1. Imaging Sequences part II
Gradient Echo
Spin Echo
Fast Spin Echo
Inversion Recovery

2. Spin Echo Refresher 900 RF pulse followed by 1800 RF pulse
least artifact prone sequence
moderately high SAR

3. Spin Echo gradient frequency encode readout  RF pulse  RF pulse
signal
FID
spin
echo

4. Spin Echo pulse timing

 RF
slice
phase
readout
echo
signal TE

5. Spin Echo Contrast
T1 weighted
T2 weighted

6. Multi Echo Spin Echo rationale
conventional imaging uses a multi-slice 2D technique
at a given TR time, number of slices depends on the TE time
T2 weighted imaging:
long TR
long TE
PD weighted imaging:
short TE

7. Multi Echo Spin Echo designed to obtain simultaneously multiple echos
generally used for PD and T2 weighted imaging
no time penalty for first echo
inserted before second echo
can do multiple echos (usually 4) to calculate T2 relaxation values

8. Multi Echo Spin Echo gradient  RF pulses  RF pulse signal TE 1

9. Multi Echo Spin Echo pulse timing


 RF
signal
readout
phase
slice
echo 1
echo 2

10. Spin Echo Contrast
PD weighted
T2 weighted

11. Multi Echo Spin Echo Summary
simultaneously generates PD and T2 weighted images
no time penalty for acquisition of PD weighted image
no mis-registration between echos

12. Fast Spin Echo Rationale importance of T2 weighted images
most clinically useful
longest to acquire
lowest S/N
need for higher spatial resolution

13. Fast Spin Echo historical perspective
faster T2 weighted imaging
gradient echo (T2*)
reduced data acquisition
“half-NEX”, “half-Fourier” imaging
rectangular FOV
S/N or spatial resolution penalty
altered flip angle SE imaging
“prise”, “thrift”

14. Fast Spin Echo
single most important time limiting factor is the acquisition of enough data to reconstruct an image
at a given image resolution, the number of phase encodings determines the imaging time

15. Fast Spin Echo
each phase encoding is obtained as a unique echo following a single excitation with a 90 degree RF pulse

16. Spin Echo pulse timing TR ….. ….. phase encode n phase encode n+1


echo


…..
phase encode n
phase encode n+1
echo n
echo n+1 TR
…..

17. Spin Echo Spin Echo Imaging Time = time-between-90-degrees times
total-number-of-unique-echos times
number-of-signal-averages

18. Spin Echo scan time time-between-90-degrees = TR
total-number-of-unique-echos = phase encodings
number-of-averages = NEX, NSA

19. Fast Spin Echo implementation
collect multiple echos per TR
similar to multi-echo SE
number of echos per TR referred to as the “echo train”
re-sort the data collection order to achieve the desired image contrast (effective TE time)

20. Multi Echo Spin Echo pulse timing


 RF
signal
readout
phase
slice
only 1 phase encode
per TR
echo 1
echo 2

21. Fast Spin Echo pulse timing




 RF
signal
readout
phase
slice
multiple phase encodes
per TR
echo train

22. Fast Spin Echo scan time
time-between-90-degrees = TR
total-number-of-unique-echos = phase encodings
number-of-averages = NEX, NSA
echo-train-length = ETL

23. Fast Spin Echo advantages
acquisition time reduced proportional to echo train length (ETL)
can trade-off some of the time savings to improve images
increased NEX
increased resolution

24. Fast Spin Echo advantages
image contrast similar to SE
scan parameters TR TE
echo train length

25. Fast Spin Echo disadvantages
new hardware required
ear protection may be necessary
higher SAR
many 1800 flips closely spaced
motion sensitive

26. Fast Spin Echo disadvantages
reduced number of slices for equivalent TR SE scan
MT effects alter image contrast
TE time imprecise
image blurring may occur
fat remains relatively bright on long TR/long TE scans
“J-coupling”

27. Fast Spin Echo disadvantages
TE 20
Want:
TR 3000, TE 80
Do:
TR 3000, ET 4
20 msec IES
TE 40
computer
TE 60
TE 80
TE 70ef
Get:
TR 3000, TE 70ef

