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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3734_Библиотеки_им_академика_М_И_Перельмана
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Imaging ofAortic Dissection: CT, MRI, andAngiography
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Fig. 6 (a) ECG demonstrating an inferoposterior ST elevation myocardial infarction in a patient
presenting with chest pain. (b) Invasive aortography was performed following activation of the
cardiac catheterization lab for an ST elevation myocardial infarction and non-obstructive coronary
angiography. In this case, aortography demonstrated a double lumen with an intimal ap (arrow)
and differential opacication of the two lumens within the ascending aorta. (c) ECG-gated CT
angiography of the chest (shown), abdomen, and pelvis was subsequently performed to conrm the
diagnosis and evaluate the extent of the dissection. An intimal ap is present in the aortic root
extending into the ostium of the right coronary artery
presence of a hyperattenuating (>40HU) uid collection within the pericardium,
pleural space, or mediastinum is suggestive of aortic rupture on both non-contrast
and contrast studies (Figs.7 and 8) [4]. With contrast administration, irregularity of
the aortic wall may be noted, or frank extravasation of vascular contrast into the
uid collection [20].
The identication of features suggestive of a complicated dissection with endorgan malperfusion or evidence of rupture is of almost equal importance to diagnosing the presence of an aortic dissection. In up to 30% of patients with an aortic
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Fig. 7 ECG-gated CT
angiography of the chest
with a large
hyperattenuating (~54
Hounseld units)
pericardial effusion
(asterisk), concerning for
hemopericardium and
aortic rupture in the setting
of a Type A aortic
dissection
Fig. 8 ECG-gated CT
angiography of the chest
with a large
hyperattenuating (~62
Hounseld units)
mediastinal uid collection
(asterisk) compressing the
main pulmonary artery
(arrow), concerning for
mediastinal blood products
due to aortic rupture in the
setting of a Type A aortic
dissection
A. Tower-Rader et al.
dissection, and approximately 10% of patients with a type B aortic dissection, evidence of end-organ malperfusion is apparent at the time of initial presentation [2, 5].
While the most common description is of the celiac trunk, superior mesenteric and
right renal arteries arising from the true lumen, and the left renal artery arising from
the false lumen, signicant variation has been noted in the pattern of abdominal
branch vessel involvement [15]. It is important to examine whether each branch,
including the head and neck vessels, arises from the true or false lumen, as well as
the presence and mechanism of end-organ malperfusion for each branch vessel territory. End-organ malperfusion may be due to one of four mechanisms: (1) static
occlusion of the branch artery by extension of the dissection ap into the ostium or
proximal segment, (2) dynamic due to the dissection ap prolapsing over and intermittently occluding the ostium of the non-dissected branch vessel, (3) mixed static
and dynamic, or (4) ostial disconnection and avulsion from the true lumen [10, 14,
21]. The mechanism of occlusion is important to identify since it inuences
Imaging ofAortic Dissection: CT, MRI, andAngiography
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management: static occlusion is often treated with a stent, whereas dynamic occlusion may be managed by creating fenestrations in the intimal ap to reduce the
pressure within the false lumen [10, 14]. Additionally, the evaluation of the ostia
and proximal segment of each branch vessel of the aorta is crucial to understanding
which organs may be at risk for ischemia. Regardless, careful inspection of the
abdominal organs for evidence of decreased perfusion should also be performed
(Fig.9). Delayed phase imaging may be helpful in differentiating complete versus
delayed perfusion of a vascular territory in the setting of a chronic dissection; however, in the setting of acute symptoms, organ hypoperfusion on arterial phase imaging is presumed to represent an area at risk and delayed phase imaging is typically
not included in the protocol in order to decrease both the acquisition time and radiation dose of the study. Reports should include a detailed description of whether the
aortic branch vessels are involved in the dissection, patency of the ostia and proximal aortic branch vessels, and evidence of decreased organ perfusion or infarction
(Table2).
Additional Findings onCT Imaging
In addition to obtaining data regarding the aorta itself, additional information is
obtained regarding both cardiovascular and non-cardiovascular structures, which
may give clues to an underlying syndrome or predisposition for aortic dissection,
alternative diagnosis, or mayhelp guide surgical planning. As is discussed in depth
in the sections regarding long-term imaging of the aorta and management of chronic
Fig. 9 CT angiography of the abdomen demonstrating an aortic dissection with the left renal
artery arising from the true lumen and an intimal ap extending to the ostium of the right renal
artery, resulting in occlusion by the false lumen (arrow). Additionally there is hypoperfusion of the
right kidney (asterisk), which is readily apparent, especially in comparison to the well-perfused
left kidney
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dissections, several imaging features have been identied that are predictive of
future aortic dilation and adverse aortic events (Table 3). With the use of ECGgating MDCT, it is possible to identify the aortic valve as well as the anatomy of the
sinotubular junction. The identication of a bicuspid aortic valve carries additional
implications for follow-up for the patient, as well as rst degree relatives. In the case
of syndromic connective tissue disorders, including Marfan, vascular type EhlersDanlos and Loeys-Dietz syndromes, additional associated features may be identied including pectus deformity of the chest wall, scoliosis or kyphosis, dural ectasia,
or lung bullae. One of the other main benets of MDCT in comparison to other
imaging modalities is that in the absence of an aortic dissection other etiologies of
chest pain may be identied, including pulmonary embolus, pneumothorax, orpulmonary or chest wall mass. In patients who have undergone prior sternotomy it is
important to detail the proximity of cardiovascular structures to the sternum, particularly bypass grafts and whether they cross the midline, since this may affect
planning for a redo sternotomy. Reports should include information regarding proximity of cardiovascular structures to the sternum, as well as featuresthat may suggest an underlying syndrome since the management of these patients in the acute
and chronic phases, and the implications for screening of family members varies
(Table2).
