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Introduction
 On
July 3, 1977, the first MRI exam was performed on a human
being. It took almost five hours to produce one
image. Dr. Raymond Damadian, a physician and scientist,
along with colleagues Dr. Larry Minkoff and Dr. Michael
Goldsmith, labored for seven years to reach that point.
They named their original machine "Indomitable."
This machine is now in the Smithsonian Institution. As
late as 1982, there were a handful of MRI scanners in the
United States. Today there are thousands, and images can
be created in seconds what used to take hours.
The basic
design of an MRI machine resembles a cube, typically
measuring 7 feet tall by 7 feet wide by 10 feet long,
although new models are rapidly shrinking. There is
a horizontal tube running from front to back through the
center of the machine which houses an extraordinary strong
magnet. This tube is known as the bore of the magnet. The
patient, lying on his or her back, slides into the bore on
a special table. Whether or not the patient goes in
head first or feet first, as well as how far in the magnet
they will go, is determined by the type of exam to be
performed. MRI scanners vary in size and shape, and
newer or specially designed models have some degree of
openness around the sides, but the basic design is the
same. Once the body part to be scanned is in the
exact center or isocenter of the magnetic field, the scan
can begin.
In
conjunction with radio wave pulses of energy, 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 point might be a cube that is
half a millimeter on each side. The MRI system goes
through the patient's body point by point, building up a
2-D or 3-D map of tissue types. It then integrates
all of this information together to create 2-D images or
3-D models.
MRI
provides an unparalleled view inside the human body. The
level of detail we can see is extraordinary compared with
any other imaging modality. MRI is the method of
choice for the diagnosis of many types of injuries and
conditions because of the incredible ability to tailor the
exam to the particular medical question being asked.
By changing exam parameters, the MRI system can cause
tissues in the body to assume different appearances.
This is very helpful to radiologists who read MRIs in
determining if something seen is normal or not. MRI
systems can also image flowing blood in virtually any part
of the body. This allows us to perform studies that show
the arterial system in the body, but not the tissue around
it. In many cases, the MRI system can do this
without a contrast injection, which is required in
vascular radiology.
Magnetic
Intensity
The biggest and most important component in an MRI system
is the magnet. The magnet in an MRI system is rated using
a unit of measure known as a tesla. The magnets in use
today in MRI are generally in the 0.5-tesla to 3.0-tesla
range.
Safety
 Prior
to allowing a patient or support staff member into the
scan room, he or she is thoroughly screened for metal
objects. Often however, patients have implants
inside them that make it very dangerous for them to be in
the presence of a strong magnetic field. People with
pacemakers cannot be scanned or even go near the scanner
because the magnet can cause the pacemaker to malfunction.
Aneurysm clips in the brain can be very dangerous as the
magnet can move them, causing them to tear the very artery
they were placed on to repair. Some dental implants are
magnetic. Most orthopedic implants, even though they
may be ferromagnetic, are fine because they are firmly
embedded in bone. Even metal staples in most parts
of the body are fine -- once they have been in a patient
for a few weeks, enough scar tissue has formed to hold
them in place. Each time we encounter patients with
an implant or metallic object inside their body, we
investigate thoroughly to make sure it is safe to scan
them. There are no known biological hazards to humans from
being exposed to magnetic fields of the strength used in
medical imaging today. Most facilities prefer not to image
pregnant women. This is due to the fact that there
has not been much research done in the area of biological
effects 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.
The Magnets
There are three basic types of magnets used in
MRI systems:
- Resistive
magnets consist of many windings or coils of wire
wrapped around a cylinder or bore through which an
electric current is passed. This causes a magnetic
field to be generated. If the electricity is turned
off, the magnetic field dies out. These magnets are
lower in cost to construct than a superconducting
magnet (see below), but require huge amounts of
electricity (up to 50 kilowatts) to operate because of
the natural resistance in the wire.
- A
permanent magnet's magnetic field is always there and
always on full strength, so it costs nothing to
maintain the field. The major drawback is that these
magnets are extremely heavy. They weigh many, many
tons at the 0.4-tesla level. A stronger field would
require a magnet so heavy it would be difficult to
construct. Permanent magnets are getting smaller, but
are still limited to low field strengths.
- Superconducting
magnets are by far the most commonly used. A
superconducting magnet is somewhat similar to a
resistive magnet -- coils or windings of wire through
which a current of electricity is passed create the
magnetic field. The important difference is that the
wire is continually bathed in liquid helium at 452.4
degrees below zero. This almost unimaginable cold
causes the resistance in the wire to drop to zero,
reducing the electrical requirement for the system
dramatically and making it much more economical to
operate. Superconductive systems are still very
expensive, but they can easily generate 0.5-tesla to
3.0-tesla fields, allowing for much higher-quality
imaging.
