MR Sequences and TechniquesMR Sequences and Techniques

BME595 MR Physics Lectures 2/3

Chen Lin, Ph.D.

Rev. 2/2007

The Anatomy of Basic MR Pulse SequencesThe 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

Increment Phase

Encoding

Slice/Slab Selective Excitation Slice/Slab Selective Excitation

SINC RF Pulse

Trapezoid Gradient Pulse

RF

Gz

Phase Encoding Phase Encoding

Trapezoid Gradient Pulses

Gy

Gradient Performance: Rise Time, Max. Amplitude and FOV

EchoEcho

• 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 ( ~ y Gy ~ y Gy ), is zero.

• No necessary for all three axis at the same time.

TE/2

Spin EchoSpin Echo

B1

The “Spin Echo Race”The “Spin Echo Race”

1800 Refocusing RF Pulse

Start and Finish

Slice/Slab Selective Refocusing Slice/Slab Selective Refocusing

1800 SINC RF Pulse

Trapezoid Gradient Pulse

RF

Gz

Frequency Encoding Frequency Encoding

Trapezoid Gradient Pulse

Signal

Gx

Echo

Spin Echo (SE) SequenceSpin Echo (SE) Sequence

TE/2TE/2

Excitation Refocusing

Phase Encoding

Frequency Encoding

TR

Next Excitation

Short TE, Long TR

PD Weighted ImagingPD Weighted Imaging

T1 Weighted ImagingT1 Weighted Imaging

Short TE (<<T1), Intermediate TR (~T1)

Axial T1w SEAxial T1w SE

• TR = 500 msec• TE = 15 msec

Dark CSF

T2 Weighted ImagingT2 Weighted Imaging

Intermediate TE ( ~T2), Long TR ( >> T1)

Axial T2w SEAxial T2w SE

• TR = 2000 msec• TE = 90 msec

Bright CSF

Excitation

Phase Encoding

Frequency Encoding

Gradient Recalled Echo (GRE)Gradient Recalled Echo (GRE)

TE

SE versus GRESE 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 SequenceMulti-contrast Sequence

Additional SE

TE2

k1 k2

Image 1 Image 2

Fast/Turbo SE (RARE)Fast/Turbo SE (RARE)

Rewind Rewind Rewind

k

TE = ?ETL/Turbo Factor = ?

3D Sequence3D Sequence

Slab Excitation

FrequencyEncoding in X

Phase Encoding in Z

Phase Encoding in Y

k X

Y

Ultra-fast SequencesUltra-fast Sequences

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

SS-FSE SequenceSS-FSE Sequence

k

EPI SequenceEPI Sequence

k

GRASE/TGSE SequenceGRASE/TGSE Sequence

GRE GRE GRE GRE

SE SE

Chemical ShiftChemical 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

MR Signal Frequencies at 1.5T

5.0 4.0 3.0 2.0 1.0 0.0 ppm

Water MI Cho Cr Glu NAA Lac/Lipid

Chemical Shift 1ppm = 63Hz

1H MRS

13C 23Na 31P 19F 1H

1 25 50 63 75 MHzFrequency

SaturationSaturation• 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 SaturationFat Saturation

T1w T1w + FS

Inversion Recovery (IR)Inversion Recovery (IR)

IR?

TI

Contrast vs Inversion TimeContrast vs Inversion Time

Tissue 1

Tissue 2

Null Points

Applications of IRApplications 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

SpoilerSpoiler

• 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)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 !Thank you !