MRI PULSE SEQUENCES.pptx/////////////////

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1. MRI PULSE SEQUENCES Presented by : Prativa Khanal BSc.MIT 2 nd yr NMCTH

2. INTRODUCTION TO PULSE SEQUENCES The way in which the RF coil & gradient fields are turned on & off is called pulse sequences Image quality is determined by the pulse sequence used PS is which kind of image contrast we want to see or even which kind of pathology we want to detect A sequence of radiofrequency (RF) pulses applied repeatedly during MR study to acquire MRI images

3. IMAGE CONTRAST The contrast characteristics of each image in MRI depends on many variables, It is important to understand the mechanisms that affect image contrast in MRI An image has contrast if it has high signal(white on the image), as well as areas of low signal (dark on the image) Some areas have intermediate signal(shades of grey btn white & black) NMV can be separated into the individual vectors of the tissues present in the pt i.e.fat, CSF,& muscle

4. FAT AND WATER Fat is hydrogen linked to carbon & Water is hydrogen linked to oxygen which tends to steal the electrons away from around the hydrogen nucleus In fat, the carbon does not take the electrons from around the hydrogen nucleus. They remain in an electron cloud protecting the nucleus from the effects of the main field Hydrogen in fat fecovers more rapidly along the longitudinal axis than water and loses transverse magnetization faster than in water Subsequently, fat and water appear differently in MR image

5. CONTRAST MECHANISM Images obtain contrast mainly through the mechanisms of : T1 recovery T2 decay & Proton or spin density- the proton density of a tissue is the number of protons per unit volume of that tissue

6. T1 recovery in fat Occurs due to nuclei giving up their energy to the surrounding environment The slow molecular tumbling in fat allows the recovery process to be relatively rapid The NMV of fat realigns rapidly with  B and therefore the T1 time of fat is short 

7. T1 RECOVERY IN WATER T1 recovery occurs due to nuclei giving up the energy acquired from the RF excitation pulse to the surrounding lattice In water nuclear mobility is high resulting in less efficient t1 recovery The magnetic moments of water take longer to relax and regain their longitudinal magnetization NMV of water takes longer to realign with  B and so the T1 time of water is long

8. T2-decay in fat :occurs as a rest of the magnetic field of the nuclei interacting with each other, therby changing their energy to their neighbours. As energy exchange is more efficient in the hydrogen in fat the T2 time is short. The T2 time of fat is approx. 80ms T2 decay in water :As energy exchange in water is less efficient than in fat, the T2 time of hydrogen in water is long. The T2 time of water is approximately 200 ms

9. T1 CONTRAST T1 time of fat is shorter than water, the fat vector realigns with  B faster than that of water There is less longitudinal magnetization in water after the RF pulse. Water has low signal & appears dark The longitudinal component of magnetization in fat is larger than water A short TR & short TE will result in a T1 weighted image Excellent for demonstrating anatomy

10. T2 CONTRAST T2 time of fat is shorter than that of water,the transverse component of magnetization of fat decays faster Magnitude of transverse magnetization in water is large Water has high signal and appears bright on T2 contrast image Magnitude of transverse magnetization in fat is small so has low signal and appears dark

11. BASIC PARAMATERS TR (Repetition time) :- It is the time from the application of one RF pulse to the application of next & is measured in milliseconds (ms)

12. TE (Echo Time) :- It is the time from the application of the RF pulse to the peak of the signal induced in the coil & is also measured in milliseconds (ms). It is usually the half of the TR

13. TI(Time from Inversion):It is the time from the application of the 180 ° inverting pulse to the 90 ° excitation pulse

14. AT (ACQUISTION TIME) Time of acquisition is controlled by many factors :- Signal to noise ratio ( SNR) Contrast to noise ratio (CNR) Spatial resolution Scan time

15. FLIP ANGLE The first result of the resonance is the NMV moves out of alignment away from B . The angle to which the NMV moves out of alignment is called flip angle. The magnitude of the flip angle depends upon the amplitude & duration of the RF pulse. Usually the flip angle is 90 °

16. Phase : NMV move into phase with each other. The phase is the position of each magnetic moment as the precessional path around Bo. Two types 1) Out of phase 2 ) In phase

17. TYPES OF PULSE SEQUENCES 1.Spin Echo (SE) a.Conventional spin echo ( CSE) b. Fast spin echo (FSE) c. Ultrafast spin echo (UFSE) d. Turbo spin echo (TSE) 2. Inversion Recovery (STIR , FLAIR ) 3. Gradient Echo (GRE)

