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MR Sequences and Techniques

Learn about fundamental MR pulse sequences, including Magnetization Preparation, Data Acquisition, Magnetization Recovery, and more. Understand techniques like Spin Echo and Gradient Recalled Echo. Master concepts like Chemical Shift and Saturation to improve image quality and reduce artifacts.

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MR Sequences and Techniques

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  1. MR Sequences and Techniques BME595 MR Physics Lectures 2/3 Chen Lin, Ph.D. Rev. 2/2007

  2. The Anatomy of Basic MR Pulse Sequences • Magnetization Preparation Section • Chemical Shift Selective Saturation • Spatial Selective Saturation • Inversion Recovery (IR) • Magnetization Transfer (MT), CHESS water suppression • Data Acquisition Section • Excitation • Phase Encoding • Echo Generation • Spin Echo (SE), Fast SE, Single-shot FSE (HASTE) • Gradient Recalled Echo (GRE), Fast GRE, Single-shot GRE (EPI) • Diffusion Weighting (DWI/DTI) and Gradient Moment Nulling (GMN) • Frequency Encoding and Digital sampling Increment Phase Encoding • Magnetization Recovery Section • Spoiling • Driven Equilibrium

  3. Slice/Slab Selective Excitation a SINC RF Pulse RF Trapezoid Gradient Pulse Gz

  4. Phase Encoding Trapezoid Gradient Pulses Gy Gradient Performance: Rise Time, Max. Amplitude and FOV

  5. Echo • The directions of magnetic moments in the transverse plane are re-aligned to generate a detectable signal. • The time integral of gradient pulses from excitation to echo, i.e. the accumulated phase shift (q~ y Gy t), is zero. • No necessary for all three axis at the same time.

  6. Spin Echo B1 TE/2

  7. The “Spin Echo Race” Start and Finish 1800 Refocusing RF Pulse

  8. Slice/Slab Selective Refocusing 1800 SINC RF Pulse RF Trapezoid Gradient Pulse Gz

  9. Frequency Encoding Trapezoid Gradient Pulse Gx Echo Signal

  10. Spin Echo (SE) Sequence Excitation Refocusing Frequency Encoding Phase Encoding TE/2 TE/2 Next Excitation TR

  11. PD Weighted Imaging Short TE, Long TR

  12. T1 Weighted Imaging Short TE (<<T1), Intermediate TR (~T1)

  13. TR = 500 msec TE = 15 msec Axial T1w SE Dark CSF

  14. T2 Weighted Imaging Intermediate TE ( ~T2), Long TR ( >> T1)

  15. TR = 2000 msec TE = 90 msec Axial T2w SE Bright CSF

  16. Gradient Recalled Echo (GRE) Excitation Phase Encoding Frequency Encoding TE

  17. SE versus GRE • Reverse de-phasing in the transverse plane due to: • Chemical shift • Local field inhomogeneity • T2 weighted instead of T2* weighted • Less artifacts. • Longer TR and higher RF energy deposition due to refocusing RF pulse.

  18. Additional SE TE2 k1 Multi-contrast Sequence k2 Image 1 Image 2

  19. Rewind Rewind Rewind Fast/Turbo SE (RARE) k TE = ? ETL/Turbo Factor = ?

  20. 3D Sequence Slab Excitation Phase Encoding in Z Phase Encoding in Y Frequency Encoding in X Y k X

  21. Ultra-fast Sequences • Single-shot FSE / TSE (HASTE) • Echo Planar Imaging (EPI) • Interleave of SE and GRE (TGSE, GRASE)

  22. SS-FSE Sequence k

  23. EPI Sequence k

  24. GRASE/TGSE Sequence GRE GRE GRE GRE SE SE

  25. Chemical Shift The electron density around each nucleus varies according to the types of nuclei and chemical bonds in the molecule, producing different opposing field. Therefore, the effective field at each nucleus will vary. n-CH, n-CH2, n-CH3, n-OH, n-NH

  26. MR Signal Frequencies at 1.5T MNS 13C 23Na 31P 19F 1H 1 25 50 63 75 MHz Frequency 1H MRS Water MI Cho Cr Glu NAA Lac/Lipid 5.0 4.0 3.0 2.0 1.0 0.0 ppm Chemical Shift 1ppm = 63Hz

  27. Saturation • Saturation = Selective excitation + De-phrasing (with gradient) • Chemical Shift Selective Saturation: • Suppress signal within certain resonance frequency range. i.e. Fat Sat. • Narrow bandwidth excitation with no gradient applied. • Improve contrast and conspicuity. • Spatial Selective Saturation: • Suppress signal within certain spatial range. i.e. Sat. Band. • Slab selective excitation + de-phasing to create signal void. • Reduce flow/motion/phase-warp artifacts.

  28. Fat Saturation T1w T1w + FS

  29. Inversion Recovery (IR) IR ? TI

  30. Contrast vs Inversion Time Tissue 1 Null Points Tissue 2

  31. Improve T1 contrast IR-SPGR/MP-RAGE Selective nulling based on T1 difference: STIR with TI = 150ms to suppress fat signal. FLAIR with TI = 2000ms to suppress CSF. More accurate T1 measurement. Phase sensitive IR Applications of IR STIR

  32. Spoiler • Prevent magnetization build up in the transverse plane. • Through variable crusher gradient or RF phase cycling. • Suppress artifacts due to remaining transverse magnetization from previous TR. • Reduce T2 weighting in GRE sequences. • Spoiled GRE: FLASH/SPGR • Un-spoiled/Coherent GRE: FISP/GRASS, PSIF/SSFP, TrueFISP/FIESTA

  33. Driven Equilibrium (Fast Recovery, Restore) • A 1800y + a 900-x RF pulses to focus and flip the transverse magnetization to Z axis. • Allow shorter TR for the recovery of magnetization. • Increase T2 weighting.

  34. Thank you !

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