Gradient echo sequences and SWI - PowerPoint Presentation

Slide1

Gradient echo sequences and SWI

Dr Harish K Gowda

Slide2

MR SIGNAL

Slide3

MR SEQUENCE

Carefully

co-ordinated

and timed series of events to generate particular type of image contrast.

Slide4

Classification

Spine Echo sequence

Echoes are

rephased

by 180

0

rephasing

pulse

Gradient sequence

Echoes are

rephased

by gradients

Slide5

Various types of sequences

Slide6

What is T2* Relaxation?

T2* Relaxation is combination of true T2

relaxation

and magnetic field

inhomogenities

Slide7

T2* Relaxation

Main magnetic field

inhomogeneity

,the

differences

in magnetic

susceptibility among various tissues

or materials

,

chemical shift, and gradients applied for spatial encoding

Slide8

Comparison of SE and GRE Sequences

Criterion

SE

GRE

Rephasing

mechanism

RF pulse

Variation of gradients

Flip angle

90° only

Variable

Efficiency at reducing magnetic

inhomogeneity

Very efficient (true T2 weighting)

Not very efficient (T2* weighting)

Acquisition time

Long (slow sequences)

Short (fast sequences)

Slide9

GRADIENT ECHO SEQUENCES

Slide10

GRE sequences can be

T1 W

Large flip angle (70-110)

Short TR <50ms

Short TE 5-10 ms

T2 W

small flip angle (5-20)

Long TR

Long TE 15-25 ms

Slide11

PD W

Small flip angle (5-20)

Long TR

Short TE 5-10 ms

Slide12

THE STEADY STATE

State

where

TR

is

shorter

than

T1 and T2 relaxation time of the tissue.

To

achieve

this

,

energy

given

to H2

atom

should

be

equal

to

energy

lost

by H2

atom

.

Flip angle of 30-45⁰ and TR – 20-50ms

In SS image

contrast

is

not due to

differences

in T1 and T2 relaxation times of tissues but due to ratio of T1 and T2.

Slide13

STEADY STATE

Slide14

THE STEADY STATE AND ECHO FORMATION

GE

sequences

can

be

classified

accn

to

residual

transverse

magnetization

as

-

Coherent

( tr.

Magnetization

in phase )

-

Incoherent

( tr.

Magenetiazation

out of phase)

Slide15

ECHO FORMATION IN SS

In SS ,

repeated

RF pulses

applied

at an time

interval

less

than

T1 and T2 relaxation time of all tissues,

producing

2

signals

:

* A FID signal :

occurs

as a

result

of

withdrawal

of the RF pulse and

contains

T2* and T1 info.

* A Spin

echo

:

occurs

at the

same

time of

next

RF pulse.

Contains

T2* and T2 info.

Slide16

ECHO FORMATION IN SS

Slide17

COHERENT GRADIENT ECHO

GRASS ( GE ) , FFE ( PHILIPS ), FISP ( SIEMENS)

Use variable flip angle excitation pulse

followed

by

rephasing

gradient to

produce

gradient

echo

.

Residual

transevese

magnetization

is

kept

coherent

by

rewinding

.

Slide18

Rewinding

is

achieved

by

reversing

the

slope

of phase

encoding

gradient

after

readout

.

The

rewinder

gradient

rephases

all transverse

magnetization

hence

it

contain

info

from

both

FID and

stimulated

echoes

.

Used

to

produce

T2W images

Slide19

Slide20

Adv

:

1.

Very

fast

scans,

hence

breath

holding

is

possible.

2.

Very

sensitive to flow –

Angiography

.

3.Myelography ,

arthrography

.

4.Can

be

acquired

in volume acquisition.

Disadv

:

1. Poor SNR in 2D acquisitions.

2.

Increased

magenetic

susceptibilty

.

Slide21

PARAMETERS

To

maintain

SS:

Flip angle : 30 -45⁰

TR :

20-50ms

To

maximize

T2*:

Long TE - 15-25

ms

GMR

can

be

used

to

accentute

T2* and to

reduce

flow artefacts.

Avg

Scan time : Sec for single slice , 4-15 min

for vol.

Slide22

Slide23

Slide24

INCOHERENT GRADIENT ECHO

SPGR /MPGR(GE), T1 FFE (Philips ), FLASH ( Siemens)

Similar

to

coherent

gradient

echo

except

that

the

residual

tr.

Mag

is

dephased

so

that

its

effect

on image

contrast

is

minimal.

Tr.

Mag

from

previous

excitation

is

used

only

hence

predominantly

T1W image.

Dephasing

is

done

by

either

RF

spoiling

or Gradient

spoiling

.

Slide25

Digitized RF Spoiling(SPGR)

RF

is

transmitted

at a

particular

frequency

to excite a slice and at a specific phase.The

receiver

coil

digitally

communicates

with

the transmit

coil

and

only

frequencies

from

echo

that

just

has been

created

are

digitized

.

