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DISEASES
OF
THE AORTA
0733-8651
99
$8.00
+
.OO
IMAGING
OF
THORACIC
AORTIC DISEASE
Bruce
A.
Urban, MD, David
A.
Bluemke, MD, PhD,
Kevin M. Johnson, MD, and Elliot
K.
Fishman, MD
Historically, the depiction of thoracic aortic
disease has been limited to conventional angi-
ography. Although long considered to be the
“gold-standard of vascular imaging, conven-
tional angiography is relatively invasive,
time-consuming, and costly. Over the last
two
decades, marked advances have been made in
the noninvasive imaging techniques, in many
cases making conventional angiography an
obsolete procedure for the detection, diagno-
sis, and display
of
the aorta and aortic pathol-
ogy.
Computed tomography (CT), magnetic
resonance
(MR)
imaging, and transesopha-
geal echocardiography (TEE) represent three
outstanding, commonly available noninva-
sive modalities for imaging the thoracic aorta.
The purpose of
this
article is to:
1)
describe
the protocols and techniques for each imaging
modality;
2)
demonstrate the imaging appear-
ance
of
commonly encountered vascular pa-
thologies of the thoracic aorta, including aor-
tic aneurysms and aortic dissections; and
3)
discuss the advantages and disadvantages of
these modalities.
COMPUTED TOMOGRAPHY (CT)
Overview
CT evaluation of the thoracic aorta has
been possible since the advent of computed
body tomography in the late
1970s.
Prior to
this time, visualization of the aorta and great
vessels was limited to invasive angiographic
techniques, which are associated with a small
but significant risk of morbidity and mortal-
ity. Catheter induced complications from an-
giography include hemorrhage, arrhythmia,
and stroke. CT significantly decreased risk to
the patient
by
eliminating the need for direct
arterial injection. CT could also be performed
much faster. Early generation scanners were
very slow by today’s standards but could still
obtain images
of
the aorta within several
minutes. In addition, for the first time CT
allowed direct visualization of the aorta and
surrounding mediastinal structures rather
than just the endoluminal contour visualized
by angiography. Cross-sectional CT images
provided simultaneous evaluation of the aor-
tic lumen, aortic wall, and adjacent organs
From The Russell H. Morgan Department
of
Radiology and Radiological Science (BAU, DAB,
EKF),
Divisions
of
Magnetic Resonance Imaging (DAB) and Diagnostic Imaging and Body Computed Tomography (EKF), The
Johns
Hopkins Medical Institutions, Baltimore, Maryland; and the Department
of
Diagnostic Radiology, Yale University
School
of
Medicine
(KMJ),
New Haven, Connecticut
CARDIOLOGY CLINICS OF NORTH AMERICA
VOLUME
17
NUMBER
4
*
NOVEMBER
1999
659
660
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data previously unobtainable with any tech-
nique.
Increased availability of CT scanners, com-
bined with continued improvements in CT
technology, resulted in widespread applica-
tion of CT for aortic imaging in the
1980s.
Improvements in contrast agents, contrast de-
livery systems, scanner hardware, and com-
puter software enabled rapid, quality images
of the aorta to be obtained with only
1
to 2
minutes of acquisition time. Scanners were
soon placed in close proximity to the emer-
gency department, and in many hospitals, di-
rectly within the trauma suite. By the mid-
1980s,
CT scanning had become an integral
examination for the work-up of aortic pathol-
ogy.
Spiral CT
The Birth
of
CT Angiography
The advent of spiral CT in the early
1990s
redefined the role of CT for imaging of the
thoracic aorta. Unlike conventional CT, which
obtains images sequentially in a step-wise
fashion, spiral CT scanners have the ability to
image the entire aorta in a single, volumetric
breath-hold scan.24 This is made possible by
coupling patient motion through the scanner
gantry with continuous data acquisition. Spi-
ral CT can completely visualize the entire tho-
racic aorta during peak contrast opacification,
making detection of entities such as aortic
dissection and aortic aneurysms much easier.
Newer scanners can rapidly acquire informa-
tion with images of the entire aorta now ob-
tainable in seconds.16 Furthermore, motion ar-
tifact is essentially eliminated with this
technique. The information obtained from
this motion-free, volumetric CT acquisition,
coupled with advanced computer technology,
can be used to generate images in multiple
planes and orientations, including interactive
three-dimensional (3-D) and virtual reality
interface^.^^,
47
This technique is referred to
as "CT angiography" because images can be
manipulated to mimic the appearance of a
conventional angiogram. Studies have shown
CT angiography to provide excellent accuracy
for the detection of thoracic aortic pathol-
ogYs6<
19,
44
CT Technique
CT imaging of the thoracic aorta requires
the rapid intravenous injection of iodinated
contrast material. Contrast is usually adminis-
tered via a
18
or
20
gauge IV in the antecubi-
tal vein. Power injectors are essential for the
delivery of
120
to 150 mL of contrast at the
rates of up to 3 to
4
mL/second required for
optimal aortic opacification. Nonionic con-
trast is preferred over ionic contrast because
it is better tolerated by the patient, with little
chance of inducing the patient motion or sick-
ness which can interfere with image acquisi-
tion at time of contrast delivery.
Acquisition of images during peak aortic
contrast opacification is the goal of CT im-
aging. Because contrast circulation time can
vary from patient to patient, automated tech-
niques have been introduced that result in
triggering of the CT scanner when aortic con-
trast is optimal.46 However, these triggering
devices are somewhat cumbersome and not
completely reliable. Most authors, including
ourselves, rely upon the setting of an empiri-
cal delay time of 25 to 30 seconds that results
in excellent aortic opacification in the vast
majority of patients. In older patients or those
with decreased cardiac output, increasing the
empirical delay to 35 to
40
seconds is advised.
Patients who cannot receive iodinated con-
trast are not candidates for CT imaging of the
aorta. The only absolute contraindication for
iodinated contrast is significant contrast al-
lergy, specifically anaphylaxis. Steroid pre-
medication is advised prior to imaging in
these patients. Renal insufficiency is a relative
contraindication, as iodine can exacerbate re-
nal insufficiency. In general, a serum creati-
nine level of greater than
1.8
to 2.0 mg/dL is
used as a "cut-off" value. This is particularly
important in patients with underlying diabe-
tes or dehydration, in whom the potential
toxic effects of iodine are increased.
