1.5 T MRI Scanner during installation
at Ontario Veterinary College,
University of Guelph, Canada.
The biggest and most important component in
an MRI system is the magnet.
The magnets in use today in MRI are in the 0.5-
tesla to 2.0-tesla range, or 5,000 to 20,000 gauss.
Magnetic fields greater than 2 tesla have not
been approved for use in medical imaging,
though much more powerful magnets -- up to
60 tesla -- are used in research.
The MRI magnet surrounding you, on the other
hand, is a superconducting magnet; it
conducts electricity, thereby creating a
The superconducting magnet you’re inside
could be up to 3 tesla – 60,000 times the force
of the Earth’s magnetic field 0.5 Gauss.
Limits of variables given by Food
and Drug Administration
Maximum field to 8 Tesla.
Maximum noise to 99 dB.
MRI is a true three-dimensional imaging technique,
where the object itself is active, responding in
various ways to the radiation, which is sent into it–
not only onto it.
In this way it has some resemblance to computed
tomography (CT, CAT) and positron emission
tomography (PET), where one also can get
information from small specific volumes deep inside
a human body.
MR imaging is based on nuclear magnetic resonance
(NMR) principles and discerns tissues by the
characteristics of the signal emitted by their nuclei in
a strong magnetic field when subjected to resonant
Major step was the utilization of Super
conducting magnets, which can create
magnetic fields that are a magnitude
higher than ordinary resistive coils.
A magnetic resonance imaging magnet
includes a cryocooler penetration
assembly and a superconducting coil
An essential part of the Superconducting
main magnet system is the cryocooler,
which is used to cool the radiation shields
of the main magnet system and to
recondense the helium present in the
main magnet system.
Utilizing reliable compressors
Over 20,000/yr made
Large and heavy
Intrinsic vibration from displacer
Low efficiency (valve loss)
The cryocooler has moving internal parts,
which constitute a source of distortions of
the magnetic field in the examination
The reason is that the moving parts have
conducting and/or magnetic materials,
and field distortions arise as a result of
the eddy currents induced in the
conducting materials and as a result of
the moving magnetic field of the
Furthermore, the moving parts lead to vibrations which
are transmitted to other portions of the MRI system,
and field distortions arise as a result of the eddy
currents induced in these other portions when they
vibrate in the magnetic field.
The cryocooler penetration assembly has a thermal
station within a thermal box within a vacuum vessel.
The superconducting coil assembly has a magnet
cartridge within a thermal shield within a vacuum
A flexible bellows hermetically connects the vacuum
vessel and the vacuum enclosure.
The weight of the cryocooler penetration assembly is
supported independent of the superconducting coil
assembly which, together with the flexible connections,
isolates the vibrations of the cryocooler coldhead
(which is attached to the cryocooler penetration
assembly) from the superconducting coil assembly
thereby improving imaging quality.
Overview of MRI :
Magnetic Resonance Imaging or MRI is a
modern diagnostic technique for acquiring
information from the interior of a body.
Usually this is a human body or an animal, but
MRI is also used in the industry for more
The greatest advantage of MRI is that it can
create three-dimensional images of the object
under study without hurting the object in any
way and without using any ionizing radiation.
The body must be placed in a strong magnetic
field, more than ten thousand times the
magnetic field of Earth.
A radio signal is sent into the body, where it is
absorbed by hydrogen atoms.
The hydrogen atoms in the body respond by
sending back a signal to a detector.
The strength of this signal mirrors the amount
of hydrogen in various parts of the body.
When creating an image of an organ, the signal
must be acquired from every part of the organ
point by point by a scanning procedure.
To accomplish this, the magnetic field is rapidly
varied with gradients in three dimensions.
The gradients divide the object studied into a
great number of volume elements, called voxels,
each of which has a volume of about one cubic
millimeter or less.
Every voxel gives rise to one signal with a
unique amplitude. Calculating the amplitudes of
all these signals and their space coordinates in
the studied body is more or less what MRI is
Scanner construction and operation
The three systems form the major components
of an MRI scanner:
A static magnetic field.
An RF transmitter and receiver.
Three orthogonal, controllable magnetic
An outline of a cylindrical Whole body system,
which includes the main magnet creating the
background field ,gradient system and RF body
coil, is shown in Fig.
These three subsystems compete for the same
annular space that also provides maximum
accommodation and comfort for the patient.
Need of Superconducting Magnet:
The option of creating a magnet by running electrical
current through wire coils – an electromagnet.
The problem is the electrons making up that current
are forever bumping into the fidgety atomic particles of
the material through which they are traveling, slowing
them down considerably.
Given the resistance the current encounters, providing
the vast amount of power required to overcome it and
generate a magnetic field sufficient to operate an MRI
would be prohibitively expensive.
Take special coils and surround them with
something really, really cold – liquid helium, at
452.4 degrees below zero on the Fahrenheit
scale, does quite nicely.
