Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3925_Библиотеки_им_академика_М_И_Перельмана
.pdf
42 Danias
occur in more than half of patients with dissection, but these findings are not
pathognomonic. Typically, the mediastinum bulges to the right with dissection of
the ascending thoracic aorta, and to the left with dissection of the descending aorta
(3,4). At least 10% of patients with dissection have a normal CXR (1).
Therefore, the utility of CXR alone for confirmation or exclusion of tho-
racic aortic dissection is rather limited and may even delay appropriate diagnosis.
Luker et al. (5) evaluated the effect of prospective chest radiographic diagnosis
on the delay of definitive diagnosis of aortic dissection. The authors reviewed the
records and CXRs of 75 patients who were subsequently proven to have aortic
dissection. The original reports suggested that aortic dissection was likely or that
additional imaging was necessary in only one-quarter of patients with dissection.
In the retrospective review of the chest radiographs, only about one-half of patients
had sufficient findings to suggest the diagnosis. Notably in five patients there
was a delay to diagnose aortic dissection due to failure of CXR to prospectively
suggest the diagnosis.
Other studies have also reported a low diagnostic yield of chest radiography
for diagnosis of aortic dissection. Spittell et al. (2) reported an initial radiographic
diagnosis of possible aortic dissection in only 9% of cases proven to have dissec-
tion. The most common finding in this series was aortic dilation (noted in over
three-quarters of cases); however this is considered a nonspecific finding and
frequently difficult to differentiate from the aortic ectasia encountered with aging
and hypertension. In the study by Luker et al. (5), widening of the aortic contour
was also seen in three-quarters of patients with dissection, but only in half of these
cases was it felt to be sufficiently abnormal to suggest the diagnosis in the
retrospective evaluation. In the International Registry of Acute Aortic Dissection,
Figure 2 Anteroposterior CXR of a patient with post-traumatic pseudoaneurysm of the
aorta, years after a motor-vehicle accident. There is distortion of the aortic knob, with
protuberance of the aortic contour to the left.
https://t.me/med1917
Imaging of Aneurysms and Dissections 43
widening of the mediastinum and abnormal aortic contour were noted in 60% and
50% of patients with aortic dissection, respectively (1).
Other authors have reported a higher sensitivity, specificity, and accuracy
of CXR for assessment for aortic dissection. Jagannath et al. (4) evaluated the
utility of CXR to differentiate aortic dissection from normal aortas in age-matched
controls, and reported a sensitivity, specificity, and accuracy of 81% to 85%, but
associated with a rather low interobserver agreement. These investigators
noted that the radiologist’s impression was a better predictor of the presence
of dissection than any single radiographic finding alone. Hartnell et al. (6) also
reported that plain radiographic evaluation had an accuracy of 81% in differen-
tiating between aortic dissection and myocardial infarction. However, both
these studies involved a focused comparison between aortic dissection and a
specific other disease, condition, or patient population. Therefore, findings
from these studies might not be applicable to patients first presenting with
symptoms suggestive of aortic dissection, a population closer to that evaluated
by Luker et al. (5).
ECHOCARDIOGRAPHY
Echocardiography relies on the reflection of ultrasound waves that are transmitted
from an appropriate transducer to the organ of interest. The amplitude, phase,
and time delay of the reflected echoes due to their relative distance from the
imaging transducer, and differences in acoustic properties between adjacent
structures are decoded to provide images in one dimension (M-mode) or
2-dimensions (2D-echo), thereby providing time-resolved anatomic information.
More recently, 3D-echo has been developed by combining several 2D images
that are acquired simultaneously or in rapid succession to cover an entire volume-
of-interest. Taking advantage of the Doppler effect, blood flow can also be assessed
and characterized as laminar or turbulent, while the direction of flow and blood
flow velocity can be quantified. Depending on the mode of acquisition, the
ultrasound Doppler beam may be pulsed or continuous, providing spatial flow
information versus measurement of high velocity flows, respectively. If pulsed
Doppler is color-coded and combined with the anatomic 2D information,
color images are obtained and can easily demonstrate normal and abnormal
intracardiac and intravascular flow patterns. With a combination of M-mode,
2D- and 3D-echo, Doppler, and color Doppler techniques, imaging of the
anatomy and function of the cardiovascular system, including the thoracic aorta,
may be obtained.
