136
sometimes causing compression of the true lumen. Moreover, the
characteristics of blood flow vary in the true and false lumens. In
the true lumen, antegrade systolic flow is rapid enough to create
brighter shades of red or blue on color Doppler. In contrast, flow in
the false lumen is generally slower, producing duller colors. In fact,
flow in the false lumen may be absent or in the opposite direction
(retrograde) to that of the true lumen. The sluggish flow in the false
lumen may result in the presence of spontaneous echo contrast,
sometimes referred to as ‘‘smoke.’’ The false lumen may also contain
variable degrees of thrombus. Additional findings in patients with
aortic dissection include dilatation of the aorta, compression of the
left atrium, AR, pericardial and/or pleural effusion, and involvement
of the coronary arteries.Table7 summarizes the main echocardio-
graphic findings in aortic dissection. Three-dimensional TEE may
provide information beyond what can be obtained with 2D TEE.
151
For example, the size of the entry tear size and its relationship to
surrounding structures may be shown in greater detail, allowing
better morphologic and dynamic evaluation of aortic dissection
(Figure30). Such information may be particularly helpful when the
flap spiralsaround the long axisof theaorta. Moreover, 3D TEE dem-
onstrates the dissection flap not as a linear structure but as a sheet of
tissue of variablethickness in the long, short, orobliqueaxis. This may
make it possible to distinguish a true dissection flap from an artifact
when it is relatively immobile. In addition, multiplane 3D TEE
provides a more rapid and accurate evaluation of the aortic arch
than 2D TEE.
b. Detection of Complications.–AR occurs in approximately 50%
of patients with type A aortic dissection. The presence, severity, and
mechanism(s) of AR may influence surgical decision making and
aid the surgeon in deciding whether to spare, repair, or replace the
aortic valve.
148,152,153
The mechanisms of AR are listed inTable8,
and several of these are illustrated inFigure31. These mechanisms
will be discussed in greater detail in section III.B.6, ‘‘Use of TEE to
Guide Surgery for Type A Aortic Dissection.’’
Apericardialeffusion in an ascending aortic dissection is an indica-
tor of poor prognosis and suggests rupture of the false lumen in the
pericardium. Echocardiography is the best diagnostic technique for
estimating the presence and severity of tamponade. Periaortic hema-
toma and pleuraleffusion arebest diagnosed byCT. The presence of
periaortic hematoma has also been related to increased mortal-
ity.
154,155
TEE is capable of imaging the ostia and proximal segments of the
coronary arteriesin nearlyallpatients and maydemonstratecoronary
involvement due to dissection (flap invagination into the coronary
ostium and origin of coronary ostium from the false lumen).
148
Color Doppler is useful for verifying normal or abnormal or absent
flow into the proximal coronary arteries. Detection of segmental
wall motion abnormalities of the left ventricle by TTE or TEE may
also help identify this complication. Color Doppler also reveals
reentry sites (often multiple, as in n Figure 32), which explain why
the false lumen often remains patent over time.
c. Limitations of TEE.–Thelimitations of TEE for evaluating patients
with aortic dissection are few but deserve mention. Interposition of
the trachea between the ascending aorta and the esophagus limits
visualization of thedistalascending aorta and proximalarch. In a small
numberof patients, the dissection may belimited to this area, making
detection more difficult. In addition, the cerebral vessels (especially
thebrachiocephalic and left common carotid arteries) can be difficult
to image by TEE. Moreover, the celiac trunk and superiormesenteric
artery cannot be consistently imaged by TEE, and CT is considered
the gold standard for detecting complications below the diaphragm.
Last, TEE depends largely on operator skill for image acquisition
and interpretation. Reverberationartifacts, especially in theascending
aorta, can mimic a dissection flap and result in a false-positive diag-
nosis.
156-159
Knowledge of mediastinal and para-aortic tissues (e.g.,
the hemiazygos sheath, the thoracic venous anatomy and common
anatomic variants) is essential.
3. CT. Data from the IRAD published in 2000 showed that among
464 patients with acute aortic dissections (62% with type A), nearly
two-thirds underwent CTA as the initial diagnostic imaging. The
computed tomographicdata in this studywereacquired on oldergen-
eration scanners, whichmayexplainthefactthat most patientsunder-
went several imaging tests (average, 1.8 tests).
129
Amore recent IRAD publication, now including 894 patients,
showed that the ‘‘quickest diagnostic times’’ were achieved when
the initial test was CT, whereas the initial use of MRI or catheter-
based aortography resulted in significantly longer diagnostic
times.
160
Today, newer generation modern multidetector computed tomo-
graphic scanners are ubiquitous even in remote-area hospitals
throughout the United States and Europe and are usually staffed
and readily available 24 hours a day. In 2007, according to 2011
health data from the Organisation for Economic Co-operation and
Development, there existed 34.3 computed tomographic scanners
per million population in the United States, and 185computed tomo-
graphic examinations were performed per 1,000 patients in US
hospitals.