18. SPIN ECHO It has at least two RF pulses,an excitation pulse and one or more 180 ° refocusing pulses that generate spin echo Utilizes 90 ° excitation pulse to flip the NMV into transverse plane.NMV precesses in the T Plane induces voltage in the receiver coil The paths of precession of the nuclei within the NMV are translated into the T plane When the 90 ° RF pulse is removed an FID signal is produced, T2 dephasing occurs immediately & the signal decays A 180 ° RF pulse is then used to compensate this dephasing

19. 180° pulse has sufficient energy to move NMV through 180° The T2 dephasing causes the magnetic moments to fan out in the TP The magnetic moments becomes out of phase with each other The magnetic moments that slow down, form the trailing edge of the fan (S) & that speed up, forms the leading edge of the fan (F),

20. The 180° RF pulse flips these individual magnetic moments through 180° They are still in the TP ,but now the magnetic moments that form the trailing edge before the 180 ° pulse, form the leading edge Conversely, previously formed leading edge becomes trailing edge. So the trailing edge begins to catch up with the leading edge after specific time both edges superimposed At this instant –transverse magnetization in phase –max. signal induced in the coil which is called spin echo

21. TIMING PARAMETERS T1 WEIGHTING short TE 10-20ms short TR 300-600ms Typical scan time 4-6min PROTON DENSITY/T2 WEIGHTING Short TE 20 ms/long TE 80ms+ Long TR 2000 ms+ Typical scan time 7-15min

22. Advantages Good image quality Very versatile True T2 weighting sensitive to pathology Disadvantages Scan times relatively long

23. In most of the SE PS , more than one 180 ° RF pulse can be applied after the 90 ° excitation pulse Each 180 ° pulse generates a separate SE . One two or four 180 ° RF pulses can be used to produce either one two or four Images

24. Spin echo using single echo Used to produce T1 weighted images if short TR &TE r used One 180 ° RF is applied after 90 ° excitation Generates single spin echo

25. Spin echo using two echoes Used to produce both PD & a T2 weighted image in TR time The 1st spin echo generated early by selecting short TE A little T2 decay has occurred & T2 diff, btn the tissues are minimized

26. Two 180 ° pulses are sent after each 90 ° pulse to obtain dual echoes per TR The first echo has a short TE (TE1) and a long TR and results in a set of proton density weighted image The second echo has a long (TE2) and a long TR and results in a T2 weighted set of images. This echo has less amplitude than the first echo because more T2 decay has occurred by this time

27. FAST SPIN ECHO (TURBO SPIN ECHO) A regular SE seq requires as many repetitions as there are lines in the K-space to complete a slice acquisition Instead of acquiring K-space lines of other slices at diff. positions during the wasted time,it is possible to acquire several K-space lines for the same slice Such fast spin echo sequences use one 90 ° excitation pulse & 2 or more 180 ° pulses in the same repetition time & with different phase encoding gradient steps, to acquire multiple echoes that will fill the K-space

28. The NO. of echoes acquired after a single 90 ° excitation is called the turbo factor or ECHO TRAIN LENGTH As each echo undergoes more & more T2 decay, the image contrast is modified The effective echo time,detrmined by when the central lines of K-space are acquired,indicates approxmately what the image contrast obtained is like (rather T1-weighted or T2 weighted)

29. Advantage and disadvantage Clinical use :- Billiary & urinary tract exploration, myelography ADVANTAGE DISADVANTAGE Short scan times Some flow artifacts increased High resolution imaging Incompatible with some imaging options Increased T2 weighting Some contrast interpretation problems Magnetic suspectibilty decreases Image blurring possible

30. WEIGHTING To demonstrate either T1 proton density or T2 contrast, specific values of TR & TE are selected for a given pulse sequence The selection of appropriate TR& TE weights an image so that one contrast mechanism predominates over the other two

31. T1 weighting image TR controls how far each vector can recover before it is excited by the next RF pulse, to achieve T1 weighting TR must be short enough so that neither fat nor water return to B   . If the TR is too long both fat & water return to Bo & recover their Longutidinal Magnetization fully TR controls the amount of T1 weighting For T1 weighting the TR must be short If the TE is also short,there is little time for T2 relaxation & image contrast will not be very dependent on T2

32. T1 DIFFERENCES BETN FAT & WATER

33. T1 WTD IMAGE If the TR is very long, the longitudinal magnetization of all the tissues will have recovered completely(complete T1 relaxation) If the TR is short,tissue signal & image contrast will depend on the T1 characteristics of tissues as not all the tissues will have completely recovered their longitudinal magnetization When both the TR and the TE are short, the image is said to be T1 weighted

