MRI Systems 101: Part 1

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MRI Systems 101 Part 1
Sensible Solutions for Refurbished Radiology
By Vikki Harmonay
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There are nearly 40 million MRI scans performed
every year in the United States, making it a true
workhorse of medical imaging systems. They also
account for well over 100 billion in revenue.
With so much at stake, its important to
understand how MRIs work. Lets take a look at
the components of MRI equipment.
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The Magnet
At the heart of every MRI system is a large
magnet, producing a very strong magnetic field.
To create the image, the patients body is placed
in that magnetic field during the imaging
procedure. There are two distinct effects that
work together to create the image Tissue
Magnetization and Tissue Resonance. Tissue
Magnetization As the patient is placed in the
magnetic field of an MRI, the tissue becomes
magnetized temporarily because of the alignment
of the protons. This very low-level effect
disappears when the patient is removed from the
MRIs magnetic field.
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Because the normal and pathologic tissues become
magnetized to different level or changes their
level of magnetization at different rates, the
MRI can distinguish between the different types
of tissues. Tissue Resonance The MRIs magnetic
field causes tissue to tune in or resonate at a
very specific radio frequency. Which is why the
procedures is called Magnetic Resonance Imaging.
The nuclei, typically protons within the tissue,
resonate. In the presence of a strong magnetic
field the tissue resonates in the RF range, which
causes tissue to function as a tuned radio
receiver and transmitter during the imaging
process. Two-way radio communication between the
tissue in the patients body and the equipment
results in the MRI image.
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The Magnetic Field
At any point within a magnetic field, youll find
two primary characteristics field direction and
field strength. Field Direction On the surface
of the earth, the direction of the earths
magnetic field is specified with reference to the
north and south poles. However, this north-south
designation is not usually applied to magnetic
fields used for imaging. Most electromagnets used
for imaging produce a magnetic field that runs
through the bore of the magnet and parallel to
the major patient axis.
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As the magnetic field leaves the bore, it spreads
out and encircles the magnet, which creates an
external fringe field. This can be a source of
interference with other devices and is usually
contained by some form of shielding. Field
Strength Each point within a magnetic field has
a particular strength or intensity. Its
expressed in either units of tesla (T) of gauss
(G), with 1.0 T being equal to 10,000 G or 10 kG.
At the earths surface, the magnetic field is
relatively weak and has a strength of less than 1
G. For imaging, magnetic field strengths are the
range of 0.15 T to 1.5 T.
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Homogeneity
MRI requires a very uniform magnetic field with
respect to strength. Field homogeneity is
affected by magnet design, adjustments and
environmental. conditions. Imaging usually
requires a homogeneity (field uniformity) on the
order of a few parts per million (ppm) within an
imaging area. High homogeneity is obtained by the
process of shimming.
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Magnets
Several different types of magnets can be used to
produce a magnetic field, each with its own
advantages and disadvantages
Superconducting Magnets
Most MRI systems utilize superconducting magnets.
A superconducting magnet is capable of producing
a much stronger and stable magnetic field than
resistive fields and permanent fields.
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A superconducting magnetic is an electromagnet
that operates in a superconducting state. A
superconductor is an electrical conductor (wire)
that has no resistance to the flow of an
electrical current, which means that tiny
superconducting wires can carry very large
currents without overheating. This is typical of
more conventional conductors like copper. Strong
magnetic fields are made possible by the combined
ability to construct a magnet with many loops or
turns of small wire and the use of large
currents.
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To achieve super-conductivity, the conductor or
wire must be fabricated from a special alloy and
then cooled to a very low temperature. The
typical magnet consists of small niobium-titanium
(Nb-Ti) wires imbedded in copper, which has
electrical resistance and functions as an
insulator around the Nb-Ti superconductors. The
electrical current flows through the
superconductor without dissipating any energy or
producing heat. If, however, the temperature of
the conductor should ever rise above the critical
superconducting temperature, the current will
begin to produce heat and the current will be
rapidly reduced, resulting in the collapse of the
magnetic field. This undesirable event is known
as a quench.
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Superconducting magnets are cooled with liquid
helium however the coolant must be replenished
periodically. Most superconducting magnets are in
the form of cylindrical or solenoid coils with
the strong field in the internal bore. Because of
the relatively small diameter and the long bore,
it can produce claustrophobia in some patients.
To reduce this concern, superconducting magnetic
design is evolving to more open patient
environments.
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Resistive
A resistive type magnet is made from a
conventional electrical conductor like copper.
The name resistive refers to the inherent
electrical resistance that is present in all
materials except for superconductors. When a
current passes through a resistive conductor to
produce a magnetic field, it produces heat, which
limits this type of magnet to relatively low
field strengths.
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Permanent
MRI can be performed with a non-electrical
permanent magnet, which doesnt require
electrical power or coolantsbut this type of
magnet is limited to relatively low field
strengths. Both resistive and permanent magnets
are designed to produce vertical magnetic fields
that run between the two magnetic poles. Possible
advantages include a more open patient
environment and less external field than
superconducting magnets.
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Gradients
When an MRI system is in a resting state and not
producing an image, the magnetic field is uniform
or homogenous over the targeted region of a
patients body. During the imaging process, the
filed must be distorted with gradients. Think of
a gradient as a change in field strength from one
point to another in a patients body. These
gradients are produced by a set of gradient coils
contained within the magnet assembly. The
gradients are turned on and off many times during
an imaging procedure, which creates the noise
that comes from a magnet.
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Gradient Orientation
Imaging magnets contain three separate sets of
gradient coils. The coils are oriented so that
gradients can be produced in three orthogonal
directions (x, y and z directions). Two or more
of the gradient coils can also be used together
to produce a gradient in any desired direction.
Gradient Functions
Gradients are used to perform a variety of
different functions during an image acquisition
process. These gradients create the spatial
characteristics by producing slices and voxels.
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All gradient echo imaging methods use a gradient
to produce an echo events and signal. Gradients
are also used to produce a type of image contrast
(phase contrast angiography) for vascular
imaging. They are also used as part of techniques
to reduce image artifacts.
Gradient Strength
Gradient strength is expressed in terms of the
change in field strength per unit of distance.
These typical units are millitesla per meter
(mT/m). The maximum gradient strength that can be
produced is a design characteristic of a specific
imaging system.
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Risetime and Slew-Rate
he gradient has to be able to change rapidly for
certain function. The time required for a
gradient to reach its maximum strength is called
the risetime and the rate at which the gradient
changes with time is called the slew-rate.
Eddy Currents
These are electrical currents that are induced or
generated in metal structures or conducting
materials that are within a changing magnetic
field.
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Eddy currents are produced in some metal
components of the magnet assembly by strong,
rapidly changing magnetic fields. Because eddy
currents create their own magnetic fields that
interfere with the imaging process they are
undesirable. Gradients are designed to minimize
eddy currents by using special gradient shielding
or electrical currents that control gradient
currents in a way that can compensate for the
eddy current effects. LOOK FOR PART TWO OF THIS
MRI COMPONENT SERIES NEXT WEEK
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Talk To An Expert
Are you in the market for medical imaging
equipment like MRI or CT Scanners? Talk to the
experts at Atlantis Worldwide. For more than 30
years, weve helped hospitals, clinics, medical
practices and urgent care centers find the
medical imaging systems they need on the used or
refurbished market. Youll get the performance
you want at prices that are kinder to your
budgetand with great warranties! Contact Us!
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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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Sensible Solutions for Refurbished Radiology