Title: Magnetic Resonance Imaging
1Magnetic Resonance Imaging
- Basic Principles
- V.G.Wimalasena Principal
- School of Radiography
2Introduction
Modern 3 Tesla MRI unit
Bore of the magnet
RF Coil (for head)
Main magnet body
Patient Couch
3What 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.
4MRI 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.
5Uses 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.
6History
- 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
7Brief 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.
8- 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.
10- 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.
11Use 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.
12Safety 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.
13Uses 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.
14System 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
15Explaining 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
16Atomic structure
- Central nucleus orbiting electrons
- Nucleus
- Nucleons
- (Protons neutrons)
- Atomic number
- Mass number
- Electrically stable
17Motion 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
18- The principles of MRI rely on the spinning motion
of specific nuclei present in biological tissues - These are called (MR active nuclei)
19MR 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
20MR 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
21The 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
22The 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
23The 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
24The 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
25Alignment 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)
26Alignment of the magnetic moments
Parallel
Anti-parallel
Random alignment in the absence of external
magnetic field
Alignment
External magnetic field
27The 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
28Energy levels field strength
Low energy population
Energy difference depends on field strength
high energy population
29Energy 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
30Alignment 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
31The net magnetization vector
B0
Net Magnetization Vector (NMV)
32Summary
- 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)
33Summary 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 )
34Precession
- 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
35Precession 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
36The 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
37Precessional 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
38Resonance
- 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.
39Nuclear 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.
40RF 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
41Excitation 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
42Energy transfer during excitation
Low energy population
Some nuclei gain energy to join the high energy
population
High energy population
43The 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
44The 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
45In 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
46Phase of magnetic moments around the precessional
path
Out of phase
In phase
47Summary
- 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
48The MR signal
- Formation of MR signal after removal of RF pulse
49The 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.
50MR 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
51- 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
52Generation of the MR signal in the receiver coil
B0
Precession of NMV
Receiver coil
NMV
53Relaxation The free induction decay signal
- Switching off RF pulse
- Relaxation
- Recovery decay
- FID
54Relaxation
- 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
55Recovery 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
56The 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
57Result 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
58T1 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
59T1 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
60Recovery 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
61T2 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
62Time constant of decay T2
100
- T2 is the time it takes 63 of the transverse
magnetization to be lost
Signal intensity
37
T2
Time
63Dephasing 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)
64Pulse timing parameters
- The magnitude and timing of the RF pulses form
the basis of MRI and are discussed in Next lesson