Three-Dimensional CT Imaging and
Volume Rendering
Spiral CT scanners generate a tremendous
volume of data, which can be processed and
viewed in a wide variety of formats. Options
IMAGING
OF
THORACIC AORTIC
DISEASE
661
include standard axial images, multiplanar
reconstructed images (in any plane), or
3-D
images. These techniques provided new and
improved formats for displaying of CT data,
capable of ”angiographic” equivalent
3-D
im-
ages.44
Surface rendering was one of the earliest
methods available for
3-D
display. With this
method, the ”surface” of the object (the aorta)
is defined by a threshold intensity value arbi-
trarily set by the user. The resulting aortic
image is displayed with surface shading to
give a
3-D
appearance. Today, surface render-
ing remains a simple computation readily
performed on most scanners, but quite lim-
ited in diagnostic use. Another method of
3-
D
display, maximum intensity projection
(MIP), is also available on most CT scanners.
The MIP algorithm views the CT data along
a ray projected from the viewer’s “eye” and
selects the maximum value along this ray as
the display value. This results in images that
can be displayed from various projections.
Although impressive in their display, nu-
merous studies found the early generation
3-D
techniques of surface rendering and MIP
to be no more accurate than standard axial
CT images. Only recently have advances in
3-D
imaging, using a technique called volume
rendering, reached diagnostic levels superior
to that of standard axial imaging and of suf-
ficient quality to provide a serious challenge
to conventional angiography in evaluation of
the vascular system.22 Originally developed
in the
1980s,
volume rendering represents the
most advanced
3-D
rendering algorithm cur-
rently available. The initial application of this
technique, combined with advanced imaging
computers, occurred in the movie and enter-
tainment industry. Although this method was
spectacular in display, data processing was
somewhat cumbersome and volume render-
ing found little application outside of major
academic centers. Fortunately, advances in
high-speed computing combined with sig-
nificant decreases in the cost of computers
have finally made volume rendering widely
available for diagnostic medical application?*
The major advantage of volume rendering
is the incorporation of all of the acquired CT
data into the resulting
3-D
image, overcoming
many of the shortcomings of surface render-
ing and MIP techniques. Potentially all of the
available data and diagnostic information can
be conveyed on the volume rendered
3-D
im-
age. Computers can perform very rapid edit-
ing of the data using an interactive clip-plane,
which can target in on the area of interest
(Fig.
1).
This
enables the user to manipulate
Figure
1.
Spiral computed tomography (CT) with volume rendering. Current state-of-the-art three-
dimensional imaging with spiral CT angiography allows for real-time, interactive viewing of the aorta
and aortic pathology. Volume rendered images are extremely useful for visualizing the aorta, and can
be rotated and positioned into any orientation for real-time viewing, as shown in the left anterior
oblique
(A),
and posterior positions
(B).
In this case, an intimal flap from aortic dissection is easily
appreciated
(arrowheads).
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the dataset in ”real-time” at the workstation,
creating images of virtually any organ and
vessel from any perspecti~e.~~ This also
allows the image to be customized to the
varying levels of contrast enhancement that
can be present in the aorta and great vessels.
In addition, the inner contours of the vascula-
ture can be visualized, much like a camera
looking inside a vessel (virtual angioscopy)
(Fig.
2)j7
Using volume rendering, data can be dis-
played at varying levels of opacity and
brightness depending on the area of interest.
These options allow the user to produce an
optimal image for diagnosis and display. At
the workstation, each factor operates inde-
pendently of others in producing the final
image. Depth shading can be employed to
make distant objects appear darker than those
close to the viewer. Lighting models are used
to produce depth perception from one or mul-
tiple sources. Rotation of the image can also
be employed to render depth perception and
visualize overlapping structures.
Indications for
CT
Evaluation
of
the
Thoracic Aorta
CT scanning provides accurate diagnostic
information for most conditions affecting the
thoracic aorta.16,
39
Common indications for
CT scanning of the thoracic aorta include pre-
operative planning for aneurysm repair, sus-
pected aortic dissection, and aortic injury fol-
lowing blunt trauma. CT is also useful for
evaluation of suspected arteritis, the postop-
erative aorta, and congenital variations of
anatomy.
CT scanning has many advantages in eval-
uation of the aorta. One very important ad-
vantage is availability. CT scanners far out-
number MR imaging scanners available for
imaging in the emergent setting, and scans
can be performed by on-call technologists
Figure
2.
Spiral
CT
angioscopy. By altering the editing param-
eters
of
the volume rendered three-dimensional dataset, spiral
CT
images can demonstrate the inner contours
of
the aorta
much like a camera looking inside a vessel. This can be
especially helpful for evaluating the extent
of
intimal flaps in
patients with aortic dissection.
In
this patient with Marfan’s
syndrome,
CT
angioscopy
cathedral
view looking
up
and into
the aortic arch reveals a dissection
flap
(arrowheads)
involving
the great vessels. Aneurysmal dilatation is noted in the de-
scending aorta (d).
IMAGING
OF
THORACIC
AORTIC DISEASE
663
without the need for an on-site radiologist or
cardiologist. Images can be quickly trans-
ferred via teleradiology to the interpreting
physician. Many hospitals now have CT scan-
ners in close proximity to the emergency
ward and trauma suite, providing around-
the-clock availability for the diagnosis and
detection of a vast spectrum of emergent pa-
thologies, including aortic dissections and an-
eurysms.
A
second advantage of CT scanning
is speed. Modern spiral CT scanners are capa-
ble of producing images of the entire aorta in
a matter of seconds. Patients are usually
on
and off the table within a few minutes.
A
third advantage is patient convenience. CT
scanning is minimally invasive, requiring
only a peripheral venous injection of contrast.
CT scanning is also suitable for critically ill
or unstable patients. Lastly, CT can diagnose
other potentially important thoracic pathol-
ogy
that can mimic the clinical presentation of
aortic dissections and aneurysms, including
pulmonary embolism, pneumothorax, and
pneumonia.
MAGNETIC RESONANCE (MR)
IMAGING
Overview
MR imaging has been widely used for eval-
uation of the aorta since the 1980s.2 Like CT,
MR provides
a
noninvasive alternative to
conventional angiography and provides an
excellent diagnostic tool for evaluation of the
thoracic aorta. Unlike CT images, which de-
pend on tissue and contrast media x-ray at-
tenuation for visualization, MR imaging relies
on the mobile hydrogen concentration of
blood and tissues to generate an image. When
placed in an external magnetic field, the indi-
vidual magnetic moments of all hydrogen
atoms
in
the body, including the blood ves-
sels, align themselves with the external field.
Energy in the form of a radiofrequency
(RF)
pulse is used to energize the aligned nuclei
to
varying
degrees. It
is
the relaxation of these
hydrogen atoms that produces a characteristic
signal that is used to generate an image. The
final
MR
"signal"
is
a very complex function
of the concentration of hydrogen atoms, the
relaxation time, motion, blood flow, scanning
parameters, and the imaging protocol. T1 and
T2
are commonly used terms that describe an
MR signal. T1 describes the time required for
magnetization buildup, and T2 describes the
time for relaxation. There are literally hun-
dreds
of
ways to obtain an MR image, many
of which have been developed to evaluate the
heart, aorta, and peripheral vessels. Different
pulse sequences can produce dramatic
changes in the appearance of the MR image
(Fig. 3). In the past, MR angiography has been
limited by long acquisition times and motion.
