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Magnetic Resonance Imaging
Contributors: Sarah Ezzell
Abstract – This paper explores the past, present, and future of
magnetic resonance imaging (MRI).
The history of such
technology is first described, followed by the basics of MRI and a
description of how it works. Two main ideas discussed are the
magnetic field and the radio frequency (RF) pulse. The advantages
and disadvantages of using an MRI system to look inside a person’s
body are explored. Finally, the future of MRI in medicine as well
as industrial applications is explained.
I.
HISTORY
Figure 1 Patent drawing2
Magnetic resonance imaging (MRI) is a means of
looking inside the human body. This method is achievable
without the use of surgery, harmful dyes, or x-rays.
MRI makes use of nuclear magnetic resonance (NMR), a
physics phenomenon that was discovered in the 1930s1 by
Isidor I. Rabi and his colleagues at Columbia University 2.
This is a phenomenon in which tiny radio signals are given
off by atoms due to magnetic fields and radio waves, which
are both harmless. In the 1940s, research physicists learned
more. They found that depending on the substance being
examined, variations were evident in the length of time the
response signals were emitted after the atom was stimulated
by radio waves. Biological tissue is no exception to this
phenomenon.
The gap between researchers familiar with NMR and
doctors anxious for better diagnostic methods was bridged by
Dr. Raymond Damadian.
Dr. Damadian studied
mathematics, but then earned a medical degree and became a
professor shortly after. He became interested in NMR and
through testing normal mouse tissues against tumors
removed from the animals, concluded that NMR signals in
cancerous cells endured much longer than those in healthy
cells. His paper, published in 1971, entitled “Tumor
Detection by Nuclear Magnetic Resonance” described these
results.
He then filed his idea, using magnetic resonance for
medical diagnosis, with the U.S. Patent Office less than two
years later. His idea, entitled “Apparatus and Method for
Detecting Cancer in Tissue”, was granted a patent in 1974 by
the Patent Office and was the world’s first patent given in the
field of MRI. A patent drawing from Dr. Damadian’s 1972
filing can be seen in Figure 1.
By 1977, Dr. Damadian’s idea had become a physical
reality. He constructed a whole-body MRI scanner, which he
called “Indomitable.” The machine had been built despite
those who believed it was impractical and foolish. Dr.
Damadian and his associates produced the first whole-body
magnetic resonance image on July 3, 1977 with Indomitable.
This scan lasted four hours and 45 minutes. Today
Indomitable can be found on permanent display in the
Smithsonian Institution in Washington, DC.
II.
BASICS OF MRI
The basic design used in most MRI machines is a large
cube3. New models are shrinking in size, but the typical
system is two meters by two meters by three meters. A
horizontal tube known as the bore runs through the magnet.
Newer models may also have some openness around the
sides. A MRI scanner can be seen in Figure 2.
Figure 2 MRI scanner4
The patient slides into the bore while on his or her back
on a special table. The type of exam determines how far into
the magnet the patient goes as well as whether the patient
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goes in head first or feet first. The scan can begin once the
patient is in the exact center, isocenter, of the magnetic field.
Accompanied by radio-wave pulses of energy, the MRI
scanner is able to select a point inside the patient’s body and
ask the question, “What type of tissue are you?” This
question is asked to the entire area, point by point, that is
being examined and the information is integrated into a 2-D
image or a 3-D model. Each point that is examined is
approximately a cube that is half a millimeter on each side.
III.
ferromagnetic material: alloys of iron and cobalt (ALNICO)
or rare earth alloys such as Neodymium Iron Boron (Ne-FeB) or Samarium Cobalt (Sm-Co)5. Rare earth alloys are
more efficient; a 2.0 T magnet weighs 23 tons with ALNICO
and only 4 tons with Ne-Fe-B.
