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 Magnetization Recovery Section Spoiling Driven Equilibrium Incremen t Phase Encoding
Phase Encoding Trapezoid Gradient Pulses Gy Gradient Performance: Rise Time, Max. Amplitude and FOV
Echo The directions of magnetic moments in the transverse plane are re-aligned to generate a detectable signal. ~ y GyThe time integral of gradient pulses from excitation to echo, i.e. the accumulated phase shift ( ~ y Gy ), is zero. No necessary for all three axis at the same time.
TE/2 Spin Echo B1B1
The “Spin Echo Race” Refocusing RF Pulse Start and Finish
Frequency Encoding Trapezoid Gradient Pulse Signal Gx Echo
Spin Echo (SE) Sequence TE/2 Excitation Refocusing Phase Encoding Frequency Encoding TR Next Excitation
Short TE, Long TR PD Weighted Imaging
T1 Weighted Imaging Short TE (<
Axial T1w SE TR = 500 msec TE = 15 msec Dark CSF
T2 Weighted Imaging Intermediate TE ( ~T2), Long TR ( >> T1)
Axial T2w SE TR = 2000 msec TE = 90 msec Bright CSF
Excitation Phase Encoding Frequency Encoding Gradient Recalled Echo (GRE) TE
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.
Multi-contrast Sequence Additional SE TE2 k1k2 Image 1Image 2
Fast/Turbo SE (RARE) Rewind k TE = ? ETL/Turbo Factor = ?
3D Sequence Slab Excitation Frequency Encoding in X Phase Encoding in Z Phase Encoding in Y k X Y
Ultra-fast Sequences Single-shot FSE / TSE (HASTE) Echo Planar Imaging (EPI) Interleave of SE and GRE (TGSE, GRASE)
SS-FSE Sequence k
EPI Sequence k
GRASE/TGSE Sequence GRE SE
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-CH 2, n-CH 3, n- OH, n-NH
MR Signal Frequencies at 1.5T ppm Water MI Cho Cr Glu NAA Lac/Lipid Chemical Shift 1ppm = 63Hz 1 H MRS 13 C 23 Na 31 P 19 F 1 H MHz Frequency
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.
Fat Saturation T1w T1w + FS
Inversion Recovery (IR) IR ? TI
Contrast vs Inversion Time Tissue 1 Tissue 2 Null Points
Applications of IR 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 STIR
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
Driven Equilibrium (Fast Recovery, Restore) A y + a x RF pulses to focus and flip the transverse magnetization to Z axis. Allow shorter TR for the recovery of magnetization. Increase T2 weighting.