Magnetic Resonance Imaging
Mary Holleboom
Abstract This paper investigates the development, principles, and
applications of magnetic resonance imaging. As a widely used
medical technique, this type of imaging utilizes the inherent
magnetic properties of protons in human cells. Superconducting
magnets are generally enclosed in a large housing unit to scan
various parts of the body. Several types of magnetic resonance
imaging have been developed to better scan specific organs and
tissues. These include functional magnetic resonance imaging,
nuclear magnetic resonance, and echo-planar imaging. Although
there are no biological hazards yet associated with this technique,
extreme caution must be taken when working with such large
magnets. They produce strong magnetic fields that can pull both
small and large metal objects into the machine. Overall this is an
important medical breakthrough that is continually developing to
diagnose and prevent serious illnesses.
Keywords Magnetic resonance imaging, Medical applications,
Image processing techniques, Protons, Resonance, Gradient
I.
INTRODUCTION
Magnetic resonance imaging, commonly referred to as
MRI, is the use of a very strong magnetic field to scan an
object and produce an image. It is most frequently used for
medical purposes. Unlike radiology methods, MR imaging
uses no radiation. Instead, a magnet is housed in a large
scanner (Fig. 1), and current flowing through a coil of wire is
used to create a magnetic field. This field affects the
alignment of the protons in a person’s tissues. An antenna is
then used to detect signals sent from the protons, and the
signals are converted into an image.
Fig. 1 Magnetic resonance imaging scanner
This technique for imaging internal organs and tissues is
quite new. The magnetic resonance phenomenon was first
discovered by Felix Bloch and Edward Purcell in 1946.
From then until 1970, nuclear magnetic resonance (NMR)
was developed for chemical and physical molecular analysis.
Paul Lauterbur used a back projection technique to
demonstrate MR imaging on small test tube samples in 1973.
The basis of current MRI techniques didn’t appear until 1975
when Richard Ernst proposed MR imaging using phase and
frequency encoding and the Fourier Transform. Since then,
several developments have been made to reach the current
status of MRI technology. In 1977 Raymond Damadian
demonstrated MRI of the whole body, and Peter Mansfield
developed an imaging technique called echo-planar imaging
(EPI). This technique was utilized in 1987 to perform realtime movie imaging of a single cardiac cycle. A new
development, functional magnetic resonance imaging
(fMRI), came about in 1993. This allows the mapping of the
function of the various regions of the human brain. Today
MRI technology is continually improving and is used in
common medical practice.
There are several types of magnetic resonance imaging
now in use, including volume imaging, nuclear magnetic
resonance, and functional magnetic resonance imaging.
They are all based on the same principles and make use of
similar equipment.
Finally, as with most medical
procedures, there are health and safety issues to consider.
II.
IMAGING PRINCIPLES
A. Spin Physics
Magnetic resonance imaging makes use of a
fundamental property called spin. Protons, electrons, and
neutrons all posses spin, either + or – ½. Because of the
positive and negative factors, spins can pair up and cancel
each other. Unpaired, nuclear spins are utilized in NMR.
However, NMR can only be performed on isotopes whose
natural abundance is high enough for detection.
Within a magnetic field, particles with spin behave in a
specific manner. For example, a proton acts like a small
magnet with a north and a south pole. When this proton is
then placed in an external magnetic field, it aligns itself with
the field, as shown in Fig. 2. The particle can also undergo a
transition between two energy states when it absorbs a
photon. The energy of the photon is related to its frequency.
This is called the resonance or the Larmor frequency in
NMR and MRI. The signal used for NMR is produced by
the difference in energy absorbed and emitted from the spins.
-1-
MRI, the most useful type of gradient is a one-dimensional
linear magnetic field gradient, and is symbolized as Gx, Gy,
or Gz. The magnetic field gradient is used to image the
positions of the regions of spin.
Fig. 2 Proton in an external magnetic field
Spin can also be described at the macroscopic level as a
spin packet, or group of spins experiencing the same
magnetic field strength. The magnetic field due to the spins
is represented by a magnetization vector. T 1 and T2 are time
constants describing how the equilibrium magnetization is
achieved. The z component of the magnetization is referred
to as longitudinal magnetization (MZ) (Fig. 3). T1 is the time
required to change the longitudinal magnetization by a factor
of e and is called the spin lattice relaxation time. It is
governed by the equation:
Mz = Mo ( 1 - e-t/T1 ) (1)
where Mo is the equilibrium magnetization and t is the
time after displacement.
