Magnetic Resonance Imaging 4

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 After the RF pulse is withdrawn, the FID signal is immediately produced due to inhomogeneities in the magnetic field and T2* dephasing occurs.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 transversecomponent 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. 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. Dephasing Rephasing How gradient dephase & rephase 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 • The TR, TE and flip angle affect image weighting and contrast RF pulse Rephasing gradient Gradient echo TAU TAU TE TR

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 100 ms Short TR less than 50ms Short TE 5 – 10 ms Low flip angles 5 -20 Large flip angles 70 - 110

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

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