MRI combines a strong magnetic field and radio-frequency energy to study the distribution and behaviour of hydrogen protons
in fat and water. MRI systems are graded according to
the strength of the magnetic field they produce. Highfield systems
are those capable of producing a magnetic field strength
of 1–2 Tesla. MRI does not cause any recognized biological hazard,
but patients who have any form of pacemaker or
implanted electroinductive device must not be examined using
MRI because of risks to its function. Other prohibited items
include ferromagnetic intracranial aneurysm clips, certain types
of cardiac valve replacement and intraocular metallic foreign
bodies. It is generally safe to examine patients who have
extracranial vascular clips and orthopaedic prostheses,but
these may cause local artefacts. Loose metal items must be
excluded from the examination room, and beware of your credit
cards being wiped clean!The
real advantage of MRI is that it provides a vastly superior visualization
of the soft tissues than CT. It also provides especially
detailed imaging of soft tissues such as muscles,brain,
fascial planes and intervertebral discs. An intravenous injection
of contrast medium may be
given to enhance tumours and inflammatory and vascular abnormalities.
In recent years there has been an increase in the
use of MRA in which the flow of blood in vessels is picked up
without the use of contrast medium or any interventional procedure.
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