Magnetic Resonance Imaging

1
Magnetic Resonance Imaging

• Basic Principles
• V.G.Wimalasena Principal
• School of Radiography

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Introduction
Modern 3 Tesla MRI unit
Bore of the magnet
RF Coil (for head)
Main magnet body
Patient Couch
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What is MRI?

• Magnetic resonance imaging (MRI), or nuclear
  magnetic resonance imaging (NMRI), is primarily a
  Medical Imaging technique most commonly used in
  radiology to visualize the structure and function
  of the body.
• It provides detailed images of the body in any
  plane.

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MRI Vs CT

• MRI provides much greater contrast between the
  different soft tissues of the body than CT does,
  making it especially useful in neurological
  (brain), musculoskeletal, cardiovascular, and
  oncological (cancer) imaging.
• Unlike CT, it uses no ionizing radiation, but
  uses a powerful magnetic field to align the
  nuclear magnetization of (usually) hydrogen atoms
  in water in the body.

5
Uses RF fields

• Radiofrequency fields are used to systematically
  alter the alignment of the nuclear magnetization
  of Hydrogen atoms, causing the hydrogen nuclei to
  produce a rotating magnetic field detectable by
  the scanner.
• This signal can be manipulated by additional
  magnetic fields to build up enough information to
  construct an image of the body.

6
History

• MRI is a relatively new technology, which has
  been in use for little more than 30 years
  (compared with over 110 years for X-ray
  radiography).
• The first MR Image was published in 1973 and the
  first study performed on a human took place on
  July 3, 1977.
• Magnetic resonance imaging was developed from
  knowledge gained in the study of nuclear magnetic
  resonance

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Brief lay explanation of MRI physics

• The body is mainly composed of water molecules
  which each contain two hydrogen nuclei or
  protons.
• When a person goes inside the powerful magnetic
  field of the scanner these protons align with the
  direction of the field.

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• A second radiofrequency electromagnetic field is
  then briefly turned on causing the protons to
  absorb some of its energy.
• When this field is turned off the protons release
  this energy at a radiofrequency which can be
  detected by the scanner.

9

• The position of protons in the body can be
  determined by applying additional magnetic fields
  during the scan which allows an image of the body
  to be built up.
• These are created by turning gradients coils on
  and off which creates the knocking sounds heard
  during an MR scan.

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• Diseased tissue, such as tumors, can be detected
  because the protons in different tissues return
  to their equilibrium state at different rates.
• By changing the parameters on the scanner this
  effect is used to create contrast between
  different types of body tissue.

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Use of contrast agents

• Contrast agents may be injected intravenously to
  enhance the appearance of blood vessels, tumours
  or inflammation.
• Contrast agents may also be directly injected
  into a joint, in the case of arthrograms, MR
  images of joints.

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Safety precaution

• Unlike CT scanning MRI uses no ionizing radiation
  and is generally a very safe procedure.
• But Patients with some metal implants, cochlear
  implants, and cardiac pacemakers are prevented
  from having an MRI scan due to effects of the
  strong magnetic field and powerful radiofrequency
  pulses.

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Uses of MRI

• MRI is used to image every part of the body,
• But is particularly useful in
• neurological conditions,
• disorders of the muscles and joints,
• for evaluating tumors and
• showing abnormalities in the heart and blood
  vessels.

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System components
Magnet power supply
Gradient amplifiers
Shim power supply
RF transmitter
Operator consol
Magnet coils
Shim coils
Host computer
Gradient coils
RF coils
Magnet bore
Image processor
Image disk
RF receiver
Digitizer
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Explaining Basic principles

• This is an Integration of Two ways of explaining.
  i. e
• Classically
• Via quantum physics
• It describes
• Properties of atoms
• Their interaction with magnetic fields

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Atomic structure

• Central nucleus orbiting electrons
• Nucleus
• Nucleons
• (Protons neutrons)
• Atomic number
• Mass number
• Electrically stable

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Motion within the atom

• There are three types of motion within an atom
• Electrons spinning on their own axis
• Electrons orbiting the nucleus
• The nucleus spinning about its own axis

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• The principles of MRI rely on the spinning motion
  of specific nuclei present in biological tissues
• These are called (MR active nuclei)

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MR active nuclei ?

• MR active nuclei are Characterized by their
  tendency to align their axis of rotation to an
  applied magnetic field
• Due to the laws of electromagnetic induction,
  nuclei that have a net charge and are spinning
  acquire a magnetic moment and are able to align
  with an external magnetic field

20
MR active nuclei continued..

• Important Examples
• Hydrogen 1
• Carbon 13
• Nitrogen 15
• Oxygen 17
• Fluorine 19
• Sodium 23
• Phosphorus 31

• The nuclei with odd mass numbers undergoes this
  interaction
• The result of this interaction is angular
  momentum or spin

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The magnetic moment alignment

• The alignment of the magnetic moment is measured
  as the total of the nuclear magnetic moments and
  is expressed as a vector sum
• The strength of the total magnetic moment is
  specific to every nucleus and determines the
  sensitivity to magnetic resonance

