This document discusses various spin echo pulse sequences used in MRI and their applications. It begins by introducing spin echo pulse sequences and their use of RF pulses and gradients to manipulate proton behavior and generate contrast in images. It then describes conventional spin echo sequences and how different repetition times (TR) and echo times (TE) produce T1, proton density, and T2 weighting. Fast spin echo sequences are covered next, explaining how they reduce scan time by acquiring multiple echoes per TR. Inversion recovery sequences like STIR and FLAIR are also summarized, noting how they suppress signal from tissues like fat or CSF. The document concludes by listing some common pulse sequence parameters and their applications in MRI.

1. MRI SPIN ECHO PULSE SEQUENCES ITS
VARIATIONS AND APPLICATIONS
Presenter :- Yashawant Ku. Yadav
Bsc.MIT 3rd Year
NAMS (Bir Hospital )

2. Outline
• Introduction P-S
• Terminologies
• Types of Spin echo pulse sequences (Conventional spin echo ,Fast or turbo spin
echo , Inversion recovery ,Fast inversion recovery [STIR (short tau inversion
recovery), FLAIR (fluid attenuated inversion recovery) ] .
• Application of spin echo pulse sequences
• References

3. Introduction
• Pulse sequence is a series of RF pulses, gradient applications and intervening time
periods.
• 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.

4. Intro…
• In MRI, we are imaging properties of hydrogen nuclei (protons) in
tissue.
• With the protons precession around B0, nothing interesting will happen
and we cannot measure any signal.
• In order to measure parameters of the tissue, we need to perturb - excite
- the system
• This is done by applying a transient magnetic field - a radiofrequency
pulse - in a different direction from B0; the direction of this pulse is
often referred to as B1

5. Contd..
• By applying this RF for a short time, we can rotate the magnetization of
the tissue.
• If the protons are initially aligned along the z (vertical) axis, and we apply
a pulse along the x -axis for the right amount of time, we can rotate the
magnetization from the z-axis onto the y-axis.

6. Contd..
• This is referred to as a 90-degree pulse, since the magnetization
direction is turned by 90 degrees.
• Once the pulse is turned off, the protons will resume precession along
B0 and will slowly return to their initial equilibrium with the net
magnetization pointing along the z axis.
• The RF pulses are applied for excitation purposes and, in the case of
spin echo, for rephasing purposes too.

7. Why pulse sequences ??
Image contrast
• All clinical diagnostic images must demonstrate contrast between
normal anatomical features and between anatomy and any pathology.
• And MRI is consider as a high tissue contrast discriminating imaging
modalities among all.
• The contrast characteristic of each image depend on many variables,

8. Contd..
Basically it is divided into two categories.
• Intrinsic contrast parameters are those that cannot be changed because they are
inherent to the body ’ s tissues.
• Extrinsic contrast parameters are those that can be changed.
Intrinsic contrast PMs Extrinsic contrast PMs
T1 recovery time TR
T2 decay time TE
proton density FALIP ANGLE
flow TI
apparent diffusion coefficient (ADC). TURBO FACTOR / ECHO TRAIN LENGTH
B Value

9. Terminologies
The repetition time (TR) is the time
from the application of one RF pulse
to the application of the next RF
pulse for each slice and is measured
in milliseconds (ms).
• The TR determines the amount of
longitudinal relaxation that is allowed
to occur between the end of one RF
pulse and the application of the next.
• TR thus determines the amount of T1
relaxation that has occurred when
the signal is read.

10. Contd..
 The echo time (TE) is the time from
the application of the RF pulse to the
peak of the signal induced in the coil
and is also measured in ms.
• The TE determines how much decay of
transverse magnetization is allowed to
occur.
• TE thus controls the amount of T2
relaxation that has occurred when the
signal is read.

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

12. Variations
Spin echo pulse sequences (spins are rephased by a 180 ° rephasing
pulse):
• conventional spin echo
• fast or turbo spin echo
• inversion recovery.

14. Conventional spin echo
• spin echo uses a 90 ° excitation pulse followed by one or more 180 ° rephasing
pulses to generate a spin echo.
• If only one echo is generated, a T1 weighted image can be obtained using a short
TE and a short TR.

