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A. R. Benkert and J. G. Gaca
paraparesis and even paraplegia after repair is possible. Resolution of paraparesis
or paraplegia with immediate endovascular therapy of acute TBAD is less likely
but should still be considered.
Visceral Malperfusion
Visceral malperfusion should be suspected in any patient with acute dissection who
presents with absent unilateral or bilateral femoral pulses. The presence of intact
femoral pulses does not guarantee adequate visceral blood ow, but it does make the
presence of visceral malperfusion much less likely. One strategy for management of
visceral malperfusion entails taking the patient to a hybrid operating room with both
the general and cardiac surgery teams. Exploratory laparotomy is performed rst: if
the bowel is ischemic and unlikely to recover, no further intervention is performed.
If the bowel is viable, blood ow is restored via TEVAR of the thoracic aorta (and
potentially branch vessel revascularization). The abdomen is then left open, followed by a second-look operation to assess the bowel integrity. Intensive medical
management of systolic blood pressure and resuscitation to correct metabolic
abnormalities then allows for resolution of end-organ failure prior to open repair of
the acute dissection. This does place the patient at risk for rupture in the intervening
period, and is a challenging situation if the patient has severe aortic insufciency
and heart failure associated with the dissection. In a single center, retrospective
study of patients with mesenteric malperfusion syndrome, 38% died prior to open
repair: 24.4% from organ failure and 13.4% from aortic rupture. A multivariable
logistic regression revealed that independent risk factors for death from organ failure were acute stroke, gross bowel necrosis at laparotomy, and serum lactate greater
than 6mmol/L [16].
Isolated renal malperfusion without other visceral malperfusion (intact celiac
and superior mesenteric arteries) is exceedingly rare. TAAD repair should not be
delayed in this rare scenario as isolated renal malperfusion is not immediately lifethreatening. Immediate therapy for TBAD should also be considered in this
scenario.
Limb Malperfusion
Lower limb malperfusion syndrome is present in 40% of complicated dissections
and in up to 71% of patients with another malperfusion syndrome [17]. The degree
of limb ischemia and duration of symptoms are important parameters to consider. A
pulseless, cold, and insensate leg with no motor function should be revascularized
prior to dissection repair. Traditional approaches include endovascular repairs and
extra-anatomic bypass grafting. However, a leg with diminished pulse, but with
intact sensorimotor function, will often improve with repair of the dissection. In the
latter case, immediate dissection repair is recommended.
Initial Medical Management ofAcute Aortic Syndromes
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References
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10. Feldman M, Shah M, Elefteriades JA.Medical management of acute type A aortic dissection.
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12. von Kodolitsch Y, Csösz SK, Koschyk DH, etal. Intramural hematoma of the aorta: predic-
tors of progression to dissection and rupture. Circulation. 2003;107(8):1158–63. https://doi.
org/10.1161/01.cir.0000052628.77047.ea.
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14. Hiratzka LF, Bakris GL, Beckman JA, etal. 2010 ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/
SIR/STS/SVM guidelines for the diagnosis and management of patients with thoracic aortic disease: executive summary. A report of the American College of Cardiology Foundation/
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CIRCULATIONAHA.112.000327.
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acute aortic dissection. Spinal Cord. 2000;38(11):702–4.