Due to limitations of acoustic penetration, TTE can usually visualize only a
few segments of the aorta. The proximal ascending aorta can usually be examined
using the parasternal (and occasionally the suprasternal) approach. The more
distal ascending aorta can sometimes be visualized from the suprasternal window,
which is also typically used to assess the aortic arch. The descending thoracic
aorta is usually seen from the suprasternal, parasternal, and subcostal views.
https://t.me/med1917
44 Danias
However, due to the distance of the aorta from the anterior chest wall and the
imaging transducer and intervening lung tissue, the TTE image quality is usually
suboptimal, except for the proximal ascending aorta. Diagnostic images of
extended segments of the aorta can only be obtained in slender subjects with good
acoustic windows, while adequate imaging of the entire thoracic aorta becomes
impossible in patients with emphysema or other lung disease, obesity, and chest
wall abnormalities. Nevertheless, TTE frequently initiates the work-up of patients
with aortic pathology by demonstrating some abnormal finding during evaluation
for another indication.
Aortic aneurysms can be easily detected with TTE when the ascending aorta
is involved (Fig. 3). Particularly in patients with Marfan’s syndrome, their slender
body habitus and usual confinement of aortic dilation to the sinuses of Valsalva
and aortic root make TTE a valuable method for initial and subsequent evaluation.
Figure 3 Transthoracic echocardiogram of a patient with dilated thoracic aorta. The
M-mode echocardiogram from the parasternal long axis view (A) demonstrates dilation
of the ascending aorta. The two-dimensional image of the arch from the suprasternal
approach (B) demonstrates dilation of the aortic arch (arrows) and descending thoracic
aorta (arrowheads).
https://t.me/med1917
Imaging of Aneurysms and Dissections 45
TTE has significant added value in patients in whom the aortic pathology also
involves the aortic valve. The degree of aortic regurgitation is a major determinant
of the surgical planning, as degree of aortic insufficiency enters into the equation,
in addition to the goal of preventing rupture and/or dissection. Aortic aneurysms
and pseudoaneurysms in more distal segments of the aorta may be harder to
diagnose, due to limited visibility. Even when the diagnosis can be made by TTE,
additional imaging is usually required to define the exact aortic anatomy, to obtain
a more precise diameter measurement, and to visualize adequately the length of
the abnormality.
For the reasons described, mainly the incomplete vessel visualization
and large distance from the transducer, TTE is not highly accurate for diagnosis
of thoracic aortic dissection. Sensitivity of TTE for detection of aortic dissection
is best in the ascending aorta, with reported values 78% to 100% and corres-
ponding specificities 87% to 96%. Accuracy is substantially lower in cases
with type B dissections, with much lower sensitivities (7–9). Even when detected,
TTE is rarely able to fully describe the length of the dissection, and to clarify
the entry and exit points. The proposed echocardiographic criteria for diagnosis
of aortic dissection are listed in Table 2. These criteria are applicable for both
TTE and TEE.
Specificity of TTE for detection of acute aortic dissection is better than
the corresponding sensitivity but also suboptimal (7–9). False positive findings
are frequently the result of echo reverberations, occurring usually when the
ultrasound beam is directed perpendicularly to an interface of tissues with greatly
different acoustic impedance (10). Therefore, a positive TTE study may not auto-
matically establish the diagnosis, and confirmation may be sought with a more
definitive imaging modality. Similarly, when there is a strong clinical suspicion
for dissection, failure of echocardiography to detect the abnormality should not be
interpreted as absence of the disease. Instead, additional imaging should be
obtained, so that the entire aorta can be adequately imaged.
TTE can be particularly helpful in assessing patients with acute aortic
dissection when the dissection involves the aortic valve with subsequent aortic
regurgitation. This may occur in as many as 50% of patients with ascending
aortic dissection (11). In cases where hemopericardium develops from rupture
of the aorta into the pericardial sac, TTE can demonstrate the pathology, although
the acuity of this condition rarely allows for elective or very detailed assessment.
Table 2 Proposed Echocardiographic Criteria for Aortic Dissection
Aortic root dilation (type A dissections)
Widening of the aortic wall
Identification of the dissection flap
Detection of two separate flows in the aorta (true and false lumens)
Visualization of complications of dissection (aortic regurgitation,
left pleural effusion, and hemopericardium)
https://t.me/med1917
46 Danias
TEE is highly sensitive for detection of acute aortic pathology. Compared to
TTE, TEE has the advantage of unobscured images, courtesy of the immediate
proximity of the esophagus to the heart and the aorta. The descending thoracic
aorta lies immediately behind the esophagus and therefore the entire descending
aorta can be visualized in transverse, longitudinal, and oblique planes from the
upper and lower esophagus. Imaging of the ascending aorta can be obtained from
the upper esophagus, along with a thorough evaluation of the aortic valve. Finally,
as the probe is pulled further back, a good view of the aortic arch and the takeoff
of the large arch vessels can be obtained.
Aortic aneurysms can be thoroughly imaged and precise measurements can
be obtained in all directions. The thickness and texture of the aortic wall can also
provide information regarding the underlying pathology. For example, in patients
with atherosclerotic aneurysms, the presence of calcium, plaque, and intramural
thrombus can be easily detected. Regarding assessment of aortic dissection, the
sensitivity of TEE is much higher than TTE, and almost the entire thoracic aorta
can be adequately imaged. The same diagnostic criteria as those previously described
for TTE (Table 2) are used for detection of aortic dissection. The typical diagnos-
tic finding is the identification of the intimal flap inside the aortic lumen (Fig. 4).
