3contrast y học

MRI Physics 2: Contrasts and Protocols



Chris Rorden
Types of contrast: Protocols
–
–
–
–

Static: T1, T2, PD
Endogenous: T2* BOLD (‘fMRI’), DW
Exogenous: Gadolinium Perfusion
Motion: ASL

www.fmrib.ox.ac.uk/~karla/
www.hull.ac.uk/mri/lectures/gpl_page.html
www.cis.rit.edu/htbooks/mri/chap-8/chap-8.htm
www.e-mri.org/cours/Module_7_Sequences/gre6_en.html

1


MR Contrast – a definition
 We

use different MRI protocols
that are dominated by different
contrasts.
 Contrasts influence the

brightness of a voxel.
 For example, water (CSF) is
relatively dark in a T1-weighted
scan, but relatively bright in a T2
scan.

2


MR Contrast


Four types of MR contrasts:
1.
2.
3.
4.

Static Contrast: Sensitive to relaxation properties of the
spins (T1, T2)
Endogenous Contrast: Contrast that depends on intrinsic
property of tissue (e.g. fMRI BOLD)
Exogenous contrast: Contrast that requires a foreign
substance (e.g. Gadolinium)
Motion contrast: Sensitive to movement of spins through
space (e.g. perfusion).

3

Anatomy of an MRI scan
1.
2.
3.
4.
5.




Place object in strong magnetic field: atoms align to field.
Transmit Radio frequency pulse: atoms absorb energy
Wait
Listen to Radio Frequency emission due to relaxation
Wait, Goto 2
Time between set 2 and 4 is our Echo Time (TE)
Time between step 2 being repeated is our Repetition Time (TR).
TR and TE influence image contrast.
TE

Time

TR

4


T1 and T2 definitions
 T1-Relaxation:
–

–

Recovery of longitudinal
orientation.
‘T1 time’ refers to interval where
63% of longitudinal magnetization
is recovered.

 T2-Relaxation:
–
–

Recovery

Dephasing

Loss of transverse magnetization.
‘T2 time’ refers to interval where
only 37% of original transverse
magnetization is present.

5


Contrast: T1 and T2 Effects

Fat: Short T1

1

1

CSF: Long T1

0

0
0

TR (s)

3

CSF: Long T2

Signal



T1 effects measure recovery of longitudinal magnetization.
T2 refers to decay of transverse magnetization.
T1 and T2 vary for different tissues. For example, fat has very
different T1/T2 than CSF. This difference causes these tissue to
have different image contrast.
T1 is primarily influenced by TR, T2 by TE.
Magnetization





Fat: Short T2

0

TE (s)

0.2

6


T1 Effects: get them while their down
 Consider very short TR:
Fat has rapid recovery, each RF pulse will
generate strong signal.
Water has slow recovery, little net
magnetization to tip.
Before first pulse:
1H in all tissue
strongly
magnetized.

T1 effects explain why we
discard the first few fMRI
scans: the signal has not
saturated, so these scans
show more T1 than
subsequent images.

After several rapid pulses: CSF has little net magnetization,
so these tissue will not generate much signal.

Fat
CSF

7


Signal Decay Analogy
 After
–
–
–

RF transmission, we can detect RF emission

Emission at Larmor frequency.
Emissions amplitude decays over time.
Analogous to tuning fork: frequency constant, amplitude decays

8


Relaxation
 After

RF absorption ends, protons
begin to release energy
–

–
–
–

Emission at Larmor frequency.
Emissions amplitude decays over time.
Different tissues show different rates of
decay.
‘Free Induction Decay’ (FID).

 Strongest

signal immediately after
transmission.
 Therefore, do we always want a short
TE?
9


TE and T2 contrast
1.
2.
3.

Signals from all tissue decays with time.
Signal decays faster in some tissues than others.
Optimal contrast between tissue when they emit
relatively different signals.

Optimal

GM/WM
contrast

White Matter: Fast Decay

Signal

Signal

Gray Matter: Slow Decay

0

TE (s)

.2

0

Contrast: difference
between GM and
WM signal

TE (s)

.2

10

Optimal contrast

Optimal TE will depend on which
tissues you wish to contrast
–
–

Gray matter vs White matter
CSF vs Gray matter

Signal



0

TE (s)

.2

11


T2: Dephasing
 RF
–
–
–

pulse sets phase.

Initially, everything in phase: maximum signal.
Signals gradually dephase = signal is reduced.
Some tissue shows more rapid dephasing than other tissue.

Fat

…
CSF
Time

12


T1 and T2 contrasts



Every scan is influenced by both T1 and T2.
However, by adjusting TE and TR we can determine which effect dominates:
–

T1-weighted images use short TE and short TR.


–

T2-weighted images use long TE and long TR: they are dominated by the T2


–

Fat bright (fast recovery), water dark (slow recovery)
Fat dark (rapid dephasing), water bright (slow dephasing).

Proton density images use short TE and long TR: reflect hydrogen concentration.

13


T2 vs T2*
 T2
–
–

only one reason for dephasing:

Pure T2 dephasing is intrinsic to sample
(e.g. different T2 of CSF and fat).
T2* dephasing includes true T2 as well
as field inhomogeneity (T2m) and tissue
susceptibility (T2ms).
 Due

 T2*

leads to rapid loss of signal:
images with long TE with have little
coherent signal.

