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MRI uses magnetic energy and radio waves
to create cross-sectional images or "slices" of the human body. The main
component of most MR systems is a large tube-shaped or cylindrical magnet. Also, now
available are MR systems with a C-shaped magnet or other type of open design. The strength
of the MR system's magnetic field is measured in metric units called "Tesla".
Most of the cylindrical magnets have a strength between 0.5 and 1.5 Tesla and most of the
Open or C-shaped magnets have a magnetic strength between 0.01 and 0.35 Tesla. A 1.5 Tesla
MR system has a magnetic field 30,000 times stronger than the pull of gravity on the
earth's surface.
The MR technologist positions a chest surface coil on the patient prior to the MR
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The patient aperture or bore of the
cylindrical magnet is usually between 55 cm and 65 cm wide (21.6" to 25.6") and
they have a total end to end length of 160 cm to 260 cm (5‘ 3" to 8' 6").
Approximately 3% of MR patients suffer from claustrophobia and may not tolerate an MR exam
in a traditional cylindrical MR system. These patients may now have the option of having
an MR study with an Open MR system where typically many sides of the system are open and
claustrophobic anxiety is lessened.
To begin the MR examination, the
patient is positioned on a special table and positioned inside the MR system opening where
the magnetic field is created by the magnet. Each total MR examination typically is
comprised of a series of 2 to 6 sequences, with each sequence lasting between 2 and 15
minutes. An "MR sequence" is an acquisition of data that yields a specific image
orientation and a specific type of image appearance or "contrast." Thus a
typical exam can last for a total of ten minutes to an hour, depending on the type of exam
being run and the MR system being used.
During the examination, a radio signal
is turned on and off, and subsequently the energy which is absorbed by different atoms in
the body is echoed or reflected back out of the body. These echoes are continuously
measured by the MR scanner and a digital computer reconstructs these echoes into images of
the body. The tapping heard during the MR exam is created when "gradient coils"
are switched on and off to measure the MR signal reflecting back out of the patient's
body. A benefit of MRI is that it can easily acquire direct views of the body in almost
any orientation, while CT scanners typically acquire images perpendicular to the long body
axis.
The following diagram
shows the three main planes of acquisition used in MR imaging.
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Medical images taken of the human body
are acquired or displayed in three main orientations:
- Coronal orientation: in a cross section
(plane), for example, across the shoulders, dividing the body into front and back halves
- Sagittal orientation: in a cross section
(plane), for example, down the middle, dividing the body into left and right halves
- Axial orientation: in a cross section
(plane), perpendicular to the long axis of the body, dividing the body into upper and
lower halves
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Views which are combinations of the
above three orientations (called oblique views) can also be directly acquired with the MR
system. These oblique views are especially important for orthopedic and sports medicine
applications when tendons and ligaments that run at oblique angles need to be clearly
imaged.
Updated: June 10, 2008
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