Blood flow is usually slower in the false lumen compared to the true lumen, and
thrombosis of the false lumen is not uncommon. The lumen diameter may be
increased in patients with dissecting aneurysm and, in cases with aortic valve
involvement, aortic insufficiency can be clearly visualized. TEE can usually identify
the entry and exit points (12) but identification of multiple entry and exit sites may
be challenging. Compared to TTE, TEE has enhanced sensitivity due to improved
visualization. However, the incremental increase in specificity is not similar.
Reverberation and linear artifacts are a common source of erroneous
interpretation in TEE (13). These artifacts in the ascending aorta occur in as many
as one-quarter of patients. Linear artifacts are less common in the descending
thoracic aorta and account for <10% of cases (14). Vignon et al. (14) evaluated
retrospectively 351 patients at high risk for aortic dissection or traumatic rupture,
all of whom underwent TEE, to quantify the occurrence of linear artifacts and to
formulate criteria for their correct categorization. Subsequently, the authors
applied these criteria prospectively to a cohort of 121 consecutive patients,
demonstrating an increase in the specificity of TEE for detection of aortic
dissection. The criteria used to identify artifacts were: displacement of the line in
a pattern parallel to the aortic wall, and superimposition of blood flow and similar
blood flow velocities in both sides of the line within the aorta (i.e., blood flow
does not “respect” the reverberation line). The presence of an anomalous inferior
vena cava may also be misinterpreted as an aortic dissection. Extensive, chronic,
and complicated dissections may be misinterpreted for other aortic pathology.
A major advantage of both TTE and TEE is that both tests can be performed
quickly at the bedside or en route to the operating room, even in hemodynamically
unstable patients. It is important that appropriate sedation be used for TEE in
patients with suspected aortic aneurysm and/or dissection, to avoid an abrupt
https://t.me/med1917
Imaging of Aneurysms and Dissections 47
increase in blood pressure that may worsen a dissection or precipitate rupture.
Moreover, because of the semi-invasive nature of TEE, this is usually not the
best alternative for pathology that requires sequential follow-up evaluations, for
example, in patients with aortic aneurysms for whom repeat imaging is warranted
to determine the optimal timing for surgical correction.
COMPUTED TOMOGRAPHY
Even at early stages of development, CT proved to have great value for assessment
of aortic disease. As CT evolved from conventional to helical to multislice, with
evergrowing number of radiation detectors, the examinations became considerably
Figure 4 Transesophageal echocardiogram of the descending thoracic aorta of a young
patient with Marfan’s syndrome. The dissection flap is clearly identified within the aortic
lumen (arrows, panels A and B ), establishing the diagnosis. Mapping demonstrates differ-
ent flow velocities in the smaller true lumen (small star, panel B ) and the larger false lumen
(large open star, panel B ). Source: Courtesy of Drs. Kiaffas and Kirvasilis, Onasseio
Cardiac Surgery Center, Athens, Greece.
https://t.me/med1917
48 Danias
faster, with higher resolution and fewer artifacts, and thus more accurate and user-
friendly. With modern scanners the entire thoracic aortic volume can be imaged
with submillimeter in-plane resolution within the timespan of a single breath-hold
of a few seconds’ duration.
The inherently high resolution of CT, combined with the fact that images
are obtained in the transverse orientation while the aorta is positioned vertically in
the body, make cross-sectional evaluation of the vessel possible with high degrees
of accuracy. The very high time-efficiency of CT is another major advantage,
allowing for even the sickest patients to be imaged quickly, without compromise
of their medical status. Moreover, the relatively low cost of CT and its widespread
applications in all other areas of diagnostic imaging, along with the simplicity of
image acquisition, have made CT one of the most easily available tests in virtually
every medical facility. Accordingly, CT is the most frequently used initial test for
patients suspected to have acute aortic pathology (15).
CT is not free of drawbacks. The examination involves exposure to ionizing
radiation, making it a suboptimal test for younger patients and, particularly,
women of reproductive age. There is a considerable radiation exposure from a
single chest CT (16), which is even higher for the newer multi-detector CT
ECG-gated cardiac studies (17). More importantly, the organ-specific absorbed
dose for the breast is rather high, potentially increasing the risk for malignancies
in the subsequent years of life. The issue of radiation exposure becomes pertinent
for patients who need to have frequent repeat evaluations (e.g., those with aneu-
rysms), as there is an additive effect of ionizing radiation on biologic tissues.
