1. The document discusses the basics of magnetic resonance imaging (MRI). It explains that MRI uses strong magnetic fields and radiofrequency waves to align hydrogen proton spins in the body and produce signals that are used to generate images. 2. The static magnetic field polarizes proton spins. Radiofrequency waves excite the spins, causing them to emit radio signals that are measured to form an image. Gradient magnetic fields are used to spatially encode the signals. 3. Hydrogen protons are ideal for MRI because they are highly abundant in the body and have a simple structure. When placed in a magnetic field, their spins can exist in either a high or low energy state. The energy difference between the states produces the signal measured in

1. ‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬
‫ال‬ ‫فضل‬ ‫وكان‬‫ال‬ ‫فضل‬ ‫وكان‬
‫عظيما‬ ‫عليك‬‫عظيما‬ ‫عليك‬

2. Basics Of Magnetic
Resonance
Imaging (MRI)
Lecture One
GGamalamal FFathallaathalla MM..
MMahdalyahdaly
gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com

3. Magnetic Resonance Imaging (MRI)Magnetic Resonance Imaging (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 strongPatient is put into the bore of a strong
magnet.magnet.
2)2) Radiofrequency waves are transmitted intoRadiofrequency waves are transmitted into
the patient for a extremelythe patient for a extremely shortshort time [2~10time [2~10
ms], then turned off.ms], then turned off.
3)3) Radio waves re-transmitted by the patientRadio waves re-transmitted by the patient
are then received in the Receive coil.are then received in the Receive coil.
4)4) Measured RF data are then transformed toMeasured RF data are then transformed to
image.image.

4. MRI Uses Three Principal Magnetic FieldsMRI Uses Three Principal Magnetic Fields
 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 signalOn the order of O.1 G but in resonance with MR signal
– 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 signalUsed to image & determine spatial position of MR signal

5. Static Magnetic Field (BStatic Magnetic Field (B00))
Bore
(55 – 60 cm)
Shim
(B0 uniformity)
Magnetic field (B0)
Body RF
(transmit/receive)
GradientsGradients

6. Back to MRI, Let's dissect this titleBack to MRI, Let's dissect this title,,
MagneticMagnetic::
 Any moving charged particle generates aAny moving charged particle generates a
magnetic dipole moment (field) around.magnetic dipole moment (field) around.
 Applying this concept to the atom excludesApplying this concept to the atom excludes
neutrons from magnetic field production (onlyneutrons from magnetic field production (only
protons & electrons are charged). Being of anprotons & electrons are charged). Being of an
extremely small mass, the electron isextremely small mass, the electron is
negligible, so only protons will be considerednegligible, 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 aLet's, then, consider the proton as a simplesimple
bar magnetbar magnet

7. A Single ProtonA Single Proton
++
++
++
There is electric chargeThere is electric charge
on the surface of the proton,on the surface of the proton,
thus creating a small currentthus creating a small current
loop and generatingloop and generating
magnetic momentmagnetic moment µµ..
The proton also hasThe proton also has
mass whichmass which
generates angenerates an
angular momentumangular momentum
JJ when it is spinning.when it is spinning.
JJ
µµ
Thus the proton “magnet” differs from the bar magnet in that itThus the proton “magnet” differs from the bar magnet in that it
also possessesalso possesses angular momentumangular momentum caused by spinning.caused by spinning.

8. Which nuclei are NMRWhich nuclei are NMR
active?active?
 To be NMR active, a nucleus needs to have 2To be NMR active, a nucleus needs to have 2
properties:properties:
– SpinSpin
– chargecharge
 Pairs of spins tend to cancel, so only atomsPairs of spins tend to cancel, so only atoms
with an odd number of protons or neutronswith an odd number of protons or neutrons
have spinhave spin
– Good MR nuclei areGood MR nuclei are 11
H,H, 1313
C,C, 1919
F,F, 2323
Na,Na, 3131
PP

9. What protons are we talking about?What protons are we talking 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 mostThe most abundantabundant protons in our bodiesprotons in our bodies
are the hydrogen protons which are presentare the hydrogen protons which are present
in almost all our body chemistry e.g., water,in almost all our body chemistry e.g., water,
fat, carbohydrates, proteins, vitamins,…etc.fat, carbohydrates, proteins, vitamins,…etc.
If imaged, hydrogen is a good reflection ofIf imaged, hydrogen is a good reflection of
our body anatomy & pathology.our body anatomy & pathology.

