short TE to minimize T2* decay
Typical values
Long TR ms
Short TR less than 50ms
Short TE – 10 ms
Low flip angles
Large flip angles

11. Advantages of gradient echo
Since gradients can rephase faster than 180 RF pulses the minimum TE is much shorter
As the flip angle is small TR also can be reduced
So the scan time is reduced

12. Disadvantages of gradient echo
There is no compensation for magnetic field inhhomogeneities.
Therefore very susceptible to magetic field inhomogeneities and produce artefacts.
T2 weighting is termed T2* weighting because T2* effects are not eliminated.

13. Comparison TR TE Flip angle Spin echo Long 2000ms+ Long 60ms+
90 (usually)
Short ms
Short 10 – 25 ms
Gradient echo
Long 100ms+
Long 15 – 25 ms
Small 5 – 20 large
Short
Less than 50 ms
Short 5 – 10 ms
Small 5 – 20 large

14. Questions Define the term weighting What is meant by:
A T1 weighted image?
A T2 weighted image?
A proton density weighted image?
When saturation occurs:
The NMV is pushed beyond the transverse plane
The magnetic moments dephase
The MR signal is received
What values of TR and TE are needed for T1 weighting and why?
Why do we use a 1800 RF pulse in spin echo?
List three main factors that make gradient echo sequences different from spin echo.
What parameters control T1 and proton density weighting in gradient echo?
What type of contrast will the following produce?
TR 400 ms, TE 5ms, flip 120
TR 50 ms, TE 15 ms, flip 35