Computed tomographic angiographic protocols are robust and
relatively operator independent. Computed tomographic angio-
graphic protocols that are designed to exclude dissections typically
begin with low-dose noncontrast CT to exclude the possibility of
IMH, followed by contrast-enhanced computed tomographic angiog-
raphy. The coverageincludes the entire thorax, abdomen, and pelvis
to allow delineation of the extent of a flap and its extension into
branch vessels and to evaluate for end-organ ischemia (e.g., bowel
or kidneys), and possible extravasation.
1
Examples of computed
tomographic angiography are illustrated inFigures33and34.
Diagnostic accuracy is extremely high for the exclusion of aortic
dissection (98%–100%).
122,161,162
However, false positives for the
detection of type A dissection near the aortic arch may infrequently
occur with older generation computed tomographic scanners,
which may lead to unnecessary operations.
163-166
Single-slice spiral
computed tomographic scanners and early-generation multidetector
computed tomographic scanners frequently demonstrate pulsation
artifact in the ascending aorta, which occasionally may mimic type
A dissection (pseudoflaps).
80,164,165
However, aortic pulsation
artifact and pseudoflaps can be completely eliminated with the use
Table 8 8 MechanismsofARintypeAaorticdissection
1. Dilatation of the aorticroot leadingto incompleteaortic leaflet
coaptation
2. Cusp prolapse(asymmetric dissectiondepressingcusp[s] below
annulus)
3. Disruptionof aorticannularsupport resulting in flail leaflet
4. Invagination/prolapse of dissectionflap throughtheaortic valve
in diastole
5. Preexisting aorticvalve disease (e.g., bicuspid valve)
Journal of the AmericanSociety of Echocardiography
Volume 28 Number2
Goldstein et al 141
105
of
electrocardiographically
gated
computed
tomographic
angiographic acquisitions.
167,168
Therefore, it is advisable to use
electrocardiographic gating or triggering if ascending aortic
pathology is suspected.
80,167,169,170
False-positive results on CT lead-
ing to unnecessary surgery for aortic dissection have not been
reported to date with the use of newer generation electrocardio-
graphically gated multidetector computed tomographic angiographic
scans.
Surgery or transcatheter intervention in type B dissection may be
indicated if there is occlusion of major aortic branches leading to
end-organ ischemia or expansion of the aortic diameter or interval
extension of thedissection flap.
171
MDCTallowsimaging of theentire
aorta and iliac system within seconds and allows delineating the
intimalflap extension into aortic arch vessels andthe abdominalaorta
and its branches as wellas the iliac system, which maydeterminethe
feasibility of stent-graft repair.
170,172
Entry and reentry sites, aortic
diameters, and the relationships between true and false lumen can
be defined using multiplanar multidetector computed tomographic
reformations. MDCT also allows the determination of end-organ
perfusion, such as asymmetric or absent enhancement of kidneys in
case of renal artery occlusion.
72,167
Given the multiplanar reformation capabilities that, unlike MRI,
can be applied post hoc, and 3D imaging capabilities, CT has
extremely high retest reliability for measurement of aortic diameters
on follow-up scans. The multiplanar reconstruction capabilities facili-
tate endovascular treatment planning and may allow the determina-
tion of proximal fenestrations that may be amenable to
endovascular repair.
173
Because determination of these features is
important, reporting of the extension of dissection and aneurysms
into branch vessels and secondary end-organ hypoperfusion are
considered ‘‘essential elements’’ of aortic imaging reports.
1
Gated
MDCT may determine proximal extent of the flap into coronary
arteryostia, or the aortic valve, as well as presence of pericardialeffu-
sion or hemopericardium.
168
Gated MDCT may simultaneously exclude the presence of
obstructive coronary artery disease in acute dissections,
174
as
well as coronary artery dissection and aortic valve tears.
167,170,175
In addition, combination of a gated or triggered thoracic
computed tomographic angiographic acquisition with a nongated
abdominal and pelvic acquisition is feasible at low radiation
doses.
172,176-178
Further dose reduction using axial prospective electrocardio-
graphic triggering (compared with spiral retrospective gating)
computed tomographic angiography at a tube potential of 100 kV
allows the further reduction of radiation doses without impairment
of image quality of the aorta or coronary arteries.
179
The ‘‘triple rule-out’’ protocol for assessing acute chest pain in the
emergency room is rarely needed and is neither technically suitable
normedicallynecessaryon a routine basis. Optimalprotocolsfor cor-
onary CT angiography, for pulmonary embolism, and for aortic
dissection differ, and ‘‘triple rule-out’’ CT is not optimal for all three.
