1. Durgesh Kumar Dwivedi Department of NMR & MRI AIIMS, New Delhi, India
FAST SPIN ECHO
Durgesh Kumar Dwivedi
Department of NMR & MRI
AIIMS, New Delhi, India
December 2, 2009

2. Contents Basic terminology of MR pulse sequences RARE/ FSE
SE (CSE) vs FSE
Contrast in FSE
Advantages
Disadvantages
SSFSE, HASTE

3. Pulse sequence and timing diagram
Four lines are needed
radio-frequency (RF) pulse
three gradients - slice, phase, frequency/ readout
From Abbreviations:
One for RF tX and others for Gx Gy Gz
Additional lines may be added to indicate other activity such

4. SPIN ECHO (SE) SEQUENCES
Manufacturer
Single echo SE
Multiple echo SE
Echo train SE
Siemens
Single SE SE
Double echo
Turbo spin echo (TSE)
Half Fourier acquisition turbo SE (HASTE) GE
Multiecho multiplanar (MEMP)
Variable echo multiplanar (VEMP)
Fast SE (FSE)
Single shot FSE (SSFSE)
Philips
SE,
Modified SE
Multiple SE (MSE)
Ultra fast Spin echo (UFSE)

5. Contrast parameters
Two key parameters : repetition time (TR) and echo time (TE) - are key to the creation of image contrast.
TR (in milliseconds) is the time between the application of an RF excitation pulse and the start of the next RF pulse
TE (in milliseconds) is the time between the application of the RF pulse and the peak of the echo detected
From MR pulse sequence page 4
Both parameters affect contrast on MR images because they provide varying levels of sensitivity to differences in relaxation time between various tissues. At short TRs, the difference in relaxation time between fat and water can be detected (longitudinal magnetization recovers more quickly in fat than in water); at long TRs, it cannot be detected. Therefore, TR relates to T1 (Fig 4b) and affects contrast on T1-weighted images. At short TEs, differences in the T2 signal decay in fat and water cannot be detected; at long TEs, they can be detected. Therefore, TE relates to T2 (Fig 4b) and affects contrast on T2-weighted images.
When the TR is long and the TE is short, the differences in magnetization recovery and in signal decay between fat and water are not distinguishable (Fig 4b); therefore, the contrast observed on the resultant MR images is predominantly due to the difference in proton density between the two tissue types.

6. Effect of TR and TE on MR image contrast
Imaging technique TR TE
T1 weighting
Short
T2 weighting
Long
PD weighting
* Short TR & long TE produces very low SNR and should be avoided

7. TR (in ms)
TE (in ms)
Sequence
Short
Long SE
>2000
10-25
>60
GRE
<50
>100
1-5
>10

8. Spin echo 90° pulse flips the net magnetization
vector into the transverse plane
A 180° pulse is applied at a time
equal to one-half of TE to rephase
the spinning nuclei
When the nuclei are again spinning
in phase (at total TE), an echo is
produced and read

9. FSE RARE (Hennig et al 1986) FSE (Fast spin echo) (Mulkern et al 1990)
(Rapid Acquisition with Relaxation Enhancement)
FSE (Fast spin echo) (Mulkern et al 1990)
TSE (Turbo spin echo)

10. FSE Fast scan (Based on principle of echo imaging)
Long TR (Multiple RF pulse)
- T2W강조
Conventional SE
Fast SE
1 NEX
4 NEX
9 min 28 sec
2 min 25 sec

11. Frequency encoding axis
Spin Echo, k-space
RF signal (Echo)
(Phase) 부호화 경사 ( Gy )
(Frequency) 부호화 (Gx)
RF Pulse
90°
180°
Frequency encoding axis
Fourier Transform
Phase encoding axis ( Ky)
+ 127
- 127
k-space
If 256개의 phase encoding을 채울 경우 TR 256번을 반복
If you fill the 256 phase encoding TR 256 iterations

12. Fast Spin echo … Echo train length (ETL) Echo train spacing (ETS)
180°
90°
Echo train length (ETL)
: Number of 180° RF pulse
: Scan time ∝ (1/ETL)
Echo train spacing (ETS)
: Space between 180° RF pulse
: Dwell time (in phase encoding direction)
Effective echo time (TEeff)
         : TE of k-space mid-line   
ETS
+ 127
Overall
contrast
0 (Ky)
Detailed
Description
- 127
K-space

13. Fast Spin Echo ETL=8 TE5 ETS TE eff Centre of k-space RF Pulse
90°
180°
180°
180°
180°
180°
180°
ETL=8
RF Pulse
TE eff
Phase-encoding
gradient
Echo
TE1
TE2
TE3
TE4
TE6
TE7
TE8
TE5
+ 127
0 (Ky)
- 127
Ky=0
Centre of k-space

