Magnetic Resonance Imaging (MRI)
Assignment
ABSTRACT
The technique of
functional magnetic resonance imaging is rapidly moving from one of technical
interest to wide
clinical application. MRI
stands for Magnetic Resonance Imaging, a noninvasive diagnostic technique that
uses harmless radio waves rather than x-rays to create images. It is
particularly useful for imaging of soft tissues such as the brain, spinal cord,
muscles and ligaments and detecting abnormal tissues.
An
MRI scanner is a cylindrical machine, used to get images of the human body. An
MRI machine consists of a round tunnel within where the patient lies on a
narrow table. An image of this is seen to the right. Surrounding the tube is a
large cylindrical magnet.
In an MRI scanner, the
human body is subjected to a very strong magnetic field and it is made to
absorb and then radiate energy at the Larmor precession frequency of the
hydrogen nuclei present in water. The intensity with which these radiations are
emitted by the body are recorded by the MRI scanner which displays an image of
the portion of the body under test on its monitor. The portions containing
larger amount of water look darker on the screen as compared to those having
little water. If different colours are assigned to different intensities, then
a colored picture can be obtained. Since water content in the bones and teeth
etc. is very small, these parts of the body are nearly invisible and therefore
it is very easy to see through the bones in the case of MRI scanners. For this purpose
along with a few others, makes MRI scanners very useful for medical science.
Introduction
Magnetic resonance
imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance
tomography (MRT) is a medical imaging technique used in radiology to visualize
detailed internal structures. MRI makes use of the property of nuclear magnetic
resonance (NMR) to image nuclei of atoms inside the body.
The property of
Nuclear Magnetic Resonance (NMR) was first described by Purcell and Bloch in 1946, work for which they received the
Nobel prize in 1952. Since then NMR has become a
powerful tool in
the analysis of chemical composition and structure. In 1973 Lauterbur and
Mansfield used
the principles of NMR to describe a technique for determining physical structure.
Since then
Magnetic Resonance Imaging (MRI) has been used in many biomedical, chemical and
engineering
applications.
An MRI machine uses a
powerful magnetic field to align the magnetization of some atoms in the body,
and radio frequency fields to systematically alter the alignment of this
magnetization. This causes the nuclei to produce a rotating magnetic field
detectable by the scanner—and this information is recorded to construct an
image of the scanned area of the body. Strong magnetic field gradients cause
nuclei at different locations to rotate at different speeds. 3-D spatial
information can be obtained by providing gradients in each direction.
During an MRI Scan, the
patient is within a stable magnetic field which is 10,000 - 30,000 times
stronger than the earth's magnetic field. Protons are tiny particles that are
present in water molecules throughout the body. These are aligned by the
incredibly strong magnetic field, noting that there are no water molecules in
the human skeleton, only in bodily tissue. Radio waves are transmitted in
pulses, and these protons produce echoes that are emitted out of the body.
These echoes are received by the MRI scanner, and are then reconstructed into
images of the body by a computer. These images are very precise and give a
clear anatomical view of the body from any angle.
Background
Magnetic Resonance Imaging (MRI) uses the magnetic properties
of hydrogen and its interaction with both a large external magnetic field and
radio waves to produce highly detailed images of the human body.
Some Basic principles of magnetism-
The magnetic properties
of the hydrogen nucleus, and its interaction with the externally applied magnetic field . In its early days, MRI was known as NMR. This stands
for Nuclear Magnetic Resonance.
Although the name has changed (primarily due to the negative connotation of the word
“nuclear”), the basic principles are the same. . We derive our images from the magnetic resonance
properties of nuclear
particles (specifically hydrogen).
In order to perform MRI, we first need a strong magnetic field. The
field
strength of the magnets used
for MR is measured in units of Tesla.
One(1) Tesla is equal to 10,000 Gauss. The magnetic field of the earth
is
approximately 0.5 Gauss. Given
that relationship, a 1.0 T magnet has a magnetic field approximately 20,000 times stronger than
that of the earth. The
type of magnets used for MR imaging usually belongs to one of three types; permanent, resistive,
and superconductive .A permanent magnet is sometimes referred to as a vertical
field magnet. These magnets are constructed of two magnets (one at each pole).
The patient lies on a
scanning table between these two plates. Advantages of these systems are:
1) Relatively low cost, 2) No electricity or
cryogenic liquids are needed to maintain the magnetic field, 3) Their more open
design may help alleviate some patient anxiety, 4) Nearly non-existant fringe
field. It should be noted that not all vertical field magnets are permanent
magnets.