Fig. 3 Longitudinal magnetization
In the x-y direction, the magnetization is referred to as
transverse (MXY). T2 is the time it takes to reduce the
transverse magnetization by a factor of e and is called the
spin-spin relaxation time. It acts according to the equation:
MXY =MXYo e-t/T2
(2)
where MXYo is the equilibrium transverse magnetization.
Both the T1 and T2 processes occur simultaneously, provided
T2 is less than or equal to T1.
Fig. 4 Magnetic field gradient
Next, frequency encoding is the basis behind all MR
imaging. If a linear magnetic field gradient is applied to the
isocenter of a magnet, where (x,y,z) = (0,0,0), the three
regions of spin each experience different magnetic fields.
This creates an NMR spectrum with multiple signals, whose
amplitude is proportional to the number of spins in a plane
normal to the gradient. Applying the linear magnetic field
gradient to obtain the NMR spectrum is known as frequency
encoding.
Back projection imaging then utilizes this spectrum to
produce the image. To do so, an object is placed in a
magnetic field, and a one-dimensional field gradient is
applied at various angle intervals. The NMR spectrum of
each gradient is recorded and backprojected through
computer space. The image is seen once the background
intensity is suppressed.
Finally, to fully utilize this technique, the spins must be
imaged in thin slices. Slice selection is the selection of spins
in a plane through an object. This is done by applying a onedimensional, linear magnetic field gradient during the period
that the RF pulse is applied. To create the backprojection
image, first an apodized sinc pulse-shaped 90o pulse and a
slice selection gradient are applied concurrently. Once this
gradient is turned off, a frequency encoding gradient is
turned on. Fourier transformation is then used to produce the
frequency domain spectrum. This process is shown in Fig. 5.
Again, backprojection is used to produce the image from the
spectrum. Although this is a well-developed technique,
Fourier transform imaging methods are usually used in most
imaging machines.
B. Fundamental Principles
Four basic principles of imaging are the magnetic field
gradient, frequency encoding, back projection imaging, and
slice selection. First, a magnetic field gradient is a variation
in the magnetic field with respect to position (Fig. 4). In
-2-
into intensities of pixels. This creates a tomographic image,
shown in Fig. 6.
Fig. 5 Backprojection imaging sequence timing diagram
C. Fourier Transform Principles
Fourier transform tomographic imaging is the most
commonly used MRI method utilized today. It uses another
type of magnetic field gradient, called a phase encoding
gradient, in addition to the slice selection and frequency
encoding gradients. The phase encoding gradient is used to
give a specific phase angle to the transverse magnetization
vector. It acts in much the same way as the frequency
encoding gradient when turned on. In this case each
transverse magnetization vector has a unique Larmor
frequency. However, the phase encoding gradient is unique
in that when it is turned off, each transverse magnetization
vector is identical. The phase angle of each vector, measured
when the phase encoding gradient is off, is the angle between
a reference axis and the magnetization vector.
Fourier tomographic imaging is done using a specific
sequence, utilizing radio frequency, magnetic field gradients,
and signals.
A simplified Fourier transform imaging
sequence includes a 90o slice selective pulse, a slice selection
gradient pulse, a phase encoding gradient pulse, a frequency
encoding gradient pulse, and a signal. The magnitude and
duration of the magnetic field gradients are represented by
the pulses. A typical imaging sequence would start by
turning on the slice selection gradient, while applying the
slice selection RF pulse. Once the pulse is done, the slice
selection gradient turns off, and the phase encoding gradient
is turned on. After that is done, the frequency encoding
gradient is turned on. At this point, a signal is recorded.
This process is repeated 128 to 256 times, varying the
magnitude of the phase encoding gradient each time, in order
to obtain sufficient data, free induction decays or signals, for
creating an image.
Before actually creating the image, however, the signals
must be Fourier transformed. This is done first in the
direction in which the spins are located to extract the
frequency domain information. Then it is done in the phase
encoding direction to obtain information about the spin
locations in that direction. The FT data finally becomes an
image when the intensities of the data peaks are converted
Fig. 6 Tomographic image created from Fourier transformed data
III.
IMAGING HARDWARE
A. Magnets
The magnet is the most expensive part of the imaging
hardware. There are three types used for MR imaging.
These are superconducting, resistive, and permanent. A
superconducting magnet is the strongest of the three. In such
an electromagnet, current flows in a circular direction in a
coil of wire in order to create a magnetic field. The wire
used in the magnet is superconducting wire. This means that
it has nearly zero resistance when cooled to a temperature
close to absolute zero, achieved by placing it in either liquid
helium or liquid nitrogen (Fig. 7). The current in the
superconducting wire continues to flow as long as it is kept
at the specific temperature. Figure 8 shows an actual
superconducting magnet without the housing.
