Basic Pulse Sequences In MRI

MRI Pulse Sequences

1. BASIC PULSE SEQUENCES IN MRI UPAKAR PAUDEL ( B.Sc MIT ) UCMSTH Bhairahawa

2. INTRODUCTON A pulse sequence is a sequence of events, which are needed to acquire MRI images. These events are: RF pulses, gradient switches and signal collecting. The way in which the RF coil & gradient fields are turned on & off is called pulse sequences. What kind of image contrast we want to see and kind of pathology we want to detect is determined by the pulse sequence used.

3. The basic MRI pulse sequences are Spin echo pulse sequence Gradient echo pulse sequence

4. BASIC PARAMETERS TR (Repetition time) :- It is the time from the application of one RF pulse to the application of next RF & is measured in milliseconds (ms).( usually time between two (90 degree pulses)

5. 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). TE is the time interval between the beginning of transverse relaxation following excitation and when the magnetization is measured to produce image contrast (Echo event)

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

7. Multi Slicing

8. The average time per slice is significantly reduced using multiple slice acquisition methods. Several slices within the tissue volume are selectively excited during a TR interval to fully utilize the (dead) time waiting for longitudinal recovery in a specific slice . The total number of slices is a function ofTR , TE, and machine limitations : Total no of slices = TR/(TE+C) where C is a constant dependent on the MR equipment capabilities (computer speed, gradient capabilities, RF cycling, etc.). Long TR acquisitions such as proton density and T2-weighted sequences provide a greater number of slices than T1 weighted sequences with a short TR. The chief trade-off is a loss of tissue contrast due to cross-excitation of adjacent slices, causing undesired spin saturation.

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

10. The 180 degree RF pulse flips these individual magnetic moments through 180 degree. They are still in the T P ,but now the magnetic moments that form the trailing edge before the 180 degree 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 magnetisation is in phase –max. signal induced in the coil which is called spin echo. The spin echo now contains T1 and T2 information as T2* decay is reduced.

11. Spin echo using 1 echo

12. Spin echo using 2 echoes

13. Multi echo spin echo

14. TIMING PARAMETERS

15. Advantages Good image quality Very versatile True T2 weighting Available on all systems Gold standard for image contrast and weighting Disadvantage Long scan times

16. GRADIENT ECHO The gradient echo pulse sequence is the simplest type of MRI sequence. The major purpose behind the gradient technique is a significant reduction in scan time Small flip angle are employed, which in turn allow very short repetition time thus decreasing the scan time. The gradient echo is generated by the frequency encode gradient, except that it is used twice in succession and in opposite direction : it is used in reverse at first to enforce transverse dephasement of spinning protons and then right after , it is used as a readout gradient to realign the dephased protons and hence acquired signals.

17. There is absence of 180 degree RF pulse in gradient echo sequence Does not compensate for T2* effect Increased sensitivity to T2* effect as there is lack of 180 degree refocusing pulse

18. Dephasing And Phasing

19. TIMING PARAMETERS

20. Advantages: Fast imaging Low RF deposits Dynamic scan possibility Low flip angle Disadvantages: Low signal Difficult to generate T2 contrast Sensitive to Bo inhomogenities Sensitive to susceptibility effects T2* related artifacts

21. INVERSION RECOVERY In inversion recovery first 180 degree RF pulse is applied which flip the magnetization vector in 180 degree i.e. –Z direction. There is no magnetization in the x-y plane yet After the 180 degree pulse there is only T1 recovery going on because there is no component in the x-y plane and therefore no T2 relaxation The T1 relaxation process would take place twice as long as when the net magnetization would have been flipped to x-y plane. T1 relaxation is allowed to happen for certain time, known as the inversion time (TI) after that normal SE sequence is applied

22. After a time TI , we apply the 90 degree pulse which flips the longitudinal magnetization into the x-y plane the contrast on the image depends on the amounts of longitudinal recovery of each vector If the excitation pulse is applied after the NMV has relaxed back through the transverse plane , the contrast will depend on the amount of longitudinal relaxation of each vector (like in spin echo) The resultant image is highly T1 weighted as the 180 degree pulse causes full saturation ensuring a large contrast between tissues. T1 WEIGHTING

23. If the 90 degree excitation pulse is not applied until the NMV had reached full recovery , a proton density weighted image results, as both fat and water have fully relaxed PD WEIGHTING

24. PATHOLOGY WEIGHTING Results an image that is predominantly T1W but where pathological processes appear brighter. It is achieved when TE is increased to give tissue with long T2 a bright signal. TI 400-800ms. TE 70ms+ TR 2000ms+

25. TIMING PARAMETERS

26. FLAIR Fluid attenuated inversion recovery (FLAIR)is an special inversion recovery sequence with long TI to remove the effect of fluids from the resultant images The signal from CSF is nulled by selecting TI corresponding to the time of recovery of CSF from180 degree to the transverse plane and there is no longitudinal magnetisetion present in CSF . When the 90 degree excitation pulse is applied the CSF vector is flipped into full saturation. So signal from CSF is nulled ( as no transverse comp) It is used to suppress the CSF signal in T2 & proton density images. So pathology adjacent to the CSF is seen more clearly A TI of 1700-2200 ms achieves CSF suppression

27. This type of sequence is particularly useful in the detection of subtle changes at the periphery of the hemispheres and in the periventricular region close to CSF. The usefulness of FLAIR sequences has been evaluated in diseases of the central nervous system such as : infarction multiple sclerosis subarachnoid hemorrhage head injuries, and others.