Resistive magnets are
constructed from a coil of wire. The more turns to the coil, and the more
current in the coil, the higher the magnetic field .These types of magnets are
most often designed to produce a horizontal field due to their solenoid design.
Some vertical field systems are based on resistive
magnets. The main advantages of these types of magnets are:
#No liquid
cryogen,
# The ability to “turn off” the magnetic field,
# Relatively small fringe field.
Superconducting magnets
are the most common. They are made from coils of wire (as are resistive
magnets) and thus produce a horizontal field. They use liquid helium to keep
the magnet wire at 4 degrees Kelvin where there is no resistance. The current
flows through the wire without having to be connected to an external power
source. The main advantage of superconducting magnets is their ability to
attain field strengths of up to 3 Tesla for clinical imagers and up to 10 Tesla
or more for small bore spectroscopy magnets.
Magnetic
Properties of Matter
Magnetism
is a fundamental property of matter. The three types of
magnetic properties
are: diamagnetic, paramagnetic, and ferromagnetic.
Outside a magnetic
field, diamagnetic substances exhibit no magnetic properties. When placed in a
magnetic field,diamagnetic substances
will exhibit a negative interaction with the external magnetic field. In other
words they are not attracted to, but rather slightly repelled by the magnetic
field. These substances are said to have a negative magneticsusceptibility
.
Paramagnetic
Substances also exhibit
no magnetic properties outside a magnetic field. When placed in a magnetic
field, however, these substances exhibit a slight positive interaction with the
external magnetic field and are slightly attracted. The magnetic field is
intensified within the sample causing an increase in the local magnetic field.
These substances are said to have a positive
magnetic susceptibility
.
Ferromagnetic
Substances are quite
different. When placed in a magnetic field they exhibit an extremely strong
attraction to the magnetic field. The local magnetic field in the center of the
substance is greatly increased. These substances (such as iron) retain magnetic
properties when removed from the magnetic field. Objects made of ferromagnetic
substances should not be brought into the scan room as they can become
projectiles; being pulled at great speed toward the center of the MR imager. An
object that has become permanently magnetized is referred to as a permanent
magnet. A
permanent magnet, such as a bar magnet, has two poles and is referred to as a
dipole.
Atomic
Structure
The nucleus of an atom
consists of two particles; protons and neutrons. The protons have a positive
charge and the neutrons have a neutral charge. The atomic number represents the
number of protons in the nucleus. The atomic mass number is the total number of
protons and neutrons. Orbiting the nucleus are the electrons, which carry a
negative charge.
All of these particles
are in motion. Both the neutrons and protons spin about their axis. The
electrons, in addition to orbiting the nucleus, also spin about their axis. The
spinning of the nuclear particles produces angular momentum. If an atom has an
even number of both protons and neutrons, then the angular momentum is zero. If
an atom has an uneven number of neutrons or protons, then the atom has a
certain angular momentum. The angular momentum is expressed as a vector
quantity having both magnitude and direction.
In addition to spin
angular momentum, certain nuclei exhibit magnetic properties. Because a proton
has mass, a positive charge, and spins, it produces a small magnetic field much
like a bar magnet. This small magnetic field of the proton is referred to as
the magnetic moment. The magnetic moment is also a vector quantity having
both magnitude and direction and is oriented in the same direction as the
angular momentum. The ratio between the angular momentum and the magnetic
moment gives us a constant known as the gyromagnetic ratio, which is specific
to each magnetically active nuclei. There are several nuclei, which are
magnetically active.Hydrogen has a significant magnetic moment and is nearly
100% abundant in the human body. For these reasons, we use only the hydrogen
proton in routine clinical imaging, and that is where we will focus our
attention from here on.The nucleus of the hydrogen atom contains a single
proton. Because ofthis, as previously mentioned, it possesses a significant
magnetic moment. The proton will behave as a tiny bar magnet.
Creating
an MR Signal
A radio wave is
actually an oscillating electromagnetic field. The RF field is also referred to
as the B1 field. It is oriented perpendicular to the main magnetic field. If we
apply a pulse of RF energy into the tissue at the Larmor frequency, we first
find the individual spins begin to precess in phase, as will the net
magnetization vector. As the RF pulse continues, some of the spins in the lower
energy state absorb energy from the RF field and make a transition into the
higher energy state. This has the effect of “tipping” the net magnetization
toward the transverse plane.
