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Imaging Sequences part I Gradient Echo Spin Echo Fast Spin Echo Inversion Recovery.

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Presentation on theme: "Imaging Sequences part I Gradient Echo Spin Echo Fast Spin Echo Inversion Recovery."— Presentation transcript:

1 Imaging Sequences part I Gradient Echo Spin Echo Fast Spin Echo Inversion Recovery

2 Goals of Imaging Sequences generate an RF signal perpendicular to  0 generate tissue contrast minimize artifacts

3 Measuring the MR Signal z yx RF signal from precessing protons RF signal from precessing protons RF antenna 00

4 Gradient Echo simplest sequence –alpha flip gradient-recalled echo 3 parameters –TR –TE –flip angle reduced SAR artifact prone

5 Gradient Echo FI D gradient recalled echo  RF pulse rephase dephase signal gradient

6 z yx z yx  0 RF t=t 0 t=t 0+ Partial Flip 00 MLML M XY M M XY = M sin(  ) M L = M cos(  )

7 Dephasing in the xy-plane view from the top y x z M xy y x z M xy  dephase phase coherencyphase dispersion

8 y x z M xy phase coherency minus t2* decay Rephasing in the xy-plane view from the top rephase y x z M xy  phase dispersion

9 MR Signal During Rephasing z yx RF signal “echo” RF signal “echo” RF antenna 00

10 T2* decay occurs between the dephasing and the rephasing gradients rephasing incompletely recovers the signal signal loss is greater with longer TEs decay generates image contrast

11 T2* decay T2* decay is always faster than T2 decay gradient echo imaging cannot recover signal losses from –magnetic field inhomogeneity –magnetic susceptibility –water-fat incoherence

12 T2 and T2* Relaxation T2 is the spin-spin relaxation time T2 M is the contribution to relaxation induced by inhomogeneities of the main magnet (predominant factor) T2 MS is the contribution to relaxation induced by magnetic susceptibility in the object

13 T2 and T2* Relaxation T2* relaxation influences contrast in gradient echo imaging T2 relaxation influences contrast in spin echo imaging

14 Gradient Echo pulse timing echo RF signal readout  phase slice TE

15 Gradient Echo advantages faster imaging –can use shorter TR and shorter TEs than SE low flip angle deposits less energy –more slices per TR than SE –decreases SAR compatible with 3D acquisitions

16 Gradient Echo disadvantages difficult to generate good T2 weighting magnetic field inhomogeneities cause signal loss –worse with increasing TE times –susceptibility effects –dephasing of water and fat protons

17 Gradient Echo changing TE TE 9 FA 30 TE 9 FA 30 TE 30 FA 30 TE 30 FA 30 susceptibility effect T2* weighting

18 Gradient Echo magnetic susceptibility post-surgical change “blooming” artifact post-surgical change “blooming” artifact

19 Gradient Echo in-phase / opposed-phase TE TE in-phaseopposed-phase

20 Water/Fat Dephasing MR signal is a composite of fat and water in the imaging voxel water and fat resonate at slightly different frequencies cyclic variation in relative phase of fat and water resonance results in signal variations dependent on TE times

21

22 In-Phase / Opposed-Phase TE Times (msec)

23 Gradient Echo image contrast depends on sequence conventional GR scan –aka GRASS, FAST –decreased FA causes less T1 weighting –increased TE causes more T2* weighting

24 Conventional GR TE 20, FA 15

25 Gradient Echo Spoiled GR –aka SPGR, RF-FAST –spoiling destroys accumulated transverse coherence –maximizes T1 contrast

26 Gradient Echo Contrast enhanced GR –aka SSFP, CE-FAST –infrequently used because of poor S/N –generates heavily T2* weighted images

27 Gradient Echo other varieties –MTC T2 - like weighting –IR prepped 180 preparatory pulse –DE (driven equilibrium) prepped preparatory pulses T2 contrast

28 MTC GR TE 13, FA 50

29 Spin Echo widely used sequence – echo 2 parameters –TR –TE generates T1, PD, and T2 weighted images minimizes artifacts

30 Spin Echo FI D spin echo   RF pulse readoutfrequency encode signal gradient   RF pulse

31 Gradient versus Spin Echo

32 90 0 Flip z yx z yx  0 RF t=t 0 t=t 0+  0 After M L =0 M XY =M Before M L =M M XY =0

33 Dephasing in the xy-plane view from the top y x z M xy y x z M xy  phase coherencyphase dispersion Dephasing begins immediately after the 90 0 RF pulse. t=0t=TE/ RF

34 y x z M xy phase coherency minus t2 decay Rephasing in the xy-plane view from the top y x z M xy  phase dispersion t=TE/2t=TE RF

35 z yx z yx z yx z yx t=TE/2t=TE RF t= RF dephased rephased Flip

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

37 WNMR Race t= RF

38 WNMR Race

39 t=TE/ RF

40 t=TE WNMR Race

41 Effects of the Pulse eliminates signal loss due to field inhomogeneities eliminates signal loss due to susceptibility effects eliminates signal loss due to water/fat dephasing all signal decay is caused by T2 relaxation only

42 Spin Echo advantages high signal to noise least artifact prone sequence contrast mechanisms easier to understand

43 Spin Echo disadvantages higher SAR than gradient echo because of 90 0 and RF pulses long TR times are incompatible with 3D acquisitions

44 Spin Echo Contrast T1 weighted –short TR ( ) –short TE (10-30) T2 weighted –long TR (2000 +) –long TE (> 60) PD weighted –long TR, short TE

45 Spin Echo Contrast T1 weighted - T1 relaxation predominates Short TE minimizes differences in T2 relaxation Short TR maximizes differences in T1 relaxation T2 weighted - T2 relaxation predominates Long TE maximizes differences in T2 relaxation Long TR minimizes differences in T1 relaxation

46 T1 weighted T2 weighted Spin Echo Contrast

47 PD weighted T2 weighted

48 Summary Detection of the MR signal only occurs in the transverse plane Gradient echo –Alpha degree pulse, dephase-rephase-echo –Contrast (T1/T2/T2*) depends on sequence type Spin echo –90 degree pulse, dephase, 180 degree pulse, rephase-echo –T1 weighted: short TR, short TE –PD weighted: long TR, short TE –T2 weighted: long TR, long TE


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