34. SE (T1 weighted image) Advantage - Anatomic Imaging Disadvantage- very long acquisition time Clinical use:- almost all the organs explored in MRI

35. T2 weighting image The contrast predominantly depends on the differences in the T2 times betweenn fat & water TE controls the amount of T2 decay that is allowed to occur before the signal is received TE must be long enough to give both fat & water time to decay A long TR is about 2000 ms or more & along TE is about 80 to 140ms With such parameters , the tissues with a long T2 time will have a stronger signal than the tissues with a short T2 time

36. T2 diff. btn fat &water

37. T2 weighted- Advantage –imaging of water/fluids ( CSF,edema,biliary MRI) Disadvantage –very long acquisition time

38. PROTON DENSITY CONTRAST Refers to differences in signal intensity between tissues which are consequences of their relative no. of protons /volume. The effect of T1&T2 contrast must be diminished If we use a long TR (>2000ms) & a short TE(10-20ms) , the tissue signal will not be very dependent on T1 & T2 relaxation Tissues with rich hydrogen content(like water)will be bright where as tissues with low hydrogen content will be dark

39. Advantage -Very long acquisition time Clinical use: -osteoarticular -paediatric Neuroradiology

40. SUMMARY OF SPIN ECHO Uses a 90 ° excitation pulse followed by one or more 180 ° rephasing pulses to generate one or more spin echoes SE produces either T1, T2 or proton Density weighting images TR controls the T1 weighting Short TR maximizes T1 weighting Long TR maximizes PD weighting TE controls the T2 weighting Short TE minimizes T2 weighting Long TE maximizes T2 weighting

41. GRADIENT ECHO PULSE SEQUENCE Gradient Echo PS utilizes an RF excitation pulse that is variable Therefore flips NMV through any angle (not just 90 ° ) When a flip angle other than 90 ° is used, only part of the longutidinal magnetisation is converted to transverse magnetisation, which precesses in the transverse plane and induces a signal in the receiver coil

42. After the RF pulse is withdrawan,FID signal is immediately produced due to inhomogenities in the magnetic field & T2 dephasing therefore occurs Magnetic moments within the transverse component of magnetisition diphase, and are then rephased by a gradient The gradient rephases the magnetic moments so that signal can be received by the coil which contains T1 and T2 information This signal is called gradient echo

43. There are basic three differences between SE and GRE sequences 1. There is no 180 ° pulse in GRE. Rephasing of TM in GRE is done by gradients 2. The flip angle in GRE is smaller, usually less than 90 ° Since flip angle is smaller there will be early recovery of longitudinal magnetization (LM) such that TR can be reduced, hence the scanning time 3. Transverse relaxation can be caused by combination of two mechanisms:- A. Irreversible dephasing of TM resulting from nuclear, molecular and macromolecular magnetic interactions with proton B. Dephasing caused by magnetic field inhomogeneity

44. Advantages & Disadvantages Advantages Rephase faster than 180 ° RF pulses TR can be shortened without producing saturation Shorter scan time than SE pulse sequence Disadvantages No compensation for magnetic field inhomogeneties Can also contain magnetic suspectively artifact

45. USES Can be used to acquire T2,T1 and proton density weighting GE allow for reduction in scan time so can be used for single slice breath-hold acquistions in the abdomen and for dynamic contrast enhacement May be used to produce angiographic type images

46. Timing parameters of gradient echo

47. SPOILED OR INCOHERENT GRE SEQUENCES If the residual TM is destroyed by RF pulse or gradient such that it will not interfere with next TR, the sequences are called spoiled or incoherent GRE sequences. These sequences usually provide T1-weighted GRE images These sequences can be acquired with echo times when water and fat protons are in-phase and out-of-phase with each other. This ‘in- and out-of-phase imaging’ is used to detect fat in the lesion or organs. It is modified to have time-of-flight MR Angiographic sequences. The 3D versions of these sequences can be used for dynamic multiphase post contrast T1-weighted imaging

48. STEADY STATE (SS) Sequences The residual TM is not destroyed. When residual transverse magnetization is refocused keeping TR shorter than T2 of the tissues, a steady magnitude of LM and TM is established after a few TRs. Once the 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. 1. Post-excitation refocused steady-state sequences :- Only FID (S+) component is sampled. Since S+ echo is formed after RF excitation (pulse), it is called post-excitation refocused.. 2. Pre-excitation refocused steady-state sequences . Only spin echo (S-) component is used for image formation. S- echo is formed just before next excitation hence the name pre-excitation refocused 3.Fully refocused steady-state sequences :- Both FID (S+) and Spin echo (S-) components are used for the image formation. These are also called ‘balanced-SSFP’ sequences as gradients in all three axes are balanced making them motion insensitive

