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Basics Of Magnetic Resonance Imaging (MRI)

Lecture One

GGamal amal FFathalla athalla MM.. MMahdalyahdaly

gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com

Magnetic Resonance Imaging Magnetic Resonance Imaging (MRI)(MRI)

A pilot look on the events occurring in MRI:A pilot look on the events occurring in MRI:1)1) Patient is put into the bore of a strong magnet. Patient is put into the bore of a strong magnet. 2)2) Radiofrequency waves are transmitted into Radiofrequency waves are transmitted into

the patient for a extremely the patient for a extremely shortshort time [2~10 time [2~10 ms], then turned off.ms], then turned off.

3)3) Radio waves re-transmitted by the patient are Radio waves re-transmitted by the patient are then received in the Receive coil.then received in the Receive coil.

4)4) Measured RF data are then transformed to Measured RF data are then transformed to image.image.

MRI Uses Three Principal Magnetic MRI Uses Three Principal Magnetic FieldsFields

Static High Field (BStatic High Field (B00) ) – Creates or polarizes signalCreates or polarizes signal– 1000 Gauss to 100,000 Gauss1000 Gauss to 100,000 Gauss

Earth’s field is 0.5 GEarth’s field is 0.5 G

Radiofrequency Field (BRadiofrequency Field (B11) ) – Excites or perturbs signal into a measurable formExcites or perturbs signal into a measurable form– On the order of O.1 G but in resonance with MR On the order of O.1 G but in resonance with MR

signalsignal– RF coils also measure MR signalRF coils also measure MR signal– Excited or perturbed signal returns to equilibriumExcited or perturbed signal returns to equilibrium

Important for contrast mechanisms to take place.Important for contrast mechanisms to take place.

Gradient FieldsGradient Fields– 1-4 G/cm1-4 G/cm– Used to image & determine spatial position of MR Used to image & determine spatial position of MR

signalsignal

Static Magnetic Field (BStatic Magnetic Field (B00))

Bore(55 – 60 cm)

Shim(B0 uniformity)

Magnetic field (B0)

Body RF(transmit/receive)

GradientsGradients

Back to MRI, Let's dissect this titleBack to MRI, Let's dissect this title,,

MagneticMagnetic:: Any moving charged particle generates a magnetic Any moving charged particle generates a magnetic

dipole moment (field) around. dipole moment (field) around. Applying this concept to the atom excludes neutrons Applying this concept to the atom excludes neutrons

from magnetic field production (only protons & electrons from magnetic field production (only protons & electrons are charged). Being of an extremely small mass, the are charged). Being of an extremely small mass, the electron is negligible, so only protons will be considered electron is negligible, so only protons will be considered as the source of magnetization in MRI. as the source of magnetization in MRI.

Let's, then, consider the proton as a Let's, then, consider the proton as a simplesimple bar bar magnetmagnet

A Single ProtonA Single Proton

++++

++

There is electric charge There is electric charge on the surface of the on the surface of the proton, thus creating a proton, thus creating a small current loop and small current loop and generating magnetic generating magnetic momentmoment ..

The proton also The proton also has mass which has mass which generates angenerates anangular angular momentummomentumJJ when it is when it is spinning.spinning.

JJ

Thus the proton “magnet” differs from the bar magnet in Thus the proton “magnet” differs from the bar magnet in that itthat it

also possesses also possesses angular momentum angular momentum caused by spinning.caused by spinning.

Which nuclei are NMR Which nuclei are NMR active?active?

To be NMR active, a nucleus needs to have 2 To be NMR active, a nucleus needs to have 2 properties:properties:– SpinSpin– chargecharge

Pairs of spins tend to cancel, so only atoms Pairs of spins tend to cancel, so only atoms with an odd number of protons or neutrons with an odd number of protons or neutrons have spin have spin – Good MR nuclei are Good MR nuclei are 11H, H, 1313C, C, 1919F, F, 2323Na, Na, 3131PP

What protons are we talking What protons are we talking about?about?  

So as to image the body, we should considerSo as to image the body, we should consider the following conditionsthe following conditions::1-Abundance:1-Abundance: The most The most abundantabundant protons in our bodies are the protons in our bodies are the

hydrogen protons which are present in almost all hydrogen protons which are present in almost all our body chemistry e.g., water, fat, our body chemistry e.g., water, fat, carbohydrates, proteins, vitamins,…etc. If carbohydrates, proteins, vitamins,…etc. If imaged, hydrogen is a good reflection of our body imaged, hydrogen is a good reflection of our body anatomy & pathology.anatomy & pathology.

