MRI spin echo pulse sequences

MRI pulse sequence and inversion recovery.
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1. SPIN ECHO PULSE SEQUENCES PRESENTED BY V.SIVA PRAKASH B.Sc. MIT [email protected]

2. INTRODUCTION Pulse sequences enable us to control the way in which the system applies pulses and gradients. In this way, image weighting and quality is determined. There are many different pulse sequences available, and each is designed for a specific purpose. [email protected]

3. TYPES Spin echo pulse sequences (spins are rephased by a 180 ° rephasing pulse): Conventional spin echo Fast or turbo spin echo Inversion recovery. [email protected]

4. Conventional Spin Echo Mechanism: Spin echo uses a 90° excitation pulse followed by one or more 180° rephasing pulses to generate a spin echo. [email protected]

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6. PARAMETERS T 1 weighting Short TE 10– 30ms Short TR 300– 700ms Typical scan time 4– 6 min Proton density/T2 weighting Short TE 20ms/long TE 80ms + Long TR 2000ms + Typical scan time 7–15min [email protected]

7. ADVANTAGES The contrast is truly based on the T1 and T2 relaxation T1 weighted images for anatomy (high SNR) with contrast enhancement - show pathology. T2 weighted images also demonstrate pathology. DISADVANTAGES Scan times relatively long [email protected]

8. Fast Or Turbo Spin Echo MECHANISM The main aim is to reduce the scan time. TR, NEX, no. of phase encoding are the function of time. Decreasing TR,NEX affect image weighting. Reducing phase encoding reduce spatial resolution. [email protected]

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10. So In Fast Spin Echo.... Several 180 ° rephasing pulses to produce train of echo called echo train With More than one phase encoding step and more lines of k Space filled per TR . At each rephasing, an echo is produced and a different phase encoding step is performed. The no:of 180 ° rephasing pulse corresponds to no:of echoes and k space lines, this number called turbo factor or echo train length [email protected]

11. How The Scan Time Reduced Higher the turbo factor shorter the scan time as more phase encoding steps are performed per TR Eg . In conventional spin echo, 256 phase matrix selected so, 256 TR elapse to complete scan In fast spin echo, using turbo factor 16, 16 phase encoding steps are performed every TR. So 256 ÷ 16, scan time reduced to 1/16 of the original Conventional one line is filled per TR FSE several lines are filled per TR [email protected]

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13. Weighting In Fast Spin Echo Different slope of gradient to phase shift the signal by different amount. Steep = less amplitude, but good spatial resolution, effective TE is away from center Shallow = maximum signal, effective TE is centered [email protected]

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16. Two Contrast Differences Occur Fat remains bright on T2 weighted images due to multiple RF pulses (J coupling) Remedy: fat saturation technique Repeated 180 ° pulse can increase “Magnetization transfer effect” so muscle appear darker. [email protected]

17. Sagittal T2 weighted fast spin echo sequence through the pelvis. Note that both fat and water have high signal intensity. [email protected]

18. Blurring may occur at edge of tissue - late echoes have low signal amplitude. Remedy Decrease the spacing between echoes or turbo factor Multiple 180˚ reduce magnetic susceptibility effect Detrimental when looking hemorrhages Artefact from metal implant is greatly reduced Respiratory artefact happen when respiratory compensation technique are not compatible. Patient holds their breath while imaging [email protected]

19. USES Generally speaking contrast in fast spin echo is similar to spin echo, and used in.. Musculoskeletal regions Central nervous system pelvis [email protected]

20. Parameters For T1 Weighting TR 300 -700ms Effective TE minimum Turbo factor 2-8 . [email protected]

21. For PD weighting TR 3000-10000ms (depending on required slice number) effective TE minimum turbo factor 2-8 . [email protected]

22. For T2 weighting TR 3000-100 00ms (depending on required slice number) Effective TE 80-140ms turbo factor 12-30 [email protected]

23. Short Turbo Factor decreased effective TE increased T1 weighting longer scan time more slices per TR reduced image blurring Long Turbo Factor increased effective TE increased T2 weighting reduced scan time reduced slice number per TR increased image blurring [email protected]

24. Advantages Scan times greatly reduced High - resolution matrices and multiple NEX can be used Image quality improved Increased T2 information Disadvantages Some flow and motion affects increased Incompatible with some imaging options Fat bright on T2 weighted images Image blurring with very long echo trains [email protected]

25. Single Shot Fast Spin Echo (SS-FSE) Scan time is much reduced in SS-FSE than fast spin echo All lines of K space is filled in one TR SS-FSE combines a partial Fourier technique. Half of lines acquired in one TR and other half are transposed [email protected]

26. However …… There is a SNR penalty. Because of longer turbo factor Specific absorption rate (SAR) is increased because of successive 180˚ pulses. Remedy: (to decrease SAR) Reduce no: of slices Reduce refocusing angle to low as 120˚. But ..., Decreasing the SAR - will Decrease the SNR [email protected]

27. Driven Equilibrium Fourier Transform Modification of FSE ( called DRIVEN, RESTORE, or FR-FSE) A reverse flip angle excitation pulse applied at end of echo train. No need to wait for T1 Relaxation to occur This drives any transverse magnetization into longitudinal so available for next TR. Water has longest T1 and T2 times, appear bright [email protected]

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29. USES Cranial nerves Inner ear MR Urogram MRCP Spinal imaging [email protected]

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31. INVERSION RECOVERY Inversion recovery is a pulse sequence that begins with a 180° inverting pulse. This inverts the NMV through 180° into full saturation. When the inverting pulse is removed, the NMV begins to relax back to B˳. A 90°excitation pulse is then applied at a time from the 180° inverting pulse known as the TI ( time from inversion) [email protected]

32. STIR (short tau inversion recovery) Uses TI that corresponds to the time it takes fat to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat. A 90˚ excitation pulse is applied, so fat signal is nulled . A TI of 100– 175ms achieves fat suppression. [email protected]

33. However.., STIR should not be used in conjunction with contrast enhancement, which shortens the T1 times of enhancing tissues, making them bright. [email protected]

34. PARAMETERS Short TI - 150– 175ms (to suppress fat depending on field strength) Long TE - 50ms+ (to enhance signal from pathology) Long TR - 4000ms+(to allow full recovery) Long turbo factor - 16–20 (to enhance signal from pathology) [email protected]

35. USES Musculoskeletal imaging Bone bruising Tumors Suppress fat in general [email protected]

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38. FLAIR (fluid attenuated inversion recovery) TI corresponding to the time of recovery of CSF from 180° to the transverse plane nulls the signal from CSF. FLAIR is used to suppress the high CSF signal in T2 weighted images so that pathology adjacent to CSF is seen more clearly A TI of 1700– 2000ms achieves CSF suppression [email protected]

39. PARAMETERS Long TI 1700– 2200ms (to suppress CSF depending on field strength) Long TE 70ms+(to enhance signal from pathology) Long TR 6000 ms+ (to allow full recovery) Long turbo factor 16– 20 (to enhance signal from pathology) [email protected]

40. USES Periventricular and cord lesions. Multiple sclerosis. Acute sub- arachnoid hemorrhage Meningitis Periventricular leukomalacia gray/white matter abnormalities [email protected]

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