Download Technical Description of an MIR Magnetic resonance imaging (MRI

Technical Description of an MIR
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Technical Description of an MIR
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Technical Description of an MIR
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Technical Description of an MIR
Magnetic resonance imaging (MRI) is a
medical imaging technique used in
radiology to investigate the anatomy and
function of the body in both health and
disease. MRI scanners use strong magnetic
fields and radio-waves to form images of the
body. The technique is widely used in
hospitals for medical diagnosis, staging of
disease and for follow-up without exposure
to ionizing radiation.
MRI scanners vary in size and shape, and
some newer models have a greater degree of
openness around the sides. Still, the basic
design is the same, and the patient is pushed
into a tube that's only about 24 inches in
diameter
The biggest and most important component
of an MRI system is the magnet. There is a
horizontal tube -- the same one the patient
enters -- running through the magnet from
front to back. This tube is known as the
bore. But this isn't just any magnet -- we're
dealing with an incredibly strong system
here, one capable of producing a large,
stable magnetic field.
The strength of a magnet in an MRI system
is rated using a unit of measure known as a
tesla. Another unit of measure commonly
used with magnets is the gauss (1 tesla =
10,000 gauss). The magnets in use today in
MRI systems create a magnetic field of 0.5tesla to 2.0-tesla, or 5,000 to 20,000 gauss.
When you realize that the Earth's magnetic
field measures 0.5 gauss, you can see how
powerful these magnets are.
Most MRI systems use a superconducting
magnet, which consists of many coils or
windings of wire through which a current of
electricity is passed, creating a magnetic
field of up to 2.0 tesla. Maintaining such a
large magnetic field requires a good deal of
energy, which is accomplished by
superconductivity, or reducing the resistance
in the wires to almost zero. To do this, the
wires are continually bathed in liquid helium
at 452.4 degrees below zero Fahrenheit. This
cold is insulated by a vacuum. While
superconductive magnets are expensive, the
strong magnetic field allows for the highestquality imaging, and superconductivity
keeps the system economical to operate.
There are also three gradient magnets inside
the MRI machine. These magnets are much
lower strength compared to the main
magnetic field. They may range in strength
from 180 gauss to 270 gauss. While the
main magnet creates an intense, stable
magnetic field around the patient, the
gradient magnets create a variable field,
which allows different parts of the body to
be scanned.
Another part of the MRI system is a set of
coils that transmit radiofrequency waves
into the patient's body. There are different
coils for different parts of the body: knees,
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Technical Description of an MIR
shoulders, wrists, heads, necks and so on.
These coils usually conform to the contour
of the body part being imaged, or at least
reside very close to it during the exam.
Other parts of the machine include a very
powerful computer system and a patient
table, which slides the patient into the bore.
Whether the patient goes in head or feet first
is determined by what part of the body needs
examining. Once the body part to be
scanned is in the exact center, or isocenter,
of the magnetic field, the scan can begin.
When patients slide into an MRI machine,
they take with them the billions of atoms
that make up the human body. For the
purposes of an MRI scan, we're only
concerned with the hydrogen atom, which is
abundant since the body is mostly made up
of water and fat. These atoms are randomly
spinning, or precessing, on their axis. All of
the atoms are going in various directions,
but when placed in a magnetic field, the
atoms line up in the direction of the field.
These hydrogen atoms have a strong
magnetic moment, which means that in a
magnetic field, they line up in the direction
of the field. Since the magnetic field runs
straight down the center of the machine, the
hydrogen protons line up so that they're
pointing to either the patient's feet or the
head. About half go each way, so that the
vast majority of the protons cancel each
other out -- that is, for each atom lined up
toward the feet, one is lined up toward the
head. Only a couple of protons out of every
million aren't canceled out. This doesn't
sound like much, but the sheer number of
hydrogen atoms in the body is enough to
create extremely detailed images.
An X-ray is very effective for showing
doctors a broken bone, but if they want a
look at a patient's soft tissue, including
organs, ligaments and the circulatory
system, then they'll likely want an MRI.
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