Top Banner
26-Jul-14 1 Basic Physical Principles Basic Physical Principles of of MRI MRI Present by M.Shafiee. MSc RSO at Urmia Nuclear Medicine Center [email protected] 2 Medical Imaging Modalities: History Medical Imaging Modalities: History 1895: X-ray ~1950: Ultrasound ~1955: Radionuclide 1972: CT ~1980: MRI Nobel Prize
25

Principle of MRI Physics. Shafiee

Aug 21, 2014

Download

Health & Medicine

Meysam Shafiee

Principle of MRI physics: Magnetic Resonance Imaging
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: Principle of MRI Physics. Shafiee

26-Jul-14

1

Basic Physical PrinciplesBasic Physical Principlesofof MRIMRI

Present by M.Shafiee. MScRSO at Urmia Nuclear Medicine Center

[email protected]

2

Medical Imaging Modalities: HistoryMedical Imaging Modalities: History

• 1895: X-ray†

• ~1950: Ultrasound• ~1955: Radionuclide• 1972: CT†

• ~1980: MRI†

†Nobel Prize

Page 2: Principle of MRI Physics. Shafiee

26-Jul-14

2

History of NMRHistory of NMRNMR = nuclear magnetic resonance

Felix Block and Edward Purcell1946: atomic nuclei absorb and re-emitradio frequency energy1952: Nobel prize in physics

nuclear: properties of nuclei of atomsmagnetic: magnetic field requiredresonance: interaction between magneticfield and radio frequency Bloch Purcell

NMRNMR MRI: Why the name change?MRI: Why the name change?

most likely explanation:nuclear has bad connotations

less likely but more amusing explanation:subjects got nervous when fast-talking doctors suggested an NMR

Page 3: Principle of MRI Physics. Shafiee

26-Jul-14

3

Strengths of MRI

••Images of softImages of soft--tissue structures of the body, such as the heart,tissue structures of the body, such as the heart,

lungs, liver, are clearer and more detailedlungs, liver, are clearer and more detailed

••MRI can help evaluate theMRI can help evaluate the functionfunction as well as the structureas well as the structure

••Invaluable tool in early evaluation of tumorsInvaluable tool in early evaluation of tumors

••MRI contrast materials are less harmful than those used in XMRI contrast materials are less harmful than those used in X--rayray

or CTor CT

••Fast, nonFast, non--invasive angiographyinvasive angiography

••Exposure to radiation is minimal (nonExposure to radiation is minimal (non--ionizing)ionizing)

Compare the detail:Compare the detail:CT (left) vs. MRI TCT (left) vs. MRI T22 (right)(right)

Page 4: Principle of MRI Physics. Shafiee

26-Jul-14

4

Risks and Weaknesses:Risks and Weaknesses:Risks and Weaknesses:Risks and Weaknesses:

• Metal implants may cause problems• Problems with claustrophobia• MRI is to be avoided during the first 12 weeks

of pregnancy• Bone is usually better imaged with X-rays• MRI typically costs more than CT

Necessary Equipment

Magnet Gradient Coil RF Coil

Source: Joe Gati, photos

RF Coil

4T magnet

gradient coil(inside)

Page 5: Principle of MRI Physics. Shafiee

26-Jul-14

5

x 80,000 =

4 Tesla = 4 x 10,000 0.5 = 80,000X Earth’s magnetic field

Robarts Research Institute 4T

The Big MagnetVeryVery strong!strong!

Continuously on

1 Tesla (T) = 10,000 Gauss

Earth’s magnetic field = 0.5 Gauss

Main field = B0

B0

MRI Clinical Safety Practice• Pacemaker• Aneurysm clip• Metal in the eye• Credit card

• Bone implant• Denture

X

Page 6: Principle of MRI Physics. Shafiee

26-Jul-14

6

Magnet SafetyThe whopping strength of the magnet makes safety essential.Things fly – Even big things!

Screen subjects carefullyMake sure you and all your students & staff are aware of hazzardsDevelop stratetgies for screening yourself every time you enter the magnet

Open Bore MRI Scanner

• Avoid claustrophobia• Lower image quality

Page 7: Principle of MRI Physics. Shafiee

26-Jul-14

7

Orthopedic MRI

Orthoscan MRI

Synopsis of MRI

1) Put subject in big magnetic field

2) Transmit radio waves into subject [2~10 ms]

3) Turn off radio wave transmitter

4) Receive radio waves re-transmitted by subject0

5) Convert measured RF data to image

Page 8: Principle of MRI Physics. Shafiee

26-Jul-14

8

Many factors contribute to MR imaging

• Quantum properties of nuclear spins• Radio frequency (RF) excitation properties• Tissue relaxation properties• Magnetic field strength and gradients• Timing of gradients, RF pulses, and signal

detection

What kinds of nuclei can be used for NMR?