-3-
Fig. 7 Internal view of magnetic resonance imaging scanner
spins of the imaged object. The transmit only and receive
only coils can be used in combination to perform the same
tasks. A variety of coils are needed for imaging to
compensate for different imaging needs.
An imaging coil consists of both inductive and
capacitive elements. This allows it to efficiently store
energy. In other words, the inductance and capacitance
allow it to resonate. The frequency (ν) at which the imaging
coil resonates is determined by the inductance (L) and
capacitance (C) as shown in (3).
Fig. 8 Superconducting magnet without housing

Resistive magnets are also electromagnets; however,
they are cooled by the air. This causes a greater resistance to
current. This in turn produces a weaker magnetic field. The
third type of magnet, permanent, is not an electromagnet, but
rather it is made of solid magnetic material. As expected, a
permanent magnet creates the weakest magnetic field.
Nevertheless, permanent magnets are useful in Open MR
scanners due to the fact that they can be arranged in any
position. This eliminates the need for the patient to be
surrounded by the magnet.
B. Coils
1
2 LC
(3)
There is a multitude of other coils used for specific
patients or organs. They include surface coils, bird cage
coils, saddle coils, phased-array coils, and litz coils.
IV.
ADVANCED IMAGING TECHNIQUES
Newer imaging techniques are continually being
developed. A few of the latest techniques will be discussed
here.
A. Volume Imaging
Two important types of coils are gradient coils and RF
coils. The gradient coil creates gradients in the equilibrium
magnetic field, Bo (Fig. 9). This is possible because they are
room temperature coils in a specific configuration. The RF
coils, on the other hand, create the B1 magnetic field. This is
the same as the magnetic field produced by alternating
current through the coil at the Larmor frequency. In
addition, RF coils detect transverse magnetization.
Volume imaging, also called 3-D imaging, obtains
magnetic resonance data from a volume instead of a
tomographic slice. In other words, a group of slices is used,
as seen in Fig. 10. As with previous techniques, an RF pulse
and gradient are used; however, the gradient only rotates in
the imaged volume. Next, phase encoding gradients in two
dimensions are varied between their maximum and minimum
values. Finally, the frequency encoding gradient is applied
to obtain the signal. Basically this technique is similar to
others, except it uses a volume, or thick slices.
Single Slice
Contiguous Slices
Fig. 10 Multiple slice selection of volume imaging
Fig. 9 Gradients in an equilibrium magnetic field
An RF coil can be of the transmit and receive, receive
only, or transmit only type. The transmit and receive RF coil
transmits the B1 field and receives the RF signal from the
B. Flow Imaging
Flow imaging is also known as Magnetic resonance
angiography (MRA). With this technique, blood flowing
-4-
through the arteries and the veins of the body are imaged.
Previous imaging methods, such as X-rays, did not have the
capability of distinguishing between flowing and static
blood. With MRA, however, the velocity of the blood flow
is determined by the intensity of the image. Figure 11 shows
an MRA image used to detect an aneurysm. Three types of
MRA are Time-of-flight angiography, Phase contrast
angiography, and Contrast enhanced angiography.
is removed and filled with water or another aqueous solution.
The signals from this solution are imaged. To test the spatial
uniformity of the transmit and receive radio frequency
magnetic fields, RF homogeneity phantoms are utilized. An
example of a homogeneity phantom is shown in Fig. 13. The
ideal situation is to have uniform rotation of the spins and
uniform sensitivity across the imaged object, which is what
the phantom tests for. Multiple phantoms can be utilized to
model specific organs.
Fig. 11 Aneurysm detected with MRA imaging
C. Echo Planar Imaging
Fig. 12 Slice of a resolution phantom
Echo planar imaging is sometimes referred to as
functional magnetic resonance imaging (fMRI). This means
that the imaging relates body function or thought to specific
locations in the brain. The basic concept behind echo planar
imaging is to produce tomographic images at video rates. In
normal imaging techniques, the Fourier transform of the
magnetic resonance image is recorded one line at a time,
each taking one period (TR). In other words, it takes TR
times the number of lines to create the image. In contrast,
echo planar imaging measures all of the image lines in a
single period. The major application of echo planar imaging
is fMRI. A rapid momentary increase in blood flow during
brain activity can be imaged at video rates. This is a very
useful technique.
V.