15. Contd…
• The SE represents regeneration of spin phase information apparently
lost during the decay of the FID. The "rebirth" of the FID as a SE is
possible… by Erwin Hahn in 1949
• In fact, most of the FID signal has not been destroyed; it has merely
become "disorganized" because the individual spins comprising it have
lost their phase coherence.
• The system is said to possess a "hidden order" or "atomic memory". By
applying a second RF pulse, certain dephased components of the
original FID can be refocused into a SE.

16. Contd…
• For proton density and T2
weighting, two RF rephasing
pulses, generating two spin
echoes, are applied.
• The first echo has a short TE and
a long TR to achieve proton
density weighting, and the second
has a long TE and a long TR to
achieve T2 weighting.

17. Contd..

20. Parameters
T1 WT
Te short :- 10-30 ms
Tr short :- 300-700ms
Typical scan time :- 4-6 min
T2 WT / PDW
Te short :- 20ms / 80+MS(LONG)
Tr long :- 2000+ms
Sacn time:-

21. Applications
• Used in any part of the body for any indication.
• To produce T1, T2 (True)and PD weighted images of good quality.
 Disadvantage
Longer scan time

22. Fast spin echo /turbo
• In this pulse sequence scan time is much shorter than conventional spin
echo .
• In fast spin echo, the scan time is reduced by performing more than one
phase encoding step and subsequently filling more than one line of K
space per TR.
• This is achieved by using several 180 ° rephasing pulses to produce a
train of echoes or echo train.
• At each rephasing, an echo is produced and a different phase encoding
step is performed.

23. Contd..
• In conventional spin echo, raw image data from each echo are stored in
K space,
• Each echo is used to produce a separate image
• In FSE data from each echo are placed into one image.
• The number of 180 ° rephasing pulses performed per TR corresponds to
the number of echoes produced and the number of lines of K space
filled.

24. Contd..
• This number is called the turbo factor or the echo train length . The
higher the turbo factor, the shorter the scan time as more phase
encoding steps are performed per TR.
• Number of echoes -- called echo train length (ETL) by GE and Canon; turbo factor by Siemens and
Philips; or shot factor by Hitachi.
• At each 180 ° /phase encoding combination, a different amplitude of
phase encoding gradient slope is applied to fill out a different line of K
space

25. Contd..
• ETL is the single most important parameter. In general, image acquisition time is
inversely proportional to ETL.
• ETL also has important effects on image quality. Longer ETLs result in more T2-
weighting because more late echoes with longer TE's contribute to the overall
signal.
• Longer ETL's are also decreases overall signal-to-noise ratio (SNR) and
contrast-to-noise ratio (CNR) because the later echoes are weaker.

26. TR :- 4000MS
OTHER PARAMETERS
UNCHANGED

27. Contd..
• Time between echoes -- called echo spacing (ESP) by GE, Philips,
Siemens, and Canon; interecho time (ITE) by Hitachi.
• Increasing echo spacing (ESP) permits the use of longer TE's but
adversely impacts SNR and CNR

28. Weighting in fast spin echo
• The echoes are generated at different TE times and therefore data
collected from them have variable weighting.
• So how is a fast spin echo sequence weighted correctly?
• The TE selected is only an effective TE
• That mean TE at which the operator wishes to weight the resultant
image.
• The system orders the phase encoding steps so that steep or shallow
slopes are applied to the various echoes produced.

31. Contd…
• fast spin echo has largely replaced spin echo especially for T2
weighting.
• There are, however, two contrast differences between spin echo and fast
spin echo,
• due to the repeated, closely spaced 180 ° pulses of the echo train
• First, fat remains bright on T2 weighted images due to the multiple RF
pulses, which reduce the effects of spin – spin interactions in fat ( J
coupling )
• Second, the repeated 180 ° pulses can increase magnetization transfer
effects so that muscle, appears darker on fast spin echo images than in
conventional spin echo.

32. Contd..
• In addition, the multiple 180 ° pulses reduce magnetic susceptibility effects,
which can be detrimental when looking for small hemorrhages.