Another limitation of CT is the use of contrast media, which are usually necessary
for assessment of suspected aortic pathology. Although imaging of aortic aneu-
rysms may be performed without contrast enhancement, imaging for suspected
aortic dissection requires administration of intravenous iodinated contrast. X-ray
contrast media are potentially nephrotoxic, particularly in diabetics, hypertensives,
and patients with pre-existing renal disease. These subgroups are also the ones
more likely to develop aortic pathology and to be referred for CT imaging. Thus,
the risk to develop contrast-induced renal complications in patients with suspected
acute aortic pathology is not negligible.
Aortic aneurysms can be readily diagnosed with CT imaging. The dimen-
sions of all segments of the vessel can be accurately measured in the transverse
orientation, and this has become the standard of quantification of aortic diameter
(Fig. 5). Following contrast administration, a 3D representation of the aorta can be
created, and the cross-sectional area perpendicular to the vessel can be obtained
through post-processing. CT can also identify mural thrombus by demonstrating
differential attenuation within the aortic lumen. When using a standardized proto-
col, the reproducibility of thoracic aortic diameter measurements has been shown
to be high (18), allowing for reliable sequential examinations.
Typical CT findings in patients with aortic dissection are listed in Table 3.
Identification of the intimal flap in the contrast-enhanced scan is diagnostic
for aortic dissection (Fig. 6). The findings from nonenhanced scans are usually
https://t.me/med1917
Imaging of Aneurysms and Dissections 49
suggestive but not diagnostic, and can be used when the intimal flap is not visible.
The displacement of intimal calcification (Fig. 7) may be difficult to discern in
small dissections and in patients with highly tortuous aortas. A thrombosed false
lumen usually presents as hyperattenuating region of the vessel wall. Similar
imaging characteristics are found in cases of periaortic hematomas, the difference
being that the hyperenhanced region is now outside the vessel wall.
In aortic dissections, CT can usually identify the true and false lumens. The true
lumen is usually smaller and, because blood flow velocity is higher than in the false
lumen, there is differential attenuation in the contrast-enhanced scan (Fig. 8). On
cross-sectional imaging, the dissection flap and the outer vessel wall of the false
lumen form an acute angle, giving the impression of a bird’s beak (the so-called “beak
sign”) (19). Cobwebs (thin filamentous fibrous strands) are also frequently seen in the
Table 3 Computed Tomography Findings in Aortic Dissection
Nonenhanced scans
Displacement of intimal calcification from the vessel wall
Thrombosed false lumen (differential attenuation)
Periaortic hematoma
Distortion of aortic contour
Enhanced scans
Intimal flap
Figure 5 Contrast enhanced computed tomography (CT) in a patient with aneurysm of
the ascending thoracic aorta. The patient was previously operated on for aortic dissection,
and developed the aneurysm several years after the initial operation. The ascending aorta
diameter is 59 mm. There are calcifications on both the ascending and descending thoracic
aortic wall (thick white arrows). The left anterior descending coronary artery is also heavily
calcified (thin white arrows).
https://t.me/med1917
50 Danias
contrast-filled false lumen. LePage et al. (20) evaluated the clinical utility of indica-
tors of true and false lumen in 51 patients with aortic dissection. The two most reliable
signs to identify the false lumen were the “beak sign” and the larger area.
CT can define the entry point in the majority of cases with confirmed
aortic dissection. In a study of 57 patients with aortic dissection or intramural
Figure 7 Non-enhanced (A) and contrast-enhanced (B) CT of a patient with post-traumatic
pseudoaneurysm of the aorta, years after a motor-vehicle accident (the patient’s CXR is shown
in Fig. 2). The nonenhanced scan demonstrates intraluminal calcification (arrow). The con-
trast-enhanced scan demonstrates homogenous signal enhancement in both the aortic lumen
and pseudoaneurysm (
*
). Abbreviations: CT, computed tomography; CXR, chest X-ray.
Figure 6 Transverse slice from a contrast-enhanced CT scan at the base of the heart in a
patient with type A dissection. The right and noncoronary cusps of the aortic valve are visu-
alized (black straight arrows). The dissection flap (curved black arrow) is visualized just
above the aortic valve, but does not involve the origin of the right coronary artery (white
arrow). Abbreviation: CT, computed tomography.
https://t.me/med1917
Figure 8 Extensive type B aortic dissection extending from the distal aortic arch to the
abdominal aorta. The transverse images of the contrast-enhanced CT at various levels
(A–D) clearly demonstrate the thrombosed false aortic lumen (
*
) as an area of lower signal
intensity. The 3-dimensional reconstruction at an oblique saggital plane (E) provides an
easier appreciation of the extent of the dissection.
Imaging of Aneurysms and Dissections 51
https://t.me/med1917
Соседние файлы в папке Библиотека им академика М.И. Перельмана