• Nucleus needs to have 2 properties:– Spin– charge

• Nuclei are made of protons and neutrons– Both have spin ½– Protons have charge

• Pairs of spins tend to cancel, so only atoms withan odd number of protons or neutrons have spin– Good MR nuclei are 1H, 13C, 19F, 23Na, 31P

Page 9: Principle of MRI Physics. Shafiee

26-Jul-14

9

Hydrogen atoms are best for MRI

• Biological tissues are predominantly 12C, 16O, 1H, and14N

• Hydrogen atom is the only major species that is MRsensitive

• Hydrogen is the most abundant atom in the body• The majority of hydrogen is in water (H2O)• Essentially all MRI is hydrogen (proton) imaging

A Single Proton

++++

++

There is electric chargeThere is electric chargeon the surface of the proton,on the surface of the proton,thus creating a small currentthus creating a small currentloop and generating magneticloop and generating magneticmomentmomentmm..

The proton also hasThe proton also hasmass which generatesmass which generatesananangular momentumangular momentumJJ when it is spinning.when it is spinning.

JJµµ

Thus proton “magnet” differs from the magnetic bar in that itThus proton “magnet” differs from the magnetic bar in that italso possesses angular momentum caused by spinning.also possesses angular momentum caused by spinning.

Page 10: Principle of MRI Physics. Shafiee

26-Jul-14

10

Polarization• Spins are normally oriented randomly.• In an applied magnetic field, the spins align with the

applied field in their equilibrium state.• Excess along B0 results in net magnetization.

No Applied Field Applied Field

B0

Precession• Spins precess about applied magnetic field, B0, that is

along z axis.• The frequency of this precession is proportional to the

applied field:

B

Page 11: Principle of MRI Physics. Shafiee

26-Jul-14

11

Magnetic Moment

II

BB

FFLL

F = IBLF = IBL

BB

LLWW

ττ = IBLW = IBA= IBLW = IBA

µµ = τ= τmaxmax / Β/ Β= Ι Α= Ι Α

tt==mmBB= µ Β= µ Βsinsinθθ

ForceForce TorqueTorque

Angular Momentum

JJ = m= mωω==mmvvrr

mm

vvrr

JJ

Page 12: Principle of MRI Physics. Shafiee

26-Jul-14

12

mm==ggJJ

ggis theis the gyromagneticgyromagnetic ratio. unit: MHz/Tratio. unit: MHz/Tggis a constant for a given nucleusis a constant for a given nucleus

The magnetic moment and angularmomentum are vectors lying along thespin axis

How do protons interact with amagnetic field?

• Moving (spinning) charged particle generatesits own little magnetic field– Such particles will tend to line up with external

magnetic field lines (think of iron filings around amagnet)

• Spinning particles with mass have angularmomentum– Angular momentum resists attempts to change

the spin orientation (think of a gyroscope)

Page 13: Principle of MRI Physics. Shafiee

26-Jul-14

13

The energy difference between the two alignment statesdepends on the nucleus

DE =mz Bo/I DE = h ν

mz =ghI D E=gh Bo

ν = γ/2p Bo

known as Larmor frequency

γ/2p== 4242..5757 MHz / Tesla for protonMHz / Tesla for proton

B

Page 14: Principle of MRI Physics. Shafiee

26-Jul-14

14

Resonance frequencies of common nuclei

Note: Resonance at 1.5T = Larmor frequency X 1.5

MRI

X-Ray, CT

Electromagnetic Radiation Energy

Page 15: Principle of MRI Physics. Shafiee

26-Jul-14

15

MRI uses a combination of Magnetic andElectromagnetic Fields

• NMR measures the net magnetization of atomic nuclei in thepresence of magnetic fields

• Magnetization can be manipulated by changing the magneticfield environment (static, gradient, and RF fields)

• Static magnetic fields don’t change (< 0.1 ppm / hr):The main field is static and (nearly) homogeneous

• RF (radio frequency) fields are electromagnetic fields thatoscillate at radio frequencies (tens of millions of times persecond)

• Gradient magnetic fields change gradually over space and canchange quickly over time (thousands of times per second)

