Gradient echo pulse sequences

1. 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)

2. Gradient echo pulse sequences 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.

3. 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

4. Conventional gradient echo

5. Uses of gradient echo 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.

6. Manipulating Parameters 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.

7. Typical values T1 weighting Large flip angle 70 -110 degrees Short TE 5-10 ms Short TR less than 50 ms Average scan time several seconds to minutes T2* weighting 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 Proton density weighting 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

8. The steady state 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.

9. 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)

10. 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).

11. Learning point 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 A FID signal which occurs as a result of the withdrawal of the RF pulse and contains T2* information A spin echo whose peak occurs at the same time as an RF pulse

12. 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.

13. A FID and a spin echo occur at each RF pulse

14. Summary 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.

15. GE-Coherent residual transverse magnetization 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.

16. 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.

17. Coherent gradient echo pulse sequence

18. Sample image T2* weighted image using a coherent gradient echo. TE 15 ms, TR 40 ms, flip 350, breath holding single slice obtained in 11s

19. Uses of coherent gradient echo 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.

20. Parameters To maintain the steady state Flip angles 30 � 45 TR 20-50 ms 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 (to minimize T2* to produce T1 or proton density weighting TE should be the shortest possible)

21. Advantages & Disadvantages Very fast scans, breath holding possible Very sensitive to flow so good for angiography Can be acquired in a volume acquisition Poor SNR in 2D acquisitions Magnetic susceptibility increases Loud gradient noise

22. Incoherent(spoiled) RTM 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

23. RF spoiling 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

24. 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.

25. Uses 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

26. Example for RF spoiled GE 5.21 T1/proton density weighted image using RF spoiling. TE 6 ms, TR 35 ms, flip 35, part of volume acqusition which took 7 minutes

27. Parameters To maintain steady state Flip angle 30-45 TR 20-50 ms 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

28. Gradient spoiling 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)

29. Steady state free prcession (SSFP) 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.

30. Composition of RF pulse 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 The average amplitude = 19600/32 = 61.250

31. 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.

32. 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.

33. 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.

35. Effective TE & actual TE 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

36. Uses of SSFP 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.

37. Parameters 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

38. Comparison between Coherent, incoherent & SSFP

39. Ultra-fast sequences 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 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.

40. 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.

41. (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)

42. Echo planer imaging(EPI) 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