14. Effective TE TEeff 40.36 msec 80.72 msec 96.86 msec 137.22 msec
T2 effect ↑, SNR ↓
TEeff 40
TEeff 100
TE 40
TE 100

15. Echo Train Length ETL 4 ETL 8 ETL 16 ETL 32 10 min 45 sec 5 min 25 sec
TR 5000msec, NEX = 2
ETL governs by: (1)T2 relaxation, (2) ETS
ETL 4
ETL 8
ETL 16
ETL 32
10 min 45 sec
5 min 25 sec
2 min 45 sec
1 min 25 sec
ETL ↑ : Time ↓
Issues : Slice number ↓
Correction: TR ↑, Slice thickness ↑

16. Scan Time Scan Time(FSE) = (TR)(Ny)(NEX) /ETL
Scan Time(SE) = (TR)(Ny)(NEX)
Scan Time(FSE) = (TR)(Ny)(NEX) /ETL Ny
NEX
ETL :
Phase-encoding steps
Number of excitation
Echo Train Length
Example: TR = 3000 msec, NEX=1, Matrix 256 X          ETL of 8.
Calculate time for CSE and FSE images?
CSE
3000(TR) * 256(Ny) * 1(NEX)
= 12.8min
FSE
3000(TR) * 256(Ny) * 1(NEX) / 8
= 1.6 min

17. Contrast in FSE FSE SE T1W T2W
(17) T1-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 500-msec TR. The images
are very similar, even though the fast spin-echo image was obtained four times faster. The latter image
shows a slight reduction in resolution due to echo decay throughout the echo train. (18) T2-weighted
images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left)
with an echo train length of four and a 2,000-msec TR. TE was 68 msec for the conventional image,
and the TE encoding the center of k space in the fast spin-echo image was also 68 msec. The images
are very similar, even though the fast spin-echo image was obtained four times faster. Blurring seen in
the Ti-weighted fast spin-echo image is not apparent in the T2-weighted fast spin-echo image because
the earlier echoes are used to sample the higher frequency phase-encoding views.
T1-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 500-msec TR.
Below image: T2-weighted images obtained with a conventional spin-echo sequence (right) and a fast spin-echo sequence (left) with an echo train length of four and a 2,000-msec TR. TE was 68 msec for the conventional image, and the TE encoding the center of k space in the fast spin-echo image was also 68 msec. Blurring seen in the T1-weighted fast spin-echo image is not apparent in the T2-weighted fast spin-echo image because the earlier echoes are used to sample the higher frequency phase-encoding views.
Images illustrate how the various regions of K space (upper row) can be reconstructed, with the corresponding images (bottom row). Reconstructions are shown for all of the data (left), the center of k space (center), and the outer regions of k space (right).

18. Phase (Phase) Signed slope
FSE vs CSE
Multiple 180° pulse
FSE
CSE TR
90°
180°
TE30
TE60
TE80
TE100
90°
180°
90°
RF Pulse
Phase (Phase) Signed slope
TE80
K-space
TEeff 80
TE 80
Time saving
No time saving 18

19. Advantage Scan time ↓↓: ∝ETL Image quality ↑
: Scan time saving; trade off – ETL and Slice thickness
Based on spin echo and similar contrast
Artifact (motion, susceptibility) ↓
: by 180° refocusing pulse

20. Disadvantage
Blurring
TEeff 50 ms
T2 weighted
TEeff 150 ms

21. Disadvantage Bright fat signal : Remedy- Fat suppression image
: J-coupling
: Remedy- Fat suppression image
Conventional SE
Fast SE
Fast SE
Fat suppression

22. Disadvantage Specific absorption rate (SAR) Use low flip angles
: Total RF energy (E) dissipated in a sample over
exposure time (texp) per unit mass(M) (watts per
kilogram)
SAR= E/ (texp*M)
Also,
SAR α Bo2 * θ (Theta)2 *Bandwidth
Use low flip angles

23. FSE의 변형 3D FSE (+ 3D) SSFSE, HASTE (+ Single shot FSE)
(+ Half fourier acquired sigle shot turbo spin
echo)

24. 3D FSE Thinner image Phase encoding : z Direction (Nz), add
Multiple slices → Slab
More scan time
Scan time = (TR*NEX*Ny*Nz)/ETL
2D Image
3D Image

25. SSFSE , HASTE Single shot FSE (Single Shot) + Half fourier acq.
Single shot FSE, Half Fourier acquired sigle shot turbo spin echo
Frequency encoding ( Kx)
Single shot
: A very long one echo train ( locations)
Ny = ETL, so, Scan time = TR* NEX
FSE (Single Shot) + Half fourier acq.
Partial fourier technique
: K-space Fill in the date part of the
Phase encoding ( Ky)
K-space
Partial fourier technique
64-128
90°
180°

26. HASTE, SSFSE Ultra-fast : 1-2초 (single breath hold)
Abdomen, chest imaging
MRCP Liver MR

27. Summary Characteristics of spin echo Fast scan ∝ETL
Advantage : Image quality ↑, Artifact↓
Disadvantage : Fat signal ↑, Slice number ↓
ETL, ETS, TE eff
Advancement due to SSFSE & HASTE- better image quality

28. THANK YOU…