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Magnetic Resonance Imaging (MRI)
How MRI Works
Dr.
Raymond Damadian, a physician and scientist, toiled for years trying to produce
a machine that could noninvasively scan the body with the use of magnets.
Along
with some graduate students, he constructed a superconducting magnet and
fashioned a coil of antenna wires.
Since
no one wanted to be the first one in this contraption, Damadian volunteered to
be the first patient.
When
he climbed in, however, nothing happened. Damadian was looking at years wasted
on a failed invention, but one of his colleagues bravely suggested that he
might be too big for the machine.
A
svelte graduate student volunteered to give it a try, and on July 3, 1977, the
first MRI exam was performed on a human being.
It
took almost five hours to produce one image, and that original machine, named
the "Indomitable," is now owned by the Smithsonian Institution.
In
just a few decades, the use of magnetic resonance imaging(MRI)
scanners has grown tremendously.
Doctors
may order MRI scans to help diagnose multiple sclerosis, brain tumors, torn
ligaments, tendonitis, cancer and strokes, to name just a few.
An
MRI scan is the best way to see inside the human body without cutting it open.
That
may be little comfort to you when you're getting ready for an MRI exam.
You're
stripped of your jewelry and credit cards and asked detailed questions about
all the metallic instruments you might have inside of you.
You're
put on a tiny slab and pushed into a hole that hardly seems large enough for a
person.
You're
subjected to loud noises, and you have to lie perfectly still, or they're going
to do this to you all over again.
And
with each minute, you can't help but wonder what's happening to your body while
it's in this machine.
Could
it really be that this ordeal is truly better than another imaging technique,
such as an X-ray or a CAT scan? What has Raymond Damadian wrought?
Let
the magnets of this mighty machine draw you to the next page, and we'll take a
look at what's going on inside.
MRI Magnets: the Major Players
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 (60 centimeters) in diameter [source: Hornak].
But
what's in there? 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 0.5-tesla 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 (269.1 below zero degrees Celsius) [source: Coyne].
This
cold is insulated by a vacuum. While superconductive magnets are expensive, the
strong magnetic field allows for the highest-quality imaging, and
superconductivity keeps the system economical to operate.
The Other Parts of an MRI Machine
MRI Developments
MRI
machines are evolving so that they're more patient-friendly.
For example, many claustrophobic people
simply can't stand the cramped confines, and the bore may not accommodate obese
people.
There are more open scanners,
which allow for greater space, but these machines have weaker magnetic fields,
meaning it may be easier to miss abnormal tissue.
Very small scanners for imaging
specific body parts are also being developed.
Other
advancements are being made in the field of MRI.
Functional MRI (fMRI),
for example, creates brain maps of nerve cell activity second by second
and is helping researchers better understand how the brain works.
Magnetic resonance angiography (MRA) creates images of flowing
blood, arteries and veins in virtually any part of the body.
Two
other magnets are used in MRI systems to a much lesser extent.
Resistive magnets are
structurally like superconducting magnets, but they lack the liquid helium.
This difference means they require a huge amount of electricity, making it
prohibitively expensive to operate above a 0.3 tesla level.
Permanent magnets have
a constant magnetic field, but they're so heavy that it would be difficult to
construct one that could sustain a large magnetic field.
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, 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.
Hydrogen Atoms and Magnetic Moments
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, like
a child's top.
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. It's these unmatched atoms that
we're concerned with now.
What Else Is Going on in an MRI Scan?
Next, the MRI machine applies a radio
frequency (RF) pulse that is specific only to hydrogen.
The
system directs the pulse toward the area of the body we want to examine.
When
the pulse is applied, the unmatched protons absorb the energy and spin again in
a different direction. This is the "resonance" part of MRI.
The
RF pulse forces them to spin at a particular frequency, in a particular
direction.
The
specific frequency of resonance is called the Larmour
frequency and is calculated based on the particular tissue
being imaged and the strength of the main magnetic field.
At
approximately the same time, the three gradient magnets jump into the act.
They
are arranged in such a manner inside the main magnet that when they're turned
on and off rapidly in a specific manner, they alter the main magnetic field on
a local level.
What
this means is that we can pick exactly which area we want a picture of; this
area is referred to as the "slice."
Think
of a loaf of bread with slices as thin as a few millimeters -- the slices in
MRI are that precise.
Slices
can be taken of any part of the body in any direction, giving doctors a
huge advantage over any other imaging modality.
That
also means that you don't have to move for the machine to get an image from a
different direction -- the machine can manipulate everything with the gradient
magnets.
But
the machine makes a tremendous amount of noise during a scan, which sounds like
a continual rapid hammering.
That's
due to the rising electrical current in the wires of the gradient magnets being
opposed by the main magnetic field.
The
stronger the main field, the louder the gradient noise. In most MRI centers,
you can bring a music player to drown out the racket, and patients are given
earplugs.
