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 orsupport 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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