49. INVERSION RECOVERY(IR) One method for manipulating contrast is called inversion recovery Is a pulse sequence which begins with a 180 ° inverting pulse . It is used to produce heavily T1 weighted images to demonstrate anatomy . Large contrast differences between fat and water can be obtained The 180 ° IR pulse changes the direction of the longitudinal Magnetisation vector to its opposite.Then it is going to recover as defined by T1 relaxation At time T1(inversion time), a regular spin echo(or gradient recalled echo or echo planar seq is performed,starting with an excitation pulse

50. The inversion recovery pulse sequence

51. T1 weighting in inversion recovery

52. PD weighting in IR

53. STIR (Short Tau Inversion Recovery ) STIR is an IR pulse sequence that uses a TI that corresponds to the time it takes the fat vector to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat. This is called the null point. As there is no longitudinal component of fat when the 90° RF excitation pulse is applied, there is no transverse component after excitation, and signal from fat is nulled A TI of 100-175 ms usually achieves fat suppression, although this value varies slightly at different field strengths Uses :STIR is an extremely important sequence in musculoskeletal imaging because normal bone, which contains fatty marrow, is suppressed, and lesions within bone such as bone bruising and tumours are seen more clearly

54. Suggested parameters Short TI (tau) 150-175 ms (to suppress fat depending on field strength) Long TE 50 ms+ (to enhance signal from pathology) Long TR 4000 ms+ (to allow full longitudinal recovery) Long turbo factor 16-20 (to enhance signal from pathology) Scan tip:When not to use STIR STIR should not be used in conjunction with contrast enhancement, which shortens the T1 recovery times of enhancing tissues, making them relatively hyperintense. The T1 recovery times of these structures are shortened by the contrast agent so that they approach the T1 recovery time of fat. In a STIR sequence, therefore, enhancing tissue may also be nulled.

55. FLAIR ( fluid attenuated inversion recovery ) FLAIR :- is the inversion recovery sequence where CSF is nullified by selecting 180 ° inverting pulse to the inverse plane . It is used to suppress the CSF signal in T2 & proton density images. So pathology adjacent to the CSF is seen more clearly In the same way, with a TI time about 2000ms, water signal is eliminated (water T1 is long)

56. USES FLAIR is used in brain and spine imaging to see periventricular and cord lesions more clearly because high signal from CSF that lies adjacent is nulled lt is especially useful visualizing multiple sclerosis plaques, acute sübarachnoid meningitis. This TI value nulls signal from normal white matter so that lesions within it appear much brighter by comparison This sequence (which requires a TI of about 300 ms) is very useful for white matter lesions such as periventricular leukomalacia and for congenital grey/white matter abnormalities

57. Suggested parameters Long TI factor1700-2200 ms (to suppress CSF depending on field strength Long TE 70 ms+ (to enhance signal from pathology Long TR 6000 ms+ (to allow full longitudinal recovery) Long turbo factor 6-20 (to enhance signal from pathology) Learning tip: FLAIR and gadoliniumSometimes gadolinium is given to enhance pathology in the FLAIR sequence. This oddity (gadolinium enhancement in T2-weighted images) may be due to the long echo trains used in FLAIR sequences that cause fat to remain bright on T2-weighted images. As gadolinium reduces the Ts recovery time of enhancing tissue so that it is similar to fat, enhancing tissue may appear brighter than when gadolinium is not given

58. THE STEADY STATE The condition where the TR is shorter than the T1&T2 times of the tissue & thus no time for TM to decay before the pulse sequence is repeated again

59. ECHO PLANAR IMAGING Ehoes can be generated by multiple 180 ° pulses termed spin echo EPI (SE EPI) or by gradients termed gradient echo (GE-EPI) It represents the fasest acquisition modes in MRI Real time ,dynamic &functional studies are possible using this technique

60. REAL TIME IMAGING Very fast sequences such as EPI ,permit real time imaging of moving structure It has proved to b very useful in interventional procedures whereas a biopsy needle ,laser probe or other instruments can be visualized in real time Biopsies thermal ablations of tumors angioplastics endoscopies & limited field surgical operation are the most promising applications of this technique

61. REFERENCES MRI Made Easy-schering MRI in practice catherine westbrook & carolyn kaut Internet sources

62. THANK YOU…….