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Overview:
Magnetic Resonance Imaging (MRI)
is a state of the art diagnostic modality with broad clinical applications. It
uses a powerful but safe magnetic field, radiowaves and a computer for
analysis. MRI produces high contrast, high resolution images of the body's
internal organs and tissues. MRI provides the highest inherent sensitivity of
all diagnostic imaging modalities, may require the injection of contrast and
does not use ionizing radiation. It therefore poses no known health hazards to
adults and children alike.
Since its introduction to clinical imaging in
the early 1980s, MRI has become the diagnostic imaging modality of choice for
most malignancies and neurological diseases of the brain and spine. The
technology's ability to generate superb anatomical detail of both bone and soft
tissues non-invasively has made MRI the preferred modality for most orthopedic
applications including imaging of the knee, hip, shoulder, wrist and elbow. MRI
has become the examination of choice in children as well. The fact that it does
not use radiation and is generally a non-invasive technique means that it will
cause minimal distress if any to children.
The introduction of MRI
contrast agents has put to rest any lingering doubts about MRI's superiority
over CT in the examination of the Brain and Spine. The advantages over CT are
attributable to MRI's inherent superior resolution of soft tissue planes,
higher contrast discrimination and multiplanar capability not easily attainable
on CT.
Patient Comfort and Convenience:
Because of its
convenience and safety, MRI is suitable for virtually everyone (refer to
contraindications for MRI). Since there are no side effects, patients can
resume their normal activities as soon as the examination is finished (their
medical condition permitting).
The examination itself involves placing
the patient on the scanner bed and positioning specially designed surface coils
around or adjacent to the specific part of the body being examined (e.g., head,
knee, spine, shoulder), and then moving the patient into the center of a large
magnet. The center of the magnet is the place where the imaging takes place.
Once the patient is inside, the technologist communicates with the patient via
intercom.
The patient experiences no physical sensation during
the scan and will only hear some gentle tapping
noises. The tapping noises are merely the changing
magnetic field which helps to produce the scan being
switched on and off very fast with the aid of a
radio frequency. The scanner is located in a room
which must be shielded both to prevent external
radio frequencies getting in and the pulses produced
leaving.
Since no ionizing radiation
is used, it is commonplace for a close friend or relative to accompany the
patient into the scan room and stay with them during the procedure. This is
especially helpful in the case of claustrophobic patients and of course those
ill enough to require the supervision of nursing staff.
How MRI
Works:
MRI is based on a fundamental principle of physics governing the
behavior of the proton in the nucleus of the hydrogen atom when subjected to a
strong magnetic field. Within the powerful magnet to protons in the nuclei of
the hydrogen atoms become polarized and align themselves with the long axis of
the magnetic field in either a parallel state or in an opposite anti-parallel
state. Then a changing magnetic field is applied using a radio frequency which
is about 25MHZ for a 0.5 TESLA machine and 63MHZ for a 1.5 TESLA machine (both
being close to the FM Band or our radios). This change is magnetic field causes
the protons to align to a different axis.
When the radio pulse is turned
off, the protons return to the alignment with scanner's main magnetic field. In
doing so the proton emits a radio frequency as it gives up the energy given to
it. This signal is received by an antenna or surface coil which feeds the
information to a computer together with the position at which it occurred. All
these readings are converted into images which appear as slices through the
body in the area under examination. Using high speed reconstruction, it is
possible to stack these two-dimensional images to produce a three-dimensional
display.
Contraindications for MRI:
Patient should be screened
for contraindication metal or electro-mechanical implants such as:
- Pacemakers & Intracerebral aneurysm clips
It is the policy of the Thumb MRI Center not
to scan any heart pacemakers or intracerebral
aneurysm clips.
- Vena Cava filters
- Some mechanical prosthetic heart valves
- Cochlear implants
- Nerve stimulators used for pain relief where the
electrode is implanted
- Shrapnel or other metal foreign body which may
move when placed in a strong magnetic field
The most common
occurrence of this is patients who work or have worked in the sheet
metal industry, where metal foreign bodies get into the eyes despite
the use of guards and shields. The patients must be screened with conventional
orbital radiography to detect potential fragments in the eyes, and
any found should be removed before MRI scanning is carried out. Alternatively,
at the clinician's decision, an alternative diagnostic imaging modality
should be used.
- Patient who are pregnant
Pregnant
patients should only be scanned after the first
trimester of pregnancy. There are no known side effects of MRI, but
non-urgent examinations are only carried out after cell division has
finished and the fetus enters the growing phase. Of course MRI in urgent
situations is far safer than other radiological techniques.
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