Recently, new approaches to "breath-hold"
MR angiography of the thoracic aorta have
been developed that allow rapid acquisition
with markedly improved image quality, mak-
ing MR an increasingly viable option for the
evaluation and depiction of aortic pathology
(see Fig. 3).
MR
Techniques
for
Vascular Imaging
Traditional
MR
Traditional MR evaluation of aortic vascu-
lopathy involves spin-echo T1-weighted im-
ages for anatomic depiction combined with
gradient-echo (GRE) flow-compensated
"bright-blood images, with flow-related en-
hancement roughly proportional to the veloc-
ity of flowing Imaging the entire
chest requires 3 to
4
minutes plus additional
time to allow the patient to breathe between
acquisitions. Traditional gated GRE acquisi-
tions also require 3 to
4
min. for a full cine
sequence containing
1
to
4
imaging slices.
Intravenous injection of exogenous contrast
agents combined with 3-D GRE has yielded
anatomic images of the aorta of excellent
quality based on the T1-shortening of blood,
rather than Fast low angle shot
(FLASH) techniques, requiring a
3
to
5
mL
gadolinium injection per imaging plane, have
also been advocated for evaluation of the
aorta.15
Time-of-Flight
MR
Angiography
Time-of-flight MR angiography relies
on
application of multiple alpha
RF
pulses at
very short repetition times
(TR)
of
5
to
50
msec. If relatively high flip angles are used,
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Figure
3.
Effect
of
magnetic resonance
(MR)
pulse sequence on appearance of
thoracic aorta in patient with right-sided aortic arch.
A,
Traditional
MR
images of the
thoracic aorta involve axial spin echo
T1
-weighted acquisitions, which typically reveal
flowing blood as relatively black. Traditional
MR
images are extremely accurate for
showing anatomical relationships. In this case, various axial images show the right-
sided position of the aortic arch (Arch) relative
to
the trachea.
AA
=
ascending
aorta; DA
=
descending aorta; a
=
accessory lung fissure.
B,
Gadolinium-enhanced
MR
angiogram in the same patient also shows the right-sided descending aorta
(arrows).
In the abdomen, the aorta shifts
to
the normal left of midline position. Note
how with the gadolinium enhanced sequences, contrast-enhanced flowing blood is
bright white.
IMAGING
OF
THORACIC AORTIC DISEASE
665
stationary tissues within the imaging plane
quickly become saturated, and generate a
minimal MR signal. Therefore, the back-
ground stationary tissue shows relatively lit-
tle MR signal. In contrast, flowing blood from
outside of the cross-sectional imaging plane
has not been exposed to the radiofrequency
pulses. This blood generates relatively large
amounts of signal as it flows into the thin
slice, termed “flow-related enhancement.”
Using this technique, either arterial or venous
signals can be selectively visualized by satu-
rating blood in the unwanted vascular terri-
tory, before it moves into the cross-sectional
imaging slice. Three-dimensional time-of-
flight imaging is similar, although scan times
are much longer. The main drawback of time-
of-flight methods outside the central nervous
system is that respiratory and cardiac motion
generate motion artifacts. Stair-step artifacts
are very apparent in the chest and abdomen
using time-of-flight MR angiography, limiting
the diagnostic use of the method.
Contrast Enhanced
MR
Angiography
Contrast enhanced MR angiography of the
thoracic aorta
is
performed following acquisi-
tion of axial
Zocalizer
images. This aids in de-
termining the maximum anterior to posterior
dimensions of the aorta, as well as the left to
right extent. If the main area of interest is
confined to the ascending and descending
aorta, as well as the great vessels, a sagittal
acquisition is recommended. This approach
would be useful, for example, in the evalua-
tion of aortic dissection. Contrast enhanced
MR angiography images can be readily re-
formatted to measure the cross-sectional size
of
aorta, and to define the extent and origin
of aortic dissection as well as branch vessel
invol~ement.~~,
42
Prince et a1 first described the use of gado-
linium-based contrast infusion in conjunction
with MR angiography
technique^.^^
In this
method, the arterial system is essentially
flooded with contrast agent, resulting in a
decreased
T1
time of blood from approxi-
mately
1000
msec to less than
100
msec. Sta-
tionary tissues are saturated, but blood-with
a very low
T1
after contrast idusion-
generates the most signal for large vessels in
the chest and is easily depicted (Fig.
4).
With
this technique, image acquisition times
ini-
tially ranged from
3
to
5
min40; however, fast
Figure
4.
Gadolinium-enhanced
MR
angiogram in patient
with coarctation
of
the aorta. Image reveals narrowing
of
the aorta
(arrow)
immediately distal to the take-off
of
the
left subclavian artery. Much like spiral
CT,
contrast en-
hanced
MR
angiograms such as this can now be rapidly
obtained during a breath-hold acquisition.
A
=
ascending
aorta;
D
=
descending aorta.
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3-D
spoiled gradient echo recalled acquisi-
tions are now available, with
TR
values as
low as 5 m~ec.~~ Aortic images for a
10
cm
slab can now be obtained with a breath-hold
in about
20
to 25 seconds. Because acquisition
times are now short, the rate at which con-
trast is infused, and the relationship between
the start of the injection and the start of im-
aging begin to become crucial, as with CT
angiography. Therefore, it is necessary to time
the gadolinium bolus
so
that the peak en-
hancement occurs during the middle of the
MR acquisition. Failure to do this will result
in suboptimal images.
Principles of
MR
Contrast
Administration
Because gadolinium imaging relies on re-
duction of the
T1
time of blood, an adequate
agent must be administered. The primary
goal of contrast infusion is to inject a suffi-
cient contrast dosage such that the
T1
time of
blood is reduced below that of fat (270 msec
at 1.5
T)
for the duration of the scan acquisi-
tion. In this manner, the blood vessels will
have significantly higher signal than the sur-
rounding soft tissues. In our experience, pa-
tients typically receive
40
mL of contrast at a
rate of
1
to 2 mL/min. For newer scanners
that have low TRs, applications in chest can
use reduced contrast dosages of 20 mL.
There are several strategies to proper tim-
ing of the contrast injection relative to the
start of
3-D
MR angiography acqui~ition.~,
l7
Starting too early results in poor or incom-
plete arterial opacification; starting too late
after injection will result in significant venous
contamination. There are
3
approaches to cor-
rect timing of the sequence:
1)
empirical
times, 2) calculated times and test bolus injec-
tion, and
3)
automatic triggering.41 All can be
useful in selected settings.