Those over-caffeinated atoms in the conducting
wire are frozen into submission. Slowed to a
virtual halt, they allow the current to sail right
through the miles of wires snaking through an
This technology allows for the construction of
hugely powerful magnets like the one
surrounding you right now.
Would you think that being in the
middle of such a powerful force
would make you feel different –
tingly or something?
It doesn’t. However, on an atomic
level, it’s quite a different story,
which takes us from the “M” of
MRI to the “R” – Resonance.
You’re made up mostly of water, which means a
large number of the atoms inside your body are
This turns out to be quite fortuitous, because
hydrogen atoms happen to be built in such a
way that they react dependably to the forces
they will be subjected to inside this scanner.
It is of special interest that the signal from
hydrogen in lesions and tumors often decay
slower than surrounding tissues and therefore
can be detected in the image.
The contrast of the image is created by the
experimental procedure and is not inherent in
the imaged body.
Forces subjected to patient
inside the scanner :
Main magnetic field
Pulses of radio waves.
In the nucleus of every hydrogen atom is a
positively-charged proton that spins (or
precesses, scientifically speaking) around an
axis, much in the same way as a child’s top.
This spinning generates its own tiny magnetic
field, giving the proton its own north and south
Now, the nuclei of other atoms spin, too, but
for a number of reasons (including, as we’ve
mentioned, their sheer quantity), MRI is
generally interested only in hydrogen atoms.
Under normal circumstances, these hydrogen
protons spin about willy-nilly, on randomly
However, when these atoms are placed in a
more powerful magnetic field, it’s as if a drill
sergeant blew a whistle: the protons line up at
attention. in the direction of the field.
Now that the magnet has gotten the hydrogen
protons lined up at attention, the scanner is
ready to subject them to the next step, the one
that will result in an actual signal.
A radio transceiver, also called an RF coil, which can
communicate with your hydrogen atoms via radio
frequency (RF) waves.
These waves are close in frequency to those of your
favorite FM station. In fact, the room in which the MRI
scanner is located is probably shielded so that the local
easy listening station doesn't interfere with your images.
Your technologist is using that coil to send RF pulses at
When the RF pulse stops, the protons release
that absorbed energy, return to their previous
alignments and, in so doing, emit a signal back
to the coil .
The signal gets turned into an electric current,
which the scanner digitizes.
The lower the water content in an area, the
fewer hydrogen protons there will be emitting
signals back to the RF coils.
Different types of MRIs display this data
differently, but in any case you get a variety of
shades of grey that reflect the different densities.
In some scans, the weaker the signal, the darker
that part of the image will be.
So bone will be fairly dark, while fat will be light.
The gradient magnets. There are three of them
in the machine (called x, y and z), each oriented
along a different plane of your body, all of them
far less powerful than the main magnet.
They modify the magnetic field at very particular
points and work in conjunction with the RF
pulses to produce the scanner’s picture by
encoding the spatial distribution of the water
protons in your body.
Using medical terminology, the transverse (or
axial, or x-y) planes slice you from top to
bottom; the coronal (x-z) plane slice you
lengthwise from front to back; and the sagittal
(y-z) planes slice you lengthwise from side to
However, the x, y and z gradients can be used in
combination to generate image slices that are in
any direction, which is one of the great strengths
of MRI as a diagnostic tool.
The technologist and radiologist have the ability
to alter imaging parameters (like the timings of
the RF pulse and gradients) to emphasize areas
of injury or disease or to acquire higher image
Shades of gray:
Here’s a picture (sagittal view) of your spine! Now it’s
clear what the trouble is.
See the dark disc that, unlike the others, protrudes into
the spinal canal? That’s a herniated disc compressing
the nerves of the spinal cord.
MRI scans can display more than 250 distinct shades of
grey, each reflecting slight variations in tissue density or
It is in those subtle shades that radiologists unlock the
secrets of the tissues.
Why Cryogenics in MRI?
The spatial resolution of MRI Tomography
improves as the strength of available magnetic
The magnetic field strength is a function of
current conducted in superconducting loops.
Conventional MRI equipment useful in diagnostic
medical imaging requires high DC magnetic
fields, such as 5000 gauss or greater.
The basic physics principle says that as you cool
electrical components and circuits, two things
The impedance and resistance of those circuits
The noise, which is a result of thermal activity in
the materials, also comes down. So, you wind up
with lower resistance and lower noise.”
The stronger the magnetic field the higher the
frequency of the RF-pulse.
The proportionality factor is called the gyromagnetic
ratio, which is γ = 42.58 MHz/T for protons of
For human imaging however, magnetic fields in the
order of 0.1–4 T are commonly used.
This means that the RF-pulses will have frequencies up
to about 170 MHz –not far from commercial TV and
FM radio stations, which can interfere with the imaging
and vice versa.