Tr.

Mag

in

another

phase or position

is

not

received

by

receiver

coil

.

Slide26

Gradient Spoiling (MPGR)

Opposite of

rewinding

.

The slice select, phase

encoding

and

frequency

encoding gradients are

used

to

dephase

the

residual

magnetization

.

Hence

it

is

incoherent

at the

beginning

of

next

repetition

.

Slide27

Slide28

Uses of SPGR

SPGR

sequences

produce

T1 or PDW images.

Can

be

used 2D or vol. acquisition.

As TR

is

short

can

be

used

for T1W

breath

holding

sequences

.

Demonstrate

good T1 W

anatomy

.

Slide29

3D SPGR

Slide30

Parameters of SPGR

To

maintain

steady

state :

Flip angle – 30 -45⁰

TR - 20- 50 ms

To

maximize T1 :

Short TE : 5-10ms

Avg

scan time :

several

sec for single slice

4-15 min for volume

Slide31

Adv

:

* Can

be

acquired

in vol or 2D.

*

Breath

holding is possible.

* Good SNR and

anatomical

details

.

Disadv

:

* SNR

poor

in 2D.

*

Loud

gradient noise.

Slide32

STEADY STATE FREE PRECESSION

SSFP( GE), T2FFE (PHILIPS), PSIF( SIEMENS)

In Gr.

Echo

sequences

TE

is

not

enough (

atleast

70ms) for

true

T2w

imaging

.

SSFP

provides

sufficiently

long TE and

less

T2*

In SSFP,

only

frequncies

from

stimulated

echoes

are

digitized

and not

from

FIDs

.

Slide33

STEADY STATE FREE PRECESSION

It

is

achieved

by

applying

rewinder

gradient which speeds up the rephasing process

initiated

by RF pulse.

Hence

stimulated

echo

occurs

just

before

the

next

excitation pulse.

Slide34

Slide35

Resultant

echo

is

more T2W and

contain

2

TEs

. - Actual TE – time betn

echo

and

next

excitaion

pulse.

- Effective TE – time

from

echo

to the excitation pulse

that

created

its

FID.

Effective TE = (2

xTR

) - TE

Slide36

Slide37

Parameters

of SSFP

To

maintain

steady

state:

Flip angle – 30 -45⁰

TR - 20- 50 ms

Actual

TE affect the effective TE.

More the

actual

TE

less

the effective TE.

Avg

scan time : 4-15 min.

Slide38

USES of SSFP

ADV:

To

demonstrate

true

T2

weighing

.

Esp in brain and joint imaging.

Can

be

used

in

both

2D and 3D acquisition.

Disadv

:

Susceptible for artefacts.

Poor image

quality

.

Slide39

GRASS/FFE/FISP (CGRE)

Slide40

SPGR/T1FFE/FLASH(IGRE)

Slide41

SSFP/T2FFE/PSIF

Slide42

BALANCED GRADIENT ECHO

FIESTA (GE), BFFE (PHILIPS), TRUE FISP(SIEMENS), CISS

Modification of

coherent

gradient

echo

.

T2W 3D

sequence

.Uses balanced gradient system to correct for phase errors

in

flowing

blood

and CSF and

alternating

RF excitation to

enhance

steady

state.

Slide43

Slide44

PARAMETERS OF BGRE

Balanced

frequency

readout

gradient.

Higher

flip angles and

shorter TR to maintain steady

state.

Complete TR

is

not

applied

in single TR but

it

is

applied

in

succesive

TR

with

alternative

polarity

.

Slide45

Uses Of FIESTA

3D

acuqisition

of

posterior

cranial

fossa

gives high resolution images showing

cranial

nerves

dark

against

background of

bright

CSF.

Esp

used

for

suspected

IAC and CP angle pathologies.

Also

used

to visualise spinal nerve

roots

and

optic

nerve.

Slide46

AX 2D FIESTA

Slide47

ULTRA FAST SEQUENCES

Fast

gradient

echo

(FGRE)

Echo

planar

imaging

(EPI)

SE- EPI

GE-EPI

Slide48

FAST GRADIENT ECHO

Fast gradient echo (FGRE) sequences (also known as turbo GE or ultrafast GE sequences) used in conjunction with state-of-the-art gradient systems (active shielding) achieve TE below 1

ms

with TR of 5

ms

or less.

Fast GRE is basically a conventional GRE sequence that is run faster imaging only part of RF pulse is applied and only part of echo is read.

.)

Slide49

FAST GRADIENT ECHO

Fast GRE - yield an excellent image quality although a slice can be acquired in only a few seconds (typically 2–3 sec).

Highly suitable for dynamic imaging, for example, to track the inflow of a contrast medium bolus.