Indications for
MR
Evaluation
of
the
Aorta
Indications for MR imaging of the thoracic
aorta are similar to those for CT scanning,
except that MR imaging is usually not a prac-
tical option in injured or unstable patients.
MR imaging has the capability to depict the
entire aorta and surrounding structures in
any desired field of view, making it useful for
preoperative evaluation of aortic aneurysms
and dissections. Reliable measurements of an-
eurysms can be made at well-defined posi-
tions. MR is also useful for monitoring the
postoperative patient, as it is easy to compare
MR images obtained at follow-up examina-
tions. MR can detect subtle changes on fol-
low-up exam, such as with chronic aneu-
rysms in patients with Marfan’s syndrome. In
addition, multiple follow-up exams can be
performed in this subgroup of patients and
others without the need for exposure to ioniz-
ing radiation or iodinated contrast adminis-
tration. Disadvantages of MR include its rela-
tively high cost and longer examination times
compared with CT scanning, although with
continued technical improvements these is-
sues are becoming less significant.
TRANSESOPHAGEAL
ECHOCARDIOGRAPHY (TEE)
Overview
Unlike CT and MR, TEE represents a porta-
ble imaging modality that can be brought to
the patient’s bedside. This advantage makes
TEE a valuable tool in the diagnosis of acute
thoracic aorta pathology, particularly in eval-
uation of the patient with suspected acute
aortic dissection of aortic trauma, the intraop-
erative patient, or the patient with acute, se-
vere chest pain.57 TEE also plays a major role
in the evaluation of aortic valvular disease
(Fig. 5).
High-resolution vascular images are possi-
ble with TEE because of the close proximity
of the esophagus to the thoracic aorta. More
than any other imaging modality,
TEE
is de-
pendent on the examiner’s skill and expertise,
and accurate results depend on a thorough
knowledge of the patient’s symptoms and
history. Detection of one abnormality should
lead to a search for related findings, particu-
larly those that would have therapeutic or
prognostic implications.
A
pitfall when per-
forming TEE is to be satisfied with an inade-
quate exam, or to be distracted by one finding
and forget to look for important associated
IMAGING
OF
THORACIC AORTIC
DISEASE
667
Figure
5.
Postoperative abscess at transesophageal echocardi-
ography
(TEE).
Hypoechoic abscess is identified surrounding
the ring of a prosthetic aortic valve
(arrows).
Plane of imaging
is just cephalad to the valve.
TEE
plays a major role in the
evaluation of pathology in and around the aortic valve, and is
ideally suited for the postoperative patient.
abnormalities. Two limitations are that
TEE
cannot visualize the entire aorta and that de-
termining the exact location of some abnor-
malities can be difficult.
Patient Selection and
Contraindications
TEE carries a small but definite risk to the
patient, and unlike the other imaging modal-
ities, proper patient selection becomes para-
mount to avoid
complication^.^^
Extreme cau-
tion is warranted in certain clinical scenarios
when the decision to perform a
TEE
is being
considered. Patients with respiratory failure
may become dangerously hypoxemic during
attempts to pass the probe. Other patients, in
particular the elderly, are very sensitive to the
sedation necessary for TEE and require close
monitoring. Obtunded or pharmacologically
paralyzed patients cannot object if the probe
enters the pyriform sinus, risking perforation.
The trachea can easily be intubated by mis-
take in these patients. Care must be taken not
to inadvertently extubate the patient upon
removal of the TEE probe. Relative contrain-
dications also include esophageal stricture,
severe esophagitis, varices, and coagulopathy.
TEE Preparation and Technique
Proper patient preparation and monitoring
is
required prior to and during
TEE.
The pa-
tient is kept nil per
0s
(i.e., nothing by mouth)
for
4
hours prior to the test to reduce the risk
of aspiration. Approximately
5
minutes prior
to the procedure, the oropharynx is anesthe-
tized with topical lidocaine and oxygen is
administered by nasal cannula. Systemic in-
travenous sedation is carefully titrated and
monitored with the aid of
2
blood pressure
cuffs and pulse oximetry. Suction, ambubag,
and code cart should be on standby in the
event of an emergency.32
The examination is usually performed in
the left lateral decubitus position. The probe
is inserted into the esophagus after lubricat-
ing the distal end of the scope with lidocaine
jelly. The
two
principal types of probes used
for TEE are biplanar and multiplanar.
Multiplanar probes with mechanically driven
annular-array transducers can rotate the im-
aging plane from a transverse position, which
corresponds to the standard plane of conven-
tional echocardiography, through all the inter-
mediate angles to a reversed transverse posi-
tion. The multiplanar probe permits more
complete evaluation.
Sections usually well-imaged include the
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left ventricular outflow tract, the ascending
aorta exclusive of its most cephalad portion,
the mid to distal aortic arch, and the entire
descending aorta. Color doppler identifies
communications between spaces, for example
between the true and false lumen of a dissec-
tion, or flow through the mouth of a pseu-
doaneurysm. Care is taken to optimize the
gain setting just below the level where flow
signal is introduced outside of vessels or onto
adjoining structures. A routine
TEE
lasts
about
15
minutes. After the procedure, the
patient is observed until they are alert; the
patient can eat after the gag reflex returns.
Procedural Pitfalls
of
TEE
Certain limitations are inherent to aortic
imaging by TEE. The right mainstem bron-
chus is interposed between the esophagus
and the ascending aorta; consequently, as
much as
40%
of the ascending aorta may not
be seen.27
A
tortuous descending aorta can
wind away from the esophagus
so
that long
portions cannot be seen. Also, brachiocepha-
lic artery origins cannot always be imaged;
however, multiplanar probes help in this re-
gard.
A
good working knowledge of the artifacts
common to all ultrasound techniques, includ-
ing
TEE,
is essential. A reverberation artifact
in the ascending aorta can easily mimic a
dissection flap. Knowledge of the range of
appearances of the mediastinal and paraaortic
tissues, the thoracic venous anatomy, and
common anatomic variants is essential. This
can be gained only by user experience; with-
out operator experience, the accuracy of
TEE
is degraded and the potential for misdiagno-
sis is markedly increased.
Transthoracic Echocardiography
Transthoracic, as opposed to transesopha-
geal echocardiography has a very limited role
in the assessment of the aorta, with sensitivi-
ties far below
TEE.37
If
aortic disease is sus-
pected, a directed transthoracic effort to im-
age the aorta can improve the views, but only
within rather severe limits. Usually the as-
cending aorta can be seen for several centime-
ters above the valve, and a limited view of
the aortic arch can be obtained from the su-
prasternal notch position. Diameters can be
measured in these locations, and, if the im-
ages are clear enough, a intimal flap may be
detectable. It is not uncommon for patients
to present with a vague history, nonspecific
symptoms, or a presumptive diagnosis of
myocardial infarction without any indication
that aortic pathology is in the differential di-
agnosis. Under these circumstances, a dissec-
tion or rupture can easily be missed. The sen-
sitivity and specificity of transthoracic
echocardiography is simply insufficient to
make these diagnoses reliably.