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is best reserved for stable patients, or those
with chronic dissections. Prior studies have demonstrated a sensitivity and specicity of 95–98% and 94–98%, respectively, for the detection of an aortic dissection
[9]. Several different types of MRI sequences are available for imaging the aorta
including cine (dynamic) imaging, MR angiography, and respiratory navigatorgated 3D acquisitions. In addition to assessment of the aorta, cine MRI imaging
allows for the assessment of aortic regurgitation, left ventricular function, and pericardial effusions, offering the potential to obtain additional data regarding cardiac
function and potential complications of a type A dissection as part of the study.
Table 3 Imaging features predictive of aortic dilation and adverse aortic events
• Partially thrombosed false lumen>patent false lumen>thrombosed false lumen
• Maximal aortic diameter≥40mm
• Fusiform index≥0.64
• False lumen diameter≥22mm in the proximal descending aorta
• Crescent shape of the true lumen
• Thrombosed false lumen with ulcer-like projections (especially in the proximal descending
aorta)
• Entry tear≥10mm in the proximal descending aorta
Fusiform index=Maximum diameter of descending aorta/(diameter of the distal aortic arch+diameter of the descending aorta at the level of the pulmonary artery)
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Unfortunately, despite these benets, the relatively long acquisition time (approximately 20–30min), as well as the inability to adequately monitor patients who are
potentially hemodynamically unstable, makes the use of MRI for diagnosing an
acute dissection far less appealing [3]. However, MRI may be used in select scenarios where an acute aortic dissection is suspected, for instance in a hemodynamically stable patient with a severe allergy to iodinated contrast, or in a facilitythat
does not have immediate access to transesophageal echocardiography. Instead, MRI
is often used for monitoring patients with a chronic dissection due to the lack of
radiation exposure, and, with newer techniques, the ability to avoid contrast administration altogether. MR angiography is also ill-suited for patients with advanced
renal dysfunction and pregnancy since gadolinium chelate contrast agents are contraindicated. Implantable medical devices may be problematic either because the
device is not MRI compatible, or due to artifacts created by the device interfering
with image interpretation.
MRI Findings
As with CT, a dissection is often identied on MRI with the presence of a doublebarrel lumen on axial images. In particular, spin-echo black-blood sequences allow
for the rapid identication of an intimal ap [22] (Fig.10). The true lumen can be
identied using the same anatomic considerations as with MDCT; the true lumen is
usually smaller and in continuity with the unaffected aorta. Thrombus within the
false lumen is hypointense on T1 and T2 imaging [22]. Again, the status of all of the
aortic arch and abdominal aortic branches should be described. Pericardial and
pleural effusions may be recognized by their high signal intensity on axial imaging.
ECG-gated gradient echo sequences can be displayed as cine images with areas of
Fig. 10 Magnetic
resonance imaging with
spin-echo black blood
imaging demonstrating an
intimal ap (arrow) with
partial thrombosis of the
false lumen (asterisk)
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A. Tower-Rader et al.
turbulent ow creating dephasing andresulting in a signal void, which may be useful in identifying aortic regurgitation or ow between the true and false lumens. MR
angiography utilizes 3D spoiled gradient echo sequences and a gadolinium-chelate
contrast agent and is rapidly acquired during a single breath-hold with high- intensity
signal localized to the intravascular space (Fig.11). Typically unenhanced, arterial
and delayed phase images are obtained. MR angiography results in the reconstruction of a 3D dataset, which may then be manipulated to allow for multiplanar reconstruction of cross-sectional sections to the lumen to measure aortic dimensions in a
similar fashion to CT [11]. Non-contrast techniques for acquisition and reconstruction of a 3D dataset, including respiratory navigator-gated, ECG-gated 3D whole
heart balanced steady state free precession, have a long acquisition time (10–12min)
and are thus ill-suited for imaging in the acute setting [23].
