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MRI Physics

Omar Moawayh

Faculty of medicine Cairo University

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1- Resonance

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H

H

H

H

H

H

K

H

H

P C

Na

N

H

O

2- Spinning

Nuclei spin abouttheir axes acting like tiny magnets

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H2O

Water represents 60 % ofthe human body

OHH

3- Target

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Radio frequency coil Hydrogen Atom [proton]

H H HHHH H H

O O O O

N

Na

MR images depend on movement of hydrogen

protons in response to applied radiofrequency

Frequency

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H

H

H

H

H

H

H

H

Basic constituent of MR

Magnet

Radio frequency coil

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=

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Refocusing pulse

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90180

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X

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T1 Loss of energy tosurrounding = Longitudinal

relaxation time = spin to lattice

T2Loss of

energy to adjacentnuclei = Transverse

relaxation time = spinto spin

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Spin echo

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Spin echo

TE

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Spin echo

TR

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Sequence time

TR 4000m sec

TE m sec

Sequence

time seconds- min

utes

The acquisition time = 4000m sec TR X60=240000 1000 60 = 4 minutes

1 60

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Number of signal averaging

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TR 600 m sec

TR 600 m sec X30 = 18000 1000 60 = 0. 3m

TE 30 m sec

T1

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TE 80-100 m sec

TR 3000 m sec

TR 3000 m sec X100 =300000 1000 60 = 5m

T2

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Bright

Dark

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PD

==

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TE 30 m sec

TR 3000 m sec

TR 3000 m sec X100 =300000 1000 60 = 5m

PD

No T1

No T2

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TE 120 m sec

TR 3000 m sec

TR 3000 m sec X100 =300000 1000 60 = 5m

Increasing the TE of a sequence weights it more heavilytoward T2So it is sensitive for water , cyst and haemangioma

Heavy T2

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Fast spin echo

Echo train length

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Acquisition time for T2SE 7 m 17s FSE echo train length of 16 was 34 s

Signal intensi

ty of fa

tis grea

ter

than

tha

tof conven

tionalimages obtained with comparable parameters

Magnetic susceptibility difference artifacts are lessened

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Inversion recovary IR

IT

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Inversion recovary IR

IT

Choice of TE also determines amount of T2

IT

TE 100120 msecTE 30 msec

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X

IR

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FLAIR

Sufficienttime to suppress waterTR 10000 msec

IT 17002200 msec

TE 100120 msec

Fat

H2O

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STIRSufficienttime to suppress fat

TE 30 msec

TI 150 msec

TR 5000 msec

Fat

H2O

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Slice selection

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slice

phase

Read

out

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Phase

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Gradien

t

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Gradient

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Gradient

T1

T2 Flip angle

T2TR 200m sec-TE 10 msec- Flip angle 30

T1TR 75 m sec-TE 6 msec- Flip angle 70

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In T2* signal decay, the transverse magnetization isdephased because of magnetic field in homogeneities.The magnetic field is not exactly the same everywhere;in some places it is a bit stronger (B0 + )for

example, 1.505 Tand in others it is a bit weaker (B0 )for example, 1.495 T.Such differences may occur because ofthe presence ofmetallic objects, air, dental implants, or calcium, or they

may be due to the limitations of magnet construction

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T2*Signal

decay

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Blooming

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Iron

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Blooming

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Beca

use gradien

ts do no

trefocus field inhomogeneities,

GRE sequences with long TEs

are T2* weighted (because ofmagnetic susceptibility) rather

than T2 weighted like SEsequences. sequences

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Steady-state sequences are a class of rapid magneticresonance (MR) imaging techniques based on fast

gradient-echo acquisitions in which both longitudinalmagnetization (LM) and transverse magnetization(TM) are kept constant. Both LM and TM reach anonzero steady state throughthe use of a repetition

time that is shorter than the T2 relaxation time oftissue. When TM is maintained as multipleradiofrequency excitation pulses are applied, two typesof signal are formed once steady state is reached:

preexcitation signal (S) from echo reformation; andpostexcitation signal (S+), which consists of freeinduction decay.