IMAGING FINDINGS
OF
THORACIC
AORTA PATHOLOGY
Aortic Dissection
Overview
Aortic dissection is the most common catas-
trophe involving the aorta.13 Typically, the pa-
tient will present with tearing chest pain. Dis-
section is most often a spontaneous event in
hypertensive middle-aged patients with ath-
erosclerotic disease, or in younger patients
with a connective tissue disorder such as
Marfan’s syndrome or other cause of cystic
medial necrosis. An aneurysmally dilated
aorta can develop a dissection or intramural
hematoma as a complication from the high
stress in its walls, but more often the aorta is
not dilated. Imaging evaluation must deter-
mine whether the dissection involves the as-
cending (Stanford type A, DeBakey class I
and 11) or descending (Stanford type
B)
aorta
as these entities are treated differently and
have different
complication^.^^
Choosing
a
Modality: Which is Best?
It is now generally accepted that all three
noninvasive imaging modalities for the tho-
racic aorta provide excellent sensitivity and
specificity for the diagnosis of aortic dissec-
tion.
As
recently as the early
1990s,
however,
MR
was considered the noninvasive modality
IMAGING
OF
THORACIC AORTIC DISEASE
669
of choice. In the landmark 1993 study by Nie-
naber et al, sensitivity and specificity in the
detection of aortic dissection were 93.8% and
87.1%, respectively, with conventional incre-
mental CT; 97.7% and 76.9%, respectively,
with monoplanar
TEE;
and 98.3% and 97.8%,
respectively, with MR.35 More recent work
with modern spiral CT scanners and
multiplanar
TEE
has achieved comparable ac-
curacy between modalities. In a 1996 study,
Sommer et a1 found that all three modalities
approach 100% sensitivity and
loo%,
94%,
and 94% specificity for spiral CT, multiplanar
TEE, and MR respecti~ely.~~ Newer tech-
niques using spiral
CT
angiography and MR
angiography have the added benefit of
multiplanar and 3-D display formats that can
beautifully demonstrate the full extent and
orientation of the dissection flap.
No
matter which imaging modality is used,
a checklist of questions needs to be addressed
when evaluating patients with suspected aor-
tic dissection. Presence and extent of an inti-
ma1 flap is most crucial. If there is no flap, is
there instead intramural hematoma, penetrat-
ing ulcer, mural thrombus, or plaque? Is there
a mediastinal hematoma? Does the flap in-
volve the ascending aorta or coronary arter-
ies, and is there a pericardial effusion? How
dilated is the aorta, and is the aortic valve
annulus dilated? Evaluation for significant
aortic insufficiency is best evaluated by MR
or
TEE.
Similarly, MR and
TEE
can best depict
regional wall motion abnormalities of the left
ventricle.
Imaging Findings
All three imaging modalities can easily vi-
sualize the intimal flap within the dissecting
aorta (Figs.
6-8).16
At times, the intimal flap
can have atypical characteristics, including
partial thrombosis, multiple false channels, or
a circular configuration (Fig. 9). CT and MR
can reveal secondary signs of dissection, in-
cluding internal displacement of intimal cal-
cium, delayed enhancement of the false lu-
men, mural thickening with increased
attenuation or signal intensity, and ishemia
or infarction of organs supplied by branch
vessels.16,
35
In fact, spiral CT and MR are
particularly suited for demonstrating and the
anatomic course of the dissection membrane
in the arch and branch vessels (Fig.
1O).l6
This
information is valuable for planning the sur-
gical approach.48
Figure
6.
Type A aortic dissection at CT. Axial image from spiral CT
scan reveals aortic dissection. lntimal flap
(arrowheads)
is demon-
strated within the ascending and descending thoracic aorta. The as-
cending aorta is moderately dilated. Type A dissections such as this
are considered surgical emergencies, as they may occlude the coro-
naty arteries or rupture into the pericardium.
670
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et
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Figure
7.
Aortic dissection at
MR.
A,
Sagittal oblique view from spin-echo
T1 -weighted image shows complex intermediate signal in the descending
aorta, and a linear line owing
to
dissection
(arrows).
B,
Breath-hold axial
gradient echo image at the mid-aortic level more readily shows a linear
signal of the dissection flap in the descending aorta
(arrowheads),
sepa-
rated
by
areas of flow of differing signal intensity.
IMAGING
OF
THORACIC
AORTIC
DISEASE
671
Figure
8.
Type
B
aortic dissection at
TEE.
Linear, echo-
genic intimal flap
(arrow)
is seen in the descending aorta.
This is the classic appearance of aortic dissection at TEE.
Imaging Pitfalls in Aortic Dissection
All three modalities have potential pitfalls
in the interpretation of aortic dissection.
Many pitfalls pertain especially to TEE be-
cause of its dependence on the user for image
acquisition and interpretation. At TEE, a false
positive diagnosis can result from ultrasound
artifacts, especially in the ascending aorta. M
mode echo may be helpful in identifying af-
fected findings in this setting." Other pitfalls
include mistaking a thrombosed false lumen
as mural clot or plaque. Femoral cardiopul-
monary bypass primer fluid layering in the
descending aorta can mimic dis~ection.~~
False negative TEE exams for dissection also
can occur but are rare. Fortunately, the
"blind area for TEE in the ascending aorta
rarely obscures a dissection, because the flap
usually extends from the root or from the
arch (Fig.
11).
However, the entry intimal tear
can occasionally be located in this blind area.
A technically inadequate exam, or mistaking
a flap for artifact in the ascending aorta, are
probably the biggest dangers at TEE.
Pitfalls of interpretation may present at spi-
ral CT and MR evaluation as well. As with
TEE, a thrombosed false lumen can be
mis-
taken for mural clot or plaque. Spiral CT can
produce a pulsation artifact in the ascending
aorta that can be mistaken for Type A dissec-
ti~n.~ Streak artifacts from patient motion and
high-attenuation superior vena cava
(SVC)
contrast can simulate a dissection flap.I6 MR
can also produce a wide variety of imaging
artifacts related to cardiac and aortic pulsa-
tion, in addition to pseudothrombosis from
stagnant or relatively slow moving blood.16
Spiral CT, and to some extent MR, are also
dependent on quality injection of intravascu-
lar contrast agents; suboptimal technique can
limit detection of dissection.
Figure
9.
