Transcript Magnetic Resonance Imaging

Magnetic Resonance
Imaging
Basic Principles
V.G.Wimalasena
Principal
School of Radiography
Introduction
Modern 3 Tesla
MRI unit
Main
magnet
body
Patient Couch
Bore of
the
magnet
RF Coil
(for head)
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
System components
Magnet power supply
Shim power supply
Gradient amplifiers
RF transmitter
Operator
consol
Magnet coils
Shim coils
Gradient coils
RF coils
Host
computer
Magnet
bore
Image
processor
Image disk
RF receiver
Digitizer
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
Atomic structure
Central nucleus &
orbiting electrons
Nucleus
– Nucleons
(Protons & neutrons)
Atomic number
Mass number
Electrically stable
Motion within the atom
There are three
types of motion
within an atom
1. Electrons spinning
on their own axis
2. Electrons orbiting
the nucleus
3. The nucleus
spinning about its
own axis
 The principles of MRI
rely on the spinning
motion of specific
nuclei present in
biological tissues
 These are called (MR
active nuclei)
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
MR active nuclei continued..
Important Examples
The nuclei with odd
mass numbers
undergoes this
interaction
The result of this
interaction is angular
momentum or spin
Hydrogen
Carbon
Nitrogen
Oxygen
Fluorine
Sodium
Phosphorus
1
13
15
17
19
23
31
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
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
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
S
N
S
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
direction
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)
Alignment of the magnetic moments
Parallel
Random alignment in the
absence of external
magnetic field
Anti-parallel
Alignment
External
magnetic
field
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
Energy levels & field strength
Energy
difference
depends on
field
strength
Low energy population
high energy population
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 antiparallel
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
The net magnetization vector
B0
Net
Magnetization
Vector (NMV)
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)
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 )
Precession
Each hydrogen nucleus
that makes up the NMV
Precessional path
is spinning on its own
axis
The influence of B0
B0
produce an additional
spin or wobble
Magnetic
Precession
This path is called the
moment
precessional path and
of the
the speed at which the
nucleus
NMV wobbles around
B0 is called the
precessional frequency
Hydrogen nucleus
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
The Larmor equation
The value of the precessional frequency is
governed by the Larmor equation i.e
The precessional frequency (ω0) = Magnetic
field strength(B0) x Gyro-magnetic ratio(γ)
ω 0 = B0 x γ
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
Precessional frequencies of
Hydrogen
γ
B0
ω
1.5 T
63.86 MHz
42.57 Mhz/T 1.0 T
42.57 MHz
0.5 T
21.28 MHz
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.
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.
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
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
Energy transfer during excitation
Low energy
population
Some nuclei gain
energy to join the high
energy population
High energy
population
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
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
Flip
angle
NMV
Flip angle
900
NMV
Transverse plane
Transverse plane
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
Phase of magnetic moments
around the precessional path
Out of
phase
In phase
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
The MR signal
Formation of MR signal after
removal of RF pulse
The MR signal
As a result of resonance the NMV is
precessing in phase in the transverse
plane.
According to Faraday’s 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.
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
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
Generation of the MR signal in the
receiver coil
B0
NMV
Receiver
coil
Relaxation & The free
induction decay signal
Switching off RF pulse
Relaxation
Recovery & decay
FID
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
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
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
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
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
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
Recovery time constant -T1
100%
63%
Signal intensity
T1 is the time
it takes 63% of
the longitudinal
magnetization
to recover in
the tissue
T1
Time
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
Time constant of decay – T2
Signal intensity
T2 is the time
it takes 63% of
the transverse
magnetization
to be lost
100%
37%
T2
Time
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)
Pulse timing parameters
The magnitude and timing of the
RF pulses form the basis of MRI
and are discussed in Next lesson