Depending on the signal sampled and used to form an image

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Depending on the signal sampled and used to form an image,steady-state sequences can be classified as (a) postexcitationrefocused (only S+ is sampled), (b) preexcitation refocused (onlyS is sampled), and (c) fully refocused (both S+ and S aresampled) sequences. All tissues with a reasonably long T2relaxation time will show additional signals due to variousrefocused echo paths. Steady-state sequences haverevolutionized cardiac imaging and have become the standard for

anatomic functional cardiac imaging and for the assessment ofmyocardial viability because oftheir good signal-to-noise ratioand contrast-to-noise ratio and increased speed of acquisition.They are also useful in abdominal and fetal imaging and holdpromise for interventional MR imaging. Because steady-state

sequences are now commonly used in MR imaging, radiologistswill benefit from understanding the underlying physics,classification, and clinical applications ofthese sequences

S d f i

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1- Coherent completely or partially

refocused (rewound) GRE sequences

A gradient (called a rewind gradient)

to rephase the T2* magnetization

while it is being dephased and thereby

preserve the T2* effects

2-Spoiled GRE sequences

A gradient has same effect as T1

or

proton-density weightingTE

Steady-state free precession(SSFP) TR is usually shorter than the T1 and

T2 ofthe tissues imaged

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1-Coherent completely or partially refocused (rewound) GRE sequencesA gradientto rephase T2* magnetizationwhile it is being dephased and

thereby preserve the T2* effects

T1T2 T1 & T2

Complete

Partialpreexcitation postexcitation

TR46 msec- TE 12 msec;- flip angle 30

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2-Spoiled GRE sequencesspoiler RF pulse or gradientis used to eradicate any remaining transversemagnetization after each echo producing same effect as T1 or PD


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2-Spoiled GRE sequencesspoiler RF pulse or gradientis used to eradicate any remaining transversemagnetization after each echo producing same effect as T1 or PD


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Fat suppresion 1-Opposed Phase

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TE = 6.4 msec. In-Phase

TE =3.2 msec. Out-Phase

Fat suppresion 1-Opposed Phase

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- ve

52

3

2-Fat SaturationFat suppresion

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Fat suppresion 3-Shorttime inversion recovery

STIR

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H

Adequately

Mobile

SIGNAL

LOW

HIGH

INTERMEDIATE

DARK, HYPOINTENSE

BRIGHT, HYPERINTENSE

GRAY, ISOINTENSE

Not Adequately Mobile

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H

Amount

Motion Minimalhydrogen [air] no signal

Non mobile hydrogen [cortical bone] no signalFasthydrogen [flowing blood] no signal

The image will depend on

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Cortical bone

Mature fibrous tissue

Calcifications

Non mobile hydrogen

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AIR

Lung &Lung &SinusesSinuses

Minimal hydrogen

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T2T1 Subacute bloodSubacute blood

Each structure [lesion] in the humanbody has a characteristic signal

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Each structure [lesion] in the humanbody has a characteristic signal

T2T1

T2T1

Fluid

Fat

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Signal Void

Black

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How to know the pulse sequence?!

T1/ PD T2 Gradient STIR

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Diffusion DWIsDepends on Brownian movement

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PULSE SEQUENCES FOR DIFFUSION-

WEIGHTED IMAGING

T2 spin-echopulse sequence

EchoplanarImaging(EPI)

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DIFFUSION WEIGHTED IMAGING USING SPIN-

ECHO T2-WEIGHTED PULSE SEQUENCE

Spin-echo T2-weighted pulse sequence with two extragradient pulses that are equal in magnitude and opposite indirection

Spin echo T2

Gradient pulses

TR 5100 m sec TE 137m sec - total acquisition time 20 s

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ISOTROPIC AND ANISOTROPIC DIFFUSION

In isotropic diffusion nopreferred direction ofwater motion

In anisotropic diffusion ,inthe white matter,consisting of dense fiberbundles, water moves

more easily parallel to thefibers than across them.

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The signal intensity decreases when the white matter tracts

run in the same direction as DW gradients

X ZY

Direction of DW

gradient

Hypointense white

matter tractSplenium

Frontal and

occipital

Corticospinal

tract

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CREATION OF ISOTROPIC DW IMAGE

Multiply the three images created with the DW

gradient pulses applied in three orthogonaldirections (Gx, Gy,and Gz). The cube root of this

product is the DW image

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Example: water molecules

moving perpendicular to the

axons in the white matter would

be slowed by crossing morecell membranes than water

moving in a direction parallel to

an axon. the degree of

anisotropy in the WM is larger

than that in the GM.

diffusion is orientation-dependant

being affected by these barriers and by

the nerve fiberorientation.