28. Fast Spin Echo disadvantages
each echo “belongs” to a different TE image
combining the echos to form a single image creates artifacts
worse with shorter effective TE times

29. Fast Spin Echo blurring
SE TE 20
FSE TE 20

30. Fast Spin Echo limitations
solutions:
use mainly for T2 weighted imaging
limit the ET length (~ 8)
many phase encodes (192 +)

31. Fast Spin Echo limitations
solutions:
choose long TE times (> 100 msec)
choose long TR times (> 4000 msec)
increases fat-fluid contrast
for PD imaging,
use shorter echo trains (4) and wider receive bandwidths (32 kHz)
alternatively, use fatsat

32. Fast Spin Echo interecho spacing
interecho spacing is the time between echos, ~ 16 msec minimum on current equipment
echo trains vary from 2 on up on current equipment
little signal is available with long echo train imaging

33. Fast Spin Echo interecho spacing, example
16 ETL, 16 msec IES results in echos at the following:
16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256 msec
last 5 or 6 echos have so little signal that there is little contribution to the final image

34. Fast Spin Echo interecho spacing, example
time of last echo determines the number of slices per TR
long echo trains greatly reduce the number of slices per TR, even if the effective TE is short

35. Fast Spin Echo interecho spacing
hardware upgrade (echo-planar capable) will decrease interecho spacing (6-8 msec)
better image quality for same echo train lengths
more slices per TR for identical echo train lengths

36. Fast Spin Echo conclusions
should be called “faster” spin echo
produces superior T2 weighted images in a shorter time than conventional SE
great innovation
artifact prone

37. Inversion Recovery
initially used to generate heavily T1 weighted images
popular in U.K. for brain imaging
1800 inversion pulse followed by a spin echo or fast spin echo sequence

38. Inversion Recovery
three image parameters TI TR TE

39. Inversion Recovery TR    TI TE inversion recovery
conventional SE or FSE sequence

41. Initial 1800 Flip inversionz y x z y x
0 RF
0
Before
ML=M
MXY=0
After
ML=-M
MXY=0
t=t0
t=t0+ 46

42. T1 Relaxation recovery z y x z y x
After
ML=-M
MXY=0
t=t0+
t=TI 47

43. 900 Flip t=t0 t=t0+ 0 RF 0 Before ML=Msin() After MXY= ML z y x z48

44. Second 1800 Flip dephased rephased 900 RF 1800 RF t=0 t=TE/2 t=TE z yx z y x
dephased z y x z y x
rephased
900 RF
1800 RF
t=0
t=TE/2
t=TE

45. STIR Short time-to-inversion inversion recovery imaging “fat nulling”
exploits the zero crossing effect of IR imaging
all signal is in XY plane after TI time and subsequent 900 pulse produces no signal

47. STIR
optimal inversion time for fat nulling dependent on T1 relaxation time

48. STIR advantages robust technique high visibility for fluid
works better than fat saturation over a large FOV (>30 cms)
better at lower field strengths
high visibility for fluid
long T1 bright on STIR
long T2 bright on STIR, given long enough TE

49. STIR disadvantages poor S/N incompatible with gadolinium
improved with multiple averages
FSE
improved with shorter TE times
incompatible with gadolinium
shorter T1 relaxation post-contrast

50. STIR disadvantages red marrow signal can obscure subtle edema
use TE=48 to knock signal down from marrow
modified IR
TE=70-100
1.5T
excellent fluid sensitivity in soft tissues

51. Summary Spin echo Multi-echo spin echo Fast spin echo
90 degree pulse, dephase, 180 degree pulse, rephase-echo
Multi-echo spin echo
90 degree pulse, dephase, 180 degree pulse, rephase-1st-echo, 180 degree pulse, rephase-2nd-echo
Fast spin echo
obtain multiple phase encoded echos with a single 90 degree pulse
echo train length determines “turbo” factor
Inversion recovery
180 degree pulse, inversion time, then SE or FSE sequence
STIR enables fat suppression over large FOVs or for open magnets