Mri basics

Felix Bloch Edward PurcellJoseph Fourier Joseph Larmor

Raymond Damadian C Lauterbur Peter Mansfield Seiji OgawaRichard Ernst

The first Nuclear magnetic resonance

experiment was conducted

independently by two scientist in 1946.

Felix Bloch working at Stanford

University

Edward Purcell working at Harvard

University

Bloch and Purcell were awarded Nobel

Prize for Physics in1952

Joseph Fourier

1768- 1830

Joseph Larmor

1857-1942

Published his

collected papers on

electromagnetism

in 1900 in a famous

book entitled

“Aether and Matter”

In 1970 Raymond Damadian found

that it is possible to characterize

different body tissues using NMR

Technology

In 1977 he completed the

construction of the first MRI scanner

In 1978 he founded the FONAR

corporation which manufactured the

first commercial MRI

In 1973 Paul C Lauterbur discovered the

possibility to create a two dimensional

picture by introducing gradients in the

magnetic field. He used the back

projection technique to reconstruct the

image. He termed his new imaging

technique zeumatography. He shared

the 2003 Nobel prize for medicine with

Peter Mansfield

Peter Mansfield

developed the technique

of detecting the emitted

signals rapidly

mathematically analyzing

them and turning them

into an image.

He evolved a very fast

imaging technique known

as Echo Planar Imaging

In 1975 Richard Ernst

introduced 2D NMR

using phase and

frequency encoding

and the Fourier

transform instead of

Lauterbur’s back

projection technique

In 1992 Functional

MRI was developed

by Seiji Ogawa.

FIRST MRI MACHINE AND MRI IMAGE

In 1890 Roy and Sherrington’s paper ‘On the regulation of

blood supply of brain’ suggested neural activity was

accompanied by a regional increase in cerebral blood flow.

In 1992 Ogawa and Lee at AT and T Bell laboratories working

on rodents discovered that oxygenation level of blood act as

contrast agent in MR images which finally led to the fMRI

Structure of Atom Magnetic Dipole moment

ELECTROMAGNETISM

Proton has it’s

own magnetic field !

Proton spins

around it’s axis

History

Ogawa and lee?Found

1. Neural activity is proportional to

oxygenation

2. O2 level acts as contrast agent

3. EP imaging technique

4. Experimented in human MR

imaging

Precession in Magnetic Field

PRECESSIONAL FREQUENCY

In our MRI (1.5 T) ,Protons precess at around 63.75 MHz

1. A vector has a force and direction

2. Net Magnetic vector of the body is in direction

of main magnet

3.precession frequency decrease with increase of

ext. magnetic field

4.Spins pointing up are always more than that is

down

Question 1

Precession frequency

1. Is the freqeuncy of the nucleus

2. Is the freqeuncy of the protons

3. Is directly proportional to strength of

main magnet(B0)

4. Is the frequency of the spins

Question 2

How to measure magnetisation?

Transversal Relaxation

RF PULSE !

RF PULSE ON!

RF PULSE OFF !

** Change in magnetic force moment

** Induces a current

** Signal captured

* Decays when RF pulse is off

Free induction decay

Question 3

RF PULSE

1. When RF pulse is off SPINS DEPHASE

2. RF pulse has same frequency as that of spins

3. In phase vector is always in transverse direction

4. A moving magnetic field induces a current

PRECESSION FREQUENCY

SPEED !

IN PHASE !

SPIN ECHO SEQUENCE

90°--TE /2 --- 180° --- TE/2 -- RECORD SIGNAL

RF PULSE OFF

PROTONS

DEPHASE

AFTER 180° PULSE

AT TE / 2, PR0TONS REPHASE

WHY ANOTHER 180° PULSE ?

WHY ANOTHER 180° PULSE ?

* REPHASES THE DEPHASING PROTONS (SPINS)

* EXTERNAL MAGNETIC FIELD INHOMOGENITIES

NEUTRALISED!

(SIGNAL INTENSITY REDUCED BY EXT.&INT.MAGNETIC FIELD

INHOMOGENITIES)

* PRODUCES AFTER TIME TE

A STRONGER ECHO !

T2* EFFECT

T1 = t(63 % of original magnetization)

1/T1 = Longitudinal relaxation rateSpin Lattice Relaxation

T1 (water) > T1 (fat)

T2= t(37 % of its initial value)

1/T2 = Transversal Relaxation Rate

T2(water) > T2(fat)

RF PULSE REMOVED!

SPIN-SPIN RELAXTION

180180° PULSE causes the following

1. Rephases the protons

2. Ext. Inhomogenity of magnetic field reduced

3. Echo becomes weak

4. Applied at time TE.

QUESTION 4

F

90 90TR

GΦ

TE/2TE

GZ

GX

IMAGE FORMATION

T1T2 PD

Fourier Transformation

Image formation

GRADIENT

COILS

RF Coils

Depending on the working

-Transmit and Receive Coil

-Transmit only Coil

-Receive only Coil

Depending on the application

-Volume Coil

-Surface Coil

-Internal Coil

INSTRUMENTATION

1) Put subject in big magnetic field

2) Transmit radio waves into subject [about 3 ms]

3) Turn off radio wave transmitter

4) Receive radio waves re-transmitted by subject

5) Store measured radio wave data vs. time

– Now go back to step(2) to get some more data

6) Process raw data to reconstruct images

NUT SHELL!