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The hydrogen nucleus

• The hydrogen nucleus is the MR active nucleus
  used in clinical MRI
• Very abundant in the body
• Solitary proton gives a relatively large magnetic
  moment

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The hydrogen nucleus as a magnet

• The nucleus contains one positively charged
  proton that spins
• The spin of the proton induces a magnetic field
  around it and acts as a small magnet

N
N
S
S
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The magnetic vector

• The magnetic moment of each nucleus has vector
  properties.
• i.e. it has size and direction and is denoted by
  an arrow

size
direction
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Alignment of the magnetic moments

• In the absence of an applied magnetic field the
  magnetic moments are randomly oriented
• When placed in a strong external magnetic field
  the magnetic moments of the hydrogen nuclei align
  with this magnetic field , parallel or
  anti-parallel (as shown in next slide)

26
Alignment of the magnetic moments
Parallel
Anti-parallel
Random alignment in the absence of external
magnetic field
Alignment
External magnetic field
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The state of alignment

• Quantum physics describes that the hydrogen
  nuclei only possesses two energy states or
  populations low high
• Low energy nuclei align their magnetic moments
  parallel to the external magnetic field
• High energy nuclei align their magnetic moments
  anti-parallel to the external magnetic field

28
Energy levels field strength
Low energy population
Energy difference depends on field strength
high energy population
29
Energy levels alignments

• The energy level and the number of nuclei aligned
  in each direction is determined by the strength
  of the external magnetic field and the thermal
  energy level of the nuclei
• Low thermal energy nuclei do not have enough
  energy to oppose the field and align parallel
• High thermal energy nuclei have sufficient energy
  to oppose and may align anti-parallel

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Alignment field strength

• Thermal energy depends on the body temperature
• The main deciding factor to increase the number
  of parallel alignments is the high field strength
  of the external magnetic field
• At thermal equilibrium the parallel population is
  higher than the anti-parallel population
• Therefore there is a net magnetic moment parallel
  to the external magnetic field

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The net magnetization vector
B0
Net Magnetization Vector (NMV)
32
Summary

• The magnetic moment (of hydrogen in this case) is
  called the Net Magnetization Vector (NMV)
• The static external magnetic field is called B0
• The interaction of the NMV with B0 is the basis
  of MRI
• The unit of B0 is Tesla or Gauss.
• 1 Tesla (T) 10000 Gauss (G)

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Summary continued

• When a patient is placed in the bore of the
  magnet the hydrogen nuclei within the patient
  align parallel and anti-parallel to B0.
• A small excess of hydrogen nuclei line up
  parallel to B0 and constitute the NMV of the
  patient.
• The energy difference between the two populations
  increases as B0 increases.
• The magnitude of NMV is larger at high field
  strengths(B0 )

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Precession

• Each hydrogen nucleus that makes up the NMV is
  spinning on its own axis
• The influence of B0 produce an additional spin or
  wobble
• This path is called the precessional path and the
  speed at which the NMV wobbles around B0 is
  called the precessional frequency

Precessional path
B0
Magnetic moment of the nucleus
Precession
Hydrogen nucleus
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Precession continued.

• Two populations
• High energy nuclei spin down
• Low energy nuclei spin up
• Their magnetic moments precess on a circular path
  around B0 as shown

Spin up nuclei
B0
Precession
Spin down nuclei
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The Larmor equation

• The value of the precessional frequency is
  governed by the Larmor equation i.e
• The precessional frequency (?0) Magnetic field
  strength(B0) x Gyro-magnetic ratio(?)
• ? 0 B0 x ?
• Gyro-magnetic ratio is a constant for a specific
  MR active nucleus and is expressed as the
  precssional frequency at 1.0 tesla. The unit is
  MHz / T

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Precessional frequencies of Hydrogen
? B0 ?
1.5 T 63.86 MHz
42.57 Mhz/T 1.0 T 42.57 MHz
0.5 T 21.28 MHz
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Resonance

• Resonance is a phenomenon that occurs when an
  object is exposed to an oscillating perturbation
  that has a frequency close to its own natural
  frequency of oscillation.
• At resonance the object can absorb energy from
  the external source
• Therefore Exchange of energy between two systems
  at a specific frequency is called resonance.

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Nuclear Resonance

• When a nucleus is exposed to an external
  perturbation that has an oscillation similar to
  its own natural frequency, the nucleus gains
  energy from the external force.
• The nucleus gains energy and resonates if the
  energy is delivered at exactly its precessional
  frequency.

