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MRI (Magnetic
Resonance Imaging)
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.
VIDEO: MRI
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