For imaging body regions where motion

artifacts

must be eliminated such as the chest (respiratory motion) and the abdomen (peristalsis).

Slide50

K-SPACE FILLING IN FGRE

CENTRIC K SPACE FILLING

-Central k

spaces

are

filled

first.

-signal and

contrast

are

maximized

as central k

spaces

are

filled

when

echoes

have

their

highest

amplitude.

KEYHOLE K SPACE FILLING:

- K

spaces

are

filled

linearly

except

central k

spaces

are

filled

during

specific

part of

seq

.

-

Used

in CEMRA.

Slide51

Slide52

ECHO PLANAR IMAGING(EPI)

Echo planar imaging (EPI) enables ultrafast data acquisition, making it an excellent candidate for dynamic and functional MR imaging.

Term given by Sir Peter Mansfield in 1977.

This method requires strong and rapidly switched frequency and phase encoding gradients.

A single echo train is used to fill all the lines of k space.

Slide53

ECHO PLANAR IMAGING(EPI)

To

fill

all K

spaces

in single TR

readout

and phase encode gradients must

rapidly

switch on and off and change their polarity .

There are

different

ways

of

filling

k

space

in EPI.

Oscillation of

frequency

,

blipping

of phase.

Spiral K

space

filling

.

Slide54

Slide55

Oscillation of frequency ,blipping of phase

Slide56

Spiral k space filling

Slide57

PARAMETERS FOR EPI

PD or T2W images

can

be

obtained

by

selecting

short or long effective TE.

EPI

can

be

done

with

spin

echo

(SE-EPI) or gradient

echo

(GE-EPI) .

Hybrid

Sequence

which

combine gradient and spin

echo

( GRASE).

Benifit

of

both

types of

rephasing

Speed of gradient

rephasing

and T2* compensation of spin

echo

rephasing

.

Slide58

SWI

Slide59

Idea of phase imaging as new contrast enhancing parameter.

Phase imaging contains information of susceptibility differences between the different tissues.

Only magnitude imaging was used clinically till now.

Other contrast parameters T1recovery and T2 and T2* decay, TR NEX.

Susceptibility Weighted Imaging

Slide60

Applications of SWI

SWI offers information about tissues with different susceptibilities from surrounding tissues.

deoxygenated blood, hemosiderin, ferritin, calcium

Numerous Clinical applications

Hemorrhages

Cerebrovascular and ischemic brain diseases

Traumatic brain injuries

Arteriovenous malformations

Neurodegenerative diseases

Breast microcalcifications

Slide61

Currently

Packaged SWI technique available on Siemens Medical Systems.

SWAN on GE scanner: T2 Star Weighted Angiography. (multi-echo GRE) uses echoes.

Slide62

Slide63

Slide64

Magnetic Susceptibility

When an object is placed in an external magnetic field H, magnetization is induced in the object.

Magnetic susceptibility is the magnetic response of a material when it is placed in a magnetic field.

M =

χH

χ = susceptibility (ppm)

M = induced magnetization

H = applied field

If diamagnetic, like Ca3(PO)4, χ < 0

If paramagnetic, like deoxygenated blood, χ > 0

E.M. Haacke et. Al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications, Part 1. AJNR Am J Neuroradiol 30:19-30. Jan 2009.

64

Slide65

Susceptibility and Phase Relations

MRI equations

ω=γB

0

larmor

equation

ψ=

ωt

phase equationΔψ=Δω*TERelating to susceptibility,

Since

Δω

=

γΔB

and ΔB=g*

Δχ

*B

0

Δψ

=-

γΔB

*TE

And so,

Δψ

=-γgΔχB

0

*TE

65

Slide66

Effect of sample shape, orientation and susceptibility

66

Schenck, JF. The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Med Phys. 1996 Jun;23(6):815-50

Slide67

Outline of SWI Processing

Acquire high-res 3D

fGRE

.

Apply HPF to phase image to obtain “SWI filtered phase data.”

Create “phase” mask.

Multiply phase mask by original magnitude image to obtain “merged SWI magnitude data.”

67

Slide68

Step 1: Acquisition

68

Magnitude

c/o Samantha Holdsworth

Slide69

Step 2: Creating HPF

Uses 64x64 low-pass filter divided into the original phase image to create a HP-filter effect.

Method

Truncate original image ρ(r) to central

n

x

n

complex image ρ

n

(r).Zero-fill elements outside central n

x

n

elements

Complex divide ρ(r) by ρ

n

(r) to obtain a new image, ρ’(r) = ρ(r)/ρ

n

(r)

69

Slide70

Step 2: Phase Images

70

Raw phase image

HP-filtered (32x32)

HP-filtered (64x64)

Fig. 3 Haacke Review 1

Slide71

Step 3: Phase Mask

Intratissue

phase and

extratissue

phase which impair the differentiation of adjacent tissues with different susceptibility

Phase mask between 0 to 1

E.M. Haacke et. al. Susceptibility Weighted Imaging. MRM 52:612-618 (2004).