Fig. 13 Homogeneity phantom
TESTING
MRI system testing is not done by humans, but rather
with an anthropogenic object called a phantom. Trying to
use a “standard human” poses many challenges, such as
availability and variability. By using phantoms, MRI
systems around the world can easily be tested. A phantom is
made of material with a magnetic resonance signal. This can
be one of many materials, such as polyvinyl alcohol,
silicone, or agarose. Within a phantom, a substance is used
to transmit the signal. This substance is usually water.
The two types of phantoms are resolution and RF
homogeneity. Several things are tested by resolution
phantoms. These include in-plane resolution, slice thickness,
linearity, and the signal-to-noise ratio as a function of
position. Fig. 12 shows a slice of a resolution phantom with
the different components. A portion of the plastic phantom
VI.
HEALTH AND SAFETY
As expected, there are some health and safety concerns
associated with the use of such large magnets.
A. Equipment
One main safety concern is that of the imaging
equipment.
The magnets used in MRI scanners are
extremely powerful and can pull large ferromagnetic items
into their bores. For example, if a metal bucket is placed too
close to the magnet it can be pulled forcefully off the ground
and into the magnet, causing major damage to or complete
destruction of the magnet and imaging coils. In an extreme
case a fully loaded pallet jacket was pulled into the bore of
the system, as shown in Figure 14. The immense force
increases exponentially as the object gets closer to the
-5-
magnet. To fix a problem, some small objects can manually
be pulled off from the magnet. Some larger objects,
however, may require more force, such as a winch. In even
more severe cases, the magnetic field must be completely
shut down. Because of this, no ferromagnetic materials are
allowed near the scanner. This even includes belt buckles
and credit cards. A credit card placed near the magnet would
erase it. Extreme caution must be taken before placing an
object in an MRI scanning room.
Fig. 15 Burn from an RF coil
VII.
Fig. 14 Fully loaded pallet jacket pulled into the bore of a magnet
B. Patients
Fortunately there have been no biological hazards
discovered in relation to exposure to the magnetic fields or
radio frequency electromagnetic pulses.
Nevertheless,
because there has not been enough research done, most
pregnant women are not allowed to undergo MR imaging. It
could possibly have detrimental effects on the fetus.
In addition to pregnant women, patients with most metal
implants are prohibited from being scanned. A person with a
pacemaker cannot be scanned. The magnet could cause the
pacemaker to malfunction, possibly leading to the death of
the individual. Also, a patient with a cerebral aneurysm clip
is not permitted to undergo MR imaging. The magnet could
move the clip, causing severe bleeding. Other problem
posing objects include implanted electromagnetic devices,
magnetically activated or supported implants, vascular coils
or filters, implanted insulin pumps, and metal fragments in
the eye. Most orthopedic implants, on the other hand, are
safe because they are firmly embedded in bone. Because of
the potential problems, every patient must be questioned and
possibly examined before undergoing MR imaging. Finally,
a failure of the RF coils can burn a patient. Fig. 15 shows a
burn on a man’s arm from such a case.
APPLICATIONS
The ability to image internal organs and tissues creates
numerous applications. One example is imaging the brain.
In doing so, tumors, aneurysms, or blood clots are more
easily detected. This is usually done with fMRI by detecting
the levels of oxygen in the blood point by point. Another
important use of magnetic resonance imaging is scanning the
spine. Each vertebrate can be seen and analyzed. Figure 16
shows the MR image of a herniated disk in a spine. Many
bones and appendages commonly scanned include wrists,
hands, knees, shoulders, hips, prostates, and breasts. Other
applications include preventing strokes and making accurate
diagnoses of Multiple Sclerosis. Overall this technique is
extremely important in the medical field.
Fig. 16 MR image of herniated disk in the spine
VIII.
CONSLUSIONS
Magnetic resonance imaging is a relatively new
technique that has made huge contributions to the
medical field. Tumors and blood flow problems can be
detected much sooner using this process. There are
various types of MR imaging whose use is dependent
upon the specific organ or tissue. These include volume
imaging, flow imaging, functional magnetic resonance
imaging, and nuclear magnetic resonance imaging.
Extreme caution must be taken, however, because of the
large force of the magnet. Most metal objects are not
allowed in the scanning room because they can cause
damage to the machine and injure patients and doctors.
This use of magnets to create images of internal body
-6-
parts is an amazing technique now in use all over the
world.
[2]
[3]
IX.
[1]
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[4]
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Drs. Groover, Christie & Merrit: Radiologists, “MRI: How
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Greg Brown, “The Adelaide MRI Website,”
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