28. TIMING PARAMETERS

29. STIR Short Tau Inversion Recovery is an IR pulse sequence that uses a short TI that corresponding to the time it takes fat to recover from full inversion to transverse plane. It is used to achieve suppression of fat in T1 weighted image Inversion time is calculated as TI=T1ln2 For fat the inversion time is approximately 140ms at 1.5T

30. TIMING PARAMETERS

31. FAST SPIN ECHO Fast spin echo (FSE) is a much faster version of conventional spin echo. In spin echo sequences, one phase encoding only is performed during each TR . The scan time is a function of TR, NEX and phase encodings. One of the ways of speeding up a conventional sequence is to reduce the number of phase encoding steps. However this normally results in a loss of resolution. FSE overcomes this by still performing the same number of phase encodings, thereby maintaining resolution, but more than one phase encoding is performed per TR, reducing the scan time.

32. FSE employs a train of 180° rephasing pulses, each one producing a spin echo. This train of spin echoes is called an echo train . The number of 180° RF pulses and resultant echoes is called the echo train length ( ETL ) or turbo factor . The spacing between each echo is called the echo spacing . After each rephasing , a phase encoding step is performed and data from the resultant echo are stored in K space . Therefore several lines of K space are filled every TR instead of one line as in conventional spin echo. As K space is filled more rapidly, the scan time decreases.

33. Typically 2, 4, 8 or 16, 180° RF pulses are applied during every TR. As 2, 4, 8 or 16 phase encodings are also performed during each TR, the scan time is reduced to 1/2, 1/4, 1/8 or 1/16 of the original scan time. The higher the turbo factor the shorter the scan time.

34. Turbo echo

35. TIMING PARAMETERS

36. Advantages: Short scan times High resolution imaging Increased T2 weighting Excellent contrast betn tissues & still fluids Very fast: useful for moving organs. Low sensitivity to magnetic susceptibility artifacts Disadvantages: Some flow artefacts increased Incompatible with some imaging options Some contrast interpretation problems Image blurring possible fat remains bright in t2 weighted image.

37. Fast Advanced Spin Echo (FASE)

38. SINGLE SHOT FSE This is the combination of FSE with partial Fourier technique. Half of lines of k-space are acquired in one TR and the other half are interpolated. Reduction in imaging time as all of the image data is acquired in one TR. However it’s disadvantage is SNR is low.

39. 3D FSE It is achieved by the excitation of slab as opposed to single slice. Helps in Acquisition of high resolution T2W image. Single breath-hold volume acquisition of the liver and for MRCP. Less susceptibility artefact than conventional 3D gradient echo.

40. CSE V/S FSE Fat remains bright on T2W image in FSE. Muscle appear darker on FSE than CSE Multiple 180 degree pulses reduce the magnetic susceptibility effect in FSE Artefact from metal implant is significantly reduced using fast SE. The TR of fast SE is much longer than CSE.

41. STEADY STATE Stage where the TR is shorter than the T1 and T2 times of the tissue. Flip angles of 30 degree- 45 degree and TR of 20-50 ms achieves this state No times for the transverse magnetization to decay before the pulse sequence is achieved so there is coexistence of both longitudinal and transverse magnetization

42. Residual transverse magnetisetion The transverse magnetization produced as a result of previous excitations is called the residual transverse magnetization(RTM) The RTM affects the contrast as it results in tissues with long T2 times appearing bright on the image Gradient echo sequence are classified whether the RTM is in phase(coherent) or out of phase(incoherent) RTM is kept coherent by a processes know as rewinding

43. Rewinding It is the process by residual transverse magnetization is kept coherent Achieved by reversing the slope of the phase encoding gradient after readout Results in RTM rephasing so that it is in phase at the beginning of the next repetition This allows rtm to build up so that tissues with a long T2 produce high signal

44. SEQUENCES GE HITACHI PHILIPS SIEMENS TOSHIBA

45. REFERENCES MRI in Practice : Catherine Westbrook and Carolyn Kaut . MRI Made Easy : Prof. Dr. Hans H. Schild The Essentials of Medical Imaging : Jerrold T. Bushberg Contrast mechanism and pulse sequences : Allen W. Song MRI pulse sequences : Jerry Allison

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