Imaging Sequences part II

1. Imaging Sequencespart II • Gradient Echo • Spin Echo • Fast Spin Echo • Inversion Recovery

2. Spin Echo Refresher • 900RF 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 spin echo FID

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

5. Spin Echo Contrast T1 weighted T2 weighted

6. Multi Echo Spin Echorationale • 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: long TR 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 TE 2

9. RF slice phase readout signal Multi EchoSpin Echopulse timing    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 Echohistorical 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. ….. TR     echo echo ….. TR Spin Echopulse timing ….. phase encode n phase encode n+1 echo n echo n+1

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

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

19. Fast Spin Echoimplementation • 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. RF slice phase readout signal Multi EchoSpin Echopulse timing    only 1 phase encode per TR echo 1 echo 2

21. RF slice phase readout signal FastSpin Echopulse timing      multiple phase encodes per TR echo train

22. Fast Spin Echoscan 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 Echoadvantages • 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 Echoadvantages • image contrast similar to SE • scan parameters • TR • TE • echo train length

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

26. Fast Spin Echodisadvantages • 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. FastSpin Echodisadvantages 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. FastSpin Echodisadvantages • 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. FastSpin Echoblurring SE TE 20 FSE TE 20

30. FastSpin Echolimitations • solutions: • use mainly for T2 weighted imaging • limit the ET length (~ 8) • many phase encodes (192 +)

31. FastSpin Echolimitations • 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. FastSpin Echointerecho 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. FastSpin Echointerecho 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. FastSpin Echointerecho 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. FastSpin Echointerecho 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 Echoconclusions • 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. z z 0 RF y x y x Initial 1800 Flipinversion 0 Before ML=M MXY=0 After ML=-M MXY=0 t=t0 t=t0+

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

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

44. z z z z y x y x y x y x Second 1800 Flip dephased rephased 1800 RF 900 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. STIRadvantages • robust technique • 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. STIRdisadvantages • poor S/N • improved with multiple averages • FSE • improved with shorter TE times • incompatible with gadolinium • shorter T1 relaxation post-contrast

50. STIRdisadvantages • red marrow signal can obscure subtle edema • use TE=48 to knock signal down from marrow • modified IR • TE=70-100 • TI=110 @ 1.5T • excellent fluid sensitivity in soft tissues