71

Slide72

Step 3: Phase Masking Process

72

Phase profile in filtered phase image

Profile of mask created from A

Fig. 6 Haacke Review 1

Slide73

Step 4: Simulated images with phase mask multiplication

73

m=1

m=4 m=8

m=16

ρ(x)

new

=

f

m

(x)ρ(x)

Fig. 1 Haacke

The smaller the phase value, the larger the multiplication required to reach the maximum CNR

Slide74

Overview: SWI Processing

74

(phase mask)

m

Phase image

Magnitude image

X

SWI image

c/o Samantha Holdsworth

SWI minIP

≥4 images

Slide75

Greyscale inversion filtered phase images are not uniformly windowed or presented equally by all manufacturers

, therefore

care must be taken to ensure correct interpretation.

A simple step to make sure that you always view the images in the same way is to look at

venous structures &

compare them with the lesions. The lesions with blood or ferritin (paramagnetic) will have the same signal as veins.

IMAGE INTERPRETATION

Mittal

. S et al.

Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications. AJNR 2009

Slide76

IMAGE INTERPRETATION

Mittal

. S et al.

Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications. AJNR 2009

Magnitude Phase CT

Note that calcifications on the phase (center) image have different signal than blood in the sagittal sinus (arrow).

Slide77

CLINICAL APPLICATIONS

Slide78

DIFFERENTIATION BETWEEN MICROBLEED FROM MICROCALCIFICATIONS

Both calcifications & iron accumulation in chronic hemorrhage are hypointense on T2-WI & show

“

blooming

”

in SWI. It is not possible to differentiate between them on conventional MR sequences & CT is usually required.

SWI - phase represents an average magnetic field of protons in a voxel, which depends on the susceptibility of tissues.

Calcium is diamagnetic in nature and the phase shift induced by it is opposite to that found with paramagnetic substances like deoxy-Hb, methemoglobin (Met-Hb), hemosiderin & ferritin.

Yamada et al. Intracranial calcification on gradient-echo phase image: depiction of diamagnetic susceptibility. Radiology. 1996

Slide79

DIFFERENTIATION BETWEEN MICROBLEEDS AND MICROCALCIFICATIONS - AMYLOID ANGIOPATHY

A B.

SWI (A) shows subcortical black spots in parietal/occipital regions. One cannot be sure if they correspond to microhemorrhages or calcifications. In SWI-Phase (B) the spots have similar signal to that of the deep venous structures (black arrow) indicating that they correspond to blood.

Slide80

DIFFERENTIATION BETWEEN MICROBLEEDS AND MICROCALCIFICATIONS -

POST-RADIOTERAPHY

A B

High grade glioma after radiotherapy.

SWI (A) cannot differentiate if the signal loss in the ventricular system & occipital lobes are due to choroid plexus microcalcifications or microhemorrhages. SWI-Phase (B) shows bright spots similar in signal to blood in the deep venous system (arrow) suggesting microhemorrhages instead of calcifications.

Slide81

Iron build up in the pulvinar in MS indicated with SWI

MS Patient

Normal Volunteer

SWI in Multiple Sclerosis

81

Haacke Review, Part 2

Slide82

Sturge-Weber Syndrome found most often in children leads to vascular malformation

Sturge Weber Syndrome in 5 y.o. girl

Haacke Review, Part 2

82

Post-contrast T

1

w

Leptomeninges (arrowhead)

Periventricular veins (arrow)

SWI

– calcification of gyri (dotted/arrowhead)

Periventricular veins (arrow)

Slide83

Occult Venous angioma

Slide84

DAI

Slide85

Stroke and Hemorrhage

Slide86

Many imaging characteristics have been suggested to predict glioma grade including heterogeneity, contrast enhancement, mass effect, necrosis, metabolic activity & high cerebral blood volume. In human glioma cells, levels of ferritin & transferrin receptors detected during immunohistochemical analysis correlate with tumor grade.

SWI-Phase helps to detect calcifications and/or blood, therefore, improving accuracy in interpretation of brain tumors & aiding in grading.

Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain

–

A pictorial review. Neuroradiology. 2008

.

TUMOR CHARACTERIZATION

Slide87

Occult tumour

Slide88

Schwannomas

, patient with NF 2 .

Post contrast T1WI (A) demonstrates bilateral vestibular

schwannomas

.

SWI-Phase (B) shows bright

intratumoral

spots compatible with

microbleeds

(black arrow) which are more frequently found in vestibular

schwannomas

than in

meningiomas

.

Note the

their signal is similar to

the

cerebellar

veins (white arrow).

TUMOR CHARACTERIZATION

A B

Slide89

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