When
the RF pulse is turned off, the hydrogen protons slowly return to their natural
alignment within the magnetic field and release the energy absorbed from the RF
pulses.
When
they do this, they give off a signal that the coils pick up and send to the
computer system. But how is this signal converted into a picture that means
anything?
MRI Images and How They're Made
The MRI scanner can pick out a very small point inside
the patient's body and ask it, essentially, "What type of tissue are
you?"
The
system goes through the patient's body point by point, building up a map of
tissue types.
It
then integrates all of this information to create 2-D images or 3-D models with
a mathematical formula known as the Fourier transform.
The
computer receives the signal from the spinning protons as mathematical data;
the data is converted into a picture. That’s the "imaging" part of
MRI.
The
MRI system uses injectable contrast, or dyes,
to alter the local magnetic field in the tissue being examined.
Normal
and abnormal tissue respond differently to this slight alteration, giving us
differing signals. These signals are transferred to the images; an MRI system
can display more 250 shades of gray to depict the varying tissue [source: Coyne].
The
images allow doctors to visualize different types of tissue abnormalities
better than they could without the contrast. We know that when we do
"A," normal tissue will look like "B" -- if it doesn't,
there might be an abnormality.
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.
And,
as we mentioned on the last page, another major advantage of MRI is its ability
to image in any plane.
Computer
tomography (CT), for example, is limited to one plane, the axial plane
(in the loaf-of-bread analogy, the axial plane would be how a loaf of bread is
normally sliced).
An
MRI system can create axial images as well as sagitall(slicing
the bread side-to-side lengthwise) and coronal (think
of the layers in a layer cake) images, or any degree in between, without the
patient ever moving.
But
for these high-quality images, the patient can't move very much at all. MRI
scans require patients to hold still for 20 to 90 minutes or more.
Even
very slight movement of the part being scanned can cause distorted images that
will have to be repeated.
And
there's a high cost to this kind of quality; MRI systems are very expensive to
purchase, and therefore the exams are also very expensive.
But
are there any other costs? What about the patient's safety?
MRI Safety Concerns
Maybe you're concerned about the long-term impact of
having all your atoms mixed about, but once you're out of the magnetic field,
your body and its chemistry return to normal.
There
are no known biological hazards to humans from being exposed to magnetic fields
of the strength used in medical imaging today.
The
fact that MRI systems don’t use ionizing radiation, as other imaging devices
do, is a comfort to many patients, as is the fact that MRI contrast materials
have a very low incidence of side effects.
Most
facilities prefer not to image pregnant women, due to limited research of the
biological effects of magnetic fields on a developing fetus.
The
decision of whether or not to scan a pregnant patient is made on a
case-by-case basis with consultation between the MRI radiologist and the
patient's obstetrician.
However,
the MRI suite can be a very dangerous place if strict precautions are not
observed.
Credit
cards or anything else with magnetic encoding will be erased.
Metal
objects can become dangerous projectiles if they are taken into the scan room.
For example, paperclips, pens, keys, scissors, jewelry, stethoscopes and any
other small objects can be pulled out of pockets and off the body without
warning, at which point they fly toward the opening of the magnet at very high
speeds.
Big
objects pose a risk, too -- mop buckets, vacuum cleaners, IV poles, patient
stretchers, heart monitors and countless other objects have all been pulled
into the magnetic fields of the MRI.
In
2001, a young boy undergoing a scan was killed when an oxygen tank was pulled
into the magnetic bore [source: McNeil].
Once,
a pistol flew out of a policeman's holster, the force causing the gun to fire.
No one was injured.
To
ensure safety, patients and support staff should be thoroughly screened for
metal objects prior to entering the scan room.
Often,
however, patients have implants inside them that make it very dangerous for
them to be in the presence of a strong magnetic field.
These
include:
· Metallic fragments in the eye, which are very
dangerous as moving these fragments could cause eye damage or blindness
· Pacemakers, which may malfunction during a scan or
even near the machine
· Aneurysm clips in the brain, which could tear the very
artery they were placed on to repair if the magnet moves them
· Dental implants, if magnetic
Most
modern surgical implants, including staples, artificial joints and stents are
made of non-magnetic materials, and even if they're not, they may be approved
for scanning.
But
let your doctor know, as some orthopedic hardware in the area of a scan can
cause distortions in the image.
About Molly Edmonds
As a precocious child growing up in the mountains of North
Carolina, Molly Edmonds would often prepare miniature school lessons for her
younger brothers. If her brothers didn't want to play school, Molly sat on them
to prevent possible escape and continued with her lesson. Fortunately for the
readers of HowStuffWorks.com and for the listeners of the "Stuff Mom Never Told You" podcast, Molly has learned over the
years that physical harm is not a good way to provide information. A graduate
of Emory University, where she majored in creative writing and political
science, Molly spends her spare time seeking out good books, live music and
people who will cook for her.
https://science.howstuffworks.com/mri.htm
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