10. Nuclear Magnetic Resonance Visible NucleiNuclear Magnetic Resonance Visible Nuclei

11.  2-Ease of study:2-Ease of study:
Being the simplest atom (the first in theBeing the simplest atom (the first in the
periodic table), hydrogen is easy to study.periodic table), hydrogen is easy to study.
We have three isotopes for hydrogen,We have three isotopes for hydrogen,
ProtiumProtium , Deuterium, & Tritium. The, Deuterium, & Tritium. The
simplest of which (simplest of the simplest) issimplest of which (simplest of the simplest) is
ProtiumProtium which is the isotope present in ourwhich is the isotope present in our
bodies. Its nucleus contains one proton withbodies. Its nucleus contains one proton with
no neutrons. We can consider this nucleusno neutrons. We can consider this nucleus
as aas a ProtonProton..

12. Periodic tablePeriodic table

13. 3-High energy content:3-High energy content:
 The energy profile of the proton falls in twoThe energy profile of the proton falls in two
types:types: Potential energyPotential energy && Kinetic energyKinetic energy ..
 The potential energy can be understoodThe potential energy can be understood
according to Blanc's Quantum Theory whichaccording to Blanc's Quantum Theory which
states that: Matter & Energy are inter-states that: Matter & Energy are inter-
convertible.convertible.
 Matter according to Blanc, contains EnergyMatter according to Blanc, contains Energy
which can be expressed as 4 quantumwhich can be expressed as 4 quantum
numbers one of which is callednumbers one of which is called SpinSpin
quantum number (I).quantum number (I). For hydrogen, theFor hydrogen, the

14.  The number of energy states, a proton canThe number of energy states, a proton can
assume when put in a magnetic field is givenassume when put in a magnetic field is given
by the equation:by the equation:
NNoo
of Energy (spin) states= 2 x spin quantum Nof Energy (spin) states= 2 x spin quantum Noo
+1+1
 For hydrogen proton, the No of spin statesFor hydrogen proton, the No of spin states
thus = (2 x 1/2) +1 =2.thus = (2 x 1/2) +1 =2.
 Hence, we have 2 spin states one calledHence, we have 2 spin states one called
high energy state & the other is called lowhigh energy state & the other is called low

15.  The number of protons in the low energy stateThe number of protons in the low energy state
should exceed that in the high energy state &should exceed that in the high energy state &
the energy diagram would be as follows:the energy diagram would be as follows:
 This looks like the water fall, that is, the energyThis looks like the water fall, that is, the energy
in the top of the water fall is greater than at thein the top of the water fall is greater than at the
bottom & the difference ∆bottom & the difference ∆EE creates electricity (increates electricity (in
our condition ∆our condition ∆EE creates the MR image).creates the MR image).
Weak
M.F.
Strong
M.F.
Low Energy State
High Energy State
NO
M.F.
∆E

16. LowEnergyState
∆E
oB
Steady State: Effect of
M
Βο
Parallel
y
x
z
Anti-Parallel
High
Energy State

17. Reference FrameReference Frame
Magnetic field (B0) aligned with Z axis
(longitudinal axis or long axis of the body)
y
x
z

18.  This energy difference is directly proportional to theThis energy difference is directly proportional to the magnetmagnet
field strengthfield strength because the stronger the magnet, the bigger thebecause the stronger the magnet, the bigger the
number of protons it can align in the low energy state (alongnumber of protons it can align in the low energy state (along
the magnet axis) & hence the energy difference between thethe magnet axis) & hence the energy difference between the
2 states increases (i.e. image data increases). This is the2 states increases (i.e. image data increases). This is the
source of thesource of the Potential EnergyPotential Energy..
 On the other hand,On the other hand, kinetic energykinetic energy can be understood bycan be understood by
considering the proton spinning which looks like the spinningconsidering the proton spinning which looks like the spinning
top. If you watch the spinning top, you see it spinningtop. If you watch the spinning top, you see it spinning
vertically.vertically.
 But by the time, it begins to wobble (precess) changing theBut by the time, it begins to wobble (precess) changing the
spinning axis (widening the rotation diameter gradually) tillspinning axis (widening the rotation diameter gradually) till
the top touches the earththe top touches the earth..