Atypical type A dissection at TEE. Aortic dissection
has resulted in an almost circumferential separation of the me-
dia, with spontaneous echo contrast identified within the outer
(false) lumen
(arrows).
Other atypical appearances for aortic
dissection can include partial thrombosis of the false lumen and
multiple dissection flaps.
672
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et
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Figure
10.
Aortic dissection at MR. Source image from a
three-dimensional gadolinium enhanced MR angiogram
of the aorta. The dissection flap
(arrowheads)
is clearly
seen to originate distal
to
the left subclavian artery, and
extend inferiorly into the descending aorta.
MR
images
can be quickly and easily manipulated to display the
origin and extent
of
an aortic dissection.
Intramural Hematoma (IMH)
IMH, in which hemorrhage accumulates
within the media of the aorta, can occur spon-
taneously or as a complication of trauma or
ulcerated plaque. Hypertension is frequently
associated with IMH, seen in
84%
of patients
in one study.36 IMH without a free intimal
flap has recently been recognized in a signifi-
cant subgroup of patients presenting with
aortic dissection and IMH may
be the harbinger of a frank dissection. As-
cending aortic involvement is very worri-
some, with a high complication rate including
pericardial effusion, mediastinal hemorrhage,
and even death. IMH should be regarded as
a variant of dissection with similar or more
serious prognostic and therapeutic implica-
tions.
Imaging studies can confuse IMH with clot
or plaque (Fig.
12).
At TEE, IMH may demon-
strate echolucent areas and displaced intimal
calcium to aid in diagnosis. Spiral CT reveals
high-density mural thickening (Fig.
13).
IMH
is probably best depicted on initial noncon-
trast CT images, as subtle cases can be
masked following dynamic contrast adminis-
tration. MR may be the best modality for
the detection of IMH. At MR evaluation, T1-
weighted images reveal a crescent-shaped
area of abnormal signal within the aortic wall
instead of a distinct intimal flap (Fig.
14).4
The signal intensity of the intramural hemor-
rhage in aortic dissection is variable and de-
pends on the age of the IMH. Bright signal is
Figure
11.
Intramural hematoma at TEE. An intramural hema-
toma is seen in the aortic arch
(arrow),
extending up from the
blind area in the ascending aorta. Hematoma involves part of
the anterior wall (farthest from the esophageal probe). One pitfall
of TEE is difficulty in determining the entire extent of aortic
dissections or hematomas, but as this case demonstrates, this
rarely results in an inability to make a diagnosis.
IMAGING
OF
THORACIC AORTIC DISEASE
673
Figure
12.
Intramural hematoma and aortic aneurysm at
TEE.
A fusiformly dilated ascending aortic aneurysm (a) is demon-
strated. Aneurysm is complicated by intramural hematoma in
the posterior wall, manifested at
TEE
by diffuse thickening
of
the wall without a frank lumen
(arrows).
This appearance can
be very difficult to differentiate from mural thrombus.
easiest to detect; however, when the signal
intensity is medium to low, it
is
difficult to
distinguish intramural dissection from mural
thrombus or slow flow. In these cases, cine
images
of
the aorta can rapidly interrogate
multiple sites along the aorta and confirm
the diagnosis of IMH by demonstrating an
absence of cyclical changes in signal intensity.
Severe atherosclerotic involvement, usually
of the descending aorta in elderly patients,
can lead to ulceration of a plaque into the
media (penetrating aortic atherosclerotic ul-
Figure
13.
Intramural hematoma at
CT.
Axial image from contrast-
enhanced spiral
CT
scan reveals high-attenuation hematoma
(ar-
rowheads)
within the wall
of
the descending aorta. Intramural
hematoma should not be mistaken for mural thrombus. Accurate
diagnosis is critical, as intramural hematoma is considered a
variant
of
aortic dissection in both diagnosis and treatment. The
three major imaging modalities are all sensitive for the detection
of
intramural hematoma, as they can accurately depict the aortic
wall in addition to the aortic lumen.
674
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et
a1
Figure
14.
Intramural hematoma at
MR.
Axial
T1
-weighted
spin echo image at the level of the mid-aorta shows cres-
centic thickening and moderately increased signal intensity
in the wall of the descending aorta
(arrow).
This can be
difficult to distinguish from mural thrombus. Cine images (not
shown) displayed no evidence of flow in this region.
~ers).~ Such ulcers typically are located in the
descending aorta above the diaphragm. Intra-
mural hematoma is most often the presenting
imaging finding, and contrast material rarely
fills the intramural space.26 Penetrating ulcers
can form a pseudoaneurysm or rupture. Im-
aging studies are helpful to search for the
underlying ulcer crater. On CT and
MR,
this
is often appreciated on the reconstructed or
3-D
images. Because of the cross-sectional na-
ture of
TEE,
images can be rather unimpres-
sive at first,
so
imaging in several planes is
particularly helpful. Angiography may dem-
onstrate the ulcer if the imaging plane is for-
tuitous.
Aortic Aneurysms
Atherosclerotic Aneurysms
Aneurysms of the thoracic aorta are an ex-
tremely common aging or degenerative phe-
n~mena.~ Aneurysms can be diffuse or focal,
chronic or acute, and can involve all layers of
the aortic wall (true aneurysm) or a contained
rupture (pseudoaneurysm). Etiology and
therapy vary considerably. Systemic hyper-
tension and collagen defects are the primary
risk factors for the development of atheroscle-
rotic aneurysms. Many aneurysms are discov-
ered incidentally when still asymptomatic, es-
pecially on routine scans done for other
reasons. Some patients present with chest
pain or compressive symptoms referable to
the bronchi or esophagus.
Most thoracic aortic aneurysms (TAA) are
fusiform in Fusiform aneurysms
usu-
ally arise in the aortic arch or descending
aorta. The less common saccular aneurysm
usually arises from the descending aorta. The
eccentric nature of the saccular aneurysm sac
and its continuity with the adjacent aorta oc-
casionally are not appreciated, and misdiag-
nosis as a mediastinal mass may occur. The
detection of peripheral calcification in the
wall of the aneurysm is a helpful diagnostic
finding. Most degenerative aneurysms show
a significant amount of mural thrombus,
which is usually shaped like a crescent
against the outer wall of the aorta, and lumi-
nal dilatation at the level of the thrombus is
,
almost always present.
I
Imaging diagnosis of thoracic aneurysm is
~traightforward.~9 In particular,
MR
and
CT
can accurately and reliably identify diffuse or
focal aortic dilatation, quantify the diameter
and length of the aneurysm, and detect the
presence of mural thrombus and calcification
(Fig.
15).
Branch vessel involvement is best
I
IMAGING
OF
THORACIC
AORTIC DISEASE
675
Figure
15.
Aneurysm of the aortic arch at
MR.
