Basic Imaging and Contrast

8/8/2019 Basic Imaging and Contrast

1/26

Basic MR imaging & contrast

Steffen Ringgaard

Sept. 2003


2/26

Spatial signal encoding After excitation, signal is

emitted from completeobject

Linear field gradients for

introducing spatially

varying frequency

Fieldgradient

Z

Bandwidth

G

Larmor frequency

= B0


3/26

Definition of coordinate axes

Fieldgradient

Z

Slice thickness

Bandwidth

G

y, phase direction

x, frequency direction

z, slice direction


4/26

Slice excitation Simultaneous gradient

and shaped RF-pulse Gradient induces

linearly varying field

Shaped RF-pulse

excites frequency band

Field

gradient

Z

Slice thickness

Bandwidth

G


5/26

Shaped RF-pulse Rectangular slice profile

requires sinc functionshaped pulse

Slice thickness inversely

proportional to pulse

extension

Position determined by

frequency

FourierTransform

Frequency

Time

( )x

xx

sinsinc =


6/26

Slice interference

Slicegap

Slicegap

Halfmaximum

height

Halfmaximum

height

Slicethickness

Slicethickness

Interferencebetween slices

No sliceinterference


7/26

Frequency encoding

Signal from different positions has different frequency Position determined by invers Fourier Transform

Gradient

Higherfield

Frequency

Z

Signal after Fouriertransformation

X

Magnetic fieldperpendicularto slice

TimeFrequency0


8/26

Phase encoding With gradient applied, the

phase of themagnetization changes

With gradient turned off

the frequency is again the

same, but phases aredifferent

When signal is read out

(sampled) it containsmultiple frequencies and

multiple phases

1

2

3

z z

z z

z z

x x

x x

x x

B0

RF

Gs

ACQ

Gp

Gm

Time


9/26

Pulse sequence

RF

Gs

ACQ

Gp

Gm

Excitation Phase

encoding

Acquisition


10/26

Complete pulse sequence

Total scan time: TR*Matrix(y)

Typical values:

TE: 2-100 ms

TR: 10-1000 ms

Matrix: 128-512

Scan time: 1-512 s

RF

Gs

ACQ

Gp

Gm

TRTE

K-space


11/26

Contrast in MR images

Two images with different contrast


12/26

Excitation of magnetization

z

B0

B1

B

1

x'

y'

z'

x

y

M

Initial

magnetization M0

Magnetizationafter 90 pulseo


13/26

Longitudinal and transversal

components

Transverseplane (xy)

Transverseplane (xy)

Longitudinalaxis (z)

Longitudinalaxis (z)

Mxy

Mxy

yy

z z

zM

zM

xx

B0 B0

=0

M

(=M)


14/26

Relaxation T1 relaxation:

longitudinal relaxation

re-growth of longitudinalmagnetization

T2 relaxation: transverse relaxation

disappearance oftransversal magnetization

T2* relaxation: as T2, but taking

inhomogeneity into account

T2 < T1

T2* < T2

Time

Longitudinal relaxation63%

T1

Mz

0

Transverse relaxation

Time37%

Mxy

T2

0

M0

M0


15/26

T1 and T2 values Tissues have different relaxation values

Tissue T1 (ms) T2 (ms)

Gray matter 950 100

White matter 600 80

Muscle 900 50

CSF 4500 2200

Fat 250 60Blood 1200 100-200


16/26

Contrast: long TRVerticalmagnetization (M )

Signal

+M0

0TR

90 pulseo

TE

PD weighted

CSF, long T2

T weighted2

Parenchyma,short T2

CSF, long T1

Parenchyma,short T

1

z


17/26

Contrast: short TRVerticalmagnetization (M )

Signal

+M0

0

90 pulseo

TE

CSF, long T2

T weighted1

short T2

CSFlong T

1

short T1

Parenchyma

Mixed influence

z

TR


18/26

Three types of weighting Proton density weighting

TR long TE short

T1 weighting TR short (~ T1 of tissue)

TE short long T1 tissue is dark

T2 weighting TR long

TE long (~ T2 of tissue)

long T2 tissue is bright


19/26

Calculating signal strengthSignal in spin echo sequence:

Contrast between tissue A and B:

BAABSSC =

( ) 211, 0T

TT

T

ER

ER

eeTTS

=


20/26

Signal and contrastSignal strength and contrast, varying TR

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000 2500

Repetition time

Signal S1

S2

DIFF

Signal strength and contrast, varying TE

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300

TE, ms

Signal S1

S2

DIFF


21/26

k y

B C

A

kx0

0

Spin echo and gradient echo

A

C

0

k x

ky

90 90180

TE

TR

RF

Gz

Gy

G

xsignal

o o

o

Gradient echo Spin echo


22/26

Contrast for spin and gradient echo

sequences

Spin echo:

Gradient echo:

( ) 211, 0T

TT

T

ER

ER

eeTTS

=

( )

*21

1, 0T

T

T

T

ER

ER

eeTTS

=


23/26

MRI contrast agents Reduces T1 and T2

Increases signal strength Improves image contrast

Used for angiography, perfusion and myocardial

viability Gadolinium (Gd) based contrast most common

Gd inserted in large molecule (DTPA)

Used in approx. 30% of clinical scans Injected intra-venously


24/26

Contrast agents, structure


25/26

Contrast agents, signal strength

Increases signal in T1-weighted scans

Decreases signal in T2-weighted scans

Gd-DTPA of the kidney parenchyma

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30 35 40 45 50 55 60

Time [s]

MRIsignalintensity

T2-weighted

T1-weighted


26/26

Example of

contrastbased

angiography(Mobitrak)

Documents

Basic Imaging and Contrast