Given the increased radiation and contrast exposure and the lack of
accurate diagnostic data for aortic dissection, there are no grounds
to recommend triple rule-out CT for this condition. If there is a
reasonable clinical suspicion for aortic dissection, then the highest
quality study for this specific indication should be performed.
180,181
In summary, CT angiography is readily available throughout the
United States and Europe; is most often the first imaging test when
acute aortic dissection is suspected; has extremely high diagnostic
accuracy; allows the evaluation of the entire aorta and its branches,
the coronary arteries, the aortic valve, and the pericardium; and re-
sults in the shortest time to diagnosis compared with other imaging
modalities, therefore allowing rapid initiation of therapy.
Disadvantages of CT includethe need for iodinated contrast material
and ionizing radiation, although substantial dose reductions have
recentlybeen achieved withnewer hardwaretechnologyandimaging
protocols, and this issue may be of less concern in thesetting of AAS.
Figure 31 1 MechanismsofARinthesettingofaorticdissection.(A)Transesophagealechocardiogramdemonstratingabsenceof
coaptation of aortic leaflets due to dilatation of the aortic root (the most common mechanism of aortic insufficiency associated
withtype Adissection). Arrow designates the dissection flap. (B) Transesophageal echocardiogram of the aortic rootillustratingpro-
lapse of the aortic valve (smallarrow) due to extensionof the dissectionto the annulus causing AR (not shown). FL, False lumen; LA,
left atrium; TL, true lumen. (C) Transesophageal echocardiogram of the aortic root and ascending aorta (Ao) illustrating a dissection
flap (arrow) prolapsing through the aortic valve into the left ventricular outflow tract (LVOT), resulting in AR in this patient.
Figure 32 2 Longitudinalviewofatransesophagealechocardio-
gramwithcolor Doppler illustratesmultiple reentry sites(arrows)
demonstrating flow from true lumen (TL) to false lumen (FL).
Reentry sites are the major reason the false lumen remains pat-
ent over time.
142 Goldstein et al
Journal of the American Society of Echocardiography
February 2015
.NET RasterEdge XDoc.PDF Purchase Details PDF Print. License Agreement. Support Plans Each RasterEdge license comes with 1-year dedicated support (email, online chat) with 24-hour response time (working
how to add text to a pdf document; add text pdf acrobat
36
4. MRI of Aortic Dissection. Early identification of aortic dissec-
tion and precise characterization of anatomic details are critical for
clinical and surgical management of this condition.
182
Imaging of sus-
pected dissection shouldaddressnot onlythepresenceof a dissection
flap and itsextent but also theentryand reentrypoints, presence and
severity of aortic insufficiency, and flowinto arch and visceralbranch
vessels. MRI, which can address all of these issues noninvasively,
provides high spatial and contrast resolution and functional assess-
ment with an imaging time of 20 to 30 min. Specifically, MRI has
very high sensitivity (97%–100%) and specificity (94%–100%) for
diagnosing dissection.
161,183,184
MRI also provides imaging without
the burden of ionizing radiation, an important consideration for
patients who undergo serialassessments of a known aortic dissection.
MRI does have potential limitations in this patient population.
Although the scan times for MRI are relatively short, they are signifi-
cantly longer than the scan times for CT angiography. Additionally,
physiologicwaveforms arechallenging to obtain withinthe MRI scan-
ner environment.
185,186
Although cardiac rhythm, blood pressure,
and oximetry can be monitored with MRI-appropriate equipment,
caring for patients within an MRI scanning area can be difficult in
Figure 33 3 Axialsourceimagesfromthecomputedtomographicaortogram(left)andthelate-phasecomputedtomographicstudy
(right) performed in a patient with AAS. The additional late acquisition rules out false lumenthrombosis, showing late enhancement
and retention of contrast-enhanced blood in the false lumen.
Figure34 EvolutivechangesinatypeBchronicaorticdissection.Thecomparisonisperformedbysynchronizingthin(0.75-mm)axial
imagesof the baselineand follow-up computed tomographic aortograms.The imagesshowanexpansionofthefalse lumen(asterisk)
with compression of the true lumen, with an overall mild external expansion of the dissected descending thoracic aorta. Note the
similarity of mediastinal and posterior thoracic wallanatomic markers.
Journal of the AmericanSociety of Echocardiography
Volume 28 Number2
Goldstein et al 143
105
emergent or unstable clinical scenarios that may be associated with
aortic dissection.
Acombination of dark-blood and bright-blood images in axial and
oblique planes oriented to the aorta allows the detection and charac-
terization of intimal flaps. Trueand falselumens can bedifferentiated
bypatterns of flow and by anatomic features (Figures35and36).
185
The false lumen can often be identified on spin-echo images by a
higher intraluminal signal intensity attributable to slower flow and
may be characterized by web-like remnants of dissected media.
