AJR 2000; 174:1119-1125
© American Roentgen Ray Society
Diagnostic Imaging of Thoracic Aortic Atherosclerosis
Paul A. Tunick1,
Glenn A. Krinsky2,
Vivian S. Lee2 and
Itzhak Kronzon1
1
Department of Medicine, New York University School of Medicine, 560 First
Ave., New York, NY 10016.
2
Department of Radiology, New York University School of Medicine, New York, NY
10016.
Received August 3, 1999;
accepted after revision September 17, 1999.
Address correspondence to G. Krinsky
Introduction
As recently as 1989, large stroke databanks listed up to 40% of strokes as
cryptogenic [1]. More recently,
the thoracic aorta has been implicated as a source of cryptogenic strokes and
peripheral organ damage because imaging techniques, including transesophageal
echocardiography, CT, MR imaging, and MR angiography, have allowed the
visualization, characterization, and quantification of atherosclerotic lesions
in the thoracic aorta. We reviewed the imaging findings of thoracic aortic
atherosclerosis and their clinical significance, the understanding of which
has previously been derived from transesophageal echocardiography. We also
reviewed the related developments in CT and MR imaging of atheromatous
disease, including the characterization of plaque components on MR
imaging.
Transesophageal Echocardiography
First developed in the 1970s
[2], transesophageal
echocardiography provides high-resolution images of the entire thoracic aorta
except for a small part of the ascending aorta, which is blocked by the air
column in the trachea.
Currently, transesophageal echocardiography is the procedure of choice for
the detection, measurement, and characterization of thoracic aortic atheromas.
Direct sonography has been used in the operating room to view atheromas in the
ascending aorta [3], and
transthoracic echocardiography has been used to view plaque in the ascending
aorta and the aortic arch. In one study, 20 patients were examined with
transthoracic and transesophageal techniques
[4]. However, the resolution of
images obtained with the transthoracic approach is worse than that of those
obtained with the transesophageal approach. Higher resolution images can be
obtained with intravascular sonography using high frequency transducers, and
intravascular sonography has been successfully used in conjunction with
thoracic aortic interventional procedures
[5]. However, it is too
invasive to be used as a diagnostic test for atheromatous disease and could
result in iatrogenic embolization.
Transesophageal echocardiography is a safe minimally invasive procedure
with a low risk of complications. It can be performed in various settings,
from the operating room to the bedside. Moreover, it may be performed in
conscious patients with local anesthesia, and a thorough examination of the
thoracic aorta and the heart can be performed in 10 min or less. In 1990,
researchers reported viewing atheromas in the thoracic aorta on
transesophageal echocardiography in patients with embolic disease
[6].
Plaque Thickness on Transesophageal Echocardiography
The normal aorta has a smooth intimal surface (
1 mm thick) and can be
seen on transesophageal echocardiography
(Fig. 1). Atherosclerosis
causes intimal thickening with the accumulation of lipid-laden foam cells and
smooth muscle cells, which migrate from the media. Other features of
atherosclerosis include ulceration (Fig.
2), calcification (Fig.
3), and a fibrous cap. The thickness of an intimal plaque,
ulcerations, calcifications, and super-imposed mobile thrombi can be measured
on transesophageal echocardiography. One study graded aortic plaque thickness
in patients undergoing cardiac surgery with cannulation of the aorta for
cardiopulmonary bypass [7]. The
study included a five-grade ranking system: grade 1 = normal aorta, grade 2 =
flat intimal thickening, grade 3 = protruding atheroma in the aortic lumen
(< 5 mm) (Fig. 4), grade 4 =
protruding atheroma (
5 mm) (Fig.
5), and grade 5 = atheroma with a mobile thrombus
(Fig. 6). Patients with grade 5
disease had the highest risk of intraoperative stroke during cardiac surgery,
which probably resulted from the dislodgment of atheroma and thrombus by the
bypass cannula [7].
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Fig. 2. —59-year-old man with left main coronary artery stenosis and
transient ischemic attack. Transesophageal echocardiogram shows severe
atherosclerosis of thoracic aorta with large ulcerated plaque (U).