1
T2
Signal

to these artifacts, Larmor frequency
varies between locations.

1
1
1
1
=
+
+
*
T 2 T 2 T 2 M T 2 MS

T2*

0
0

TE (s)

14
0.2


Susceptibility artifacts
 Magnet

fields interact with material.
 Ferromagnetic (iron, nickel, cobalt)
–
–

Strongly attracted: dramatically increases
magnetic field.
all steel has Iron (FE), but not all steel is
ferromagnetic (try putting a magnet on a
austenitic stainless steel fridge).

 Paramagnetic
–

Weakly attracted: slightly increases field.

 Diamagnetic
–

(Gd)

(H2O)

Weakly repelled: slightly decreases field.

15


Tissue Susceptibility

 Due

to spin-spin interactions, hydrogen’s resonance
frequency differs between materials.
–

E.G. hydrogen in water and fat resonate at slightly different
frequencies (~220 Hz; 1.5T).
Macroscopically: These effects can lead spatial distortion
(e.g. ‘fat shift’ relative to water) and signal dropout.
Microscopically: field gradients at boundaries of different
tissues causes dephasing and signal loss.

16


Field Inhomogeneity Artifacts










When we put an object (like someone’s head)
inside a magnet, the field becomes non-uniform.
When the field is inhomogeneous, we will get

artifacts: resonance frequency will vary across
image.
Prior to our first scans, the scanner is ‘shimmed’ to
make the field as uniform as possible.
Shimming is difficult near air-tissue boundaries
(e.g., sinuses).
Shimming artifacts more intense at higher fields.

17


Spin Echo Sequence
echo sequences apply a
180º refocusing pulse half
way between initial 90º pulse
and measurement.
 This pulse eliminates phase
differences due to artifacts,
allowing measurement of
pure T2.
 Spin echo dramatically
increases signal.

Actual Signal

1

T2
Signal

 Spin

T2*

0

0.5 TE

0.5 TE

Time

18


Spin Echo Sequences
 The

refocusing pulse allows us to recover true T2.
 Image from
–
–

www.e-mri.org/cours/Module_4_Signal/contraste1_en.html
Web site includes interactive adjustment of T1/T2

T2
T2*

19

Analogy for Spin Echo
Consider two clocks.
–
–




Simultaneously,set both clocks to read 12:00. (~
420º
send in 90º RF pulse).
Wait precisely one hour
–




Clock 1: minute hand takes 70 minutes to make a
revolution.
Clock 2: minute hand takes 55 minutes to make a
revolution.

Minute hands now differ: out of phase.

Minute hand rotation



Reverse direction of each clock (~ send in 180º RF
pulse).
Wait precisely one hour
–
–

Minute hands now identical: both read noon.
They are briefly back in phase

0

1 hour

1 hour

20


T2*: fMRI Signal is an artifact
 fMRI

is ‘Blood Oxygenation Level Dependent’
measure (BOLD).
 Brain regions become oxygen rich after activity: ratio
of Hbr/HbrO2 decreases

21


BOLD effect

 Deoxyhemoglobin

(Hbr) acts as contrast agent
 Frequency spread causes signal loss over time
 Effect increases with delay (TE = echo time)

But, overall signal
reduces with TE.
Optimal BOLD TE
~60ms for 1.5T,
~30ms at 3T.

www.fmrib.ox.ac.uk/~karla/

0

Fera et al. (2004) J MRI 19, 19-26
Low

Frequency

TE (s)

0.2

High

22

BOLD artifacts
 fMRI

is a T2* image – we will have all the artifacts that a
spin-echo sequence attempts to remove.
 Dephasing near air-tissue boundaries (e.g., sinuses)
results in signal dropout.

Non-BOLD

BOLD

www.fmrib.ox.ac.uk/~karla/
23


Optimal fMRI scans






More observations with shorter TR, but slightly
less signal per observation (due to T1 effects and
temporal autocorrelation).
When you have a single anatomical region of
interest use the fewest slices required for a very
short TR.
For exploratory group study, use a scan that

covers whole brain with minimal spatial distortion
(for good normalization).
–
–

Typical 3T: 3x3x3mm 64x64 matrix, 36 slices, SENSE
r=2, TE=35ms, TR= 2100ms
Typical 1.5T: 3x3x3mm 64x64 matrix, 36 slcies,
TE=60ms, TR= 3500ms.

•Shorter TR yields better SNR
•Diminishing returns
•G.H. Glover (1999) ‘On Signal to
Noise Ratio Tradeoffs in fMRI’

24


Diffusion Imaging
 Diffusion

imaging is an endogenous contrast.
 Apply two gradients sequentially with
opposite polarity.
 Stationary tissue will be both dephased and
rephased, while spins that have moved will
be dephased.
 Sensitive to acute stroke (DWI, see lesion
lecture)
 Multiple directions can measure white matter

integrity (diffusion tensor imaging, see DTI
lecture)

water diffuses
faster in
unconstrained
ventricles than
in white matter

25