MAGNETIC FIELD
The magnet is the largest and most important element of
the MRI system. Magnets are rated using units of measure
known as Tesla (T) or gauss. One Tesla is equal to 10,000
gauss. The earth’s magnetic field is 0.5 gauss. The magnets
used in the MRI system are extremely powerful in
comparison. They range from 0.5 Tesla to 2.0 Tesla, which
is equivalent to 5,000 to 20,000 gauss. MRI scanners are
frequently categorized as low-, mid-, or high-field. Lowfield is considered under 0.2-Tesla, mid-field from 0.2 to 0.6
Tesla, and high-field from 1.0 to 2.0 Tesla.
Radiologists debated for years the diagnostic
effectiveness of the different field strengths. An editorial by
noted radiologist Dr. David Stark in Applied Radiology
stated, “The great field strength debate lasted one
decade…Increasing field strength was an obvious, and
expensive, approach to improve image quality. Although it
is unarguable that increasing field strength increases image
quality by increasing image signal-to-noise ratios (SNR)
achievable during a given scan time, over the past few years
it has become apparent that increasing field yields only
fractional gains in SNR, not the exponential bonanza touted
in the 1980s1.”
MRI systems use one of three basic types of magnets:
resistive magnet, permanent magnet, or superconducting
magnet. Resistive magnets are composed of many windings
of wire looped around a bore through which electric current
is delivered. The result is the generation of a magnetic field.
However, the electricity is required to maintain the magnetic
field. On account of the natural resistance in the wire, these
magnets require large amounts of power, up to 50 kilowatts,
to operate. Operation above the 0.3-Tesla level would be
unreasonably expensive.
A permanent magnet requires nothing additional to
maintain the magnetic field. It is always there and is always
on full strength. This means that there is no cost to maintain
the field. However, these magnets are extremely heavy. At
the 0.4-Tesla level, they weigh many tons. It would be
challenging to construct a permanent magnet with a stronger
field because of the tremendous weight. These magnets are
consequently typically limited to low field strengths.
Permanent magnets are constructed from magnetized
Certainly the most commonly used magnets for this
application are superconducting magnets. These have some
similarity to resistive magnets in that the magnetic field is
created by coils of wire in which a current is passed. The
windings are composed of a Niobium-Titanium alloy (Nb-Ti)
embedded in a copper core. The significant difference is that
the wire is incessantly bathed in liquid helium that is at a
temperature of 4 degrees Kelvin, or negative 452 degrees
Fahrenheit. This ensures superconductivity of the Nb-Ti,
which becomes superconductive at 10 degrees Kelvin. A
vacuum insulates it though, so the patient is not exposed to
that incredible cold.
Because of the extremely cold
temperature, the resistance in the wire drops to zero, making
operation of the system much more economical. These
magnets can generate high-fields and higher quality images,
but they are still very expensive.
In Figure 3, a
superconducting MRI magnet without the outer housing can
be seen.
Figure 3 Superconducting MRI magnet4
High-quality imaging requires a uniform magnetic field
of tremendous strength and stability. Such a field is possible
with the magnets described above. Another type of magnet
that is found in every MRI system is a gradient magnet. In
comparison to the main magnetic field, these magnets are
incredibly low-strength, ranging from 180 gauss to 270
gauss. The gradient magnets create a variable field as
opposed to the stable field created by the main magnet. The
gradient magnets are designed to alter the main field on a
local level. This allows the image of any exact “slice” of any
part in any direction to be generated.
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IV.
CREATING AN IMAGE
The basics in creating an image include the
understanding of atoms in the human body as well as their
interaction with radio frequency pulses.
Atoms, of which the human body is made up of billions,
are the fundamental building blocks of matter. The nucleus
of an atom spins on an axis, and billions of nuclei are
randomly spinning in every direction in the body. Although
there are many types of atoms in the body, MRI is concerned
with the hydrogen atom. Hydrogen has a large magnetic
moment, meaning that in the presence of a magnetic field,
the atom has a strong inclination to line up with the direction
of the field. This makes the hydrogen atom good for MRI.
The magnetic field runs down the center of the tube, the
bore, in which the patient is placed. Figure 4 shows the
orientation of the main magnetic field, which is shown in
red. The patient is surrounded by gradient coils, an RF
shield, and the main magnet coils.
