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is a relatively new and special examination of the human body, which uses
electromagnetic waves and sophisticated computers to produce images of the
internal organs of the body. For this to be possible, the patient will be
placed within the bore of a large magnet, which produces a magnetic field
several thousand times stronger than that of the earth.
Radio
waves are used to produce images of the internal structure of the body.
This is an excellent non-invasive way of looking into the human body
without the need for needles, catheters or operations. Structures as small
as one millimeter in diameter can be visualized. Excellent
characterization of soft tissue is obtained. It is possible to look at the
major blood vessels of the body without injection of contrast medium. The
structure and function of the heart can also be assessed. The images that
are obtained will be viewed by the radiologist to look for evidence of
disease. There is no radiation involved and there is no known risk to
children or adults.
There are several different types of MR scanners. Those
that use a stronger magnet generally produce better images. The MR scanner
comprises a magnet with a hollow center called the bore. The patient will
be placed within the bore of the magnet. The bore has a variable diameter
some of which may be as small as 60 cm. The length of the bore is
approximately 1.5m. However there are MR scanners which have an open
magnet, where there is no bore. This would be suitable for patients who
may not be able to tolerate being in the closed space of the narrow MR
scanners. Interventional procedures under MR guidance is also possible
using the open magnet MR scanner.
The
majority of patients can undergo an MRI scan. It is safe for women who are
more than four months pregnant to undergo this examination. Those who are
less than four months pregnant should discuss the need for the examination
with their doctor and the radiologist who will be supervising the
examination.
Patients weighing more than 140 kilograms cannot be
scanned. MRI can cause serious injury and even death in those with metal
embedded in their bodies. Patients with the following objects inside the
body generally cannot undergo an MRI examination:
Depending on which part of the body is to be examined, the
patient may be required not to drink or eat for at least six hours before
the examination. At the time of examination, female patients should not
use any facial make-up as this may interfere with the image. The patient
will be required to change into a hospital gown and remove all removable
metallic objects (e.g. jewellery, belt, buckles, pins etc) before entering
the MR scan room. After interviewing the patient, a radiographer will
position the patient on the scanner bed.
Close
relatives are usually allowed to be in the scanner room with the patients
during the examination, if it is required. Earplugs are supplied to
patients, as there will be a loud knocking noise during the examination.
This scanner bed will then be moved into the magnet tunnel. It is very
important for the patient to lie still during the examination. Sedation
may be required for children and for patients who are irritable or
agitated. In a few MRI units, scanning is done under general anaesthesia
in children.
It will be possible
to communicate with the radiographer via an intercom throughout the
procedure. The radiographer will be able to observe the patient through a
glass window from the control room or through a TV monitor via a camera
positioned in the scanner. The radiologist may decide to give the patient
an injection of a special dye for MRI scanning. This may sometimes cause a
feeling of warmth for a short while. The total duration of the examination
varies from 30-90 minutes depending on the part of the body being
examined. Generally, there are no
side effects from the examination.
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