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RF signal Nuclear magnetic Resonance

• Energy at the precessional frequency of hydrogen
  at all field strengths in clinical MRI
  corresponds to the radio frequency (RF) band of
  the electromagnetic spectrum
• For resonance of hydrogen to occur, an RF pulse
  of energy at exactly the Larmor frequency of the
  hydrogen NMV must be applied
• Other MR active nuclei that have aligned with B0
  do not resonate because their precessional
  frequencies are different to that of hydrogen

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Excitation RF frequency

• The application of an RF pulse that causes
  resonance to occur is termed excitation.
• The absorption of energy causes an increase in
  the number of spin down hydrogen nuclei
  populations as some of the spin up nuclei gain
  energy via resonance and become high energy
  nuclei (next slide)
• The energy difference corresponds to the energy
  required to produce resonance via excitation

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Energy transfer during excitation
Low energy population
Some nuclei gain energy to join the high energy
population
High energy population
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The results of resonance

• The first result is the NMV moves out of
  alignment away from B0
• The angle to which the NMV moves out of alignment
  is called the flip angle
• The magnitude of the flip angle depends upon the
  amplitude and duration of RF pulse
• Usually the flip angle is 900 (see next slide).
  The transverse NMV rotates at the Larmor frequency

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The flip angle Transverse plane

• B0 is now termed the longitudinal plane
• The plane at 900 to B0 is termed the transverse
  plane

Longitudinal plane
Longitudinal plane
B0
NMV
Flip angle
Flip angle 900
NMV
Transverse plane
Transverse plane
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In phase / out of phase

• The second result of resonance is that the
  magnetic moments within the transverse NMV move
  into phase with each other
• Phase is the position of each magnetic moment on
  the precessional path around B0
• Magnetic moments that are in phase are in the
  same place on the precessional path around B0 at
  any given time
• MM that are out of phase are not in the same
  place on the precessional path

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Phase of magnetic moments around the precessional
path
Out of phase
In phase
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Summary

• For resonance of hydrogen to occur, RF at exactly
  the Larmor frequency of hydrogen must be applied
• The result of resonance is an NMV in the
  transverse plane that is in phase
• This NMV precesses in the transverse plane at the
  Larmor frequency

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The MR signal

• Formation of MR signal after removal of RF pulse

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The MR signal

• As a result of resonance the NMV is precessing in
  phase in the transverse plane.
• According to Faradays laws of induction,
• When a receiver coil (a conductive loop) is
  placed in the area of moving magnetic field a
  voltage is induced in it.
• This Signal is produced when coherent (in phase)
  magnetization cut across the coil.

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MR signal continued.

• Therefore the moving NMV produces magnetic field
  fluctuations inside the coil
• As the NMV precesses at the Larmor frequency in
  the transverse plane a voltage is induced in the
  coil.
• This voltage constitutes the MR signal

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• The frequency of the MR signal is the same as the
  Larmor frequency
• The magnitude of the MR signal depends on the
  amount of magnetization present in the transverse
  plane

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Generation of the MR signal in the receiver coil
B0
Precession of NMV
Receiver coil
NMV
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Relaxation The free induction decay signal

• Switching off RF pulse
• Relaxation
• Recovery decay
• FID

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Relaxation

• When the RF pulse is turned off the NMV is again
  influenced by B0 , and, it tries to realign with
  it.
• To do that it must lose the energy given to it by
  the RF pulse.
• The process by which the NMV loses energy is
  called relaxation

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Recovery Decay

• As relaxation occurs the NMV returns to align
  with B0
• When this happens,
• The amount of magnetization in the longitudinal
  plane gradually increases this is called
  recovery
• The amount of magnetization in the transverse
  plane gradually decreases this is called decay

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The free induction decay signal

• As the magnitude of transverse magnetization
  decreases so does the voltage induced in the
  receiver coil.
• The induction of this reduced signal is called
  the free induction decay (FID) signal

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Result of relaxation

• During relaxation
• The NMV gives up absorbed energy and returns to
  B0
• The magnetic moments of the NMV lose the
  transverse magnetization due to dephasing

Looking down on to transverse plane
In phase
Dephasing
Out of phase
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T1 Recovery T2 Decay

• Relaxation results in
• recovery of magnetization in the longitudinal
  plane
• and
• decay of magnetization in the transverse
  plane.
• The recovery of longitudinal magnetization is
  caused by a process called T1 recovery
• The decay of transverse magnetization is caused
  by a process called T2 decay

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T1 Recovery

• T1 recovery is caused by the nuclei giving up
  their energy to the surrounding environment or
  lattice and it is often termed spin - lattice
  relaxation
• The rate of recovery is an exponential process
  with a recovery time constant called T1

60
Recovery time constant -T1

• T1 is the time it takes 63 of the longitudinal
  magnetization to recover in the tissue

100
63
Signal intensity
T1
Time
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T2 decay

• This is caused by nuclei exchanging energy with
  neighbouring nuclei.
• The energy exchange is caused by the magnetic
  fields of each nucleus interacting with its
  neighbour.
• It is often termed spin-spin relaxation results
  in a decay or loss of transverse magnetization
• The rate of decay is also an exponential process
  so that the T2 relaxation time is its time
  constant of decay

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Time constant of decay T2
100

• T2 is the time it takes 63 of the transverse
  magnetization to be lost

Signal intensity
37
T2
Time
63
Dephasing of the FID

• A signal or voltage is only induced in the
  receiver coil if there is magnetization in the
  transverse plane that is in phase

Dephasing (T2)
Signal (FID)
64
Pulse timing parameters

• The magnitude and timing of the RF pulses form
  the basis of MRI and are discussed in Next lesson