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Basic Principles of MRI
During the course of
the exam, the patient is moved into the center of a large cylinder which forms
the core of the magnet. Depending on the MRI system in use, the magnet may be
one of three types: permanent, restrictive (an electromagnet), or super
conductive (an electromagnet with higher magnetic field strength obtained
through the use of super conductors and cryogens, such as liquid helium or
nitrogen). All three types operate under the same principles, and the
differences among them are inconsequential to the patient.
Once inside
the magnet, the patient must lie perfectly still.
Several different scans comprise an exam, each ranging
from 30 seconds to 30 minutes. As the patient is
exposed to the magnetic field, many of the randomly-oriented
protons in the body will orient themselves with
the magnetic field. When imaging begins,
radio-frequency (RF) pulses of a specific frequency
are directed at the patient through the imaging
coil – causing the
protons to absorb the energy and become excited.
(The "gentle tapping" sound heard
by patients is a result of the changing magnetic
field and is physically the result of the gradients
vibrating.) After the RF pulses are turned off,
the protons release the energy at the same frequency.
the imaging coil also acts as a receiver coil to
detect the energy that is returned. This signal
is measured by the computers and is used to create
an image.
General Patient Preparation
A
patient sent for an MRI scan typically requires
no preparation, dietary restrictions, shielding,
or injections. A simple explanation of the procedure
and thorough check for contraindication are all
that is required prior to the exam. Electromachanical
implants – such
as cardiac pacemakers, cochlear implants, and neurotransmitters – may
be "frozen" or switched off in the
presence of the magnetic field. Ferromagnetic clipsare
highly dangerous because the torque induced by the
magnetic field could potentially dislodge a recently
implanted clip. Similar concern
should be expressed for any patient with a history
of steelworking, because tiny metal fragments may
be imbedded in their eyes. To prevent any possibility
of optic damage, these patients should be screened
with plain films prior to their MRI
exam.
IT IS VERY IMPORTANT THAT YOU INFORM YOUR REFERRING PHYSICIAN AND
THE THUMB MRI CENTER STAFF IF YOU HAVE ANY OF THE
FOLLOWING:
- Shrapnel
- Pregnancy
- Pacemaker
- Hearing aid
- Metal implants
- Dental bridges
- Aneurysm clips
- Surgical staples
- Cochlear implant
- Dorsal column stimulator
- Inferior vena cava umbrealla
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