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Magnetic
Resonance Imaging
MRI
Magnetic Resonance Imaging
MRI uses the
interaction between the
magnetic properties of
hydrogen nuclei,
external magnetic
fields and
electromagnetic
radiation to obtain data
used produce the
image.
To produce the strong
magnetic field required
for MRI, the scanners
require the use of a
superconducting
magnet that needing a
liquid helium coolant.
Magnetic Resonance Imaging
MRI machines look similar to CT and PET scanners
…but they operate on totally different principles
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MRI was originally called nuclear magnetic
resonance imaging (NMRI). The word nuclear was
dropped for two reasons …. Why do you think this
was?
• The term “nuclear” suggests that ionising radiation
is involved - this is not the case. Since the term is
potentially misleading, it was dropped.
• There are negative associations for many people
with the word “nuclear”. The term was dropped as a
marketing strategy to make MRI more acceptable to
the public (and to save doctors the time of having to
explain to patients that the process is perfectly safe.
•
Magnetic Resonance Imaging
The strong magnetic fields produced
by MRI machines results in unusual
occupational risks!
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Question
Identify the three interacting
factors essential to the
principle used in MRI.
• The magnetic property of the protons
• The strong external magnetic field
• An electromagetic wave
Magnetic Resonance Imaging Advantages
 MR does not involve the use of ionising radiation with its
associated risks to the patient and the medical staff
 It is non-invasive
 MRI provides excellent soft tissue imaging, providing better
contrast than CT or conventional x–rays and much better
resolution than ultrasound
 MRI data can be processed to produce a tomographic image
or a 3-D image
 Except in the case of a few patients who experience anxiety
in the MR tube because of the confined space, there is no
discomfort to the patient
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• Both protons and neutrons
in the nuclei of atoms have
a property called spin
• This spin property can have
one of two possible directions
for any given nucleon and
alignment of the axis
• If there are even numbers of
protons or neutrons, then their
spins of each pair align in
opposite directions so that the
net spin of the pair is zero
Magnetic Resonance Imaging
• If there is an odd number of protons or neutrons,
then the nucleus must possess a net spin since
pairing cannot occur with one of the nucleons
• Nuclei having a
net spin include
hydrogen,
phosphorus 31,
fluorine 19,
nitrogen 15 and
carbon 13
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• In addition to the spin of the
nucleons, electrons also
have the property of spin
• The electron has a spin
associated with both its
orbital motion and its axial
rotation
• The spin of the electron is
small in magnitude
compared with that of the
much larger and more
massive nucleons
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Magnetic Resonance Imaging
I
A current loop creates a magnetic field
Magnetic Resonance Imaging
A spinning charge behaves
like a current loop,
creating a north
and a south
magnetic pole
Protons spin on their
axis, creating a pair
of magnetic poles
The proton’s behaviour
in an external magnetic
field is determined by its
magnetic moment
Magnetic Resonance Imaging
In atomic nuclei,
paired protons
having opposite
spins result in a zero
net magnetic effect.
Hydrogen having
one proton must
have a non-zero
magnetic moment.
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In materials containing many protons, such as
hydrogen rich compounds including water, the
protons have randomly oriented magnetic fields.
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A strong external
magnetic field
causes the
magnetic moments
of the protons to
become aligned
Magnetic field
Magnetic Resonance Imaging
The alignment of the magnetic moment of the protons
is not exactly in the same direction as the external
magnetic field
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Proton spins usually become aligned parallel or the
external magnetic field
A few proton spins become
aligned antiparallel to the
external field
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The angle between the
magnetic moment of
the proton and the
external magnetic field
produces a torque on
the proton.
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The effect of the
torque is to cause the
axis of rotation of the
proton to precess.
This is the same effect
that occurs with a
spinning top if the axis
of rotation is not
parallel to the Earth’s
gravitational field.
Magnetic Resonance Imaging
Precession is the motion that
results in the axis of rotation
of a body sweeping out a
conical motion when a torque
acts to affect a change in the
axis of rotation of that body.
Precession is a consequence
of the law of conservation of
angular momentum.
The frequency of precession is
called the Larmor frequency.
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The Larmor frequency depends on
• composition of the nucleus (only hydrogen is used in MRI)
• magnitude of the external field
For a proton in a 2 T
magnetic field, the Larmor
frequency is 85.2 MHz.
