mri part III - unibas.ch

Part III:Sequences and Contrast

Contents

T1 and T2/T2* RelaxationContrast of Imaging Sequences

T1 weightingT2/T2* weightingContrast Agents

SaturationInversion Recovery

Basic Sequences and Contrast

JUST WATER? (i.e., proton density PD)

Proton density(PD)

Spin-LatticeRelaxation

Spin-SpinRelaxation

Chemical ShiftImaging (water-fat)

SusceptibilityImaging (SWI)

Diffusion weightedImaging

TemperatureMapping

Elastography(MRE)

MR Angiography

Flow / Motion Imaging

Contrast enhancedMRI, cell tracking,SPIOs, USPIOs,…

Perfusion Imaging

Spectroscopy

FunctionalImaging (BOLD)

…and many more…!

Relaxation fromMacroscopic Fieldinhomogeities

MRI Contrast Mechanisms

Equilibrium

2

2

1

/,0

/,0

/,0 0 0

( )

( )

( ) ( )

t Tx x

t Ty y

t Tz z

M t M e

M t M e

M t M M e M

0M PD

Basic Contrast Mechanism: PD, T1, T2

After excitation, the magnetization returns back to thermal equilibrium

PD


2/( ) (0)xy xyt TM t M e

01/( ) 1z

t TM t M e

PD


( Analysis under the Assumptions: T2* = T2 )Consider the following simple sequence scheme

TE: Echo-Time (time between excitation and signal acquisition)


90° pulse 90° pulse

An MRI sequnce consists of a series of (i) excitation pulses (RF pulses), (ii) gradients and (iii) signal readout events (ADC: analog

digital converter).

TR: Repetition time of RF pulse (time between excitations)

waiting time >> T1 (full recovery)

1 2

1 exp( ) exp( )TR TES PDT T


PD


1 2


PDT2T1

TE

TR

…image weighting…

Basic Contrast Mechanism: PD, T1, T2se

q. p

rope

rty

PDT2T1

TE

TR TR >> T1 (~5000ms)no effectTR ~ T1

(~1000ms)



q. p

rope

rty

1 2


PDT2T1

TE

TR TR >> T1 (~5000ms)no effectTR ~ T1

(~1000ms)

no effect


TE << T2 (~10ms)

TE ~ T2(~70ms)


q. p

rope

rty

1 2


A Gradient Echo (GRE) Sequence

TR

TE

TR: Repetition timeTE: Echo time: excitation (flip) angle

Case I: = 90°, TR >> T1 (full recovery)

(saturation recovery)

1 1( ) exp( / )E t t T 1 1: exp( / )E TR T * *2 2( ) exp( / )E t t T * *

2 2: exp( / )E TE T

*2 0xyM E M

0zM M

xy zM M

*2S PD E

Convention: The uppercase „+“ („-“) denotes the magnetization immediately after (before) the RF pulse

Signal of the Gradient Echo Sequence

Case II: = 90°, TR ~ T1 (partial recovery), TR >> T2 (full decay)

(saturation recovery)


2 2: exp( / )E TE T

*1 2(1 )S PD E E

1 0(1 )zM E M

*2 0xyM E M

xy zM M



Case III: < 90°, TR ~ T1 (partial recovery), TR >> T2 (full decay)


2 2: exp( / )E TE T

1 zE M 0 1(1 )M EzM

T1 decay polarizationT1 recovery

cosz zM M


Action of RF pulse Action of TR

sinxy zM M

10

1

11 cosz

EM ME

1

0 1

1 sin1 cos

xyxy

M EmM E


TR=3·T1

TR=0.2·T1

PDw T1w

Ernst angle (max. signal): 11cosE E

saturation

1

1

1sin1 cos

ES PDE

Contrast Behaviour of the Gradient Echo Sequence: PD, T1

TR [msec]

[deg]

Contrast Behaviour of the Gradient Echo Sequence: PD, T1

k-space

Contrast of the Gradient Echo Sequence: T2*

01

2 3

GRE reads FID: T2*-weighted

T2‘: dephasing from field inhomogeneities

T2: loss of transverse magnetization

Contrast of the Gradient Echo Sequence: T2*

Gx

ADC

TE

TE = 10 ms TE = 20 ms TE = 40 ms TE = 60 ms

GE: TR=200ms, =35°

T2* weighted

A Spin-Echo Sequence

Parameters: Repetition time (TR) & Time to Echo (or echo time TE)

k-space

Echo time TE: time between excitation (0) and arrival a k-space center (3)

1

234

0


A Multi-Echo Spin-Echo Sequence

*2 2 2

1 1 1T T T

Contrast of the Spin-Echo Sequence: T2 or T2*

microscopic field fluctuations

macroscopic field inhomogeneities

Interactions have very short correlation times:c ~ 1011 – 107 [sec]

changes are in the range of sec and thus muchlonger than the typical TE of the sequence

phase changes from macroscopic field inhomogeneities are thus deterministic

phase changes from microscopic field fluctuations are thus deterministic

What happens to the spin echo ?

Contrast of the Spin-Echo Sequence: T2 or T2*

90° 180° spin-echo

Phase evolution of a single spin (magnetic moments)

Macroscopic field inhomogeneities are rephased by the 180° pulse!