34. Contd…
• Image blurring may occur in fast spin echo images at the edges of
tissues with different T2 decay values.
• This is because each line of K space filled during an echo train contains
data from echoes with a different TE.
•
• When using long echo trains, late echoes that have a low signal
amplitude contribute to the resolution of K space.
• If these echoes are negligible, then resolution is lost from the image and
blurring occurs.

35. TYPICAL PARAMETERS
Dual echo
• TR 2500–4500 ms (for weighting and slice number)
• effective TE1 17 ms
• effective TE2 102 ms
• ETL 8 – This may be split so that the PD image is acquired with the first four
echoes and the T2 with the second four.
Single echo
• T2 weighting
• TR 4000–8000 ms
• TE 102 ms
• ETL 16
Single echo
T1 weighting
TR 600 ms
TE 17 ms
ETL 4

36. USES
• brains, spines , joints, extremities and pelvis

37. SS-FSE / HASTE
Other vendors have similar sequences under slightly different names: GE
(Single-shot fast spin echo, SS-FSE), Philips (Single-shot turbo spin
echo, SSH-TSE; ultra-fast spin echo, UFSE), HItachi (Single-shot fast
SE), and Canon (Fast Advanced Spin Echo, FASE, SuperFASE).
• This sequence even faster than FSE ,
• In this technique all the lines of K space are acquired in one TR.
• SS - FSE combines a parti al Fourier technique with fast spin echo.
• Half of the lines of K space are acquired in one TR and the other half are transposed.

38. Contd..
• Currently the highest turbo factor used in single shot imaging is 728
• Another consideration when using long echo trains is that the specific
absorption rate ( SAR ) is significantly increased by applying so many
successive 180 ° pulses.
• slice number can be reduce to manifest SAR but that cause difficult to
get required coverage area … so,
• Refocusing angle is reduced to 120°
• This reduces the SAR significantly (which is proportional to the square
of the flip angle) but also reduces the SNR.
• the benefi t of being able to obtain more slices per acquisition

39. Contd…
Representative HASTE/SS-FSE pulse sequence.
Note asymmetric sampling in phase-encode direction. The peripheral lines on one side
of k-space are not sampled but estimated by conjugate symmetry.

41. Application
• routine scout images
• images of the head or body in children or uncooperative patients
• fetal imaging
• non-breath-hold abdominal imaging
• MR cholangiopancreatography
• MR myelography, and
• non-contrast MR angiography.

43. Examples of conventional SSFSE
compared to half-Fourier vrfSSFSE.
(A) Conventional SSFSE and
(B) half-Fourier vrfSSFSE images
from a patient with renal cell
carcinoma of the left kidney (arrow).
(C) Conventional SSFSE and
(D) half-Fourier vrfSSFSE images
from a patient with a mildly dilated
pancreatic and common bile duct.
Images are mildly cropped from the
full field of view. Note the equivalent
image quality.

44. IR (Inversion recovery )
• Inversion recovery sequences were
initially designed to produce very heavy
T1 weighting.
• However at present they are mainly
used in conjunction with a FSE
sequence to produce T2 weighted
images.
• It is usually used to suppress the signal
from certain tissues in conjunction with
long TEs and T2 weighting, although at
low field it is still used for T1 contrast.

45. Contd..
• 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 0 .
• A 90 ° excitation pulse is then applied at
a time from the 180 ° inverting pulse
known as the TI ( time from inversion )

46. Contd…
• A further 180° RF pulse is applied which rephases spins in the
transverse plane and produces an echo at time TE after the excitation
pulse.
• The contrast of the resultant image depends primarily on the length of
the TI.
• If the TI is sufficiently long to allow the NMV to pass through the
transverse plane, the contrast depends on the degree of saturation that is
produced by the 90° pulse (as in spin echo), i.e.
• if the 90° pulse is applied shortly after the NMV has passed through the
transverse plane, heavy saturation and T1 weighting results.

47. Contd…
TIs of 300 ms to 700 ms result in this type of heavy T1 weighting

49. FIR (FAST INVERSION RECOVERY )
• It is a combination of inversion recovery and fast spin echo.
• It is usually used to suppress the signal from certain tissues in conjunct on with
long TEs and T2 weighting, so that water and pathology return a high signal.
• Although at low field it is still used for T1 contrast.
• The two main sequences in this category are STIR and FLAIR.