Nuclear Magnetic Resonance Visible Nuclear Magnetic Resonance Visible NucleiNuclei

2-Ease of study:2-Ease of study:

Being the simplest atom (the first in the Being the simplest atom (the first in the periodic table), hydrogen is easy to study. We periodic table), hydrogen is easy to study. We have three isotopes for hydrogen, have three isotopes for hydrogen, ProtiumProtium , , Deuterium, & Tritium. The simplest of which Deuterium, & Tritium. The simplest of which (simplest of the simplest) is(simplest of the simplest) is Protium Protium which is which is the isotope present in our bodies. Its nucleus the isotope present in our bodies. Its nucleus contains one proton with no neutrons. We can contains one proton with no neutrons. We can consider this nucleus as a consider this nucleus as a ProtonProton..

Periodic tablePeriodic table

3-High energy content:3-High energy content: The energy profile of the proton falls in two types: The energy profile of the proton falls in two types:

Potential energyPotential energy & & Kinetic energyKinetic energy. . The potential energy can be understood The potential energy can be understood

according to Blanc's Quantum Theory which according to Blanc's Quantum Theory which states that: Matter & Energy are inter-convertible. states that: Matter & Energy are inter-convertible.

Matter according to Blanc, contains Energy which Matter according to Blanc, contains Energy which can be expressed as 4 quantum numbers one of can be expressed as 4 quantum numbers one of which is called which is called Spin quantum number (I).Spin quantum number (I). For For hydrogen, the spin quantum number=1/2hydrogen, the spin quantum number=1/2

The number of energy states, a proton can assume The number of energy states, a proton can assume when put in a magnetic field is given by the equation:when put in a magnetic field is given by the equation:

NNoo of Energy (spin) states= 2 x spin quantum N of Energy (spin) states= 2 x spin quantum Noo +1 +1

For hydrogen proton, the No of spin states thus = (2 x For hydrogen proton, the No of spin states thus = (2 x 1/2) +1 =2.1/2) +1 =2.

Hence, we have 2 spin states one called high energy Hence, we have 2 spin states one called high energy state & the other is called low energy state according state & the other is called low energy state according to their energy threshold. to their energy threshold.

The number of protons in the low energy state should exceed The number of protons in the low energy state should exceed that in the high energy state & the energy diagram would be that in the high energy state & the energy diagram would be as follows:as follows:

This looks like the water fall, that is, the energy in the top of This looks like the water fall, that is, the energy in the top of the water fall is greater than at the bottom & the difference the water fall is greater than at the bottom & the difference ∆∆EE creates electricity (in our condition ∆ creates electricity (in our condition ∆EE creates the MR creates the MR image).image).

Weak

M.F.

Strong

M.F.

Low Energy State

High Energy State

NOM.F.

∆E

Low

Ener

gy St

ate

∆E

oB

Steady State: Effect of

M

Para

llel

y

x

zAnti-ParallelHigh

Energy State

Reference FrameReference Frame

Magnetic field (B0) aligned with Z axis (longitudinal axis or long axis of the body)

y

x

z

This energy difference is directly proportional to the This energy difference is directly proportional to the magnet field strengthmagnet field strength because the stronger the because the stronger the magnet, the bigger the number of protons it can align magnet, the bigger the number of protons it can align in the low energy state (along the magnet axis) & in the low energy state (along the magnet axis) & hence the energy difference between the 2 states hence the energy difference between the 2 states increases (i.e. image data increases). This is the increases (i.e. image data increases). This is the source of the source of the Potential EnergyPotential Energy..

On the other hand, On the other hand, kinetic energykinetic energy can be can be understood by considering the proton spinning which understood by considering the proton spinning which looks like the spinning top. If you watch the spinning looks like the spinning top. If you watch the spinning top, you see it spinning top, you see it spinning vertically.vertically.

But by the time, it begins to wobble (precess) But by the time, it begins to wobble (precess) changing the spinning axis (widening the rotation changing the spinning axis (widening the rotation diameter gradually) till the top touches the earthdiameter gradually) till the top touches the earth..