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Imaging ofAortic Dissection: CT, MRI,
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andAngiography
AlbreeTower-Rader, LarsG.Svensson, andVenuMenon
Imaging is integral to the initial diagnosis, management, and follow-up of patients
with a suspected aortic dissection. In contrast to acute coronary syndrome where an
ECG and presence of biomarkers indicative of myocardial necrosis can be diagnostic, acute aortic syndrome requires imaging to conrm the diagnosis. The patient
with a suspected acute aortic syndrome should undergo immediate imaging with
multi-detector computed tomography (MDCT), transesophageal echocardiogram
(TEE), or magnetic resonance imaging (MRI) based on the stability of the patient
and institutional availability and expertise since there are advantages and disadvantages to each modality [
test of choice to evaluate the aorta. A well-performed study enables immediate conrmation of the suspected diagnosis, as well as information regarding the pathophysiology and prognosis. The use of echocardiography in the evaluation of patients
with acute aortic syndrome is discussed in depth in subsequent chapters. The majority of acute aortic syndromes, 80–90%, are aortic dissections [4], with the minority
classied as intramural hematoma or penetrating aortic ulcers, the imaging for both
of which will be discussed in subsequent chapters. An ideal imaging study would
provide a rapid and accurate, noninvasive diagnosis of the presence and extent of an
aortic dissection allowing for classication under the DeBakey or more commonly
used Stanford system. Patients with involvement of the ascending aorta, Stanford
type A, are managed as a surgical emergency; however, it is equally important to
identify patients with a complicated type B dissection who are often managed by
stent-graft endovascular repair (TEVAR) in the acute setting [5, 6]. Imaging can
detect some features of a complicated type B dissection, such as the involvement of
aortic branch vessels resulting in end-organ malperfusion, or signs of rupture, such
as a pericardial effusion, hemothorax, or mediastinal blood products. Additionally
1–3] (Table1). For most patients MDCT is the diagnostic
A. Tower-Rader · L. G. Svensson · V. Menon (*)
Division of Cardiovascular Medicine, Heart and Vascular Institute, Cleveland Clinic,
Cleveland, OH, USA
e-mail: svenssl@ccf.org; menonv@ccf.org
J. S. Coselli et al. (eds.), Aortic Dissection and Acute Aortic Syndromes,
https://doi.org/10.1007/978-3-030-66668-2_10
131© Springer Nature Switzerland AG 2021
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Table 1 Comparison of available imaging techniques of the aorta
Multidetector
computed
tomography
(MDCT)
Advantages • Rapid
• Easily accessed
and performed
• May reveal
alternative/
additional
diagnoses
• Able to assess
for evidence of
end-organ
malperfusion
Disadvantages • Radiation
exposure
• Uses iodinated
contrast
• Pulsation
artifact in aortic
root and
ascending aorta
without
ECG-gating
Magnetic
resonance imaging
(MRI) Angiography
• No radiation
exposure
• Potential to be
performed without
gadolinium
contrast
• Able to assess
for aortic
regurgitation
• Able to assess
for left ventricular
function and wall
motion
abnormalities
• Able to assess
for evidence of
end-organ
malperfusion
• Longer
acquisition time
• Difcult to
perform
hemodynamic
monitoring during
study
• May not be
readily accessible
• Implanted
devices may not
be compatible or
may create
artifacts
• May require
sedation for
claustrophobic
patients
• Often
performed during
endovascular
treatment of a
dissection
• Able to assess
for aortic
regurgitation
• Possible to
evaluate patency
of aortic branches
• Invasive
• Need for an
experience
operator
• Longer
acquisition time
• Radiation
exposure
• Uses iodinated
contrast
• False negatives
in setting of
intramural
hematoma and
thrombosed false
lumens
A. Tower-Rader et al.
Transesophageal
echocardiogram
(TEE)
• May be performed
at bedside for
unstable patients
• Immediate
interpretation
• Able to assess for
aortic regurgitation
• Able to assess for
pericardial effusion
with tamponade
physiology
• Can assess left
ventricular function
and wall motion
abnormalities
• Used periprocedurally in the
operating room
• Semi-invasive
• Need for an
experienced operator
• Requires sedation
• Distal ascending
aorta obstructed
from view by
airways
• Limited
assessment of
abdominal aorta and
branches
• Contraindicated in
patients with
cirrhosis,
gastrointestinal
bleed or dysphagia
• Reverberation
artifacts may mimic
an intimal ap
the status of blood ow in the false lumen, as well as involvement of other structures
including the aortic valve and coronary arteries, or proximity of cardiovascular
structures to the sternum for patients with prior sternotomy, is often useful in determining the management strategy. Current guidelines recommend that measurements of the aorta by either MDCT or MRI should be taken at reproducible
landmarks in a plane perpendicular to the axis of the ow of blood utilizing either
multiplanar reconstruction or the centerline of ow [1, 2, 7] (Fig.1a, b). The 2015
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ab
Fig. 1 (a) An example of measurement of the descending thoracic aorta at the level of the diaphragmatic hiatus on axial imaging. (b) Measurement of the dimensions of the descending thoracic
aorta at the level of the diaphragmatic hiatus following multiplanar reconstruction to align the
plane of the image to be perpendicular to the axis of blood ow. In this case, failure to reconstruct
the plane for measurement of aorta dimensions would result in an erroneous measurement
Multimodality Imaging of Diseases of the Thoracic Aorta guidelines do not specify
which technique should be undertaken for measuring the aortic dimensions, though
prior guidelines including the 2010 ACC/AHA and the 2014 ESC guidelines had
suggested measuring outer edge-to-outer edge on MDCT and MRI, as opposed to
the recommendation for echocardiographic measurements of leading edge-toleading edge [1, 2, 7]. Care should be taken to access available prior imaging, especially for patients whohave had prior repairs, because it may be difcult for the
medical team to interpret whether acute changes are present without direct comparison to prior images potentially leading to false positive ndings of an acute dissection or concerning new aortic dilation [8]. Patients with a conrmed aortic dissection
may be transferred to a specialized center and the images obtained at the primary
facility need to be able to be rapidly accessible to the receiving care team, either by
CD or secure digital image transfer.
