Gradient echo pulse sequences hjn.pptx

The gradient echo pulse sequence is the simplest type of MRI sequence.
The major purposes behind the gradient technique is a significant reduction in scan time. Small variable flip angle are employed , usually less than 90 degrees. which in turn allow very short repetition time thus decreasing the scan time.
Gradient echo pulse sequence differ from spin echo pulse sequence . There is no 180 degree pulse in GRE. T2 relaxation in GRE is called as T2* relaxation. Gradient can be used to either dephase or rephase the magnetic moments of nuclei.

1. GRADIENT ECHO PULSE SEQUENCES ASSISTANT PROFESSOR INCHARGE OF RADIODIAGNOSIS FATHIMA INSTITUE OF MEDICAL SCIENCES

2. GRADIENTS 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 .

3. 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.

4. Basic three differences between SE and GRE sequences 1. There is no 180 degree pulse in GRE. Rephasing of TM in GRE is done by gradients; particularly by reversal of frequency encoding gradient. Since rephasing by gradient gives signal this sequence is called as Gradient echo sequence. 2. Flip angle in GRE are smaller, usually less than 90 degree . Since flip angle is smaller there will be early recovery of LM such that TR can be reduced hence scanning time. 3. Transverse relaxation in SE sequence is caused by combination of two mechanisms A. Irreversible dephasing of TM - SPIN SPIN RELAXATION. B. Dephasing caused by magnetic field inhomogeneity . In SE sequence the dephasing caused by inhomogeneity is eliminated by 180 degree pulse. Hence there is ‘true’ relaxation in SE sequence

5. CLASSIFICATION CONVENTIONAL GRADIENT ECHO PULSE SEQUENCE COHERENT (IN-PHASE) GRADIENT ECHO INCOHERENT (SPOILED) GRADIENT ECHO STEADY STATE FREE PRECESSION (SSFP) BALANCED GRADIENT ECHO FAST GRADIENT ECHO

6. INTRODUCTION TO CONVENTIONAL GRADIENT ECHO PULSE SEQUENCE The gradient echo pulse sequence is the simplest type of MRI sequence. The major purposes behind the gradient technique is a significant reduction in scan time. Small variable flip angle are employed , usually less than 90 degrees. which in turn allow very short repetition time thus decreasing the scan time. Gradient echo pulse sequence differ from spin echo pulse sequence . There is no 180 degree pulse in GRE. T2 relaxation in GRE is called as T2* relaxation. Gradient can be used to either dephase or rephase the magnetic moments of nuclei.

7. A basic gradient echo sequence gradient

8. TYPICAL VALUES IN GRE IMAGING Long TR: 100ms+ Short TR: less than 50ms Short TE: 15-25ms Low flip angles: 5-20 degree Large flip angles: 70- 80 degree

9. USES: Acquire T2 * , T1 and proton density weighting. Reduction in the scan time. Single - slice or volume breath - hold acquisitions in the abdomen, and for dynamic contrast enhancement Used to produce angiographic - type images (TOF).

10. WEIGHTING AND CONTRAST IN GRADIENT ECHO Three different processes affect weighting in gradient echo pulse sequences. 1. Extrinsic parameters ( TR, TE and flip angle affect image weighting and contrast ). 2. The steady state 3. Residual transverse magnetization

11. STEADY STATE It is defined as the stable condition that does not change over time . The overall energy of hydrogen remains constant as the energy ‘ in ’ as determined by the flip angle equals the energy ‘ out ’ as determined by the TR. Energy in = RF excitation pulse.(FLIP ANGLE) Energy out= energy lost through spin lattice energy transfer (TR) Stage where the TR is shorter than the T1 and T2 times of the tissue. Flip angle of 30-45 degree and TR of 20- 50 ms achieves this state. There are, therefore, critical values of flip angle and TR to maintain the steady state . The steady state is a state in which not only LM and TM are present at the same time but also their value or magnitude is maintained steady during data acquisition .

12. The steady state

13. RESIDUAL TRANSVERSE MAGNETISATION The transverse magnetisation produced as a result of previous excitations is called the residual transverse magnetization(RTM ). but remains over several TR periods in the transverse plane. It is called the residual transverse magnetization and it affects image contrast as it induces a voltage in the receiver coil . It affects image contrast as it results in tissues with long T2 times (such as water), appearing bright on the image

14. How is steady state achieved? Steady state is achieved by keeping TR shorter than T1 and T2 times of tissues .Since TR is shorter than T2 there is no time for TM to decay completely, before next RF pulse excitation. So, there will be some residual TM left over. Second factor contributing for SS is low flip angle usually 30-45 degree. Since NMV is flipped by less than 90 degrees there is always some residual LM left.

