1. Pulse sequences

2. Categorization Spin echo Inversion recovery Gradient echo
Conventional spin echo
Fast spin echo
Inversion recovery
Gradient echo
Coherent
Incoherent
Steady state free precession
Ultra-fast imaging

3. Conventional spin echo
Illustration
The situation before the patient is placed inside the magnet

4. The patient is now inside the bore of the magnet
The patient is now inside the bore of the magnet. The magnetization of the patient’s protons M0 is aligned along the Z axis

5. The 900 pulse is now applied
The 900 pulse is now applied. M0 is now completely removed from the Z axis and lies along the Y axis

6. Relaxation is now taking place
Relaxation is now taking place. Spin-lattice (T1)relaxation caused the magnetization to re-grow along the Z axis. Spin-spin relaxation causes the magnetization vectors to dephase (move apart) while still in the X-Y plane

7. 1800 pulse is now given. All the vectors now point in the opposite direction. The magnetization vectors rephase (come together) in the X-Y plane. As they come together the echo is being formed

8. The magnetization is now completely rephased in the X-Y plane and points along the Y axis. This causes the full height of the echo. The actual MRI signal is taken here.

9. Relaxation continues to take place
Relaxation continues to take place. The magnetic vectors again dephase in the X-Y plane while the regrowth along the Z axis continues

10. Complete relaxation has taken place
Complete relaxation has taken place. There is no net vector in the X-Y plane, and the magnetization is full grown along the Z axis. This is identical to the situation before the 900 pulse was applied

11. The coordinate system in relation to the magnet

12. Short TE and short TR gives T1 weighted image
Use two RF rephasing pulses generating two spin echoes to produce T2 and proton density weighting
First echo has short TE and long TR – produce proton density weighting
Second echo has a long TE and a long TR – produce T2 weighting

13. uses Gold standard for most imaging May be used for every examintion
T1 images useful for demonstrating anatomy – because high SNR
With contrast enhancement T1 images show pathology
T2 images also demonstrate pathology
Diseased tissues are generally more oedematous and/or vascular. They have increased water content and, have a high signal on T2 images

14. Parameters T1 weighting Proton density/T2 Short TE 10-20 ms
Short TR 300 – 600 ms
Typical scan time 4-6 min
Proton density/T2
Short TE 20 ms/long TE 80 ms+
Long TR 2000 ms+
Typical scan time 7-15 min

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

16. Fast spin echo
In contrast to conventional spin echo, fast spin echo applies a train of 1800 pulses per TR and different phase encoding steps are used.
Each 1800 pulse produce an echo (proton density & T2)
This drastically reduce the scan time
The number of 1800 pulses in the train called the turbo factor or train length

17. E.g. if in conventional SE 256 phase matrix and 1 NEX is used, the scan time is 256TR
In FSE if the turbo factor is 16, the scan time is 256TR/16 =16TR

18. Uses Useful in most clinical applications
Central nervous system, pelvis, musculoskeletal regions
Note
Fat remains bright on T2
Unless fat saturation techniques are used
Muscles appear darker in FSE images
Artefacts from metal implants is significantly reduced

19. Parameters T1 weighting T2 weighting Short effective TE less than 20ms
Short TR 300 – 600 ms
Turbo factor 2-6
Typical scan time 30s to 1 min
T2 weighting
Long effective TE 100 ms
Long TR 4000 ms+
Turbo factor 8-20
Typical scan time 2 min

20. Advantages & Disadvantages
Reduced scan time
High resolution matrices and multiple NEX can be used
Image quality improved
Increase T2 information
Some flow and motion effects increased
Incompatible with some imaging options
Fat bright on T2
Reduces magnetic susceptibility effect, so should not be used when haemorrhage is suspected

21. Inversion Recovery Starts with a 1800 inversion pulse.
This inverts NMV through 1800 into full saturation.
When inverting pulse removed NMV begins to relax back to B0
A 900 excitation pulse is then applied at a time TI (Time from Inversion) from the 1800 inversion pulse

22. TI

23. TI determines the weighting & contrast
Short TI gives T1 contrast
Long TI gives proto density contrast
After the 900 excitation pulse 1800 rephasing pulse is applied at a time TE
This produces the spin echo
TR is the time between each 1800 inverting pulse

24. Uses
conventionally used to produce heavily T1 weighted images to demonstrate anatomy & in contrast enhanced imaging
Now more widely used in conjunction with fast spin echo to produce T2 weighted images

25. Parameters

26. STIR (short TI 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.
As a result the signal from fat is nulled.
Used to achieve suppression of fat in T1 weighted images.
TI 150 – 175 ms

27. FLAIR (Fluid attenuated inversion Recovery)
The signal from CSF is nulled by selecting a TI corresponding to the time of recovery of CSF from 180 to the transverse plane and there is no longitudinal magnetization present.
Used to suppress signal from CSF in T2 weighted images
TI ms