MRI Systems 101: Part 2

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MRI Systems 101 Part 2
Sensible Solutions for Refurbished Radiology
By Vikki Harmonay
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MRI is the true workhorse of medical imaging
devices. In last weeks blog, we took a first
look at the components of MRI equipment. Lets
continue our exploration.
Shimming
For good imaging, you need a homogenous magnet
field in which there is a uniform field strength
over the image area. Shimming adjusts the
magnetic field to make it more uniform.
Magnetically susceptible materials located in the
magnetic field usually produce inhomogeneities.
The presence of these materials are responsible
for distortions in the magnetic field that are in
the form of inhomogeneities.
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These can occur in internal and external areas of
the field. When a different patient is placed in
the magnetic field, some inhomogeneities are
produced. External field items like building
structures and equipment can produce
inhomogeneities. When the external field is
distorted, the distortions can be transferred to
the internal field where they interfere with the
imaging process. These can produce a variety of
issues.
Its not possible to eliminate all of the sources
of inhomogeneities, so shimming must be used to
reduce them. Some shimming can be done by placing
metal shims in appropriate locations when a
magnet is manufactured and installed.
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In addition, magnets can contain a set of shim
coils, with shimming being produced by adjusting
electrical currents in these coils. As a rule,
general shimming is done by engineers when the
magnet is installed or serviced. Additional
shimming can be automatically done for individual
patients.
Magnetic Field Shielding
The external magnetic field surrounding the
magnet can be the source of two different types
of problems.
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• The field is subject to distortions by metal
  objects like building structures and vehicles.
  These distortions produce inhomogeneities in the
  internal field.

• The field can interfere with lots of types of
  electronic equipment like imaging equipment and
  computers.

Magnetic field shielding provides a more
attractive return path for the external field as
it passes from one end of the magnetic field to
the other. This happens because air is not a good
magnetic field conductor and can be replaced by
more conducive materials like iron.
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Two Types Of Shielding Passive and Active

• Passive Shielding is produced by surrounding the
  magnet with a structure that consists of large
  pieces of ferromagnetic materials like iron.
  These materials are a more attractive path for
  the magnetic field than air. The magnetic field
  is concentrated through the shielding material
  located near the magnet, which reduces the size
  of the field.

• Active Shielding is produced by additional coils
  that are built into the magnet assembly. They are
  designed and oriented so that the electrical
  currents in the coil produce magnetic fields
  designed to oppose and reduce the external
  magnetic field.

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Radio Frequency System (RF)
The RF system provides a communications link with
a patients body so an image can be produced. All
medical imaging modalities use a form of
radiation (like X-ray, gamma-ray) or other energy
like ultrasound to transfer the image from the
patients body. An MRI process uses RF signals to
transmit the image from the body. This RF energy
is a form of non-ionizing radiation. When RF
pulses are applied to the patients body, they
are absorbed by tissue and converted to heat. The
body emits a small amount of the energy as
signals to produce an image.
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The RF signals provide data from which the image
is reconstructed by the computer, because the
image itself is not formed within and transmitted
from the body. The image is a display of RF
signal intensities produced by these different
tissues.
RF Coils
Within the magnet assembly are RF coils, which
are relatively close to the patients body. The
coils serve as the antennae for transmitting
signals to and receiving signals from the tissue.
There are three different types of coil designs
for different parts of the body body, head and
surface coils. Sometimes the same coil is used
for both transmitting and receiving and at other
times, separate coils are used for transmitting
and receiving.
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Surface coils produce better image quality than
body and head coils can. These surface coils can
be in the form of an array of several coils or
just single coils. Each has its own receiver
circuit operated in a phased array configuration,
which produces the high image quality obtained
from small coils but with the added advantage of
being able to cover a larger anatomical region
with faster imaging.
Transmitter
RF energy is generated by the RF transmitter,
which is applied to the coils and then
transmitted to a patients body. A series of
discrete RF pulses generate the energy.
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A transmitter consists of several components like
power amplifiers and RF modulators and must be
capable of producing high power outputs of
several thousand watts. The RF power required is
determined by the strength of the magnetic field
and is proportional to the square of the field
strength. That means a 1.5 T system would require
nine times more RF power applied to a patient
than a 0.5 T system. The transmitter has a power
monitoring circuit, which is a safety feature
that prevents excessive power being applied to a
patients body.
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Receiver
After a sequence of RF pulses are transmitted to
a patients body, the resonating tissue will
respond by returning an RF signal. These are
picked up by the coils and then processed by the
receiver. The signals get converted into digital
form. They are then transferred to a computer for
temporary storage.
RF Polarization
An RF system can operate in a circularly
polarized mode or linear mode. Quadrature coils
are used in a circularly polarized mode,
consisting of two coils with 90 degrees
separation.
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This will produce improved excitation efficiency
by producing the same effect with half the RF
energy or heating to the patients, as well as
producing a better signal-to-noise ratio for the
received signals. By surrounding it with an
electrically conducted enclosure of sheet metal
and copper screen wire, an area can be shielded
against external RF signals. The principle behind
RF shielding is that RF signals cant enter an
electrically conducive enclosure. However, the
thickness of the shielding isnt a factor. Even
thin foil can serve as a good shield. However,
the room has to be completely enclosed by the
shielding materials without any holes whatsoever.
In addition, the imaging room doors are
considered part of the shielding and must be
closed while images are acquired.
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Computer Functions
An integral part of any MRI system is a digital
computer, which performs and controls the
production and display of MR images. Here are the
specific steps that are controlled and performed
by your computer.
Acquisition Control
During the acquisition process, there are many
repetitions of an imaging cycle. During each
cycle, a sequence of RF pulses is transmitted to
the patients body. The gradients are then
activated and RF signals collected. Its
important to note than one imaging cycle doesnt
produce enough signal data or information to
create an image, so the cycle must be repeated
multiple times.
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The time needed to acquire images is determined
by the duration of the imaging cycle or
repetition time. This adjustable factor is known
as TF. The number of cycles used affect the image
quality, with more cycles producing better images
as a rule.The acquisition process is controlled
by protocols stored in the computer and the
operator can select from preset protocols.
Image Reconstruction
During the acquisition phases, the RF signal data
is not in the form of an image but the computer
can use this collected can be data to
reconstruct an image. This is known as a
Fourier transformation. Its a mathematical
process that is relatively fast and usually
doesnt have a significant effect on the total
imaging time.
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Image Storage and Retrieval
The reconstructed images are stored in the
computer, where the images can be used for
additional processing and viewing. Your storage
media determines how many images can be displayed
or stored.
Viewing Control and Post Processing
Your computer system controls the display of the
images, making it possible for a user to select
specific images and control viewing factors like
windowing (contrast) and zooming (magnification).
This allows the user to reformat an image or set
of images or change the display in order to
produce specific views of anatomical regions.
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Much post processing is performed on a
workstation computer in addition to the computer
system in the MRI system.
Talk To An Expert
Does your hospital, urgent care, clinic or
veterinary practice need an MRI system? If so,
oftentimes a refurbished MRI system can provide
the performance you want for a much lower price
point. Talk to one of the experts at Atlantis
Worldwide and see if this can be a solution for
you. Contact Atlantis Worldwide today.
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Some other blogs you may have missed

1. The Future of Helium MRIs
2. MRI Cold Head Tips
3. Service Contracts for Imaging Systems Penny Wise
   and Pound Foolish?
4. Radiologists, Healthcare Social Media
5. Should your business lease or buy medical imaging
   equipment?
6. Free MRI Resources

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