15. How magnitudes of LM and TM are maintained steady? Residual TM is flipped by gradients through 180 degree from along positive side of Y-axis to negative side of Y-axis . When next RF excitation is done this residual TM along negative side of Y-axis is moved towards Z-axis. This adds up into magnitude of residual LM, which now increases . At the same time some part of the residual LM is flipped by same flip angle so that TM is formed along the positive side of Y-axis. This cycle is repeated with every RF excitation and values of LM and TM are maintained.

16. Why do we want steady state? With steady state shortest TR and scan time are achieved. It also affects image contrast as it causes tissues with long T2 appear more bright on the image.

17. Types of Steady State Depending on what is done to residual TM after gradient echo is received, SS can be of three types- Coherent (in-phase) residual TM Incoherent (spoiled) residual TM Steady state free precession (SSFP)

18. Coherent (in-phase) residual TM Uses variable flip angle excitation pulses followed by gradient repahsing to produce a gradient echo . These sequences keep this residual magnetization coherent by a process known as rewinding. Rewinding is achieved by reversing the slope of the phase encoding gradient after readout. This results in the residual magnetization rephasing, so that it is in phase at the beginning of the next repetition. Since residual TM is rephased by gradients and it is in phase, it is called as coherent residual TM. Examples—FISP, GRASS

19. The coherent gradient echo sequence.

20. The rewinder gradient rephases all transverse magnetization regardless of when it was created. Therefore the resultant echo contains information from the FID and the stimulated echo. These sequences can therefore be used to achieve T1 or T2 * weighted images. USES This pulse sequence produce T2* weighted image. As fluid is hyper intense ,they give an angiographic , myelographic or orthographic effect. Can be acquired slice by slice or in a 3D volume Acquisition. As the TR is short, a slice can be acquired in a single breath hold.

21. Advantages Very fast scans, breath - holding possible Very sensitive to flow (angiography) can be acquired in a volume acquisition Disadvantages Reduced SNR in 2D acquisitions Magnetic susceptibility increases Loud gradient noise

22. Coherent Gradient Echo GE: GRASS Siemens: FISP Philips: FFE

23. INCOHERENT OR SPOILED GRADIENT ECHO Uses variable flip angle excitation pulses followed by gradient repahsing to produce a gradient echo . The steady state is maintained so that residual transverse magnetization is left over from previous repetitions. These sequences dephase or spoil this magnetizati on so that its effect on image contrast is minimal. Spoiling can be done by RF pulses or gradients. RF spoiling Gradient spoiling

24. RF spoiling in the incoherent gradient echo sequence.

25. USES T his pulse sequences produce T1 or proton density weighted images . Image contrast is mainly influenced by the FID that contributes T1 and proton density contrast.

26. Advantages can be acquired in a volume or 2D breath holding possible good SNR and anatomical detail in volume can be used after gadolinium contrast injection Disadvantages SNR poor in 2D loud gradient noise

27. Incoherent Gradient Echo GE: SPGR Siemens: FLASH Philips: TIFFE

28. Steady state free precession (SSFP) In gradient echo sequences the TE is not long enough to measure the T2 time of tissues as a TE of at least 70ms is required for this. In addition , gradient rephasing is so inefficient that any echo is dominated by T2 * effects and therefore true T2 weighting cannot be achieved. The SSFP sequence overcomes this problem to obtain images that have a sufficiently long TE and less T2* than in other steady state sequences . However, in SSFP we need to digitize frequencies only from this stimulated echo and not from the FID. To do this, the stimulated echo must be repositioned so that it does not occur at the same time as the subsequent excitation pulse. This is achieved by applying a rewinder gradient, which speeds up the rephasing so that the stimulated echo occurs sooner Example: PSIF

29. SSFP sequence

30. By this way, the stimulated echo can be received and data from it is collected and which is used to form the image. In reverse echo gradient echo ,there are usually two TEs; 1. THE ACTUAL TE- the time between the peak of the gradient echo and the next RF excitation pulse. 2. THE EFFECTIVE TE- the time from the peak of the gradient echo to a previous RF excitation pulse.