187
Cine bright-blood imaging can also be used to directly visualize
flow patterns within true and falselumens. Associated anatomic find-
ings outside of theaorta on MRI may also be of interest, such as high
signal intensity within pericardial effusion on dark-blood imaging,
indicating thepossibilityof theascending aorta rupturing into theperi-
cardial space.
188
Phase-contrast imaging can provide flow quantifica-
tion of aortic insufficiency associated with dissection and can also
allow definition of entry and reentry sites and differentiation of
slow flow and thrombus in the false lumen. Newer 3D phase-
contrast approaches have shown promise in further defining the
flow characteristics and associated parameters of aortic dissection,
such as wall stress.
189
Contrast-enhanced 3D MRA provides 3D data, results that allow
postprocessing and detailed assessment of aortic and large-branch
vessel anatomy in cases of dissection.
190
The dynamics of aortic
flow can also be evaluated with time-resolved MRA.
191
Imaging
with blood-pool contrast agents allows steady-state phase scanning,
which can improve spatial resolution and better demonstrate the
amount of thrombus within the false lumen.
192
5. Imaging Algorithm. Aortic dissection is a life-threatening condi-
tion that is associated with high early mortalityand thereforerequires
prompt and accurate diagnosis. Numerous publications have sought
to establish the relativemerits of CT, TEE, and MRI as first-line imag-
ing modalities. In truth, each diagnostic method has its strengths and
weakness, as previouslydiscussed. Theoptimalchoiceofimaging mo-
dality at a given institution should depend not only on the proven
accuracy (all three are highly accurate) but also on the availability of
the techniques and on the experience and confidence of the physi-
cian performing and interpreting the technique. CT has become the
most commonly used first-time imaging modality partly because it
is more readily available on a 24-hour basis.
129
TEE may be the
preferred imaging modalityin theemergencyroom, if anexperienced
cardiologist is available, because it provides immediate and sufficient
information to determine if emergency surgery will be required.
Although CT may be less accurate for determining the degree and
mechanismof AR,this can beevaluated byTTE and/orintraoperative
TEE. Therelative advantages and disadvantages of the various imag-
ing modalities are summarized inTable9.
There aresituations in which a single imaging test is insufficient to
confidently confirm or exclude the diagnosis of aortic dissection. A
strong clinical suspicion accompanied by a negative initial imaging
test shoulddictatea second test, asshoulda situation in which thefirst
test is nondiagnostic. This may bedueto technicallimitations orinter-
pretative difficulties (e.g., distinguishing an artifact from a true flap).
Because of the importance of establishing a correct diagnosis in this
potentially life-threatening condition, obtaining a second or even a
third imaging modality should be considered.
In summary, CT is an excellent imaging modality for diagnosing
aortic dissection and is most often the initial modality when aortic
dissection is suspected because of its accuracy, widespread availabil-
ity, and because it provides rapid evaluation of the entire aorta and
its branches. TTE may be useful as the initial imaging modality in
the emergency room, especially when the aortic root is involved.
Contrast may improve its accuracy. TTE may also complement CT
by adding information about the presence, severity, and mecha-
nism(s) of AR, pericardial effusion, and left ventricular function.
TEE may be a second-line diagnostic procedure when information
from CT is limited (sometimes not certain if the ascending aorta is
involved). TEE can define entry tear location and size, mechanism(s)
and severityof AR, and involvement of coronaryarteries. TEE should
be performed immediately beforesurgery in theoperating roomand
should be used to monitor the operative results. All of these modal-
ities may be helpful for identifying associated lesions at the aortic
valve level (e.g., bicuspid aortic valve [BAV]) that may require a spe-
cific surgical strategy.
6. Use of TEE to Guide Surgery for Type A Aortic
Dissection. TEE should be performed in the operating room in
all patients during repair of typeA aortic dissection. Even if the diag-
nosis has been ‘‘established’’ with a preoperative imaging modality,
confirmation byintraoperativeTEEbeforeinitiating cardiopulmonary
bypass willminimize thepossibility of a false-positive diagnosis. Once
the diagnosis of aortic dissection has been confirmed, the primary
purpose of the intraoperative TEE is to detail the anatomy of the
dissection and to better define its physiologic consequences. The
origin and proximal extent of the dissection flap and the dimensions
of theaorta at the annulus, sinuses of Valsalva, and STJ areimportant
Figure 35 MRimage extractedfrom adynamic cine steady-
state free precession (SSFP) sequence in a patient with type B
aortic dissectionarising justafter the originof the leftsubclavian
artery. The arrow shows the entry tear. FL, false lumen; TL, true
lumen.
144 Goldstein et al
Journal of the American Society of Echocardiography
February 2015
103
for determining whether to replace the ascending aorta alone or to
also replace the root.
Up to 50% of typeA aortic dissections are complicated bymoder-
ateorsevere AR, and thereareseveralmechanisms bywhich this may
occur.