Angiography
Prior to the advent of non-invasive techniques for evaluating the aorta, aortography
was considered the standard for diagnosis for an aortic dissection. As a primary
diagnostic tool angiography has fallen out of favor compared to other modalities
because it is invasive, time-consuming, requires an experienced operator, and
exposes the patient to both iodinated contrast and radiation. Digital subtraction
angiography (DSA) is still utilized in a few clinical scenarios and thus, it is still
important to understand the appearance of a dissection on DSA, as well as the disadvantages of the technique. For instance, DSA is often utilized during endovascular treatment of a dissection, and may be useful as a diagnostic tool for patients
presenting to the cardiac catheterization lab for chest pain with a suspected acute
coronary syndrome who are found to have non-obstructive coronary arteries and an
alternative diagnosis of aortic dissection is suspected. Aortography may also be
Fig. 11 Magnetic
resonance imaging with
contrast enhanced
angiography, revealing an
intimal ap (arrow) and
double barrel lumen in the
abdominal aorta
Imaging ofAortic Dissection: CT, MRI, andAngiography
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necessary to diagnose an aortic dissection for institutions without access to noninvasive imaging (MDCT or MRI) or transesophageal echocardiography. Aortography
involves the placement of a pigtail catheter placed within the aortic lumen, with
injection of iodinated contrast initially by hand to conrm catheter placement followed by injection via a hydraulic power injector with digital subtraction cine uoroscopic imaging [24]. Initially aortography was thought to be associated with a risk
for propagation of the dissection, but further study demonstrated the procedure to be
relatively safe, thus becoming the standard for diagnosis around 1970 [25]. The
sensitivity and specicity of angiography are also lower than with MDCT and MRI
at 88% and 94%, respectively [26].
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Angiography Findings
Following injection of iodinated contrast within the aorta lumen, the lumen of the
aorta, as well as its branches, become opacied. The presence of double lumen with
an intimal ap and communication between the two lumens is diagnostic for an
aortic dissection [24] (Fig.6a–c). Either ow reversal or stasis of contrast within the
false lumen is often seen. Indirect signs of an aortic dissection include compression
of the true lumen, thickening of the aortic wall, out-pouchings along the aortic wall,
failure of aortic branches to ll, and aortic regurgitation [24, 26–28]. Injection of
contrast alters the pressure dynamics between the true and false lumen and thus the
assessment of malperfusion syndromes proves more complicated. False positives
may occur when the true and false lumens opacify simultaneously [29]. Additionally,
angiography is not capable of identifying intramural hematoma or patients with a
completely thrombosed false lumen, especially if there is no aortic branch vessel
involvement [27].
Conclusions
In conclusion, the sensitivity and specicity of MDCT and MRI for the detection of
an aortic dissection are similar [9], though the relative length of MRI and inability
to fully monitor patients hemodynamically make it less ideal for the evaluation of
an acute aortic dissection unless the patient is stable and unable to receive iodinated
contrast. Angiography, previously the standard for diagnosis, is less sensitive and
specic for aortic dissection, though it does still play a role during endovascular
procedures, or for further evaluation of patients with chest pain who are already in
the cardiac catheterization laboratory for assessment of chest pain. Choice of imaging modality is inuenced by institutional accessibility and expertise, though the
advantages and disadvantages of each modality should be recognized by providers.
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A. Tower-Rader et al.
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Imaging ofIntramural Hematoma
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andPenetrating Atherosclerotic Ulcer
byCT andMRI
ZacharyHartley-Blossom, SaurabhAgarwal, andMichaelK.Atalay
Introduction
Intramural hematomas (IMH) and penetrating atherosclerotic (or aortic) ulcers
(PAU) are two lesions on the spectrum of acute aortic syndromes (AAS), and their
timely, accurate diagnosis is paramount for appropriate triage and management.
Among all AAS, IMH constitutes ~4–11% of cases in North American and European
populations and~28–32% of cases in Asian populations—highlighting geographic
differences—while PAU accounts for less than 10% of such injuries [1–5]. Aortic
dissection remains the leading cause of AAS, accounting for 65–75% of all cases
[4–7], and is discussed in another chapter.
Imaging is essential for the prompt diagnosis of IMH and PAU. This chapter
aims to describe the imaging protocols, lesion characteristics, and diagnostic
challenges regarding both IMH and PAU, specically focusing on computed
tomography (CT) and magnetic resonance imaging (MRI). These two crosssectional imaging modalities provide complementary approaches to the diagnosis
and management of AAS including the identication of associated ndings and
complications. CT is generally preferred in the acute setting due to its high accuracy,
ease of use, speed, and ready-access in most emergency departments.
This chapter will: (1) briey review normal aortic anatomy, (2) discuss the basic
pathophysiology of IMH and PAU, (3) describe pertinent MRI and CT techniques
and the salient imaging ndings of PAU and IMH—along with potentially useful
prognostic features and diagnostic pitfalls—and nally (4) briey mention
management options that are covered in greater detail elsewhere.
Z. Hartley-Blossom · S. Agarwal · M. K. Atalay (*)
Department of Diagnostic Imaging, Rhode Island Hospital, Warren Alpert School of
Medicine of Brown University, Providence, RI, USA
J. S. Coselli et al. (eds.), Aortic Dissection and Acute Aortic Syndromes,
https://doi.org/10.1007/978-3-030-66668-2_11
149© Springer Nature Switzerland AG 2021
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