SPIN ECHO SEQUENCES MRI PRESENTED BY SEHRISH

1. SEHRISH MANZOOR
MRIT 2nd YEAR
SGT UNIVERSITY GURUGRAM

2. INTRODUCTION
 The spin echo pulse sequence commonly uses a 90°
excitation pulse to flip the NMV into the transverse plane.
 The NMV precesses in the transverse plane inducing a
voltage in the receiver coil.
 The precessional paths of the magnetic moments of the
nuclei are translated into the transverse plane. When the
90° RF pulse is removed, a free inducti on decay signal
(FID) is produced. T2 * dephasing occurs almost
immediately, and the signal decays. A 180 ° RF pulse is
then used to compensate for this dephasing.

3. TYPES
Spin echo pulse sequences (spins are rephased by a 180
rephasing pulse):
1. Conventional spin echo
2. Fast or turbo spin echo
3. Inversion recovery

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.
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

5. 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

7. 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

8.  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

10. How The Scan Time is 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

11. 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

12. 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
 Sagittal T2 weighted fast spin echo sequence through the
pelvis. Note that both fat and water have high signal
intensity.

13.  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

14. USES
 Generally speaking contrast in fast spin echo is
similar to spin echo, and used in..
 Musculoskeletal regions
 Central nervous system
 Pelvis
 Parameters For T1 Weighting
 TR 300 -700ms
 Effective TE minimum
 Turbo factor 2-8

15.  For PD weighting
 TR 3000-10000ms (depending on required slice number)
effective TE minimum
 turbo factor 2-8 .

17.  For T2 weighting
 TR 3000-100 00ms (depending on required slice
number)
 Effective TE 80- 140ms
 turbo factor 12- 30

18. 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

19. 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

20. 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

21.  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

22. 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

23. uses
 Cranial nerves
 Inner ear
 Mr urogram
 Mrcp
 Spinal imaging

24. Inversion recovery
 Inversion recovery is a pulse sequence begins with 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 degree excitation pulse is then applies at the time
from the 180 degree inverting pulse known as the
TI(time from inversion )

26. 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

27.  STIR should not be used in conjunction with contrast
enhancement, which shortens the T1 times of
enhancing tissues, making them bright.

28. 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)

29. USES
 Musculoskeletal imaging
 Bone bruising
 Tumors
 Suppress fat in general

30. 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

32. 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)

33. USES
 Periventricular and cord lesions.
 Multiple sclerosis.
 Acute sub- arachnoid hemorrhage
 Meningitis
 Periventricular leukomalacia
 gray/white matter abnormalities