31. Advantages can be acquired in a volume and in 2D true T2 weighting achieved than in conventional GE Disadvantages susceptible to artefacts image quality can be poor loud gradient noise

32. Reverse Gradient Echo GE: SSFP Siemens: PSIF Philips: T2FFE

33. SUMMARY Sampling Weighting Coherent gradient echo FID and the stimulated echo T1 or T2 * In Coherent gradient echo FID T1 SSFP stimulated echo more T2 weighted

34. BALANCED GRADIENT ECHO This sequence is a modification of the coherent gradient echo sequence that uses a balanced gradient system to correct for phase errors in flowing blood and CSF both the FID and the spin echo are collected within a single readout. This results in images where fat and water produce a higher signal, greater SNR and fewer flow artefacts than coherent gradient echo in shorter scan times . Balanced gradient -echo is a steady state sequences in which longitudinal during the acquisition , thereby preventing saturation . As the area of the gradient under the line equals that above the line, moving spins accumulate a zero phase change as they pass along the gradients. As a result, spins in blood and CSF are coherent and have a high signal intensity. This gradient formation is the same as flow compensation or gradient moment rephasing .

35. BALANCED GRADIENT ECHO In balanced gradient echo this gradient is applied in the slice and frequency axes. In addition , higher flip angles and shorter TRs are used than in coherent gradient echo, producing a higher SNR and shorter scan times This is achieved by selecting a flip angle of 90° , for example, but in the first TR period only applying half of this, i.e. 45° . In successive TRs the full flip angle is applied but with alternating polarity so that the resultant transverse magnetization is created at a different phase every TR The resultant images display high SNR, good CNR between fat, water and surrounding tissues , fewer flow voids and in very short scan times. A balanced gradient scheme is used to correct for flow artifacts .

36. BALANCED GRADIENT ECHO

37. USES Balanced gradient echo was developed initially for imaging the heart and grerat vessels . It is also sometimes used in joint and abdominal imaging.

38. Advantages Disadvantages Fast shorter scan times Reduced artifacts from flow Good SNR and anatomical details in 3D Images demonstrate good contrast Reduced SNR in 2D acquisitions Loud gradient noise Susceptible to artifacts Requires high performance gradients

39. FAST GRADIENT ECHO Fast gradient echo pulse sequences acquire a volume in a single breath hold. Ultrafast scanning techniques can be SE, GRE or combination of both. Major ultrafast scanning techniques include RARE, STEAM (SE ) FLASH and EPI (GRE).

40. FLASH (fast low angle shot) FLASH is possible because of availability of high gradient strengths of more than 25 mT /m. With this gradient strength TRs of 2-5 ms and TEs of 1-3 ms are easily achieved so that k-space can be filled very fast with one gradient echo per TR. FLASH can be used in body imaging where physiological motion needs to be overcome, e.g . peristaltic motions . cardiac perfusion.

41. ECHO PLANNER IMAGING Echo planner imaging is an MR acquisition method that either fills all the lines of k-space in a single repetition(single shot-SS) or in multiple section( multishot -MS). In EPI, gradient echoes are typically generated by oscillation of the readout gradient. There are many types of EPI; 1. GE-EPI uses a variable flip angle followed by EPI readout in K-space. 2. SE-EPI uses a 90-180 degree followed by EPI readout in K-space K- SPACE:- K-space is an imaginary space which represents raw data matrix.

42. USES Diffusion weighted imaging Functional imaging Real time cardiac imaging Perfusion imaging Interventional techniques Breath hold techniques

43. Advantage s Disadvantages Fast shorter scan times Reduced artifacts from respiratory and cardiac motion All three types of weighting can be achived Functional information acquired Scan time savings can be used to improve phase resolution. Chemical shift artefact is common Peripheral nerve stimulation due to fast switching of gradients Susceptible to artifacts

44. ADVANTAGES DISADVANTAGES OF GRE Much shorter scan times than spin echo pulse sequences. Minium TE is much shorter than in spin echo pulse sequences. TR can also be decreased because other than 90 degree are used. No comparision for magnetic field inhomogeneities . Very susceptible to magnetic field inhomogeneities Contain magnetic susceptibility artefact.

45. SPIN ECHO AND GRE COMPARISION

46. QUESTION Q. what are advantages of using gradient echo sequences? Q. what is the difference between spin echo and gradient echo sequences?

47. REFERENCE MRI in practice ,catherine westbrook,carolyn kaul Roth,john Talbot 4th Edition, 4th Editon.

48. THANK YOU