Gadolinium-
enhanced
MR
angiogram demonstrates moderate dilatation
of the ascending aorta (a). The descending thoracic aorta
and great vessels are normal in appearance. At the works-
tation, images such as this can be rotated in real-time,
providing views of the aneurysm from any perspective.
appreciated on
MR
or
CT.
In addition,
MR
and
CT
can clearly demonstrate the relation-
ship of an aneurysm to adjacent structures,
including the vertebrae, tracheobronchial tree,
pulmonary vessels, and
SVC
(Fig.
16).39
Aortic
insufficiency complicating an ascending aor-
tic aneurysm is best evaluated with
TEE
or
cine
MR.5
Figure
16.
Aortic aneurysm at CT. Axial image from spiral CT scan
reveals large aneurysm with extensive mural thrombus in the de-
scending thoracic aorta (d). The ascending aorta is moderately dilated
(a). Extensive erosion is seen into a vertebral body of the adjacent
thoracic spine
(arrow).
One benefit of the noninvasive imaging tech-
niques, especially CT and
MR,
is an ability to obtain additional useful
information about the structures surrounding the aorta, as in this case.
676
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et
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Unless an old film is available for compari-
son,
aneurysms must be considered acute
un-
til proven otherwise, because the underlying
entities can be immediately life threatening.
Acute diffuse dilatation of the aorta occurs in
aortic dissection and intramural hematoma,
as previously described. Chronic aneurysmal
dilatation without dissection can involve the
ascending aorta, descending aorta, aortic
arch, or all three. Chronic aortic aneurysms
have a variety of causes. Inflammatory dis-
eases can lead
to
destruction of the mechani-
cal integrity of the aortic wall with subse-
quent dilatation, as in syphilis. Atheros-
clerosis can become superimposed. Cystic
medial necrosis, a common pathologic diag-
noses in dilated aortas, is not detectable by
radiologic imaging. Cystic medial necrosis is
seen in Marfan’s disease, as well as in associa-
tion with aortic dissection (Fig.
17).
Dilatation
of the aortic root can accompany bicuspid
aortic valve or aortic stenosis.
Mycotic
Aneurysms
Mycotic aneurysms are secondary to infec-
tion. Mycotic aneurysms of the aorta are usu-
Figure
17.
Ascending aortic aneurysm at
CT.
Patient
with Marfan’s syndrome presented with retrosternal chest
pain. A three-dimensional volume rendered spiral
CT
re-
veals a large
10
cm aneurysm involving the ascending
aorta (a). Dissection flap is identified in the descending
aorta
(arrowheads).
Spiral
CT
with three-dimensional ren-
dering permits interactive display of aortic pathology, and
is helpful for planning surgical approach. The patient un-
derwent successful aortic root replacement.
ally confined to patients with predisposing
causes: intravenous drug use, valvular or con-
genital disorders of the heart, peri-cardiac in-
fections, or compromised immunity.39 My-
cotic aneurysms may occur at any site. Unlike
other aneurysms, they usually present with
pain as an early symptom. Mycotic aneu-
rysms are highly prone to rupture and are
considered a surgical emergency.
A
key to
diagnosis is the appreciation of the eccentric
or atypical location of the mycotic aneurysm,
which is often best demonstrated on 3-D im-
aging using CT or MR (Fig.
18).
Pseudoaneurysms
Pseudoaneurysms are focal dilatations and
can be acute or chronic. They are most com-
monly a consequence of surgery, trauma, or
inflammation. Late presentation in trauma
patients, often years following initial injury,
is not unusual, and patients can remain
asymptomatic for many years.43 Posttrau-
matic aneurysms frequently arise near the lig-
amentum arteriosum (Fig.
19).
Presentations
can include dyspnea, hoarseness, dysphagia,
massive hemoptysis (aortobronchial fistula),
massive hematemesis (aortoesophageal fis-
tula), superior vena cava syndrome, dia-
phragmatic paralysis, and herald bleeding
from the sternotomy in the postoperative pa-
tient.@ TEE is especially useful in demonstra-
ting the narrow neck and undermining char-
acteristic of these lesions. Pseudoaneurysms
can rupture into nearby structures such as
the mediastinum or pericardium, and TEE is
particularly suited to demonstrate these ab-
normal connections.
Aortic
Trauma
Transection of the aorta is a relatively com-
mon complication of high-speed deceleration
Deceleration forces can cause marked
shearing stress, particularly in zones where a
structure changes from being relatively fixed
to relatively unsupported.
In
the aorta, this is
commonly observed just distal to the origin
of the left subclavian artery in the region of
attachment of the ligamentum arteriosum.
Ninety-five percent of aortic transections oc-
cur at this point. Below this point, the aorta
IMAGING
OF
THORACIC AORTIC DISEASE
677
Figure
18.
Mycotic aneurysm at
CT.
A,
Left anterior oblique and
(B)
axial orientations from three-
dimensional volume rendered spiral
CT
reveal characteristic findings of mycotic aneurysm. The
aneurysm
(straight arrows)
is eccentric and arises from an unusual location
off
the antero-lateral
ascending aorta. Small ulceration
(curved
arrow)
is seen within the mural thrombus. Surgery revealed
mycotic aneurysm from
Staphylococcus aureus
infection.
is tethered by the intercostal arteries. Tears
also occur at the origins of the brachiocepha-
lic arteries, above the diaphragm, and in the
descending aorta.8 Transection of the aorta
is a major cause of death in motor vehicle
accidents. In survivors of aortic injury, the
adventitial layer maintains the continuity
of
the aorta, at least temporarily. The risk of
ultimate rupture is high;
50%
rupture within
the first
24
hours and most of the rest within
the next few weeks. Only 2% of patients sur-
vive with a chronic pseudoaneurysm.21
Supine portable chest films are unreliable
for the assessment of the presence
of
an aortic
tear.53 If a reasonable mechanism for aortic
tear exists, a proper study must be done to
rule it out, because the prognosis for undiag-
nosed tears is very poor. TEE and spiral CT
are excellent modalities for evaluation of aor-
tic trauma.lg,
33
Recent studies have shown
that a normal spiral CT examination
of
the
mediastinum in the setting of trauma virtu-
ally excludes an aortic transe~tion.3~ Spiral
CT can readily detect periaortic soft tissue
thickening and mediastinal hematomas. The
presence of periaortic hematoma is extremely
sensitive for predicting traumatic aortic rup-
ture and renders spiral CT of the chest a
reliable screening technique in this patient
pop~lation.~~ Spiral CT is also sensitive for
diagnosing intimal injuries and pseudoaneu-
rysms. In a study of
71
patients with docu-
mented blunt aortic injury, Fabian et a1 docu-
mented sensitivity and specificity of
100%
and
83%,
respectively, for spiral CT; and 92%
and 99%, respectively, for conventional angi-
ography.12
Tears may appear on imaging studies as
small intimal flaps, a blood clot localized to
the intima or the media, pseudoaneurysm,
mediastinal hemorrhage, abnormal contour of
the aortic lumen, or some combination
of
these (Fig.