Multidetector Computed Tomography
Protocol Considerations
As technology has developed over the past few decades, multidetector computed
tomography (≥64 detector rows, MDCT) angiography has evolved as the preferred
modality for imaging due to the widespread availability, quality of studies, ease of
interpretation, and rapid speed of study acquisition. A meta-analysis examining 16
studies found a very high sensitivity of 100% and specicity of 98–99% for the
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A. Tower-Rader et al.
diagnosis of aortic dissection by MDCT angiography [9]. Standard acquisition
times for MDCT with short gantry rotation times are 3–4s for the chest and <10s
for the chest, abdomen, and pelvis, often accomplished in a single breath-hold [7].
Though protocols vary by institution, ideally they are designed to allow for adequate diagnosis while minimizing radiation if possible. During the cardiac cycle,
the aortic root (including the valve and sinuses) and proximal ascending aorta are in
motion and thus prone to motion artifacts, which may be erroneously interpreted as
intimal aps [10] (Fig.2a, b). In fact, studies have shown that the most frequent
reason for a false positive diagnosis of an aortic dissection was the use of a nonECG gated CT [8]. For this reason, synchronizing image acquisition of the chest to
the cardiac cycle via ECG-gating should be performed to allow for careful assessment of the aortic root. ECG-gating may be performed either prospectively or retrospectively. With prospective gating, images are acquired only during the desired
portion of the cardiac cycle, typically diastole as cardiac motion is limited; however,
this requires a fairly regular rhythm. Retrospective gating, however, allows for
image acquisition throughout the entire cardiac cycle with subsequent reconstructions at different phases of the cardiac cycle if necessary, which is useful in the
setting of arrhythmias, though at the expense of increased radiation exposure. Care
should be taken with automated bolus tracking to ensure proper opacication of the
aorta. With automated bolus tracking, a region of interest (ROI) is placed on reference image on the descending thoracic aorta and the acquisition is triggered when
the predetermined threshold Hounseld unit (HU) is surpassed. If the ROI spans the
true and false lumens, and particularly if the false lumen is thrombosed, the threshold might be reached too late if the operator waits for complete opacication of the
false lumen, thus resulting in inadequate opacication of the true lumen. Instead it
is recommended that the operator carefully monitor to ensure the location of the
ROI is correctly placed over the true lumen as the aorta begins to enhance and be
Fig. 2 (a) Non-gated CT angiography of the chest demonstrating a central contrast-lled lumen
in the ascending aorta with a component of the lumen both anterior and posterior (arrows), which
is less intensely opacied. This may be mistaken for a dissection, but the presence of both anterior
and posterior segments of the ascending aorta lumen, which are less intensely opacied, is characteristic of a motion artifact. (b) ECG-gated CT angiography of the chest performed in the same
patient demonstrating the absence of a dissection of the ascending aorta
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prepared to manually trigger the scan, if necessary, when the true lumen is well
opacied with a goal of ≥250HU [3]. Use of a saline ush is recommended to help
tighten the contrast bolus, resulting in a higher peak contrast opacication [3, 4].
Contrast should be administered via the right arm to limit the appearance of streak
artifacts obscuring the head and neck vessels, including the left subclavian artery.
