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A Brief Journey into Parallel Transmit

A Brief Journey into Parallel Transmit. Jason Su. Description. Goal: expose myself to some of the basic techniques of pTx Replicate in-class results Explore some possibilities Learn about spokes design – 3D design Implement “inversion” design, as detailed in class notes.

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A Brief Journey into Parallel Transmit

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  1. A Brief Journey into Parallel Transmit Jason Su

  2. Description • Goal: expose myself to some of the basic techniques of pTx • Replicate in-class results • Explore some possibilities • Learn about spokes design – 3D design • Implement “inversion” design, as detailed in class notes

  3. Small Tip-Angle Inversion Design • Freedoms in design: • Number of spatial points • Number of k-space points • Regularization and smoothing • Traditional design is sometimes too fast for RF coil to keep up

  4. Basic Design • Jinc-weighted spiral design as in HW3 • Resolution = 0.2cm • FOV = 10cm • Gradient amplitude max = 6 G/cm • Gradient slew rate max = 20 G/cm/ms • Desired slice width = 4cm • Pulse length = 18ms for full FOV, 9ms for FOV/2

  5. Spiral Design

  6. Spiral Design – FOV/2

  7. Parallel Transmit System • 4 channels 24cm 16cm

  8. Parallel Transmit Design • Design target profile • Based on full FOVspiral • Cut out sidelobes • Flattened profile • 2x accelerated • This is actually infeasible • 64x64 spatial points, 512 spiral k-space points

  9. Result • Good sidelobe cancellation • Uniformity of mainlobe is poor

  10. Finer K-Space Sampling • We can improve the design by depositing more energy into k-space over the same trajectory • i.e. sample more finely in k-space • The penalty is that we need a better RF coil to keep up with the sampling rate

  11. Finer K-Space Sampling

  12. Side Note: Receive Oversampling • The excited profile is calculated as: Efbf • The spatial resolution of Ef can be as fine as desired • Sample an 80x80 grid: • The profile is quitedifferent. Matchingwith Tx required?

  13. RF Coil Considerations • RF coils have finitebandwidth • With hardest caseof 2048 k-space pts.,how does this affectexcited volume?

  14. Effects of RF Coil • Modeled coil response as that of a windowed sinc • Somewhat arbitrarily: BWhalf the k-space samplingrate = 100KHz

  15. Resulting Profile • Poor cancellation outside main lobe • Main lobe profile is worsened • Sidelobes reappearat edges • Should be able todo better, consider1024-sample

  16. Smoothing w/ Regularization • Try to minimize the high frequency energy • J = ||Eb-M||^2 + μ||Fb||^2, where F is the DFT matrix for high frequencies • Changed to 32x32,512-sample due to memory limitations

  17. Comparison

  18. Conclusion • Here, there is a knee in the RMSE vs. # k-space pts. profile at around 1024 pts., may not be always be the case • RF coil bandwidth greatly affects the quality of the resulting excitation profile • Regularization improves profile, especially out-of-mainlobe uniformity

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