Magnetic resonance imaging (MRI) is a technique for obtaining
high resolution images of various organs within the human body
by mapping the distribution of hydrogen nuclei.
The patient is placed in a strong external magnetic field, causing
the atomic nuclei of the part of the body being studied to line
up and create a magnetic field. These nuclei are exposed to
a radiofrequency (RF) electromagnetic wave, and absorb energy
from it. This absorption of energy is called resonance.
When the RF wave stops,the atoms relax back to equilibrium and
release the absorbed energy to the environment as RF wave emissions,
which can be detected with sensors.
To create a two dimensional picture, two magnetic field gradients
are applied to the organ to be studied, one perpendicular to
the other. The RF is applied after each magnetic field.
A
computer detects the RF emission and reconstructs it into a
picture (figures 23b, 122, 123, 124, 125 ).
Recent studies have shown that MRI after the intravenous administration
of a gadolinium-based contrast agent such as gadopentetate dimeglumine
can distinguish between reversible and irreversible myocardial
ischemic injury, regardless of the extent of the resultant wall
motion abnormality or the age of the infarct or heart attack
(see illustrations).
These results can be used to predict whether regions of abnormal
ventricular contraction due to the infarct will improve after
revascularization (bypass surgery or angioplasty ) in patients
with coronary artery disease.
It has been shown that areas of the myocardium which are hyperenchanced
are nonviable and do not recover to contract normally after
revascularization (figures 124, 125 ). While areas which do
not hyperenhance from the gadolinium injection are viable and
do recover and contraction improves after revascularization.
The
advantage over other methods like echocardiography and positron
emission tomography is that the hyperenhanced MRI (due to high
spatial resolution) shows the transmural extent of viable myocardium
with the nonviable myocardium being highlighted and has greater
accuracy in muscle segments with the most severe dysfunction
(while nuclear scintigraphy and echocardiography have reduced
predictive accuracy if more severe dysfunction of the myocardial
contractionis present).
Reference:R.Kim and others.The New England
Journal of Medicine, Vol.343, Nov.16, 2000, No.20, Pp.1445-1453.
