FREQUENCY SELECTIVE EXCITATION

Transcription

1 PULSE SEQUENCES

2 FREQUENCY SELECTIVE EXCITATION RF Grad 0 Sir Peter Mansfield

3 A 1D IMAGE Field Strength / Frequency Position

4 FOURIER PROJECTIONS MR Image Raw Data FFT of Raw Data

5 BACK PROJECTION Image Domain Gradient Encoding 2D Fourier Transform real imaginary Fourier Domain Paul Lauterbur

6 EQUIVALENT STRATEGIES IN K-SPACE* Gradient Samples Time Gradient Gradient Samples Gradient Time *Ignoring effects of signal decay and sample motion Samples

7 GRADIENT PRE-ENCODING Signal Gradient Samples Signal Gradient Samples Time Signal Gradient Samples

8 INTERLEAVED SPATIAL ENCODING Gradient 1 Gradient 2 A2 A1 Samples Points indicated in black are affected by Gradient 1 but NOT by Gradient 2 Points indicated in Blue are affected by both gradients

9 EPI K-SPACE TRAJECTORY k-space plots the integral of the gradient encoding. k-phase k(x,y,t) = γ T 0 G(x,y,t)dt Its Fourier transform is the image. k-frequency

10 CONVENTIONAL SPATIAL ENCODING tr RF Grad 0 Grad 1 te Grad 2 A2 A2 A2 A1 A1 A1 Samples

11 CONVENTIONAL K-SPACE TRAJECTORY +Kphase tr -K frequency

12 SPIRAL ky Gx kx Gy

13 3D K-SPACE k y kz k x

14 3D K-SPACE Gz tr Gy Gx Imaging time = tr * Nz * Ny

15 MULTI-SLICE MRI tr RF Slice 1 Slice 3 Slice 2 Slice 4 Slice 1 te Gz Gy Gx t slice N = slices tr / tslice

16 SPATIAL ENCODING tr RF Grad 0 Grad 1 te Grad 2 A2 A2 A2 A1 A1 A1 Samples

17 CONTRAST ENCODING tr tr RF Grad 0 Grad 1 te te Grad 2 A2 A2 A2 A1 A1 A1 Samples

18 B0 SPIN DEPHASING

19 T2 AND TE 1 Signal 0.5 S(t) = M xy (t) = M 0 e te T 2 CSF Brain Fat Time (milliseconds) 120

20 PARTIAL SATURATION tr tr Sequence of 90 Pulses NMR Signal

21 EFFECTS OF TE AT LONG TR

22 EFFECTS OF TR AT SHORT TE te=17 nex=1 thick=3mm Matrix=256x256 BW=16kHz

23 CONTRAST, TR AND TE Long Proton Density T2-Weighted tr Short T1-Weighted Short Long te

24 CONTRAST, TR AND TE Density T2 Long tr T1 Short Short te Long

25 T2-WEIGHTED EPI SCAN Metastatic (cancer) lesions, and many others, typically appear bright on T2- weighted MR images

26 OBSERVED RELAXATION RATE The Observed Transverse Relaxation Rate, T2*, is the sum of several components: = + + T2* T2 T2 T2 D Molecular Field Inhomogeneity Diffusion

27 HAHN SPIN ECHO pulse 90 pulse 5. rephasing dephasing 6. spin echo

28 B0 SUMMARY ANIMATION

29 MULTI-ECHO 180 T2* T2* T2 TE1 TE2

30 INVERSION RECOVERY Mz CSF Brain Fat 1 Mxy Time (ms) Time (ms) TI=700ms

31 3D T1 Images TE = 3.2 TR = slices 1.25 mm thick 1 NEX Flip Angle 20 TI = 500

32 Sample Data Set (normal) Fast Spin Echo 3 mm Slices 3D IR-SPGR TE = 3.2, TI = 700 SAMPLE DATA SET (NORMAL)

33 CONTRAST TO NOISE RATIO (GRAY-WHITE) Gray White 0.2 te tr 6 tr, te in seconds -5% 0% +3% -5% 0% 3% Contrast = [(1 e tr /1.2 )e te /.08 ], gray matter [(1 e tr /1.0 )e te /.07 ], white matter