193
Most commonly, aortic dilatation, be it acute or chronic,
leadstoaortic leaflettethering that, in turn, resultsin incompleteaortic
valve closureand secondary AR.
194
When the dissection flap extends
proximallyintothesinuses of Valsalva (i.e., belowthelevelof theSTJ),
it can effectivelydetach oneor more of theaortic valve commissures
from the outer aortic wall; the aortic valveleaflets arethen no longer
suspended from the STJ and therefore prolapse in diastole, causing
significant AR. Less commonly, the dissection process is extensive
and results in a long, complex dissection flap, a piece of which may
itself prolapsethrough theaorticvalveinto theleftventricular outflow
tract in diastole, preventing normal leaflet coaptation and causing
AR.
195
Remarkably, in some patients, the dissection causes prolapse
of the aortic leaflets, which would otherwise produce severe AR,
yet a lengthy piece of the dissection flap falls back against the aortic
valve in early diastole and essentially smothers the orifice and pre-
vents regurgitation. In such cases, Doppler may reveal only mild AR
Figure 36 6 Imagesfroma55-year-oldwomanwithchronictypeBaorticdissection.Thetruelumen(yellowarrow)ischaracterizedby
lack of signal in the dark blood image (left), bright signal in the single-shot steady-state free precession(SSFP) image (middle), and
brightsignal(caused bycontrastfilling)inthe MR angiographicimage(right). False lumen(red arrow)isnotable for intermediate signal
on dark blood and single-shot SSFP sequences, and lack of signal is noted inthe thrombosed false lumen on MRA.
Table 9 9 Recommendationforchoiceofimagingmodalityforaorticdissection
Modality
Recommendation
Advantages
Disadvantages
CT
First-line
Initial test in >70%of patients*
Widely available, quickest diagnostic times
Very highdiagnosticaccuracy
Relatively operatorindependent
Allows evaluationof entire aorta,including arch
vessels, mesenteric vessels and renal arteries
Ionizing radiation exposure
Requires iodinated contrast material
Pulsation artifact in ascending aorta (can be
improved withECGgating)
TEE
First- and
second-line
Very highdiagnosticaccuracy in thoracicaorta
Widely available, portable, convenient, fast
Excellent forpericardial effusion, and presence,
degreeand mechanism(s) of AR and LVfunction
Can detect involvement of coronary arteries
Safely performed on critically ill patients, even
those on ventilators
Optimal procedure for guidancein OR
Operatordependent (depends on skill of operator)
‘‘Blind spot’’ upperascending aorta, proximal arch
Not reliableforcerebral vessels,celiactrunk,SMA,
etc.
Reverberation artifacts can potentially mimic
dissectionflap (can be differentiated from flaps in
vast majority)
Semi-invasive
TTE
Second-line
Ofteninitial imaging modality inER
Provides assessment of LV contractility, pericar-
dial effusion, RVsizeand function, PA pressure
Presence and severity of AR
Sensitivity not sufficient distal to aorticroot
Descending thoracic aortaimaged less easily and
accurately
Misses IMHand PAU
MRI
Third-line
3D multiplanar, and high resolution
Very highdiagnosticaccuracy
Does not requireionizing radiation or iodinated
contrast
Appropriatefor serial imaging over many years
Less widely available
Difficult monitoring critically ill patients
Not feasible in emergent orunstable clinical situa-
tions
Longerexamination time
Caution with useof gadolinium inrenal failure
Angiography
Fourth-line
Rarely necessary
Often misses IMH (up to 10%–20% of ADs)
Long diagnostic time
Requires ICM
Morbidity
Less sensitivity than CT, TEE, and MRI
AD, Aorticdissection; ECG, electrocardiographic;ER, emergency room; ICM, iodinated contrast media; IMH,intramural hematoma; LV, left ven-
tricular;OR, operatingroom;PA, pulmonaryartery;PAU, penetratingatheroscleroticulcer; RV, rightventricular;SMA, superiormesenteric artery.
*In IRAD.
Journal of the AmericanSociety of Echocardiography
Volume 28 Number2
Goldstein et al 145
152
despite significant disruption of the aortic valve. Alternatively, a
circumferential dissection of the ascending aorta can tear away and
produce a tubular proximal dissection flap that prolapses the aortic
valve in diastole, essentially akin to ‘‘intussusception,’’ producing
severe AR.
196
These patients may not require repair or replacement
of the aortic valve.
Some patients with aortic dissection have more than one of these
anatomic processes occurring simultaneously. Fortunately, most of
these anatomic causes of AR are correctable during surgery, so in-
forming thesurgeon in detail about the anatomic findings and mech-
anisms of AR maypermit successfulrepairratherthan replacement of
the aortic valve. TypeA aortic dissection can sometimes compromise
flow to one of the coronary arteries, the right coronary artery more
often than the left. Although coronary involvement may be evident
preoperatively with ischemic changes on electrocardiography, the
process may be dynamic, so the echocardiographer should examine
both coronary ostia to determine if they are compromised
(Figure37). ColorDoppler is useful to document normalordisturbed
or absent flow in each coronary artery.