The gyromagnetic ratio γ, is the radio frequency
at which the nucleus absorbs energy in a
magnetic field of 1 Tesla.
In MRI still almost only hydrogen is utilized. In
special cases one can make use of Phosphorous-
3114 or of Fluorine-1915, Helium-3 and Xenon-
189 , which do not occur naturally in humans.
Simple MRI magnet design with
horizontal bore and magnetic field
The diameter of the bore is usually about 60–80
cm, which limits the size of objects (patients) to
The effective length of the bore can be almost
any size but it is usually limited to less than one
A magnet of this size may be used for imaging
any part of a human being, as long as he/she is
not too large, or an animal .
Sketch of two MRI magnets in series for
imaging larger objects
Risks of MRI
1) External Projectile Effects
2) Internal Projectile Effects
3) Other magnetic Effects
4) Radiofrequency Energy
5) Gradient field changes
6) Acoustic Noise
7) Quenching of the Magnetic Field
Objects categories :
MR-Safe: The device or implant is completely non-magnetic,
non-electrically conductive, and non-RF reactive, eliminating
all of the primary potential threats during an MRI procedure.
MR-Conditional: A device or implant that may contain
magnetic, electrically conductive or RF-reactive components
that is safe for operations in proximity to the MRI, provided
the conditions for safe operation are defined and observed
(such as 'tested safe to 1.5 teslas' or 'safe in magnetic fields
below 500 gauss in strength').
MR-Unsafe: Nearly self-explanatory, this category is
reserved for objects that are significantly ferromagnetic and
pose a clear and direct threat to persons and equipment
within the magnet room.
The largest object I know of being pulled into a magnet is
a fully loaded pallet jack.
Few Psychological effects
observed during MRI
A slight increase of body temperature, a few
tenths of degrees, caused by the RF pulses.
Nerve stimulation in limbs, when the
magnetic fields are switched rapidly.
Safety in MRI Imaging
MRI is in most cases assumed to be a very safe
It is not invasive, contrast agents can often be
avoided and it does not use ionizing radiation.
The strong magnetic field has probably no
impact on the human body, but many metallic
implants are not compatible with MRI and
deaths have occurred.
Limitations of MRI :
High-quality images are assured only if you are able to remain
perfectly still while the images are being recorded. If you are
anxious, confused or in severe pain, you may find it difficult to
lie still during imaging.
A person who is very obese may not fit into the opening of a
conventional MRI machine.
The presence of an implant or other metallic object often makes
it difficult to obtain clear images and patient movement can have
the same effect.
Breathing may cause artifacts, or image distortions, during MRIs
of the chest, abdomen and pelvis. Bowel motion is another
source of motion artifacts in abdomen and pelvic MRI studies.
Assessment of the lungs is limited.
MRI may not always distinguish between tumor
tissue and edema fluid. It cannot detect calcium
present in a tumor. Detection of calcium (in
tumors or other issues) is limited with MRI.
MRI typically costs more and may take more
time to perform than other imaging modalities.
The next breakthrough could be the
construction of “high temperature” super
conducting magnets, which would need only
liquid nitrogen at 77 K and not expensive liquid
helium at 4 K in order to be cooled to super
conducting condition, i.e. to conduct an electric
current without any resistance at all.
Gifford-McMahon cryocoolers with ever-growing
capacity have contributed significantly toward achieving
this goal—this first allowed the elimination of liquid
nitrogen as a thermal shield coolant and more recently
the implementation of zero boil-off designs (0BO)
using a helium recondenser dramatically increased
helium refill intervals.
Pulse tube coolers with lower vibration and fewer
moving parts hold potential for the next step in
reliability and patient comfort.
He (L) Recondensing system for MRI superconducting magnet
(Utilizing 4 K cryocooler as well as cooler for radiation shield)
MRI technology is still in its infancy.
Manufacturers are constantly improving
machine designs, and scientists are discovering
new applications, from monitoring wine quality
to detecting lies; one MRI study revealed that
people used twice as many regions of the brain
to tell lies as they did to tell the truth.
WIPs in Toshiba America
Medical Systems, Tustin, Calif.
The Excelart Vantage 3T
The Excelart Vantage Plus powered by Atlas
technology—a 1.5T large-bore system. It provides a
field of view up to 55 cm.
Other WIPs include an
8-channel knee coil,
a 6-channel wrist coil,
an elliptical-shaped bore to better accommodate obese
JET motion-correction software.
The Excelart Vantage at both 3T and 1.5T is
powered by Atlas technology, and the Vantage 1.5T
with Atlas received FDA clearance during
Radiological Society of North America (RSNA).
The Excelart Vantage 3T features a new magnet
design and a short-bore combination as well as
Pianissimo Technology, the last of which reduces
Goals of the system are to enable whole-body
imaging and spectroscopy that more aggressively