Principles of MRI EE225E / BIO265. Name That Artifact. RF Interference During Readout. RF Interference During Readout. Lecture 19

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1 Name That Artifact Principles of MRI EE225E / BIO265 Lecture 19 Instructor: Miki Lustig UC Berkeley, EECS RF Interference During Readout RF Interference During Readout 1D FFT 1D FFT 1D FFT readout readout 3 4

2 Name That Artifact Which direction is readout? How can you find frequency of RF? Off -Resonance Effects on Imaging m xy (~r, t) =m xy (~r, 0)e i! E(~r)TE e i2 k x(t) phase/dephasing An on-resonance spin at position: x 0 = x +! E(~r) G x x+! E (~r) Gx displacement d~r Produces the same signal as a spin at x, with off-resonance! Examples: Examples: Phase vs Echo Time signal loss water and lipids in phase water and lipids out-of-phase shape readout direction? 7 8

3 Example: Chemical Shift x 0 = x +! E(~r) G x Increasing Gx reduces chemical shift artifact Example: Let, 2 G x =1 E(~r) =3 ppm KHz/cm! E (~r) =2 200 Hz (@1.5T) x = x 0 x =! E(~r) G x = Hz 2 1 KHz/cm =0.2cm At: x =1mm this is a shift of two pixels! 9 10 Effects in Spin-Warp Off-Resonance Modest spatial distortions ( few pixels) Relatively benign artifacts Reduce artifacts with large Gx Chemical-Shift Fat shift of 1.5T, 1.5T Fat image is displaced from Water In practice F/W shift limited to ~2pixels Off-Resonance in EPI Example: Lipids phase accumulation 0.44cyc 1ms 1ms 0.44cyc 0.44cyc 1ms 96ms 42.24cyc point-spread function 2 pixels shift are two cycles of linear phase across k-space 2 cyc 9.1 ms 220 Hz 1ms 0.44cyc 11 12

4 Off-Resonance in EPI point-spread function simulation Leg Off-Resonance in Spiral phase accumulation point-spread function real imag 2 ms 0.88cyc Off-Resonance in Spiral Readout 2ms 5ms 13ms Echoes Early in NMR people noticed the following odd behavior (Hann, 1950) Spiral scan with linear off-resonance Two 90 excitations, separated by T cause large signal to form at 2T, Why? 15 16

5 Spin Echo Pulse Sequence RF Spin Echo Pulse Sequence RF T2 *? T2 * T2 Any signal loss due to dephasing is recovered at the spin-echo * T2 If we(r,t) changes over time, refocusing is not perfect Provides probe to measure molecular motion Multi-Spin Echo - CPMG Non uniform dephasing Any signal due to dephasing ωe(r)t is refocussed If ωe(r,t) refocussing not perfect time-varying dephasing shows as T2 T2 * Huge! but, constant dephasing 19 20

6 T T T T2(T) Large vessels Low Flip Angle Spin Echoes Any pulse can produce a spin-echo 180 maximally refocuses magnetization Lower refocusing power with lower flip angle 90 refocusses 1/2 (see homework) small vessels Low Flip Angle Spin Echoes Refocusing RF result can be decomposed to several components: Pass through Refocused Parasitic excitation Magnetization stored in Mz Low Flip Angle Spin Echoes α α M = R z (! E )R x ( )R z (! E )R x ( )M 0 For 90 degrees: M xy = 1 2 sin(! E2 )M 0 i sin 2 (! E )M 0 In general... I think M xy = A 1 cos(2! E )+A 2 sin(2! E ) {z } + B 1 cos 2 (! E )+B 2 sin 2 (! E ) {z } + C 1 cos(! E )+C 2 sin(! E ) {z } passthrough refocused parasitic 23 24

7 Phase Graphs Phase Graphs Useful to find echo formations α α passthrough parasitic excitation longitudinal magn. t refocused echo t * from document by Matthias Weigel Extended Phase Graphs Each component has different magnitude and phase Echo consist of contribution from many pathways Can track to calculate magnitude & Phase of echo (Beyond scope... see document by Matthias Weigel) Spin Echo Imaging Spin echo negates phase -- Conjugation How to incorporate in pulse sequence? M xy (~r, t) =M xy (~r, 0)e i2 (k x(t)x+k y (t)y) 27 28

8 Spin Echo Imaging Spin echo negates phase -- Conjugation How to incorporate in pulse sequence? imperfect 180 causes parasitic which is not phase encoded (HW) 31 32

9 33 34 what s the phase graph for all pathways? 35 36

10 Fast/Turbo Spin-Echo (RARE) Other non-idealities RF Inhomogeneity Affects Excitation Spatially varying flip-angle Spatially varying phase Can cause problem for fat saturation high field (wave effects) Affects receive Hennig 86 figure from Bernstein, King and Zhou RF excitation inhomogeneity 39 40

11 Gradient non-idealities More later! Concomitant Gradient (Maxwell Terms) Concomitant Gradient (Maxwell Terms) 43 44

12 Concomitant Gradients king et. al, MRM king et. al, MRM 41: (1999) a b a b Gradient Finite Rise-Time Eddy Currents Even finite-rise-time rate! (often ignored) 47 48

13 Eddy Currents Pre-emphasis 51

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