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About Magnetic Resonance Imaging (MRI)
 MRI
provides a wealth of structural and biochemical information
about tissue (Hilal SK, Mohr JP, 1992.
Dunbabin DW, Sandercock PAG, 1991.
Caplan LR. Stroke:
a Clinical Approach.
Boston, Butterworth-Heinemann, 1993, pp 99-150].
The technique is based on the interaction between radio
frequency waves and the nuclei of different atoms in the body
in the presence of a strong magnetic field. Hydrogen (proton) is the most common nuclear magnetic
resonance-observable nucleus within the human body. Clinically, water protons and fat protons are the most
extensively imaged nuclei.
In
the presence of the powerful magnetic field, the protons are
susceptible to excitation by selective radio frequency pulses. The energy from these pulses is absorbed and then released
until the tissue being scanned has completely reemitted the
energy absorbed and has undergone complete relaxation.
The energy released from the excited tissue occurs over
a short period of time according to two relaxation constants
known as T1 and T2. In
T1-weighted images, cerebrospinal fluid has a low-density
relative to brain, while the fat has a high-signal intensity.
In T2-weighted images, cerebrospinal fluid has an
increased signal relative to brain, and fat has almost no
signal.
Conventional
MRI shows tomographic sections of the brain in multiple plans
of proton distribution modified by T1 and T2 relaxation times
as well by a third type of image with a balanced T1-and
T2-weighting.
Ischemia
alters water content in the brain cells, changing their response
to a magnetic field.
Infarction prolongs the T1 and T2 relaxation films.
Intracerebral hemorrhage (ICH) has different appearances
that are complex and depend on the duration of time since
the bleed, the strength of the magnetic field, and the settings
used to obtain the images [Bruno A. Geriatrics.
1993; 48;26].
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Stroke Education