Diffusion Tensor Imaging

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Vessels

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Vessels

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Signal to noise ratio

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Signalto noise ra

tio

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To increase the SNRNEX

TR

FOV

Slice Thickness

Slice Gap

Phase Encoding steps

Frequency Encoding steps

Band Width

TE

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To increase the Spatial

ResolutionFrequency Encoding steps

Phase Encoding steps

FOV

Slice Thickness

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K-space

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K

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K space

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MR Angiography

MRA

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MRA

TOF

Phase contrast

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No signals

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TOF

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45

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+10 -100

-10 +100

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Arterial Spin

Labeling = ASL

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BBB is highly permeative to water

The unidirectional clearance of water can be usedas a measure ofCBF.

Proposed by Detre et al

(Magn Reson

Med. 1992 Jan;23(1):37-45)CONTINUOUS

labeling

CASL

PULSED

labeling

PASL

Arterial Spin Labeling = ASL

CONTINUOUS

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Imagingplane

Labelingplane

RF inversion pulse spins inverted

s

pin s

labeling

CASL

The labeled spins (water protons) flowing into the imaging planeand exchanging withtissue protons, cause signal lossmeasuring signal changes between tagged images and baseline

images (qualitative / quantitative)CBFuntagged images

PULSED E cho

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sp

in s

Labelingplane

Imagingplane

sp

in s

labeling

PASL

F low-sensitiveA lternating

I nversionR ecovery

P lanarI maging and

S ignalT argeting byA lternating

R adioequency

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Blood Oxygen

LevelDependent

Bold

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Magnetic susceptibility is also used in blood oxygenationleveldependent (BOLD) imaging T2*

The relative amount ofdeoxyhemoglobin in the cerebral

vasculature is measured as a reflection of neuronal activity

BOLD MR imaging is widely used for mapping of

Brain function

Bases of the BOLD effect

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Commercial pulse

sequences

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slice

phase

Read

out

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slice

phase

Read

out

FLASH

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slice

phase

Read

out

GRASS

/FISP

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phase

Readout

slice

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Contrast

C t t

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Contrast

Short T1

C t t

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Contrast

Short T1

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Short T1

GdDTPA

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GdDTPA

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MagnatizationTransvere

Magnatization transvere

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Magnatization transvere

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After 1m

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After 2 m

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After 3 m

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MR spectroscopy

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What is MRS ?

MRS is a non-invasive method which can provide in vivo dataon human bio-chemistry & pathophysiology. Thus it is anon-invasive probing of the Underlying biochemistry of cells

MRS is a new MR technique that can help to dd between

benign and malignant bony and soft tissue tumor, also playsan important role in diagnosis of bone infection andmetabolic disorders.

MR spectroscopy

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H2O+ C

H2O+ Na

What are the types of

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H-MRS

yp

Spectroscopy ?

Limitations

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Limitations

FALSE POSITIVE PEAKS

Benign lesions with high cellular concentrationInflammatory lesion with excessive inflammatory cells

FALSE NEGATIVE STUDY

Malignant lesions of low cell population will give lowcholine levels

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100 5

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100 5

100 5

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100 5

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180

Phase

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Phase

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Artifacts

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Wrap around

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90

-90

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Chemical shift artifacts

After 2 min = augmentation

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After 1 min cancellation

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100010

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Asymmetric brightness

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Inhomogeneous brightness

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Zero line artifact

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Truncation artifacts- Gibbs phenomenon

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Metal artifacts

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Iron

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Bloomingartifact

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Field inhomogenity

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Zebra stripe artifact

Cross talk

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Cross talk

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Entrance/Exit slice phenomenon

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FlowFlow

compansationcompansation

FlowFlow compansationcompansation

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pp

Stationary

Moving

Flow compensationFlow compensation

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pp

Presaturation

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Spat

ial presatu

rat

ionAntiphase aliening Suppress wrapped artifacts in

phase encoding

Antifrequency aliening

wrapped artifact in the readout direction

Flow suppression Sat blood flow and CSF flow

Leading slice

MovingS

atFollowing sliceMoving Sat

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Anti

phase

Antifrequency

Antifrequency

Antiph

ase

Flowsaturation

Skipping presaturation

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Quadiscan

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GradientGradient

50 100

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0

0

100

50

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Thank you

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