The emergent surgical treatment of type A dissection is limited to
proximal aortic segments in the majority of patients. However, when
the dissection extends into the abdominal aorta, patients are at risk
for malperfusion, which occurs from either of two mechanisms: static
obstruction occurs when the dissection flap extends into a branch ar-
tery and limitsantegrade arterialflow, and dynamic obstruction occurs
because of marked compression of thetruelumen bya distended false
lumen, resulting in impaired forward flow through the true lumen to
feed the otherwise patent branch arteries. Because TEE is usually un-
able to visualize the abdominal branch arteries themselves, the pres-
ence of static obstruction cannot be readily assessed. However, TEE
can identify true luminalcompression in the distaldescending thoracic
aorta and confirm impaired systolic flow by Doppler. Although such
findings do not necessarily indicate clinical malperfusion, at the very
least they represent the substrate for dynamic malperfusion, and it is
therefore important to bring this to the attention of the surgeon.
On occasion, distension of the false lumen will compress the true
lumen and produce malperfusion of organs orlimbs.
197
Usually, stan-
dard surgical repair of the ascending aorta restores flow to the true
lumen partially or fully. If the true lumen remains compressed and
is associated with malperfusion, further intervention (e.g., endovascu-
lar stent grafting orpercutaneous fenestration of thedissection flap to
decompress thefalselumen)maybenecessary.
198
Onoccasion, in pa-
tients with organ malperfusion, endovascular stent grafts may be
placed before repair of the ascending aorta.
PreoperativeTEE should also evaluatethepericardialspace forthe
presence of an associated pericardial effusion. In some cases, aortic
dissection may be accompanied by a small serous effusion, but
more often, the presence of an effusion is due to bleeding into the
pericardial space. In such cases, the blood pooling acutely in the peri-
cardial space will typicallyclot and appear echocardiographically as a
mass sliding back and forth within a layer of pericardial fluid. This
finding of a clot within the pericardial fluid heralds potential cata-
strophic aortic rupture and should therefore be communicated
promptly to the surgeon.
After the repair of a type A aortic dissection, the echocardiog-
rapher should systematically reexamine the anatomic features of
the aortic valve and proximal aorta to make sure that the surgical
correction has been adequate (including exclusion of the entry tear
and exclusion of all proximal communications) and that the aortic
valve is competent. In addition, when the dissection has extended
to the distal aorta, the echocardiographer should reexamine the de-
scending thoracic aorta to determine the presence of adequate flow
through the true lumen.
7. Use of Imaging Procedures to Guide Endovascular
Therapy. The success of TEVAR is critically dependent on
high-quality, accurate imaging before, during, and after stent-graft
placement.
199
Although invasive catheter-based angiography
(Figure14) is the method of first choice for the guidance of aortic
stent-graft placement,
42
TEE offers definite advantages in the hands
of an experienced examiner,
59,60,62,200,201
TEE is particularly useful
in the operating room and provides contributions at various phases
of the procedure. In patients with type B aortic dissection,
guidewire advancement and positioning can be guided by both
fluoroscopy and TEE. However, unlike fluoroscopy, TEE can
differentiate between true and false lumens and can confirm
correct guidewire placement in the true lumen and prevent
misplacement of a catheter or wire before deploying any device. In
atherosclerotic aneurysms, protruding aortic plaques at the
Figure 37 7 Transesophageal echocardiogram m from a patient
with type A aortic dissection that illustrates the dissection flap
(arrow) entering the ostium of the right coronary artery (RCA).
LA, Left atrium.
Table 10 0 PrevalenceofIMH(aspercentageofaortic
dissection or nontraumatic AAS)
Author
Year
n
%
Source
Mohr-Kahaly
1994
27/114
23%
420
Nienaber
1995
25/195
12.8%
421
Keren
1996
10/49
20%
422
Harris
1997
19/84
23%
423
Vilacosta
1997
15/88
17%
229
Nishigami
2000
59/130
45%
424
Ganaha
2002
66/725
9%
425
Evangelista
2003
68/302
22%
154
Attia (meta-analysis)
2009
—
17%
426
Totals
289/1,687
17%
146 Goldstein et al
Journal of the American Society of Echocardiography
February 2015
105
proximal neck may impede tight adhesion between the stent-graft
and aortic wall, leading to dangerous proximal leaks. These plaques
are easily detected by TEE and not by angiography or fluoroscopy.
Therefore, just before proximal stent-graft deployment, TEE is essen-
tial for selecting an aortic wall segment without protruding plaques
and confirming selection of the stent-graft diameter.