Following acquisition of an ECG-gated chest, a continuous scan extending from the
lung apices to the groin should be performed in order to obtain a continuous dataset,
which is often necessary when planning for an endovascular repair, instead of subsequently obtaining a separate acquisition of the abdomen and pelvis, effectively
splitting the aorta into two separate datasets. Acquisition of thin slice axial images
(0.5–2.0mm) is recommended [11]. A triple-rule-out (TRO) protocol is intended to
assess the aorta, coronary arteries, and pulmonary arteries in a single ECG-gated
scan utilizing a biphasic contrast injection designed for simultaneous arterial (>300
HU) and pulmonary arterial (>200HU) enhancement [12]. In practice, TRO scans
have been shown to be associated with higher radiation exposure and contrast doses
and non-diagnostic images, and thus for patients with a high-risk feature of aortic
dissection, a dedicated study should be performed for instead [7, 13]. In general,
careful attention should be paid to adjusting the tube voltage and current, as well as
any other scanner-specic features in order to minimize the radiation dose to the
patient.
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CT Findings ofAortic Dissections
Non-contrast CT Findings
Non-contrast CT is not diagnostic for aortic dissection, though it may be performed
in some centers prior to CTA to aid in the detection of an intramural hematoma or
may have been performed for another indication. Findings on non-contrast CT,
which are suggestive of aortic dissection, include displacement of intimal calcications into the lumen (Fig.3a, b), as well as a hyperattenuating uid collection within
the pericardium, pleural cavity, or mediastinum suggestive of aortic rupture.
CTA Findings
Diagnosis of an aortic dissection includes identication of a true and false lumen
separated by an intimal ap. The true lumen is often smaller, while the false lumen
is often crescent-shaped with a “beak” sign, or acute angle between the intimal ap
and the aortic wall. The typical appearance of an intimal ap or “double barrel” is
noted in approximately 70% of cases, thoughif there is circumferential separation of
the intima, the true lumen may have a more cylindrical or “windsock” appearance
due to intussusception of the ap [14] (Fig.4a, b). The true lumen typically will run
along the inferomedial aspect of the arch and descending thoracoabdominal aorta,
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ab
Fig. 3 (a) Non-contrast gated CT chest with a dilated ascending aorta and evidence of intimal
calcications within the lumen of the descending thoracic aorta (arrow) concerning for possible
aortic dissection. (b) ECG-gated CT angiography of the chest in the same patient conrming the
presence of a Type A dissection with an intimal ap present in the ascending and descending thoracic aorta (arrows)
ab
Fig. 4 (a) ECG-gated CT angiography of the chest demonstrating a Type A aortic dissection with
an intimal ap present in the ascending and descending thoracic aorta with a typical “double barrel” lumen appearance of the descending thoracic aorta with a less intensely opacied false lumen
with a “beak” appearance at the intersection of the intimal ap and aorta wall (arrow). (b) ECGgated CT angiography of the chest demonstrating a Type A aortic dissection with an intimal ap
present in the ascending and descending thoracic aorta with near complete separation of the intima
within the descending thoracic aorta, which results in a “windsock” appearance
though variation is common [15]. Identication of the true lumen may be difcult
in the setting of an extensive dissection, but can be determined with the aid of the
pattern of intimal calcications and tracking the lumens. By scrolling through the
axial images, or using reconstructions, a reader can identify and track a portion of
unaffected aorta to identify the true lumen since they remain in continuity.
Additionally, the false lumen may be less intensely opacied than the true lumen
due to either differential timing of contrast opacication, a nding thatcan be conrmed by delayed imaging. Thrombus may be present within the false lumen at the
time of presentation in approximately 40% of patients with a type B dissection [16].
Depending on the pressure differential between the true and false lumens, the false
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lumen may compress the true lumen. Compression of the true lumen by an expanding false lumen is more likely to occur in the absence of distal reentry tears. The
location of calcications in relation to the intimal ap may also aid in differentiating
the true and false lumens because in the acute setting, intimal calcications will
remain on the true lumen side of the displaced intimal ap, whereas in the setting of
a chronic dissection, calcications may be seen on either side because mural calcications may form within the false lumen in the setting of a chronic dissection [14].
Additionally, the intimal ap is often curved in the setting of an acute dissection due
to mobility, whereas in chronic dissections, the intimal ap is often at and xed
[14]. These features, which can help to distinguish an acute from a chronic dissection, are important to note, since it is possible for patients to present with a different
etiology for their symptoms in the setting of either a known chronic dissection, or a
previously unknown chronic dissection.
