Pulse sequences in MRI

Pulse sequence: RFPulse sequence: RF

Bloch Bloch equationequation

Pulse sequence: RFPulse sequence: RF

Lab reference frameLab reference frameRotating reference frameRotating reference frame

Pulse sequence: RFPulse sequence: RF

Gradients onGradients on

Pulse sequence: RFPulse sequence: RF

Slice selective pulsesSlice selective pulses TruncationTruncation

Sin

c p

uls

eS

inc

pu

lse

FT

FT

Pulse sequence: RFPulse sequence: RF

Spectral pulses (chemical shift)Spectral pulses (chemical shift)

Fat suppressionFat suppression

Pulse sequence: RF pulse designPulse sequence: RF pulse design

Pulse sequence: Spin EchoPulse sequence: Spin Echo

Spin echo - “working horse of MR”Spin echo - “working horse of MR”

Pulse sequence: Spin EchoPulse sequence: Spin Echo

contrastscontrasts

Pulse sequence: sequence diagramPulse sequence: sequence diagram

RRFF

Slice Slice selectiveselective

Phase Phase encodingencoding

Frequency Frequency encodingencoding

MR signalMR signal

TETE

TTRR

90

180

90

Pulse sequence: contrastsPulse sequence: contrasts

Returning to equilibrium – T1 Returning to equilibrium – T1

TimeTime

Mag

neti

zati

on

Mag

neti

zati

on

Pulse sequence: contrastsPulse sequence: contrasts

Spin dephasing - T2Spin dephasing - T2

TimeTime

Mag

neti

zati

on

Mag

neti

zati

on

Pulse sequence: Turbo spin echoPulse sequence: Turbo spin echo

Pulse sequence: Turbo spin echoPulse sequence: Turbo spin echo

T2 relaxation time of fat is longer due to suppressedT2 relaxation time of fat is longer due to suppressedJ-coupling by turbo spin echo (refocusing 180 pulses)J-coupling by turbo spin echo (refocusing 180 pulses)

Pulse sequence: SE EPIPulse sequence: SE EPI

Pulse sequence: SE EPIPulse sequence: SE EPI

The sequence is very sensitive to susceptibilityThe sequence is very sensitive to susceptibilityartefacts, to gradient imperfection.artefacts, to gradient imperfection.Very fast acquisition.Very fast acquisition.

Pulse sequence: GEPulse sequence: GE

Pulse sequence: GEPulse sequence: GE

Allows shot TE/TR; easy adopt to 3D;Allows shot TE/TR; easy adopt to 3D;Modification for high resolution structural image:Modification for high resolution structural image:Magnetisation prepared rapid acquisition by gradientMagnetisation prepared rapid acquisition by gradientecho (MP-RAGE) echo (MP-RAGE)

Pulse sequence: Inversion recoveryPulse sequence: Inversion recovery

Spectral selective RF pulseSpectral selective RF pulseIs ineffective in the case of B0Is ineffective in the case of B0inhomogeneityinhomogeneity

Sequence with IR allows one to solve itSequence with IR allows one to solve it

Pulse sequence: Inversion recoveryPulse sequence: Inversion recovery

Fat suppressionFat suppression

Pulse sequence: AngiographyPulse sequence: Angiography

Gradient echo: time-of-flight (ToF)Gradient echo: time-of-flight (ToF)

Spin echo: time-of-flight (ToF)Spin echo: time-of-flight (ToF)

Pulse sequence: AngiographyPulse sequence: Angiography

Pulse sequence: MR SpectroscopyPulse sequence: MR Spectroscopy

PRESS: Point resolved spectroscopyPRESS: Point resolved spectroscopy

Pulse sequence: DiffusionPulse sequence: Diffusion

Spatial encoding of Brownian motion in Spatial encoding of Brownian motion in a given directiona given direction

Pulse sequence: DiffusionPulse sequence: Diffusion

Literature

● Berstein, King, Zhou, Handbook of MRI pulse sequences.

● McRobbie, Moore, Graves, Prince, MRI: from picture to proton.

Problems

● How to estimate a flipping angle knowing the strength of applied B1 and duration of the pulse?

● What is sinc pulse and why it is applied often in imaging?● Show FT for differently cut sinc pulse. What is difference?● Why do we need a gap in slice excitation?● Write the Bloch equation with relaxation times T1 and T2. What is

a solution of the Bloch equation?● What is a spoiler gradient?● You have to learn the basic acronyms of pulse sequences: SE,

GE, TSE, EPI, FLASH, MPRAGE etc.

Next topic is Next topic is image artefactsimage artefacts