19. Top analogyTop analogy
Spins in a magnetic field: analogous to a spinning top in theSpins in a magnetic field: analogous to a spinning top in the
Earth’s gravitational field.Earth’s gravitational field.
gravity
Top precesses about the force caused by gravity
Dipoles (or spins) will precess about the static magnetic field
Axis of top

20. How can we understand what happens?How can we understand what happens?
 TThe top spins by act of centrifugal force generated by pullinghe top spins by act of centrifugal force generated by pulling
the rope wrapped around & continues to spin vertically bythe rope wrapped around & continues to spin vertically by
inertia till air resistance annihilates it enabling earth gravity toinertia till air resistance annihilates it enabling earth gravity to
attract it.attract it.
 Earth gravity causes the top to wobble (changes the axis ofEarth gravity causes the top to wobble (changes the axis of
rotation or in more clear words draws a helix till it touches therotation or in more clear words draws a helix till it touches the
earth. Each loop of this helix is called aearth. Each loop of this helix is called a precession.precession. TheThe
number of precessions/sec is callednumber of precessions/sec is called precessionalprecessional
frequency.frequency.
 Hydrogen protons put in a magnetic field behave just like aHydrogen protons put in a magnetic field behave just like a
spinning top put in the earth's magnetic field. The precessionalspinning top put in the earth's magnetic field. The precessional
frequency of the hydrogen proton is said to be high & thatfrequency of the hydrogen proton is said to be high & that
means that the number of precessions per second is high i.e. itmeans that the number of precessions per second is high i.e. it
possesses a largepossesses a large kinetic energykinetic energy..

21. Now, let’s consider the human body as a sample ofNow, let’s consider the human body as a sample of
hydrogen nuclei put in the bore of a powerful magnethydrogen nuclei put in the bore of a powerful magnet
oB
M
X
Y
Z

22.  The protons aligned along the Z- axisThe protons aligned along the Z- axis
(long axis of the couch) are said to be(long axis of the couch) are said to be
parallelparallel to the main magnetic fieldto the main magnetic field BBoo
direction.direction.
 The protons aligned perpendicular to thatThe protons aligned perpendicular to that
direction would lie in the XY plane aredirection would lie in the XY plane are
said to besaid to be anti-parallelanti-parallel to the main field.to the main field.
 The energy difference between the 2The energy difference between the 2
states (states (∆∆EE ) depends on the nucleus &) depends on the nucleus &
BBoo
..

23.  Of course protons in the high energy stateOf course protons in the high energy state
would produce inductive current in thewould produce inductive current in the
circuit as the magnetic flux of thesecircuit as the magnetic flux of these
protons when flowing from the north to theprotons when flowing from the north to the
south pole traverses the circuit inducingsouth pole traverses the circuit inducing
an alternating current of variablean alternating current of variable
frequency depending on the protonfrequency depending on the proton
density (tissue chemistry).density (tissue chemistry).
What protons are supposed to induceWhat protons are supposed to induce
electric current in an electric circuitelectric current in an electric circuit
wrapped around the patient?wrapped around the patient?

24. ++
++
++
There is electricThere is electric
chargecharge
on the surface of theon the surface of the
proton, thus creatingproton, thus creating
a small current loopa small current loop
and generatingand generating
magnetic momentmagnetic moment µµ..
The proton alsoThe proton also
has mass whichhas mass which
generates angenerates an
angularangular
momentummomentum
JJ when it iswhen it is
spinning.spinning.
JJ
µµ
The magnetic moment (The magnetic moment (µµ) and angular momentum (J)) and angular momentum (J)
are vectors lying along the spin axisare vectors lying along the spin axis
µµ == γγ JJ
Where (Where (γ)γ) 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 moment.This means that the magnetic moment is directly proportional to the angular moment.

25. The energy difference between the two energy states
depends on the nucleus & the magnetic field strength
∆ E = 2 µz Bo
∆ E = h ν
ν = γ/2π Βο
Where ν is the larmor frequency
γ/2π = 42.57 MHz / Tesla for proton= 42.57 MHz / Tesla for proton

26. Hydrogen atoms are best for MRIHydrogen atoms are best for MRI
 Biological tissues are predominantlyBiological tissues are predominantly 1212
C,C, 1616
O,O, 11
H, andH, and
1414
N but Hydrogen is theN but Hydrogen is the most abundantmost abundant atom in theatom in the
body.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) imagingEssentially all MRI is hydrogen (proton) imaging

27. Any Question???.
Take your Tiiime!!! .
Again Any Question???.
Otherwise, I’m going to ask!!!.
Should I Ask???.

28. Have a nice dayHave a nice day