34 REDUCED FLIP ANGLE IMAGING Outline Determinants of Imaging Time TR, Saturation and Image Quality Reduced Flip Angle Techniques FLASH (=SPGR) FISP (=GRASS) Gradient Echoes Applications of Shallow Flip Imaging Ultra-Fast Imaging

35 DETERMINANTS OF IMAGING TIME Scan Time = Repetition Time (TR) x Number of Phase Encodes x NEX (Averages) x Number of 3D Steps

36 TR AND IMAGE QUALITY Reduced TR Yields: Decreased Scan Time Increased T1 Contrast Reduced (Useable) T2 Contrast Reduced Signal to Noise Ratio Increased Power Deposition Reduced Slice Coverage 36

37 SIGNAL AND FLIP ANGLE Small Flip Angle Large Flip Angle α α 37

38 SMALL AND LARGE FLIP ANGLE Loss of Longitudinal Magnetization Small Flip Angle Large Flip Angle

39 FLIP ANGLE AND TR/T

40 Contrast and Flip Angle Large Flip Angles Short Long Long Short Proton Density T1 Weighted T2* Weighted Small Flip Angles Short Long Long Short Proton Density Proton Density T2* Weighted T2* Weighted

41 A 180 PULSE IS NOT USED IN FLASH IMAGING z x z After 180 Pulse z x y Initial Magnetization y After Small RF Pulse y x

42 T2 AND T2* T2: Transverse Magnetization Decay from Spin-Spin Interactions T2*: Transverse Magnetization Decay from Local Magnetic Field Variations

43 SIGNAL AND TE GRADIENT ECHO TE=20 TE=40 TE=60 TE=80 TE=100

44 MAGNETIC SUSCEPTIBILITY The Extent to Which a Substance Becomes MAGNETIZED when Placed Within a Magnetic Field

45 MAGNETIC SUSCEPTIBILITY Objects with Susceptibility Different than Air Distort the Magnetic Field Applied Magnetic Field

46 T2* 100ms 0ms

47 SIGNAL LOSSES FROM SPIN DEPHASING B Inhomogeneous Magnetic Fields Within Voxels Result in Spin Dephasing and Signal Loss in Gradient Echo Sequences Capillary Gradients of several Gauss/cm may exist near deoxy-hb-filled capillaries.

48 CONTRAST OPTIMIZATION Contrast 43 ms TE >> T2a=40; T2b=45; te=0:150; >> contrast = exp(-te/t2b) - exp(-te/t2a); >> plot(te,contrast,'linewidth',3); >> find(contrast==max(contrast)) ans = 43

49 SPIN ECHO RF Gz te Gy Gx 10 msec

50 FLASH RF te Gz Gy Gx 10 msec

51 FLASH RF te Gz Gy Gx 10 msec

52 FLASH MAGNETIZATION CYCLE α Longitudinal Recovery 3. α RF pulse followed by data collection Spoiling of transverse magnetization

53 FISP (GRASS) RF te Gz Gy Gx 10 msec

54 SSFP MAGNETIZATION CYCLE α Longitudinal Recovery and T2* relaxation α degree RF pulse and data collection α α degree RF pulse and data collection Longitudinal Recovery and T2* relaxation 54

55 SSFP RF te Gz Gy Gx 55

56 RF RF RF te te te Gz Gz Gz Gy Gy Gy Gx Gx Gx FLASH GRASS SSFP

57 3D MP-RAGE RF Gz 180 Repeat Ny times ti tr tr tr Gy Gx

58 PHASE MAPS RF te slice select readout Time shift in data collection amounts to a phase offset Spins precessing at different rates (different magnetic fields) will acquire different phase shifts

59 TRADEOFFS Volume Coverage tr slice thickness te

60 TRADEOFFS SNR tr te flip angle voxel volume contrast imaging time (e.g., averaging)

61 TRADEOFFS Imaging Time tr resolution total slices (due to vendor optimization)

62 TRADEOFFS SAR flip angle echo train length number of slices / tr total scan duration