Figure 5 Hydrogen protons align with magnetic field3
A radio frequency (RF) pulse that is specific to
hydrogen is then applied by the MRI machine to the area of
the body being examined. This pulse drives the protons that
are not canceled out to spin at a certain frequency in a certain
direction. The tissue being examined and the magnetic field
strength determine the resonance frequency, called the
Larmour frequency.
Application of the RF pulses generally occurs through a
coil. MRI machines are equipped with multiple coils,
designed for different parts of the body. The coils either
conform to the form of the body part or are located very
close during the scan.
Figure 4 Magnet placement and field orientation6
The hydrogen protons of a patient lying on his or her
back will therefore line up either in the direction of the head
or feet. Most of these protons will cancel each other out.
When there is one in the direction of the head and one in the
direction of the feet, they cancel each other out. Out of every
million protons, there are only several that are not canceled
out. Because of the huge number of hydrogen atoms in the
body, this is enough to create fantastic images. Figure 5
shows how the hydrogen protons align with the magnetic
field and most cancel each other out.
Upon termination of the RF pulse, the hydrogen protons
return to their natural alignment within the magnetic field,
sometimes called relaxation. Their excess energy is thereby
released and a signal is given off and picked up by the coil.
From there it is sent to the computer system and the data is
transformed by use of the Fourier transform into an image
that can be put onto film.
The magnetic resonance relaxation that occurs after RF
stimulation in the presence of a strong magnetic field is
measured both by values T1 and T2. Every tissue in the
body has a unique T1 and T2 value. When constructing an
image, the MRI machine can control the brightness of the
image pixels with either the T1 or T2 value. The resulting
image is accordingly either a T1 image or a T2 image. A T1
image displays tissues with low T1 values as bright elements
and a T2 image displays tissues with high T2 values as bright
elements. Therefore, what may be white in a T1 image could
be gray in a T2 image. T1 images demonstrate anatomic
detail while T2 images show good contrast between normal
and abnormal tissues. Figure 6 shows a T1 and T2 image of
the brain.
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Figure 6 T1 and T2 MRI of the brain4
V.
ADVANTAGES OF MRI
There are many advantages to performing a MRI scan.
Both the fantastic image produced and also the harmlessness
of the procedure contribute to the excellence of the idea.
MRI is a great way to see inside the human body. There
are many situations in which performing an MRI scan is
ideal. Some of them are the following:
 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
 Diagnosing strokes in their earliest stages3
The ability of MRI to image in any plane is of great
significance. This is one advantage of MRI over computed
tomography (CT) imaging, which is limited to one plane.
Moreover, the imaging in various planes is produced without
the patient moving through use of the gradient magnets.
There are three standard views that are typically used:
transverse (axial), coronal, and sagittal. The orientation of
these slices can be seen in Figure 7.
Figure 7 Axial, coronal, and sagittal planes3
To perform the procedure, the emission of ionizing
radiation, used for X-ray and CT scans, is not necessary.
This is comforting to many people. The biological effects
resulting from exposure to magnetic fields and RF
electromagnetic pulses, which are necessary for magnetic
resonance imaging, have been studied multiple times; no
significant biological hazards have been found.
Certain types of MRI scans do require the injection of
intravenous contrast, as do other imaging methods7. The
most commonly used dye is gadolinium. MRI contrast alters
the local magnetic field in the tissue being examined,
resulting in differing signals from normal and abnormal
tissue as they respond differently to the alteration. The CT
dye rarely produces complication, but even rarer are
complications from gadolinium.
VI.
DISADVANTAGES OF MRI
MRI does have drawbacks though. The most significant
is the extreme care that must be taken due to the extreme
power of the magnet. Strict precautions must be observed in
order to maintain a safe environment. Metal objects of any
kind cannot be brought into the scan room. Objects such as
paperclips, pens, keys, scissors, and stethoscopes can be
drawn out of the pockets or off the body and pulled toward
the opening of the magnet at high speeds.
