mri part iii - unibas.ch
TRANSCRIPT
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
Basic Contrast Mechanism: PD, T1, T2
PD
Basic Contrast Mechanism: PD, T1, T2
PD
Basic Contrast Mechanism: PD, T1, T2
PD
Basic Contrast Mechanism: PD, T1, T2
PD
Basic Contrast Mechanism: PD, T1, T2
PD
Basic Contrast Mechanism: PD, T1, T2
2/( ) (0)xy xyt TM t M e
01/( ) 1z
t TM t M e
PD
Basic Contrast Mechanism: PD, T1, T2
( Analysis under the Assumptions: T2* = T2 )Consider the following simple sequence scheme
TE: Echo-Time (time between excitation and signal acquisition)
Basic Contrast Mechanism: PD, T1, T2
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
Basic Contrast Mechanism: PD, T1, T2
PD
Basic Contrast Mechanism: PD, T1, T2
1 2
1 exp( ) exp( )TR TES PDT T
Basic Contrast Mechanism: PD, T1, T2
1 2
1 exp( ) exp( )TR TES PDT T
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)
…image weighting…
Basic Contrast Mechanism: PD, T1, T2se
q. p
rope
rty
1 2
1 exp( ) exp( )TR TES PDT T
PDT2T1
TE
TR TR >> T1 (~5000ms)no effectTR ~ T1
(~1000ms)
no effect
…image weighting…
TE << T2 (~10ms)
TE ~ T2(~70ms)
Basic Contrast Mechanism: PD, T1, T2se
q. p
rope
rty
1 2
1 exp( ) exp( )TR TES PDT T
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)
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
*1 2(1 )S PD E E
1 0(1 )zM E M
*2 0xyM E M
xy zM M
Convention: The uppercase „+“ („-“) denotes the magnetization immediately after (before) the RF pulse
Signal of the Gradient Echo Sequence
Case III: < 90°, TR ~ T1 (partial recovery), TR >> T2 (full decay)
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
1 zE M 0 1(1 )M EzM
T1 decay polarizationT1 recovery
cosz zM M
Convention: The uppercase „+“ („-“) denotes the magnetization immediately after (before) the RF pulse
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
Signal of the Gradient Echo Sequence
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 Spin-Echo Sequence
A Spin-Echo Sequence
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 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
MR Signal Intensity: PD, T1, T2
Contrast Agents (CA)
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
MR Signal Intensity: PD, T1, T2
Contrast Agents (CA)
CA Paramagnetic agents: 2 2, 21/ 1/ [ ]nativT T CA r
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)
MR Signal Intensity: PD, T1, T2
Contrast Agents (CA)
CA Paramagnetic agents: 2 2, 21/ 1/ [ ]nativT T CA r
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)
Contrast Modification
Tissue properties (nativ): PD, T1, T2
Contrast Modification
Tissue properties (nativ): PD, T1, T2
Contrast Modification: Saturation
Magn. Preparation Host – Sequence: ANY
spoiler
SpatialSaturation, FatSuppression,
WaterSuppression,
…
Contrast Modification: Inversion Recovery
Magn. Preparation Host – Sequence: ANY
1 (gray, white matter)TI T
Inversion time (TI)
Magn. Preparation Host – Sequence: ANY
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.
Topics: imaging, k-space, gradient-echo, spin-echo
Exercises: Part II & Part III
Topics: imaging, k-space, gradient-echo, spin-echo
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.
Exercises: Part II & Part III
Topics: imaging, k-space, gradient-echo, spin-echo
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
Exercises: Part II & Part III
Topics: imaging, k-space, gradient-echo, spin-echo
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.
Exercises: Part II & Part III
Topics: imaging, k-space, gradient-echo, spin-echo
How is proton-density weighting achieved?
• Short TR, short TE.
• Short TR, long TE.
• Long TR, short TE
Exercises: Part II & Part III