Page 16: Principle of MRI Physics. Shafiee

26-Jul-14

16

Radio Frequency FieldsRadio Frequency Fields

• RF electromagnetic fields are used to manipulate the

magnetization of specific types of atoms

• This is because some atomic nuclei are sensitive to magnetic fields

and their magnetic properties are tuned to particular RF frequencies

• Externally applied RF waves can be transmitted into a subject to

perturb those nuclei

• Perturbed nuclei will generate RF signals at the same frequency –

these can be detected coming out of the subject

The Effect of Irradiation to the SpinSystem

Lower

Higher

Basic Quantum Mechanics Theory of MRBasic Quantum Mechanics Theory of MR

Page 17: Principle of MRI Physics. Shafiee

26-Jul-14

17

Spin System After Irradiation

Basic Quantum Mechanics Theory of MRBasic Quantum Mechanics Theory of MR

Net magnetization is the macroscopic measure of many spins

BoM

T

BcM o

Page 18: Principle of MRI Physics. Shafiee

26-Jul-14

18

Net magnetization

• Small B0 produces small net magnetization M• Larger B0 produces larger net magnetization M, lined

up with B0

• Thermal motions try to randomize alignment ofproton magnets

• At room temperature, the population ratio of anti-parallel versus parallel protons is roughly 100,000 to100,006 per Tesla of B0

Quantum vs Classical Physics

One can consider the quantum mechanicalproperties of individual nuclei, but to considerthe bulk properties of a whole object it is moreuseful to use classical physics to consider netmagnetization effects.

Page 19: Principle of MRI Physics. Shafiee

26-Jul-14

19

To measure magnetization we mustperturb it

• We can only measure magnetization perpendicular tothe B0 field

• Need to apply energy to tip protons out of alignment• Amount of energy needed depends on nucleus and

applied field strength (Larmor frequency)• The amount of energy added (duration of the RF pulse

at the resonant frequency) determines how far the netmagnetization will be tipped away from the B0 axis

A Mechanical Analogy: A Swingset

• Person sitting on swing at rest is “aligned” with externallyimposed force field (gravity)

• To get the person up high, you could simply supply enoughforce to overcome gravity and lift him (and the swing) up– Analogous to forcing M over by turning on a huge static

B1

• The other way is to push back and forth with a tiny force,synchronously with the natural oscillations of the swing– Analogous to using a tiny RF B1 over a period of time to

slowly flip M over

gg

Page 20: Principle of MRI Physics. Shafiee

26-Jul-14

20

If M is not parallel to B, then it precesses clockwise aroundthe direction of B.“Normal” (fully relaxed) situation has M parallel to B, and therefore does notprecess

This is like a gyroscope

Derivation of precession frequencyDerivation of precession frequency

This says that the precession frequency is the SAME as the larmor frequency

==mm×× BBo = dJ / dtJ =m/γ

dm/dt = γ (m× Bo)

mm(t) = ((t) = (µµxocoscos γγBBott ++ µµyosinsin γγBBott)) xx + (+ (µµyocoscos γγBBott -- µµxosinsin γγBBott)) yy ++ µµzozz

Page 21: Principle of MRI Physics. Shafiee

26-Jul-14

21

Page 22: Principle of MRI Physics. Shafiee

26-Jul-14

22

Recording the MR signal

• Need a receive coil tuned to the same RF frequency asthe exciter coil.

• Measure “free induction decay” of net magnetization• Signal oscillates at resonance frequency as net

magnetization vector precesses in space• Signal amplitude decays as net magnetization

gradually realigns with the magnetic field• Signal also decays as precessing spins lose coherence,

thus reducing net magnetization

• T1 relaxation – Flipped nuclei realign with the magnetic field

• T2 relaxation – Flipped nuclei start off all spinning together, but

quickly become incoherent (out of phase)

• T2* relaxation – Disturbances in magnetic field (magnetic

susceptibility) increase the rate of spin coherence T2 relaxation

• The total NMR signal is a combination of the total number of

nuclei (proton density), reduced by the T1, T2, and T2* relaxation

components

Page 23: Principle of MRI Physics. Shafiee

26-Jul-14

23

Page 24: Principle of MRI Physics. Shafiee

26-Jul-14

24

T2* decay

• Spin coherence is also sensitive to the fact that the

magnetic field is not completely uniform

• Inhomogeneities in the field cause some protons to

spin at slightly different frequencies so they lose

coherence faster

• Factors that change local magnetic field (susceptibility)

can change T2* decay

Different tissues have different relaxation times.These relaxation time differences can be used togenerate image contrast.

Different tissues have different relaxation times.These relaxation time differences can be used togenerate image contrast.

• T1 - Gray/White matter• T2 - Tissue/CSF• T2* - Susceptibility (functional MRI)

Page 25: Principle of MRI Physics. Shafiee

26-Jul-14

25

THANK YOU FORYOUR NICEATTENTION