20).8
Dissection of the media over
more than a centimeter or
so
is rare in trau-
matic tears. The presence of pericardial effu-
sion is a bad prognostic sign, and the heart
and aortic root should be thoroughly exam-
ined for evidence of injury. There is some
evidence that local mural hemorrhage sec-
ondary to trauma can resolve, and there is
currently a discussion in the surgical litera-
ture over the role of conservative treatment
in apparently minor injuries.52,
55
Although the role of spiral CT is well-estab-
lished in the trauma patient, the use of
TEE
as
the primary diagnostic method for traumatic
tears is not universally endorsed; both false
positives and negatives have been reported.31
678
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et
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Figure
19.
Posttraumatic pseudoaneurysm at CT.
A,
Axial image
from spiral CT reveals aneurysm (p) immediately distal to the
left subclavian artery. A calcified rim is present
(arrowheads),
an
indicator
of
chronicity. The patient had a history of a motor
vehicle accident 8-years earlier. History, location, and CT ap-
pearance are compatible with diagnosis
of
posttraumatic pseu-
doaneurysm.
6,
Conventional angiogram provides excellent cor-
relation in this case. Because they are readily available, safe,
and inexpensive, noninvasive imaging modalities such as CT
have replaced conventional angiography in the detection and
display of aortic pathology at most institutions.
Aortic arch branch injury must be carefully
sought. In a study by Ahrar et all of
89
trauma patients with aortographic evidence
of aortic or branch injury,
14
had branch injur-
ies without aortic injuries;
70%
of these injur-
ies were not suspected on physical exam. This
implies that branch injury is common among
patients with vessel injury, and raises the
question of the suitability of TEE as the stan-
dard exam in trauma, because the branches
are difficult to see well by
TEE
in a sizable
minority
of
patients. Also, TEE has a few
potential problems involving image acquisi-
tion. Nasogastric tubes are frequently present
and can result in artifact. The tube can usually
be pulled up into the neck and readvanced
after the exam. Occasionally the patient is
too combative or the aorta
tortuous,
and a
nondiagnostic study
is
obtained. The echocar-
diographer must then be ready to recommend
a different imaging test.
Some of the pitfalls that aortography is
IMAGING
OF
THORACIC AORTIC DISEASE
679
Figure
20.
Aortic transection at CT. Acute extravasation of
IV
contrast is
seen at site
of
aortic tear in the distal descending thoracic aorta
(arrow).
A small periaortic hematoma is present. Deceleration forces typically
result in aortic injury at fixed points, most commonly involving the aorta
just distal to the origin
of
the left subclavian artery. This case represents
a much less common site at the level of the diaphragmatic hiatus. (From
Davis KF, Urban BA,
Ott
M,
et al: Computed tomographic demonstration
of aortic transection at the diaphragmatic hiatus. Emerg Radio1
5253-
255,
1998;
with permission.)
prone to are shared by CT and
TEE,
specifi-
cally ductus diverticulum, Kommerell's di-
verticulum, and large intrathoracic branches
mimicking tears. Doppler ultrasound and
careful imaging in multiple planes can be
helpful during
TEE.
Ductus diverticula typi-
cally have smooth transitions with the rest of
the aorta; abrupt transitions are suspect. At
the insertion of the ductus into the aortic wall,
a small focus of calcification can be seen even
in young patients.
This
has the typical bright
echogenicity of calcium at TEE and should
not be confused with pathology.
Imaging the Aorta in the Stroke
Patient
With the advent of
TEE,
the thoracic aorta
has been recognized as an important source
of emboli.34 Protuberant atheromatous
plaques and associated clots can be impres-
sive, sometimes with large mobile compo-
nents (Fig.
21).
Mobile or thick
(25
mm) le-
sions predict a high incidence of stroke.34
These lesions were not diagnosed before the
heavy use of
TEE
in stroke patients. Epiaortic
ultrasound is more accurate than TEE for
evaluation of the ascending aorta precannula-
tion for cardiopulmonary bypass, because
TEE
cannot image a portion of the ascending
aorta.',
27
Recently, unenhanced spiral CT has
been evaluated as a noninvasive alternative
technique to TEE for detecting protruding
atheromas in patients with embolic events.
One advantage of spiral CT is the ability to
evaluate areas that are
blind
to
TEE,
including
the upper ascending aorta and the proximal
arch.5l
Spontaneous echo contrast in the aorta is
an independent predictor of myocardial in-
farction and possibly of other ischemic events
as weIl.l4,
49
Young patients can be devastated
by emboli from clot adherent to minimal ath-
erosclerotic plaque in the aorta. Clot in the
aortic root in lupus erythematosis leading to
stroke has been reported.20 Aortic dissection
can sometimes present as a stroke, although
it usually presents with other signs as well.
CONCLUSION
CT, MR imaging, and transesophageal
echocardiography represent reliable, rela-
680
URBAN et a1
Figure
21.
Atheromatous plaque at
TEE.
A large irregular ather-
oma
(curved
arrow)
occupies the aortic arch. Adherent clot and
atheroma contents pose
a
significant embolization risk, and are
well-evaluated at
TEE.
tively noninvasive modalities that have essen-
tially replaced conventional aortography for
imaging the thoracic aorta. Each provides a
unique methodology for evaluation of vascu-
lar pathology, including aortic aneurysms,
aortic dissection, and blunt aortic injury.
Con-
tinued software advances in
3-D
processing
and virtual reality display techniques, most
notably in newer
CT
and
MR
techniques, en-
sure an exciting and ever-changing role for
medical imaging in the detection, diagnosis,
and display of thoracic aortic pathology for
years to come.
References
1.
Ahrar
K,
Smith DC, Bansal RC, et al: Angiography in
blunt thoracic aortic injury. J Trauma 92665469,1997
2. Barentsz JO, Ruijs JH, Heystraten FM, et al: Magnetic
resonance imaging of the dissected thoracic aorta. Br
J
Radiol 60:499-502, 1987
3. Bickerstaff LK, Pairolero PC, Hollier LH, et al:
Tho-
racic aortic aneurysms: A poplation-based study. Sur-
gery 92:1103-1108, 1982
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Address reprint requests to
Bruce
A.
Urban, MD
The Russell H. Morgan
Department of Radiology and Radiological Science
The
Johns
Hopkins Medical Institutions
600
N.
Wolfe Street
Baltimore. MD 21287
e-mail: burban@rad.jhu.edu