 A
very uniform, or homogeneous, magnetic field of incredible
strength and stability is critical for high-quality
imaging. It forms the main magnetic field. Magnets
like those described above make this field possible.
Another
type of magnet found in every MRI system is called a
gradient magnet. There are three gradient magnets inside
the MRI machine. These magnets are very, very low
strength compared to the main magnetic field; they may
range in strength from 180 gauss to 270 gauss, or 18 to 27
millitesla (thousandths of a tesla).
The main
magnet immerses the patient in a stable and very intense
magnetic field, and the gradient magnets create a variable
field. The rest of an MRI system consists of a very
powerful computer system, some equipment that allows us to
transmit RF (radio frequency) pulses into the patient's
body while they are in the scanner, and many other
secondary components
Understanding
the Technology
The MRI machine applies an RF (radio frequency)
pulse that is specific only to hydrogen. The system
directs the pulse toward the area of the body we want to
examine. The pulse causes the protons in that area
to absorb the energy required to make them spin, or
precess, in a different direction. This is the
"resonance" part of MRI. The RF pulse forces
them (only the one or two extra unmatched protons per
million) 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.
 These
RF pulses are usually applied through a coil. MRI
machines come with many different coils designed 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. 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 are turned on and
off very rapidly in a specific manner, they alter the main
magnetic field on a very local level. What this
means is that we can pick exactly which area we want a
picture of. In MRI we speak of "slices."
Think of a loaf of bread with slices as thin as a few
millimeters -- the slices in MRI are that precise. We can
"slice" any part of the body in any direction,
giving us 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.
When the
RF pulse is turned off, the hydrogen protons begin to
slowly return to their natural alignment within the
magnetic field and release their excess stored energy.
When they do this, they give off a signal that the coil
now picks up and sends to the computer system. What
the system receives is mathematical data that is converted
into a picture that we can put on film. That is the
"imaging" part of MRI.
Visualization
Most imaging modalities
use injectable contrast, or dyes, for certain procedures.
MRI is no different.
MRI
contrast works by altering the local magnetic field in the
tissue being examined. Normal and abnormal tissue
will respond differently to this slight alteration, giving
us differing signals. These varied signals are
transferred to the images, allowing us to visualize many
different types of tissue abnormalities and disease
processes better than we could without the contrast.
The fact
that MRI systems do not use ionizing radiation is a
comfort to many patients, as is the fact that MRI contrast
materials have a very low incidence of side effects.
Another major advantage of MRI is its ability to image in
any plane. CT 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 images in the
sagitall plane (slicing the bread side-to-side lengthwise)
and coronally (think of the layers of a layer cake) or any
degree in between, without the patient ever moving.
If you have ever had an X-ray, you know that every time
they take a different picture, you have to move. The
three gradient magnets discussed earlier allow the MRI
system to choose exactly where in the body to acquire an
image and how the slices are oriented.
Advantages
MRI is ideal for:
- Diagnosing
multiple sclerosis (MS);
- Diagnosing
tumors of the pituitary gland and brain;
- Diagnosing
infections in the brain, spine or joints ;
- Visualizing
torn ligaments in the wrist, knee and ankle;
- Visualizing
shoulder injuries ;
- Diagnosing
tendonitis ;
- Evaluating
masses in the soft tissues of the body ;
- Evaluating
bone tumors, cysts and bulging or herniated discs in
the spine; and
- Diagnosing
strokes in their earliest stages.
Disadvantages
Although MRI scans are ideal for diagnosing and
evaluating a number of conditions, it does have drawbacks
as follows:
- There
are many people who cannot safely be scanned with MRI
(for example, because they have pacemakers);
- The
machine makes a lot of noise during a scan. The
noise sounds like a continual, rapid hammering.
Patients are given earplugs or stereo headphones to
muffle the noise (in most MRI centers you can even
bring your own cassette or CD to listen to). The
noise results from 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;
- MRI
scans require patients to hold very still for extended
periods of time. MRI exams can range in length
from 20 minutes to 90 minutes or more. Even very
slight movement of the part being scanned can cause
very distorted images that will have to be repeated;
and
- Orthopedic
hardware (screws, plates, artificial joints) in the
area of a scan can cause severe artifacts
(distortions) on the images. The hardware causes
a significant alteration in the main magnetic field.
The
Future of MRI
The future of MRI seems limited only by our imagination.
This technology is still in its infancy, comparatively
speaking. It has been in widespread use for less than 20
years (compared with over 100 years for X-rays).
Very small
scanners for imaging specific body parts are being
developed. Functional brain mapping (scanning a
person's brain while he or she is performing a certain
physical task such as squeezing a ball, or looking at a
particular type of picture) is helping researchers better
understand how the brain works. Research is under
way in a few institutions to image the ventilation
dynamics of the lungs through the use of hyperpolarized
helium-3 gas. The development of new, improved ways to
image strokes in their earliest stages is ongoing.
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