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The magnetic force that an object experiences is
exponentially proportionate to the distance from the magnet.
A much stronger force attracts an object with more mass.
Examples of objects that have been pulled into the magnetic
fields of MRI machines are mop buckets, vacuum cleaners,
oxygen tanks, patient stretchers, and heart monitors. In
Figure 8, a fully loaded pallet jack that has been sucked into
the bore of an MRI system can be seen. The removal of
large objects may require a winch or the magnetic field to be
shut down.
implants are also generally acceptable because they are
securely embedded in the bone.
Although there are no adverse biological affects known
from exposure to magnetic fields of the strength used in
imaging today, most facilities choose not to scan pregnant
women. The reasoning behind such a preference is the lack
of research that has been done in the area of biological
effects on a developing fetus. The decision whether to scan a
pregnant woman or not is made on a case-to-case basis. The
risk to the fetus and mother must be enormously outweighed
by the benefit of the MRI scan. Generally, doctors use other
imaging methods such as ultrasound on pregnant women9.
Several other aspects of the MRI scan can make it
uncomfortable for the patient. Obese patients may have
trouble fitting well or even at all into the bore. For
claustrophobic people, the scan can be difficult in typical,
enclosed constructions. Another discomfort is the noise,
which sounds like a continual hammering, present during the
scan. The rising electrical current in the gradient magnet
wires is the source of this noise. Earplugs or stereo
headphones often mitigate the noise. A requirement of the
MRI scan is that patient hold very still. Distorted images can
be the result of slight movements.
Figure 8 Pallet jack sucked into bore of magnet
3
One recent tragic incident of a metal object being pulled
into the magnetic field left a child having a MRI scan dead8.
The six-year-old boy was in the MRI machine at Westchester
Medical Center in New York when, according to the
hospital, a metal oxygen tank was accidentally “introduced
into the exam room8.” In a news release, the center said that
the tank was “immediately magnetized and drawn to the
center of the machine, causing head trauma to the child 8.”
Besides external objects, patients may have objects
inside themselves, which make the presence of a magnetic
field dangerous. There is no danger associated with an MRI
scan for the vast majority of people, but for some it could
cause serious injury or even death. People with pacemakers
or other mechanically, electrically, or magnetically activated
implanted devices cannot be scanned. In the case of the
pacemaker, the magnet could cause malfunction. Another
dangerous object is an aneurysm clip in the brain, while the
magnet can move it and cause tearing of the artery that it was
meant to repair. Eye damage or blindness could result in a
patient with metallic fragments in his or her eye. Unlike the
rest of the human body, the eyes do not form scar tissue, so a
fragment of metal that has been there 25 years is just as
dangerous today. Metal staples in the body are usually
acceptable since enough scar tissue has formed after
approximately six weeks to hold them in place. Orthopedic
One last disadvantage is the cost of these systems. A
superconducting magnet is extremely expensive, costing
approximately $500,00010. This brings the total cost of the
system with imaging and controls to about $1 million.
VII.
FUTURE OF MRI
The future of magnetic resonance imaging is very
exciting. The medical industry is beginning to see MRI
scanners that are much more compact, less expensive, and
more patient-friendly11. The magnets are not as strong, so
some images still should be taken with the traditional scanner
like the one seen in Figure 2. The scanner in Figure 9 is a
Siemens Medical Systems Magnetom Open, 0.2 T MRI, and
is based on a permanent magnet. Shown positioned over the
patient is the RF coil.
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Technische Hochschule in Germany, a cost of about $50,000
per unit would be economically sensible in industry.
Blümich and colleagues developed a hand-held MRI
probe called the NMR Mouse, which stands for nuclear
magnetic resonance mobile universal surface explorer. The
probe is effectively a sensor that is placed over the surface of
the material being tested. Seen in Figure 11, the probe
utilizes a permanent magnet and can sense a volume
restricted to 3 mm across the gap between poles, 8 mm along
the gap, and 1 mm deep into the material. The device is
more than adequate for its intended use, and Blümich noted
“there are companies in the elastomer [rubber] business that
have acquired these devices for quality control9.”