50. STIR (Short tau inversion recovery )
• This sequences uses short TIs such as 100– 180 ms, depending on field
strength .
• TIs of this magnitude place the 90° excitation pulse at the time that
NMV of fat is passing exactly through the transverse plane.
• At this point (called the null point) there is no longitudinal component
in fat.
• Therefore the 90° excitation pulse produces no transverse component
in fat and therefore no signal.
• In this way a fat suppressed image result.

52. 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 tumors
are seen more clearly.

53. FLAIR (fluid attenuated inversion recovery )
• It uses long TIs such as 1700–2200 ms,
depending on field strength, to null the
signal from CSF in exactly same way as
STIR.
• Because CSF has a long T1 recovery
time, the TI must be longer to
correspond with its null point.

54. T1-FLAIR brain image at 3T with TR=2100,
TE=9, TI=880.
T2 FLAIR :TI 2000-2500 very long TRs
(8000) and TEs (140)
For T1-FLAIR both the TR and TE values are kept relatively short to
minimize T2-weighting. Considering these restrictions, the TInull for a T1-
FLAIR sequence is typically in the range of 800-1000 ms.

55. white matter, and gray matter are
approximately400, and 650 ms
respectively.
TI= 0.69 x T1.

56. Typical parameters The required TI depends on the field
strength (higher fields).
T1 weighting TI 300–700 ms
• TE 10–20 ms
• TR long
• ETL 4
STIR
• TI 100–180 ms
• TE 70 ms+ (for T2 weighting)
• TR long
• ETL 16
 FLAIR
• TI 1500–2200 ms
• TE 70 ms+ (for T2 weighting)
• TR long
• ETL 12–16

57. ADVANTGES DISADVANTAGES
Selective Tissue Suppression Longer scan times
Up to Twice the Dynamic Range Increase in flow-related artifacts
Additive T1 and T2 contrast ?? Signal-to-noise can decrease as
tissues are suppressed
Higher specific absorption rate (SAR)
due to additional 180° pulses

58. Summary if FIR

59. DOUBLE IR
• Double Inversion Recovery (DIR) is an inversion recovery variant that
uses not one, but two nonselective 180°-inverting pulses.
• Here the first 180°-pulse suppresses CSF and the second suppresses
white matter.
Double IR sequence suppressing CSF and white
matter. TR=11000, TE=30, TI-1=3400, TI-2=325.

60. Black blood by DIR
• In this method, two 180°-pulses are applied very close together in
time.
• The first 180°-pulse is nonselective, meaning that it inverts the
magnetization for all slices within the imaging volume.
• The second 180°-pulse, following immediately on the heels of the
first 180°-pulse, is slice selective,
• Thus the myocardium and other relatively stationary tissues have their
signals preserved,
• but the blood flowing from adjacent slices has an inverted
magnetization.

61. Contd…
The TI is adjusted to null the signal from inflowing blood, which
appears "black" on magnitude reconstructed IR images.

62. Phase sensitive IR ?????
The sequence with the gray background is
called phase-corrected or phase-sensitive
inversion recovery (PSIR).
Phase-corrected IR preserves the information about Mz polarity, with more
negative values rendered increasingly darker.

63. • If TI is not perfectly chosen, detection of contrast enhancement may be
impaired using magnitude reconstruction.
• Phase-sensitive IR is less sensitive to choice of TI as illustrated in this
example of septal ischemia (arrow).

64. References
1. https://www.mriquestions.com
2. MRI at a Glance CATHERINE WESTBROOK
3. MRI in Practice Fourth Edition

65. T2 FLAIR
T1
T1 flair

66. •Long T1 materials are dark on T1-weighted images, but
long T2 materials are bright on T2-weighted images.

67. • T2 Relaxation Accompanying T1 Relaxation how ???
• Does T2 Relaxation Occurring Without T1 Relaxation??
1. where it experiences a local static field disturbance (B1oo) in addition to
the main magnetic field (Bo).
2. dipolar interaction also known as a spin-spin "flip-flop".