1. A vector has a force and direction -T

2. Net Magnetic vector of the body is in direction

of main magnet-T

3.precession frequency decrease with increase of

ext. magnetic field-F

4.Spins pointing up are always more than that is down-T

Question 1

Precession frequency

1. Is the freqeuncy of the nucleus -F

2. Is the freqeuncy of the protons-T

3. Is directly proportional to strength of

main magnet(B0)-T

4. Is the frequency of the spins-T

Question 2

Question 3

RF PULSE

1. When RF pulse is off SPINS DEPHASE-T

2. RF pulse has same frequency as that of spins-T

3. Inphase vector is always in transverse direction-F

4. A moving magnetic field induces a current-T

180180° PULSE causes the following

1. Rephases the protons-F

2. Ext. Inhomogenity of magnetic field reduced-T

3. Echo becomes weak-F

4. Applied at time TE-F

QUESTION 4

Pre contrast

T1

T2

PD

Post contrast T1

Sagittal

Axial

Coronal

Sagittal

Axial

Coronal

Sagittal

Axial

Coronal

Sagittal

Axial

Coronal

T1 PD T2

SHORT TR LONG TR LONG TR

SHORT TE SHORT TE LONG TE

• Long T1 value (1800-2500msec)

• Fluid suppression technique

(CSF appears dark)

• Fast flair sequences

Use

• Acute subarachnoid hge

• WM lesion in spinal cord

• Flair sequence in unco-operative patients

To see internal auditory canal

CP angle structures – Cr Nn

Short T1 inversion recovery

T1 – inversion time

Fat suppression

Useful in Orbit, Optic N

Bone marrow imaging – Metastasis

(BM & Fat suppressed – Met; lesion more conspicuous)

Strong T2 Wted Image

In 3D mode – High signal from fluids (CSF)

High spatial resolution

Cisternal portions of Cr.Nns

Workup of acoustic tumours

CISS 3D

STIR

1. Is a FAT SUPPRESSED SEQUENCE

2. Ideal in CSF RHINORREA

3. FAT HAS SHORT TI VALUE

4. TISSUE WITH LONG T1 APPEAR DARK

Hypo intensity in GRE1. Haemorrhage – Deoxy Hb

Meth HbFerritin (Haemosiderin other Iron forms)

2. Calcification – Diamagnetic ca saltsAssociated paramagnetic ions

3. Air containing PNS4. Normal brain iron5. Intratumoral melanin6. Paramagnetic devices, Fn bodies7. Ferromagnetic devices, Fn bodies8. Intravascular deoxygenated blood.

1. Fat (Biological / oil based contrast agent)2. Very high non-paramagnetic Protein content3. Paramagnetic or iodinated contrast agent4. Calcification5. Paramagnetic ions (associated with liver disease,

hyper alimentation, calcification, necrosis)6. Mucinous material7. Intraluminal melanin8. Hypermyelination9. Paramagnetic artefact10. Slow flow

Iron with out hemorrhage

Calcification or bone

Air

Very high paramagnetic Protein content

Deoxy Hb in patent veins

Mucinous material

Rapid or turbulent flow

Ferromagnetic artefact

No Parameters CT MRI

1 Energy X – Ray Magnet

2 Slices Axials Multi Planar

3 Time Fast Slow

4 3D Yes Yes

5 Angio Yes Yes

6 Moving Parts Yes No

7 White Matter disease + ++++

8 Resolution ++ ++++

9 Number of lesion detected +++ ++++

10 Involve adjacent structures ++ ++++

11 Location +++ ++++

12 Spine & Spinal cord ++ ++++

13 Previous Surgery / Metal ++++ +

14 Bone disease ++++ +

15 Calcification ++++ ++

16 Oedema ++ ++++

17 Guidance + - yes

No Tissues T1 T2 CT

1 Water Hypo Hyper Iso

2 Fat Hyper Hypo/Iso Hypo

3

Hematoma

(Depends on

Stage)

Hyper Hyper Hyper

4 Bone Hypo Hypo Hyper

5 Flow Hypo Hypo Iso

Type Hb Time T1 T2

Hyper Acute Oxy Hb 2 hrs Iso Hyper

Acute De-oxy Hb 2 – 12 hrs Iso Hypo

Sub acuteI/C Met Hb 12 hrs-4 days Hyper Hypo

E/C Met Hb 4 days–2 wks Hyper Hyper

Chronic Hemosiderin > 2 Wks Hypo Hypo

SUSCEPTIBILITY WEIGHTED

IMAGES

DIA MAGNETIC PARAMAGNETIC Super Paramagnetic

Disperse EXT.

Magnetic field

CONCENTRATE

EXT. Magnetic field

CONCENTRATE

EXT. Magnetic field

Water, Org. molecule Ions , simple salts, Ferric sulphate ,

Salts of non-metals Chelates of metals Alloys( Fe, Ni, Co)

Inert gases

Magnetic Susceptibility

Sections of skull vault

Sections of skull vault

MRI basicshttp://pss100.psi.ch/~kuehne/MR_Basics.html

Basics MRIhttp://howstuffworks.com/mri.htm

MRI Basicshttp://www.wellweb.com/Diagnost/arnief/mri.htm

Magnetic Resonance Advisory Panelhttp://www.giant.net.au/air/mrpanel.htm

MRI Educationhttp://www.mrieducation.com/usa.htm

MRI Review dotcomhttp://www.mrireview.com/index2.html

Hesselink Basic Principles Of MRImaginghttp://spinwarp.ucsd.edu/NeuroWeb/Text/br-100.htm

MRI Educationhttp://www.mrieducation.com/mainb.htm

A Selection of Slides on MRI Basicshttp://porkpie.loni.ucla.edu/BMD_HTML/SharedCode/slides/SlideFiles.html

Animated Basic MRIhttp://www.t2star.com/basic_mr/Basic.html

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