In addition to identifying of the presence of an acute dissection and the status of
the true and false lumens, one must next determine whether there is involvement of
the ascending aorta and the site of the entry tear. In practice, identication of the
true and false lumens often occurs at the same time as identication of the proximal
and distal aspects of the dissection. The most common sites of entry tears, accounting for ~90% of cases, are within the rst 2cm of the ascending aorta and at the
isthmus near the ligamentum arteriosum, which is thought to be because these two
areas are under the greatest hemodynamic stress [17]. The entry tear is found by
tracing the intimal ap and looking for an area where the ap is interrupted, often in
a transverse orientation to the lumen. The ends of the discontinuous intimal ap are
often visible at the site of the entry tear with the ends pointed in the direction of ow
between the lumens. Often the ends are oriented from the true into the false lumen,
andwith variation, may occur throughout the cardiac phase or due to the relative
pressure differences between the lumens. The site of the entry tear is important to
note since it can affect the type of repair, especially if the tear is located in the arch
with retrograde extension into the ascending aorta [15]. Identication of the head
and neck vessel branching pattern and involvement by the dissection ap is important because it may affect procedural planning in regards to the cannulation site for
cardiopulmonary bypass or endovascular stent graft placement (Fig.5). During cardiopulmonary bypass, antegrade cerebral perfusion is often performed by cannulating of the right axillary or subclavian artery and clamping the brachiocephalic trunk,
diverting blood ow to the right common carotid artery [18]; however, an alternative
mechanism must be considered in the presence of an aberrant right subclavian artery
since the right common carotid artery instead has a separate origin from the aortic
arch. Extension of the dissection ap into the iliac or femoral arteries should also be
noted, as this may affect arterial access for procedural planning, especially endovascular stent graft repair. Reports should include details regarding the location of the
intimal tear, extent of aortic involvement, dimensions of the aorta, status of the false
lumen (i.e., patent or thrombosed), and whether the true lumen is compressed
(Table2).
For patients with involvement of the ascending aorta, particular attention should
also be paid to the ostia of the coronary arteries, the relation of the ap to the aortic
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Fig. 5 ECG-gated CT angiography of the chest in a patient with a Type A aortic dissection demonstrating extension of the intimal ap into the aortic branch vessels, in particular with compression of the true lumen of the innominate artery by a thrombus-lled false lumen. In this case, the
operative plan was adjusted to provide antegrade cerebral perfusion via an interposition graft
Table 2 Features to include in imaging reports in an Acute Aortic Dissection
• Extent of dissection
– Proximal and distal aspects of the dissection
– Involvement of the ascending aorta
• Dimensions of the aorta
• Status of the true and false lumens
– i.e., perfused, partial/complete thrombosis, compression
• Site of the entry tear
• Evidence of rupture
– i.e., hemorrhagic pericardial effusion, pleural effusion, mediastinal hematoma
• Involvement of the coronary arteries
• Arch and abdominal branch vessel pattern and patency
• Evidence of end-organ malperfusion
– Origin of each branch from true or false lumen
– Evidence of dissection ap extending into the ostium
– Evidence of static or dynamic obstruction
– Decreased organ perfusion
• Patency and/or dissection involvement of the iliac and femoral arteries
• Proximity of cardiovascular structures to the sternum with prior sternotomy
• Presence/location of reentry tears (if present)
• Type of aortic valve, and presence/mechanism of aortic regurgitation (if possible)
• Features associated with underlying connective tissue disease
valve, and the pericardium, all which are best examined on an ECG-gated study.
Aortic regurgitation has been reported in approximately 40–70% of cases of a type
A dissection [1]. Aortic regurgitation may occur in a dissection due to one of three
possible mechanisms: (1) acute enlargement of the aortic root, resulting in lack of
coaptation, (2) extension of the dissection into the aortic root, resulting in leaet
prolapse from disruption of the commissures, or (3) prolapse of the dissection ap
through the aortic valve in diastole, preventing valve closure [19]. Myocardial ischemia may occur due to extension of the dissection into the coronary artery ostia, or
due to external compression by the false lumen (Fig.6a–c). As previously noted, the
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