Top analogyTop analogy Spins in a magnetic field: analogous to a spinning top Spins in a magnetic field: analogous to a spinning top

in the Earth’s gravitational field.in the Earth’s gravitational field.

gravity

Top precesses about the force caused by gravityDipoles (or spins) will precess about the static

magnetic field

Axis of top

How can we understand what happens?How can we understand what happens? TThe top spins by act of centrifugal force generated by pulling he top spins by act of centrifugal force generated by pulling

the rope wrapped around & continues to spin vertically by the rope wrapped around & continues to spin vertically by inertia till air resistance annihilates it enabling earth gravity to inertia till air resistance annihilates it enabling earth gravity to attract it.attract it.

Earth gravity causes the top to wobble (changes the axis of Earth gravity causes the top to wobble (changes the axis of rotation or in more clear words draws a helix till it touches the rotation or in more clear words draws a helix till it touches the earth. Each loop of this helix is called a earth. Each loop of this helix is called a precession.precession. The The number of precessions/sec is called number of precessions/sec is called precessional precessional frequency.frequency.

Hydrogen protons put in a magnetic field behave just Hydrogen protons put in a magnetic field behave just like a spinning top put in the earth's magnetic field. like a spinning top put in the earth's magnetic field. The precessional frequency of the hydrogen proton is The precessional frequency of the hydrogen proton is said to be high & that means that the number of said to be high & that means that the number of precessions per second is high i.e. it possesses a largeprecessions per second is high i.e. it possesses a large kinetic energykinetic energy..

Now, let’s consider the human body as a sample Now, let’s consider the human body as a sample of hydrogen nuclei put in the bore of a powerful of hydrogen nuclei put in the bore of a powerful

magnetmagnet

oBM

X

Y

Z

The protons aligned along the Z- axis The protons aligned along the Z- axis (long axis of the couch) are said to (long axis of the couch) are said to be be parallelparallel to the main magnetic to the main magnetic field field BBo o direction.direction.

The protons aligned perpendicular to The protons aligned perpendicular to that direction would lie in the XY that direction would lie in the XY plane are said to be plane are said to be anti-parallelanti-parallel to to the main field.the main field.

The energy difference between the 2 The energy difference between the 2 states (states (EE ) depends on the nucleus ) depends on the nucleus & & BBo o

. .

Of course protons in the high energy Of course protons in the high energy state would produce inductive current state would produce inductive current in the circuit as the magnetic flux of in the circuit as the magnetic flux of these protons when flowing from the these protons when flowing from the north to the south pole traverses the north to the south pole traverses the circuit inducing an alternating current circuit inducing an alternating current of variable frequency depending on of variable frequency depending on the proton density (tissue chemistry).the proton density (tissue chemistry).

What protons are supposed to What protons are supposed to induce electric current in an induce electric current in an

electric circuit wrapped around the electric circuit wrapped around the patient? patient?

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++

There is electric There is electric charge charge on the surface of on the surface of the proton, thus the proton, thus creating a small creating a small current loop and current loop and generating generating magnetic momentmagnetic moment ..

The proton The proton also has mass also has mass which which generates angenerates anangular angular momentummomentumJJ when it is when it is spinning.spinning.

JJ

The magnetic moment (The magnetic moment () and angular ) and angular momentum (J) are vectors lying along the spin momentum (J) are vectors lying along the spin

axisaxis

= = JJ Where (Where (is the gyro magnetic ratio which is constant for a given nucleus.is the gyro magnetic ratio which is constant for a given nucleus.

This means that the magnetic moment is directly proportional to the angular This means that the magnetic moment is directly proportional to the angular moment.moment.

The energy difference between the two energy states depends on the nucleus & the magnetic

field strength

E = 2 z Bo

Eh /2

Where is the larmor frequency

/2= 42.57 MHz / Tesla for = 42.57 MHz / Tesla for protonproton

Hydrogen atoms are best for Hydrogen atoms are best for MRIMRI

Biological tissues are predominantly Biological tissues are predominantly 1212C, C, 1616O, O, 11H, and H, and 1414N but Hydrogen is the N but Hydrogen is the most most abundantabundant atom in the body. atom in the body.

Hydrogen atom is the simplest.Hydrogen atom is the simplest. Hydrogen nucleus possesses high energy.Hydrogen nucleus possesses high energy.

N.B.N.B.1.1. The majority of hydrogen is in water (HThe majority of hydrogen is in water (H22O).O).2.2. Essentially all MRI is hydrogen (proton) Essentially all MRI is hydrogen (proton)

imagingimaging

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Should I Ask???.

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