Dr. Bradley
is the Chairman of and a Professor in the Department of
Radiology, University of California San Diego Healthcare, San
Diego, CA.
Atherosclerosis is a topic very close to my heart. It is fitting
that this issue of
Applied Radiology
addresses cardiac imaging, given the tremendous strides made over
the past decade, particularly with magnetic resonance imaging (MRI)
and computed tomography (CT).
Much of heart disease is acquired as a result of
atherosclerosis. For the past 10 years, there has been a
fundamental shift in the way we image atherosclerotic disease, in
the coronary arteries as well as in the carotids and any other
arteries where flow may be limited by an occlusive plaque.
For the greater part of the last century, imaging was focused on
quantitating the degree of stenosis. A 70% reduction in vascular
diameter, ie, 50% reduction in cross-sectional area, was considered
"flow limiting." This 70% figure was given additional weight by the
1991 NASCET trial, which showed that at least a 70% stenosis of the
carotid artery was necessary to justify carotid endarterectomy in
symptomatic patients.
1
The NASCET trial used two numbers from a two-view cut-film carotid
angiogram: the carotid diameter on the most narrowed view of the
residual lumen divided by the normal diameter of the internal
carotid artery just distal to the carotid bulb. Although the NASCET
trial was based on cut-film angiography, the same number has come
to be used with more modern digital imaging modalities, such as
ultrasound, CT angiography, and MR angiography. Given the oft-noted
point that atherosclerotic plaque can be eccentric and that two
views from a routine angiography study may not estimate degree of
stenosis accurately, many now favor the use of a cross-sectional
area now that the newer digital modalities are being acquired in
the axial plane. Thus, instead of a 70% ratio of diameters,
stenosis would be measured in square millimeters. This should
improve quantitation of stenosis; however, degree of stenosis may
not be the most important factor in predicting stroke (as in
NASCET) or myocardial infarction.
Over the last decade, attention has been increasingly focused on
the vulnerability of the plaque itself, rather than on the degree
of stenosis. While chronic narrowing of the coronary arteries may
lead to symptoms (ie, angina with exercise) or findings (eg, ST
depression/elevation on a stress EKG), such findings, at most, may
lead to additional confirmatory testing up to an angiogram on an
elective basis.
Of much greater concern than chronic narrowing is acute rupture
of the atherosclerotic plaque. This forms a thrombogenic surface,
which leads to acute occlusion of the vessel locally by clot or to
an embolus being sent downstream. The former is more common in the
coronary arteries as a cause of myocardial infarction. The latter
is more common in the carotid arteries as a cause of cerebral
infarction. Thus, current research in atherosclerosis is less
concerned with degree of stenosis than with characterization of
plaque stability. Identification of vulnerable plaque, ie, that
which is more likely to rupture acutely, should be the goal of
imaging, as such lesions are more often the cause of fatal cerebral
or myocardial infarction.
The mature atherosclerotic plaque can be characterized simply as
a fibrous cap covering a lipid core. The thicker the fibrous cap,
the more stable the plaque and the less likely it is to rupture.
Vulnerable plaques have thin fibrous caps. In this context, "thin"
is on the order of 50 to 100 microns (0.05 to 0.10 mm), which is a
much higher spatial resolution than is currently available with
either CT or MRI. Stable plaques tend to be calcified on CT. This
poses an interesting conundrum: high coronary calcium scores may
actually represent more stable disease than low scores do. As
expected, large lipid cores are low density on CT. On MRI, a lipid
core is bright on T2-weighted images and the fibrous cap enhances
with gadolinium. It is hoped that using local coil technology and
k-space re-registration techniques, such as periodically rotated
overlapping parallel lines with enhanced reconstruction (PROPELLER)
MRI,
2
to get rid of motion artifact, the spatial resolution will improve
to the point at which the thickness of the fibrous cap can be
evaluated and disruptions can be visualized. While plaque
characterization will most likely be performed initially in the
carotid arteries (which could serve as a surrogate for coronary
disease), we should expect the technology to transfer fairly
quickly to the heart.