Contrast of the Spin Echo (SE) Sequence

SE reads echo:T2-weighted

k-space

1

234

0

T2‘: dephasing due to field inhomogeneities(between 90° and 180° pulse)

T2‘: rephasing due to field inhomogeneities(between 180° and spin echo)

T2: loss of transverse magnetization

TE = 10 ms TE = 40 ms TE = 70 ms TE = 100 ms

SE: TR=6000ms, =90°

Contrast of the Spin Echo (SE) Sequence: PD, T2

200

500

1000

3000

6000

10 40 70 100

TR [msec]

TE [msec]

Contrast of the Spin Echo (SE) Sequence: PD, T1, T2

Gra

dien

t Ech

oS

pin

Ech

o

TR=5

00m

s, T

E=1

0ms

T2* versus T2 weighting

Advanced Imaging: GRE sequences can be used for susceptibility imaging

Metallic Implants: GRE sequences show strong artifacts (signal loss)

Contrast Modification


Contrast Agents

Tissue properties (nativ): PD, T1, T2

contrast-enhanced (ce): PD, T1, T2

MR Signal Intensity: PD, T1, T2

Contrast Agents (CA)

Principle: artificial shortening of T1 and T2 with paramagnetic contrast agent

Gadolinium-DTPA



CA Paramagnetic agents: 2 2, 21/ 1/ [ ]nativT T CA r

1 1, 11/ 1/ [ ]nativT T CA r

r1 ~ r2 ~ 4500 1/Ms @ 1.5 TGadolinium-DTPA

[CA] : 0.5 M (not diluted): T1 = 0.44 ms [CA] : 0.05 M (10 x diluted): T1 = 4.4 ms [CA] : 0.005 M (100 x diluted): T1 = 42 ms

[CA] : 0.0M (native) T1 = 1000 ms




1 1, 11/ 1/ [ ]nativT T CA r

Positive CA: low concentrated Gd-chelates. Predominantly reduction in T1 (electron-proton dipolar coupling). (Positive contrast in T1w-image)




1 1, 11/ 1/ [ ]nativT T CA r

Positive CA: low concentrated Gd-chelates. Predominantly reduction in T1 (electron-proton dipolar coupling). (Positive contrast in T1w-image)

Negative CA: superparamagnetic agents (SPIO, USPIO) in small crystalline structures (iron-oxide).Predominantly reduction in T2 (increased . (Negative contrast in T2w-image)


Tissue properties (nativ): PD, T1, T2

Contrast Modification: Saturation

Magn. Preparation Host – Sequence: ANY

spoiler

SpatialSaturation, FatSuppression,

WaterSuppression,

…

Contrast Modification: Inversion Recovery


1 (gray, white matter)TI T

Inversion time (TI)


Inversion time (TI)

“Inversion Pulses” are used to induce T1-weighting onto Host - Sequence

Contrast Modification: Inversion Recovery

Contrast Modification: MPRAGE

Magn. Preparation Host – Sequence: PD weighted 3D GRE

(magnetization prepared rapid gradient echo)

1 (gray, white matter)TI T

Inversion time (TI)

Example 1: T1-weighting on GRE

Contrast Modification: MPRAGE

Magn. Preparation Host – Sequence: PD weighted 3D GRE

Inversion time (TI)

(magnetization prepared rapid gradient echo)

Induce T1-weighting onto 3D PD GRE images to allow for fast acquisition of high resolution

whole brain T1 images

PD T1

Contrast Modification: FLAIR

Magn. Preparation Host – Sequence: 2D mslc T2 (T)SE

(fluid attenuated inversion recovery)

1

: ( ) 1 2 exp( ) 0!TRTI S CSFT

Inversion time (TI)

Example 2: T1-weighting on (T)SE

Contrast Modification: FLAIR

Magn. Preparation Host – Sequence: 2D mslc T2 (T)SE

(fluid attenuated inversion recovery)

Inversion time (TI)

Supress the very strong hyperintense signal from fluids

in T2w images

T2 FLAIR

Contrast Modification: Overview

PD, T1, T2, T2*

CA

PD, T1, T2, T2*

PDw, T1w, T2w, T2*w

imaging sequences

magn. prep.

tissue

sequence

native

Summary: Part III

Contrast of gradient-echo and spin-echo sequences is modified fromT1 and T2 relaxation.

Gradient echo sequences use gradient recalled echoes, whereasspin-echo sequences use a spin-echo for signal readout.

The echo in GRE sequences is T2*-weighted, whereas the echo in spin-echo sequences is T2-weighted.

As a result, spin-echo sequences are less prone to susceptibilityeffects as compared to gradient echo sequences

Contrast in gradient echo sequences is modified by the flip angle, bythe repetition time and the echo time.

Contrast in spin-echo sequences depends on the repetition time and the echo time.

Should the slice-selection gradient be applied before, during or after the RF excitation pulse?

• Before the RF excitation.

• During the RF excitation.

• After the RF excitation.

Topics: imaging, k-space, gradient-echo, spin-echo

Exercises: Part II & Part III

How is the slice thickness increased?

• Increase the transmitted RF bandwidth or the slice selection gradient strength.

• Increase the transmitted RF bandwidth, or decrease the slice selection gradient strength.

• Decrease the transmitted RF bandwidth, or increase the slice selection gradient strength.




How do signal differences between tissue types arise from differences in T1?

• We wait for different amounts of signal decay to occur before taking a signal measurement.

• We wait for different amounts of magnetisation recovery to occur before starting the MRI signal measurement process.

• We change the T1 of certain tissues.



How do signal differences between tissue types arise from differences in T2?

• We wait for different amounts of signal decay to occur before taking a signal measurement.

• We wait for different amounts of magnetisation recovery to occur before starting the MRI signal measurement process.

• We change the T2 of tissues



What does the frequency encoding gradient do?

• It moves net magnetisations into the xy-plane.

• It reads out the MRI signal.

• It causes a range of Larmor frequencies to exist.



How is proton-density weighting achieved?

• Short TR, short TE.

• Short TR, long TE.

• Long TR, short TE


mri part III - unibas.ch

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