Figure 9 Unenclosed, permanent-magnet-based MRI11
The scanner in Figure 10 allows for most of the patient’s
body to remain outside the magnetic field during the exam.
This Lunar Corp. 0.2 T Artoscan extremity magnetic
resonance imager takes up a floor space of only nine square
meters.
Figure 11 MRI probe to monitor rubber tire hardness10
Figure 10 Extremity MRI scanner11
Not only does magnetic resonance imaging have a future
in the medical industry, but it may also be moving onto the
factory floor. Researchers are investigating its use in the
analysis of fluid content. Another possible use is the analysis
of molecular mobility, which measures the hardness of a
material. One applicable material is the elastomer; rubber
products such as automobile tires are an example of this type.
Other applicable materials are fiber products like carpet in
addition to ceramic and polycarbonate materials.
Industry cannot tolerate the enormous costs incurred in
the medical arena for magnetic resonance technology.
According to Bernhard Blümich, professor and specialist in
macromolecular chemistry with Rheinisch-Westfälische
Besides the rubber industry, another possible application
area for industrial MRI is carpet manufacturing. Researchers
at Georgia Institute of Technology with the School of Textile
and Fiber Engineering have been employing MRI off-line to
examine the water content of carpet in the course of the
drying process. Unlike the permanent magnet used in the
NMR Mouse, this system uses a cryogenically cooled
superconducting magnet, generating a flux density of 9.4 T
for increased sensitivity. Receptivity to the research results
may determine the outlook of an on-line system analogous to
the probe for the rubber industry, according to Haskell
Beckham, a leading researcher and associate professor at the
Georgia institute.
VIII.
CONCLUSION
The discovery of nuclear magnetic resonance in the
1930s has impacted society in a way never imagined before
the work of Dr. Damadian. His years of work have changed
the landscape of modern medicine. It now appears as though
the idea is branching out into other arenas besides the
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medical one. The future impact of this technology will be
exciting to follow.
IX.
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REFERENCES
MRI FAQ’s. FONAR Corporation. 13 April 2002
<http://www.fonar.com/faqs_content.htm>.
Schneider, David. “Raymond V. Damadian:
Scanning the Horizon.” Scientific American. June
1997. 13 April 2002 <http://www.sciam.com/
0697issue/0697profile.html>.
Gould, Todd. How Magnetic Resonance Imaging
(MRI) Works. Marshall Brain’s HowStuffWorks. 6
April 2002 <http://howstuffworks.com/mri.
htm/printable>.
MRI: How It Works. GCM Radiologists. 13 April
2002 <http://www.gcmradiology.com/mrworks.
html>.
Brown, Greg. Introduction to MRI Hardware. 25
April 2002 <http://www.users.on.net/vision/
papers/hardware/hardware.htm>.
Magnetic Resonance Imaging. Yale University
School of Medicine. 13 April 2002 <http://www.
med.yale.edu/intmed/cardio/imaging/techniques/mri_
diagram/>.
Magnetic Resonance Imaging (MRI).
Healthcommunities.com. 13 April 2002 <http://
www.radiologychannel.net/magneticresonance/>.
Fitzgerald, Jim. “Child Having MRI Killed in
Machine.” The Detroit News. 24 April 2002
<http://detnews.com/2001/health/0108/01/258469.htm>.
MR Imaging (MRI) – Body. American College of
Radiology (ACR). Radiological Society of North
America (RSNA). 13 April 2002 <http://
radiologyinfo.org/content/mr_of_the_body.htm>.
Kaplan, Gadi. “Industrial Electonics.” IEEE
Spectrum (2000): 104-09.
Magin, Richard, Andrew Webb, and Timothy Peck,
“Miniature Magnetic Resonance Machines.” IEEE
Spectrum (1997): 51-61.
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