For the purpose of this
explanation, we will assume sufficient energy isapplied to produce a 90-degree
flip of the net magnetization. In such an example, it is said that a 90-degree flip
angle, or a 90-degree pulse has
been applied
Application
MRIs are a relatively
new technology to hit the medical world, and have completely revolutionized
medical imaging and the diagnosing process as we know it. In-vivo images can be
taken of the human body, meaning that internal images can be seen without making
any incisions. Completely non-intrusive procedures are used, which makes MRI's
very effective, but somewhat expensive, for doctors to use.
MRIs are administered
to patients suffering from the following:
#inflammation or
infection in an organ
#degenerative diseases
#strokes
#musculoskeletal
disorders
#tumors
#other irregularities
that exist in tissue or organs in their body
High-resolution images
of organs or any area of the body can be made without the need for using x-rays
because MRIs use radio frequency (RF) light. Since they use RF light, MRIs do
not present any known health risks to the patients; however anyone with metal
implants could not receive a MRI. If a person's nervous system needed to be
studied, an MRI image would be the best imaging method to use, especially if
the brain or spinal cord needed to be investigated.
Functional MRI's are
done to determine which parts of the brain have control over which uses of the
human body. These MRIs are critical in determining motor imagery, speech
portions of the brain, and diagnosing which parts of the brain may be affected
by a tumor. Some operations are deferred because a portion of the brain that is
vital may be removed, and this is only determined via functional MRIs.
Benefits of MRI
The MRI scan is a
painless and safe scan that produces clearer images of the body and its
tissues, at any angle. This is particularly useful in detecting soft tissue
tumours throughout the body. An MRI is nearly twice as sensitive as X-ray
mammography in detecting breast cancer in women that have a high genetic risk
of the disease. It uses no radiation for scanning and therefore eliminates the
health risk of x-rays that do use radiation.
Risks of MRI
While an MRI scan is a
relatively safe procedure as there is no damaging radiation involved, there are
still several risks. If the patient is pregnant, or suspects they may be, a
doctor should be informed because the effects on an unborn baby are poorly
understood. There is also the risk of patients being injured if they forget to
remove pieces of metal from their body or their clothing. There have been cases
where patients have been injured due to metal left behind by the previous
patient. If sedation is required due to claustrophobia, then there are associated
risks of over-medication. If a contrast dye is used, which helps to show up
some parts of the body more clearly, there is a small risk of allergic
reaction.
Limitations of MRI
An MRI is a very
expensive and time consuming investigation compared to other methods such as
x-ray and CT scan. Some parts of the body, like bone, are better examined using
simpler techniques such as an X-Ray. An MRI may not always be able to tell the
difference between some disease processes. It is also not a very good investigation
for emergencies or accidents because of the long time it takes and the fact
that all equipment has to be removed from the room while the machine is
running.
Specialized Types of
MRI
Magnetic Resonance
Angiography (MRA): This type of MRI is designed to show the blood vessels. It
is most often used to examine the arteries and veins of the head, neck, brain
and heart. Usually, a contrast dye is injected, which cannot leave the blood
vessels meaning that they show up much brighter than the areas around them. An example
of this technique is seen to the right. Functional MRI: This type of MRI is
done on the brain, and not only shows the structure of the brain, but also how
much activity is taking place in each part. This has been used to find out what
parts of the brain are most active during certain situations or tasks. Cardiac
MRI: This can be used for several different conditions and is dealt with
separately.
Conclusion
It is possible that an
MRI may show that everything is completely normal; however, there are several
things that could be seen on an MRI and this will vary depending on where in
the body the scan is being done. An MRI is very good at showing up problems
with soft tissues such as muscles and ligaments and is the most sensitive
investigation for spinal and joint problems. For this reason, MRI is often used
in sports-related injuries, allowing the doctor to see even very small tears or
areas of swelling and inflammation. Almost all other organs can be examined in
some detail using an MRI, showing problems in structure and function. MRI is
most often used to show problems with the soft tissues of the body which can be
genetic or caused by some disease process like a tumor. This includes areas
like the brain, and an MRI can give a particularly clear image of the brain's
structure. This is particularly useful for conditions such as a brain tumor
where an MRI is often used to find out exactly where in the brain it is,
allowing much more effective surgery.