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The French Aortic Plaque in Stroke investigators used transesophageal
echocardiography to characterize aortic arch plaque thickness in a group of
patients with stroke [8]. They
found that increasing plaque thickness imparted increasing risk, and patients
with a plaque thickness of 4 mm or more had a significantly greater odds
ratio. The odds ratio for aortic arch plaques (<1 mm) and stroke was 1.0
(no increased risk). The odds ratio was 3.9 for arch plaques between 1- and
3.9-mm thick and 13.8 for plaques greater than or equal to 4-mm thick.
Plaque Morphology and Plaque Thrombosis
Ulceration.—In 1992, a landmark autopsy study examined the
thoracic aorta of 500 patients with stroke and other neurologic diseases
[9]. Ulcerated plaques were
present in the aortic arch of 26% of patients with cerebrovascular disease and
in 5% of patients with other neurologic diseases. Of patients with cryptogenic
stroke, 61% had aortic arch disease, the highest prevalence among all disease
groups. Ulcerations (2 mm) diagnosed with transesophageal echocardiography
are associated with a higher risk of stroke
(Fig. 2). Such ulcerations were
noted in 39% of patients with cryptogenic stroke, 8% with stroke of known
cause, and 7% without stroke
[10].
Calcification.—Although calcification represents one
manifestation of the atherosclerotic process
(Fig. 3), large high-risk
plaques are often uncalcified; therefore, palpation of the aorta by a cardiac
surgeon does not necessarily help to identify high-risk plaques
[7]. The French Aortic Plaque
in Stroke investigators examined plaque morphology on transesophageal
echocardiography in 334 stroke patients over the age of 60 years
[11]. Plaques greater than or
equal to 4-mm thick had the highest risk for and an increased prevalence of
ulceration, calcification, and hypoechoic areas. Hypoechoic areas are possibly
indicative of a lipid core. Interestingly, it was the absence of calcification
that imparted the greatest risk of stroke (odds ratio 10.3). Using an analogy
with unstable coronary artery plaques, it is possible that the lipid-laden
vulnerable plaques may be uncalcified. Support for this argument comes from a
histologic examination of human aortic plaques that were intact as compared
with those that had superimposed thrombi
[12]. Furthermore, researchers
noted the presence of plaque thrombosis as a characteristic of plaques with a
high proportion of extracellular lipid content
[12].
Superimposed thrombi.—Many protruding atheromas have mobile
components such as thrombi. (Fig.
6)
[13,14,15,16].
These mobile lesions may disappear after heparin or warfarin therapy
[17,
18] or thrombolysis
[19].
The unstable nature of plaques with thrombi.—A longitudinal
study reported the results of repeat transesophageal echocardiography in
patients with aortic atheromas
[20]. In 11 (61%) of 18
patients, repeat imaging revealed new mobile lesions on plaques that were
previously lesion free. Additionally, seven of 10 mobile lesions present
during the first examination disappeared before follow-up. Research shows that
these mobile lesions are responsible for high embolic risk and that
embolization of thrombus from aortic atheromas has been documented with
specimens removed from the femoral arteries of patients with acute limb
ischemia [13,
14]. Furthermore, it is likely
that cerebral embolization of thrombus from aortic atheromas causes stroke.
Another embolic syndrome, atheroemboli syndrome with blue toes (which may
occur in patients with aortic atherosclerosis), often accompanies renal
failure or intestinal infarction. This syndrome is relatively uncommon, and
was reported in 0.7% of patients treated with warfarin during the Stroke
Prevention in Atrial Fibrillation-III trial
[21].
One advantage of transesophageal echocardiography over other imaging
techniques is its ability to reveal high-risk mobile thrombi in the thoracic
aorta with real-time imaging.
Imaging the great vessels with transesophageal
echocardiography.—Transesophageal echocardiography does not show
the innominate artery because the tracheal air column obscures its origin. The
ability to image the innominate artery is a significant advantage of CT and MR
imaging, especially in patients with right brain events who have
transesophageal echocardiograms with negative results. However, innominate
atheromas are relatively uncommon
[22].
Transesophageal echocardiography reveals the proximal portions of the left
common carotid artery and the left subclavian artery
[23]. Usually, the two
arteries are not revealed in the same plane but each can be identified by
distinct flow patterns on Doppler sonography. The subclavian artery has a
high-resistance flow velocity pattern with relatively little or no diastolic
flow. The common carotid artery has a pattern of low-resistance flow provided
that the internal carotid artery is unoccluded.
