Transcript Introduction - Yale University

MR Sequences and Techniques
BME595 MR Physics Lectures 2/3
Chen Lin, Ph.D.
Rev. 2/2007
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
Slice/Slab Selective Excitation
a SINC RF Pulse
RF
Trapezoid
Gradient Pulse
Gz
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.
• 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.
Spin Echo
B1
TE/2
The “Spin Echo Race”
Start and Finish
1800 Refocusing RF Pulse
Slice/Slab Selective Refocusing
1800 SINC RF Pulse
RF
Trapezoid
Gradient Pulse
Gz
Frequency Encoding
Trapezoid Gradient
Pulse
Gx
Echo
Signal
Spin Echo (SE) Sequence
Excitation
Refocusing
Frequency
Encoding
Phase
Encoding
TE/2
TE/2
TR
Next
Excitation
PD Weighted Imaging
Short TE, Long TR
T1 Weighted Imaging
Short TE (<<T1), Intermediate TR (~T1)
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
Gradient Recalled Echo (GRE)
Excitation
Phase
Encoding
Frequency
Encoding
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
k1
Image 1
k2
Image 2
Fast/Turbo SE (RARE)
Rewind
k
TE = ?
ETL/Turbo Factor = ?
Rewind
Rewind
3D Sequence
Slab Excitation
Phase Encoding in Z
Phase Encoding in Y
Frequency
Encoding in X
Y
k
X
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
GRE
SE
GRE
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-CH2, n-CH3, nOH, n-NH
MR Signal Frequencies at 1.5T
13C 23Na
1
1H
19F 1H
25
Frequency
50
63
75 MHz
MRS
Water
5.0
31P
MI Cho Cr
4.0
3.0
Glu NAA Lac/Lipid
2.0
Chemical Shift 1ppm = 63Hz
1.0
0.0 ppm
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
Null Points
Tissue 2
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 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.
Thank you !