59,60,62
Orientation and navigation as guided by TEE can be comple-
mented by the use of IVUS (usually with 10-MHz transducers) over
aguidewire, thereby confirming or correcting navigation in the true
lumen even at the level of the abdominal aorta and iliac arteries. In
addition, intraprocedural IVUS may clarify the mechanism of branch
vessel compromise when malperfusion is suspected (e.g., dynamic vs
static obstruction of a branch vessel).
125,126
Thrombus formation
within the false lumen can also be visualized by spontaneous echo
contrast, and IMH is easily depicted as crescent-shaped or circular
wall thickening. Devicesizing can beverychallenging with aortic dis-
sections because of the possibility of compromising the true lumen.
After endovascular stent graft implantation, IVUS also enables dy-
namic evaluation of the success of the procedure.
200,202-204
Angiography, TEE, and IVUSareused forevaluating theexpansion
of stentgrafts, verification of branchanastomosis and thebeginning of
false lumen thrombosis, and reevaluation of improved malperfusion.
During a procedure, TEE may be superior for assessing retrograde
typeA dissection and can provideimmediate information on left ven-
tricular function. With the use of color Doppler, TEE is superior to
angiography, and especially to IVUS, in the detection of endoleaks
after stent graft implantation.
59,62,125
In several studies, TEE
provided decisive additional information to angiography and
fluoroscopy, leading to successful procedural changes in up to 40%
to 50% of patients.
59,60,62
After stent-graft deployment, color
Doppler TEE is highly useful for detecting persistent leaks that can
be promptly resolved by balloon dilatation or further stent-graft im-
plantations.
205
Most of these leaks are not visible on angiography.
To maximize sensitivity for persistent leaks, reduced Doppler scale
(25 cm/sec) can improve color signal detection. However, by itself,
this can lead to false-positivediagnosesof leaks, becauseimmediately
after implantation, Dacron porosity can create temporary low blood
flow through the stent (and seen with low-velocity color flow
Doppler), especially when systolic blood pressure is >120 mm Hg.
To prevent false-positive diagnosis of leaks, pulsed Doppler velocity
assessment permits distinction between Dacron porosity (usually
with velocity <50cm/sec) and the fasterflow of truepersistent leaks
(usually>100cm/sec)with higher sensitivity than angiography.
126
In
aortic dissection, TEE is also useful for detecting small distal reentry
tears not visible on angiography; thoracic reentry tears can subse-
quently be resolved by additional stent-graft deployment.
59,62,125
TEE is partially limited for visualizing the brachiocephalic and left
common carotid artery ostia, and this information may be crucial to
proximal positioning of the stent graft. It should be noted that TEE
is useful when a Dacron stent graft is used, whereas it is not useful
with polytetrafluoroethylene or Gore-Tex prostheses because polyte-
trafluoroethylene acts as a barrier to ultrasound.
In a recent small study, intraluminal phased-arrayultrasound imag-
ing proved to be superior to IVUSand to TEE in detecting communi-
cations between the true and false lumens of aortic dissection.
200
However, IVUS and intraluminal phased-array ultrasound imaging
catheters are disposable and therefore more expensive than TEE
and cannot be performed simultaneously with stent-graft placement,
whereas TEE is suited to parallel imaging and intraprocedural moni-
toring.
In summary, TEE and IVUS are particularly useful for guiding
endovascular procedures requiring hybrid monitoring techniques,
such as a combination of stent-graft placement and open visceral
bypass grafting.
59,62,206
TEE is crucial for selecting and monitoring
surgical treatment and detecting complications that may require
intervention. Thus, intraoperative TEE should be considered
mandatory. TEE may also be useful during endovascular procedures
in patients with descending aortic dissections by differentiating true
and false lumens, permitting correct guidewire placement in the true
lumen, helping guide correct stent-graft positioning, and identifying
suboptimal results and presence of leaks.
8. Serial Follow-Up of Aortic Dissection (Choice of
Tests). After the diagnosis and management of acute aortic dissec-
tion, imaging techniques play a major role in prognosis assessment
and in the diagnosis of complications during follow-up.
Morphologic and dynamic information may be useful for predicting
aortic dissection evolution and identifying the subgroup of patients
with a greater tendency to severe aortic enlargement. Regular assess-
ment of the aorta should be made 1, 3, 6 and 12 months after the
acute event, followed by yearly examinations.
After discharge, variables related to greater aortic dilatation were
entrytear size, maximum descending aorta diameter in the subacute
phase, and the high-pressure pattern in falselumen. Maximum aortic
Figure 38 8 (A)Diagramofclassicaorticdissectionontheleftillustratingadissectionflapseparatingatruelumen(TL)fromafalse
lumen (FL). (B) An IMH lacks a dissection flap and true and false lumens and instead appears as a thickened aortic wall, typically
with crescentic thickening as in this diagram. Notice that the aortic lumen is preserved (remains round and smooth walled).