Clinical Usefulness of Transesophageal Echocardiography in Patients
with Thoracic Aortic Plaque
As mentioned previously, aortic atheromas may be responsible for stroke and
peripheral emboli. These embolic events may be spontaneous or may result from
aortic manipulation during cardiac surgery
[24], balloon pump placement,
or cardiac catheterization
[25]. After the first report
of strokes and peripheral emboli caused by atheromas appeared in 1990
[6], other case series
[26,27,28]
were followed by retrospective (case-control)
[29,30,31]
and prospective studies
[32,33,34].
These studies reported the prevalence of significant thoracic aortic plaques
(approximately 25% in stroke patients) and the future risk of stroke in
patients with plaques (12% per year). An additional prospective study followed
patients with ascending aortic plaques seen with epiaortic sonography during
surgery [35]. The study
reported a significant increase in future events and a higher mortality rate
in patients with significant plaques. The mortality rate was 43% over 7 years
in those with the severe-grade plaques (>5 mm, ulcerated, or mobile).
Although most clinicians think of carotid artery disease and atrial
fibrillation as the most important causes of stroke, two recent studies report
the prevalence of aortic atheromas on transesophageal echocardiography
(21-27%) similar to that of carotid disease (10-13%) and atrial fibrillation
(18-30%) [8,
30]. Therefore,
transesophageal echocardiography may identify an atherosclerotic embolic
source for cryptogenic strokes.
Transesophageal Echocardiography During Heart Surgery
Intraoperative transesophageal echocardiography is used to examine left
ventricular wall motion and filling
[36] and the success of valve
reconstruction [37]. The
amount of atherosclerotic plaque in the thoracic aorta has a major impact on
the outcome of cardiac surgery. Patients with significant aortic arch
atheromas have an intraoperative stroke rate that is approximately 12%
[24]—five times higher
than the general intraoperative stroke rate. It is possible that the
identification of arch atheromas may allow surgeons to avoid cannulating
directly in the lesions, thereby avoiding embolic complications.
Patients with aortic arch atheromas should avoid the newly developed
minimally invasive technique using a port-access system. With this Heartport
technique, surgeons pass a cannula with a balloon occluder (endoclamp) through
the aorta from the femoral artery
[38]. However, high-risk
patients with significant aortic arch plaque may undergo coronary artery
bypass on the beating heart (minimally invasive direct coronary artery
bypass). During this procedure, while the heart is beating, internal mammary
grafts are placed on the epicardial coronary arteries distal to obstructions.
This procedure may avoid intraoperative strokes that can be caused by aortic
cannulation in patients with arch atheromas
[39].
The Importance of a Combined Sonographic Examination of the Carotid
Arteries and the Thoracic Aorta in Stroke Patients
High resolution real-time sonography of the carotid arteries has been used
for over 20 years [40],
although originally, some concluded that the development of venous digital
subtraction angiography would make further vascular sonography studies useless
[41]. Currently, duplex
scanning (real-time sonography combined with Doppler interrogation of carotid
flow velocity) is the standard for the diagnosis of carotid stenosis and is
often the first test performed to determine the cause of stroke in patients
with normal sinus rhythms.
Researchers have found the association of thoracic aortic atheromas with
stroke independent of carotid disease and atrial fibrillation
[29,30,31].
All patients with carotid stenosis or atrial fibrillation were excluded from
the first study, and in the other two studies thoracic aortic atheromas were
independent predictors of stroke even when carotid disease and atrial
fibrillation were controlled for multivariate analysis. However, it is not
surprising that patients with symptomatic severe carotid artery
atherosclerosis may also have severe atherosclerosis in the thoracic aorta.
This association was reported in a retrospective study that found that stroke
patients with severe carotid stenosis had a significantly higher prevalence of
severe aortic arch atheromas (38%) than stroke patients without carotid
stenosis (17%) [42]. That
study also showed that patients with high-risk aortic arch plaques (with
superimposed mobile thrombi) also had a high risk for carotid disease (>80%
diameter stenosis on duplex scanning). In fact, the presence of mobile thrombi
in the aortic arch was limited to patients with the most severe carotid
stenosis. Therefore, it is reasonable to consider transesophageal
echocardiography to examine the aortic arch in patients with carotid disease,
especially if neurologic events are contralateral to carotid stenosis, if
peripheral emboli are present, or if neurologic events occur in spite of
technically successful carotid endarterectomy because there may be more than
one source of emboli in these patients.