This corresponds to a
radio frequency (RF)
electromagnetic wave.
Magnetic Resonance Imaging
To utilise the magnetic
properties of the
proton to produce
medical images…
The patient is first
placed in a very
strong magnetic field
The magnetic field,
although very strong, is
non-ionising and low risk
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The RF oscillator and
receiver are usually a
single unit, capable of
emitting and receiving
pulses of radio frequency
electromagnetic radiation
The supercooled magnets
are electromagnets using
superconductors requiring
liquid helium to reach the
superconducting transition
temperature.
Only superconducting
electromagnets are
capable of producing the
strong magnetic fields
required to align the proton
magnetic moments.
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When protons in a strong magnetic field radio are exposed to
radio waves with a frequency equal to the Larmor frequency,
their energy is absorbed by the protons in a process called
resonance.
This causes the
proton’s magnetic
alignment to flip from
the parallel state, a
phenomenon referred
to as spin flip.
Magnetic Resonance Imaging
It is energetically more favourable for hydrogen nuclei to
return to their original state in the external magnetic field
after the RF pulse. As they do so, they re-emit the energy
absorbed from the radio wave in about 0.01 to 0.1 seconds.
The emitted energy is a
radio wave that is detected
with the same coil that
emitted the RF waves to flip
the protons.
The signals emitted by the
proton are used to create
the MR image.
No external field
Random orientation
External field
Alignment
Precession
RF pulse
Spin flip
Relaxation
RF emission
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The amplitude of the signal
produced as the nuclei relax
increases with the number of
nuclei present
Magnetic resonance imaging
results in the production of a
map of the hydrogen density
throughout a volume of the
patient.
The signal strength is greater
from tissues having a greater
density of hydrogen nuclei.
Magnetic Resonance Imaging
The use of hydrogen in MRI
MRI uses hydrogen because
• Hydrogen has a magnetic moment because the protons are
unpaired
• A strong signal from the nuclear relaxation is possible
because hydrogen is abundant in human tissues in
• Water
• Proteins
• Fats
• Carbohydrates
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Gradient coils produce small
variations in the magnetic
field across the patient’s body
so that the magnetic field
intensity has a unique value
at every point in the patient’s
body.
Thus, the hydrogen atoms at
each point have a known, and
unique Larmor frequency.
Both the exact position and
the corresponding Larmor
frequency are accurately
known.
Magnetic Resonance Imaging
The pulse of radio waves are
transmitted through the
patient’s body from the RF
coils.
This flips the magnetic axes
of the hydrogen nuclei.
As they flip back (relaxation)
the atoms within each volume
element (voxel) emit radio
waves.
The intensity is proportional
to the number of hydrogen
atoms in the voxel.
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• Data is thus gathered
relating the location
of the voxel to the
hydrogen atom
concentration.
• From this data the MR
image is computed.
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Heart and associated
blood vessels
Blood vessels in the brain
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Explain that large differences would occur in the relaxation
time between tissue containing hydrogen bound water
molecules and tissues containing other molecules
The difference in relaxation times is significantly greater
for tissues containing relatively large amounts of water
because of the hydrogen atoms present.
MRI is very sensitive to variations in water content of
tissues and this is a significant factor in its being able to
produce high resolution high contrast images.
Because tumours are characterised by rapid cell division
and high growth rates, they typically have a higher
percentage of water than similar non-cancerous tissue, and
can thus be clearly imaged using MR.
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Explain that large differences would occur in the relaxation
time between tissue containing hydrogen bound water
molecules and tissues containing other molecules
Haemoglobin molecules in red blood cells provides a
strong resonance signal, and so MRI can be used to
compare the blood content of different tissues. This is often
greater in cancerous tissue, because of the high growth
rates, and so MRI is an effective diagnostic tool for
cancerous tissue.
MRI scans of the brain show more contrast and detail than
conventional x-ray or CT scans because of the differences
in water content of the grey matter and white matter of the
brain. Excellent soft tissue resolution can also be achieved
using MRI. Colour enhancement provides clearer analysis.
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Magnetic Resonance Imaging
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Magnetic Resonance Imaging
PET SCAN
A word from the creator
This Powerpoint presentation was prepared by
Greg Pitt of Hurlstone Agricultural High School.
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