Journal of the AmericanSociety of Echocardiography
Volume 28 Number2
Goldstein et al 147
89
diameter in thesubacutephasewas a significant predictor of progres-
sive dilatation because, according to the law of Laplace, larger aortic
diameters are associated with increased wall stress.
TEE provides prognostic information in acute type A dissection
beyond that provided by clinical risk variables. A flap confined to
ascending aorta or a completely thrombosed false lumen has proved
to havea protectiverole.
207
Finally, increased falseluminalpressureis
another important factor predictive of future false luminal enlarge-
ment. In the majority of cases, high false luminal pressure relates to
alargeentrytearwithout distalemptyingfloworreentrysiteof similar
size. It may be difficult to identify the distal reentry communication;
thus, in the presence of a large entry tear, indirect signs of high false
luminal pressure such as true luminal compression, partial false
luminal thrombosis, or the velocity pattern of the echocardiographic
contrast in the false lumen should be considered.
CT is the technique most frequently used for serial follow-up of
aortic dissection. The large field of view of CT permits identification
of anatomic landmarks that allow measurements to be obtained at
identicallevels as previous measurements. CThasexcellent reproduc-
ibility for aortic size measurement, has excellent accuracy for identi-
fying entry tears and distal reentry sites, and allows the assessment
of vessel malperfusion. MRI appears to be an excellent alternative
technique for following patients treated medically or surgically in
AAS. MRI avoids exposure to ionizing radiation and the nephrotoxic
contrast agents used for computed tomographic angiography and is
less invasive than TEE. Furthermore, the integrated studyof anatomy
and physiology of blood flow can provide information that may
explain the mechanism(s) responsible for aortic dilatation.
Time-resolved MRA can provide additional dynamic information
on blood flow in entry tears. Velocity-encoded cine MR sequences
havea promising role in the functionalassessment of aortic dissection
byvirtueof quantification of flowin both lumens andthepossibilityof
identifying hemodynamic patterns of progressive dilatation risk. For
planning surgery or endovascular repair, it is very useful to demon-
strate the course of the flap, entry tear location, false luminal throm-
bosis, aortic diameter, and main arterial trunk involvement. Both
computed tomographic angiography and MRA take advantage of
postprocessing softwarecapabilities that allowmultiplanereconstruc-
tions, maximum-intensity projection (MIP) and volume-rendering re-
constructions.
9. Predictors of Complications by Imaging Techniques. a.
Maximum Aortic Diameter.–Maximum aortic dilatation after the
acute phase is a major predictor of complications during follow-up.
Both CT and MRI are superior to TEE for measuring the aortic size
distal to the aortic root. Aneurysmal dilatation of the dissected aorta
will occur in 25% to 40% of patients surviving acute type B aortic
dissection. Secondarydilatation of theaortaduring follow-upof aortic
dissection has been considered a significant predictor of aortic
rupture. A descending thoracic aortic diameter > 45 mm after the
acute phase and the presence of a patent false lumen have been
related to aneurysm development of the false lumen (>60 mm)
and surgical reintervention. A diameter > 60 mm or annual growth
> 5 mm implies a high risk for aortic rupture.
208
Other studies
have shown maximum false luminal diameter in the proximal part
of descending aorta to be a predictor of complications.
209
However, this diameter has low reproducibility, mainlydue to move-
ment of the intimal flap.
b. Patent False Lumen.–In addition to aortic diameter, a consistent
predictor of outcomes in acute type B aortic dissection has been the
hemodynamic status of the falselumen, classicallydivided into either
athrombosed falselumen or a patent false lumen. Persistence of pat-
ent false lumen in thedescending aorta is common in both dissection
types and has been strongly associated with poor prognosis. Total
thrombosis of thefalselumen,considereda precursorof spontaneous
healing, is a rare event, even after surgical repair of a type A aortic
dissection. A persistently patent false lumen can be found in most
type B aortic dissections during follow-up and in >70% of type A
Figure 39 9 Transesophageal echocardiogram
of a cross-
sectional view of the descending thoracic aorta at 35 cm from
the incisors illustrates a crescentic-shaped IMH.
Table 11 1 ImagingfeaturesofIMH
1. Focal aortic wall thickening (crescentic> concentric)
2. Preserved luminal shapewith smooth luminal border
3. Absence of dissection flap and falselumen
4. Echolucent regions may be present in the aorticwall
5. Central displacement of intimal calcium
Figure 40 0 Transesophageal echocardiogram
of a cross-
sectional view of the descending thoracic aorta (Ao) illustrating
aconcentric IMH. There is a small right pleural effusion.
148 Goldstein et al
Journal of the American Society of Echocardiography
February 2015
Documents you may be interested
Documents you may be interested