CT
The development of electron-beam CT to quantify coronary artery
calcification and coronary atherosclerotic plaque burden
[43,44,45,46]
has paralleled similar developments using conventional CT to quantify
calcifications of the aortic wall
[47]. Unlike coronary artery
imaging, the larger-sized aorta lends itself to easier quantification of
noncalcified plaque, particularly with contrast-enhanced CT. This approach has
been proposed as a valuable noninvasive method for following the progression
and regression of atherosclerotic disease
[48,
49].
Tenenbaum et al. [50] used
unenhanced dualhelical CT to assess calcium deposits and areas of
hypoattenuation adjacent to the aortic wall in 32 patients with recent stroke
or embolic events. The authors found that defining a threshold of 4-mm
thickness for protruding atheromatous plaques resulted in the best sensitivity
(13/15; 87%) and specificity (14/17; 82%) for CT when compared with
transesophageal echocardiography. This corresponds well to the threshold of 4
mm that was found in the French Aortic Plaque in Stroke study
[8] to predict a significantly
increased risk of stroke. Tenenbaum et al. also found that although unenhanced
CT is suitable for screening studies, unenhanced dual-helical CT with positive
results should be followed with contrast-enhanced CT (Figs.
7A,7B
and 8) or transesophageal
echocardiography. Notably, the authors also reported six protruding atheromas
in the upper ascending aorta and proximal arch that were overlooked on
transesophageal echocardiography. These cases highlight one advantage of CT
over transesophageal echocardiography: CT provides complete imaging of the
thoracic aorta, whereas transesophageal echocardiography does not.
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Fig. 7A. —82-year-old man with severe atherosclerotic disease.
Contrast-enhanced helical CT scan at level of right main pulmonary artery
reveals ulcerated plaque in ascending aorta (straight arrow) and
descending thoracic aorta (curved arrow).
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Fig. 7B. —82-year-old man with severe atherosclerotic disease.
Contrast-enhanced helical CT scan at level of bifurcation of main pulmonary
artery shows protruding atheromas of ascending (solid arrow) and
descending (open arrow) thoracic aortas.
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New multidetector CT scanners with image acquisition times less than 1 sec
are promising because, similar to electron beam CT, they may allow synchronous
imaging with the cardiac cycle, thereby reducing artifacts (associated with
cardiac motion) in the ascending aorta and the aortic root. Additionally, the
temporal resolution of CT fluoroscopy may provide the ability to detect the
mobile component of atheromas. However, such improvements in the detection and
characterization of atheromatous plaque remain to be revealed.
MR Imaging
As early as 1983, Herfkens et al.
[51] reported that MR imaging
could be used to view aortic atherosclerosis. Using spin-echo imaging at 0.35
T, researchers were able to view aortic wall thickening and protruding
atheromas in the abdominal aorta and pelvic vessels of patients in vivo. More
recently, one group compared transesophageal echocardiography with MR
angiography for examining protruding atheromas
[52]. They found that MR
angiography underestimated plaque thickness, probably because of difficulties
in defining the aortic wall on the contrast-enhanced MR angiograms. Without
ECG-gated cine gradient-echo images, MR imaging also provides static views of
disease without assessing clinically significant mobile thrombus. However,
similar to CT, MR imaging can reveal the complete aorta (including the blind
spots of transesophageal echocardiography) and assess great vessel
disease.
In addition to identifying morphologic features of atheromas, MR imaging
reveals contrast between different tissue types that can be used to define the
histologic components of atherosclerotic plaque. Using a range of techniques
including T1-weighted, proton density—weighted, T2-weighted, and
diffusion-weighted MR imaging, several groups describe the accuracy of MR
imaging to reveal calcification, fibrocellular tissue, lipid, and thrombus
both ex vivo and in vivo
[53,54,55,56]
(Fig.
9A,9B).
Although requiring more invasive measures, intravascular MR imaging techniques
with MR catheter coils are also under investigation for the high-resolution
imaging of the aorta [57]. For
example, Zimmermann-Paul et al.
[58] developed intravascular
MR coils that allow monitoring of plaque formation in rabbit models with
hyperlipidemia. These coils allow high-resolution imaging during intervention
under MR guidance.
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Fig. 9A. —31-year-old woman with history of peripheral embolic disease and
severe atherosclerosis of descending thoracic aorta. Representative plaques
are different in appearance and can be characterized on the basis of T1-,
proton density—, and T2-weighted MR images. Inserts represent magnified
views of descending thoracic aorta. (Courtesy of Fayad ZA, New York, NY) Axial
T2-weighted MR image shows 7-mm fibrocellular stable plaque (arrows).
Note origin of right coronary artery (RCA) from aortic root (Ao).
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Fig. 9B. —31-year-old woman with history of peripheral embolic disease and
severe atherosclerosis of descending thoracic aorta. Representative plaques
are different in appearance and can be characterized on the basis of T1-,
proton density—, and T2-weighted MR images. Inserts represent magnified
views of descending thoracic aorta. (Courtesy of Fayad ZA, New York, NY) Axial
T2-weighted MR image at different level than A shows 7-mm fibrocellular
unstable lipid-rich plaque. Hypointense area in plaque (arrow)
corresponds to lipid-rich core.
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By characterizing the components of atheromatous plaque, MR imaging can
potentially make significant distinctions between stable plaques and unstable
plaques that may be vulnerable to thrombosis or embolization, thereby
warranting intervention. As a noninvasive tool for assessing the progression
and regression of plaque components, such as the fatty core, MR imaging is
promising for the examination of antiatherosclerotic therapeutic regimens.
When combined with other MR imaging techniques to examine the cardiac
chambers, valves, morphology, and functions, and the entire thoracic aorta and
its branch vessels, MR imaging has the potential to provide a comprehensive
study of cerebrovascular atheroembolic disease. Currently, MR imaging is
useful to view atheromas in the innominate artery
(Fig. 10) and contiguous
ascending aorta (Fig. 11).
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Fig. 10. —56-year-old man with right brain transient ischemic attacks. Coronal
reformatted image from breath-hold gadolinium-enhanced three-dimensional MR
angiogram shows complex protruding atheroma originating from innominate artery
(arrow). Plaque was overlooked on transesophageal echocardiogram
because of tracheal interposition between esophagus and innominate artery.
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Fig. 11. —62-year-old man with history of stroke. Coronal reformatted image
from breath-hold gadolinium-enhanced three-dimensional MR angiogram shows
protruding atheroma of ascending aorta (solid arrow) with filiform
morphology suggesting mobility. Note innominate artery (open
arrow).
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Therapy
Researchers have not determined the correct therapeutic approach to
patients with aortic atheromas. However, anticoagulation with warfarin may be
a reasonable treatment [21]
because thrombi have been documented on aortic plaques and represent mobile
components seen on transesophageal echocardiography. Another therapeutic
approach may be the use of drugs known as statins. These cholesterol-lowering
drugs were recently found to reduce the stroke rate of patients observed in
the Cholesterol And Recurrent Events (CARE) study
[59]. By reducing plaque lipid
in patients with aortic atheromas, statins could theoretically make aortic
plaques stable or nonthrombogenic. However, this result remains to be proven.
Clearly, a large prospective study is required to determine the efficacy and
risks of various anticoagulant, antithrombotic, and lipid-lowering
strategies.
Conclusion
Thoracic aortic atherosclerosis is an important cause of severe morbidity
and mortality. Its presence in patients undergoing surgery requiring
cardiopulmonary bypass dramatically increases the risk of complications such
as stroke. Currently, the ease of performance, the detailed information
obtainable, and the cost and availability of the equipment for transesophageal
echocardiography make it the procedure of choice for examining thoracic aortic
atherosclerosis. However, technical advances in MR imaging and CT,
particularly in the characterization of plaque on MR imaging, may allow a less
invasive and more complete examination of thoracic aortic atherosclerosis in
the near future.
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Introduction
Transesophageal Echocardiography
