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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3900_Библиотеки_им_академика_М_И_Перельмана
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Fig. 1 Axial contrast-enhanced computed tomography
( CT ) image demonstrates a 5.4-cm infrarenal abdomi-
nal aortic aneurysm. Cross-sectional imaging nicely
depicts how the patent vascular lumen ( arrowhead )
comprises only a portion of the total aortic diameter
( arrow ) .
Fig. 2 ( a ) Axial contrast-enhanced computed
tomography ( CT ) image (soft tissue windowing)
demonstrates a focal saccular aneurysm involv-
ing the distal descending thoracic aorta ( arrow ).
There is abnormal attenuation in the right middle
lobe (arrowhead). ( b ) Axial contrast-enhanced
CT image (lung windowing) reveals spiculation
associated with the right middle lobe mass ( arrow-
head ). Biopsy was positive for nonsmall cell lung
carcinoma. ( c ) Axial contrast-enhanced CT image
(soft tissue windowing) demonstrates mediastinal
adenopathy (arrow) that was positive on subse-
quent PET scan
cross-sectional imaging, there remains a solid basis for catheter-based angiography.
Although it has lost its primary importance as a diagnostic modality, catheter-
based angiography must be discussed in the setting of its still irreplaceable role
in the endovascular treatment of aortic disease. As it is currently integrated with
contemporary cross-sectional imaging, catheter-based angiography entails the use
of carbon dioxide angiography and intravascular ultrasound (IVUS).
P.S. Liu and D.M. Williams36
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Chapter 3 Contemporary Imaging of Aortic Disease 37
Fig. 3 ( a ) 3D Reformatted image demonstrates abdominal aortic aneurysm ( arrow ) with atherosclerotic calcifica-
tion throughout the abdominal aorta and branch vessels. ( b ) Surface-shaded display of same data set redemon-
strates the abdominal aortic aneurysm ( arrow )
Computed Tomography
Computed tomography is an x-ray-based method of imaging which uses a
rotating gantry including an x-ray tube and detector plate to generate tomo-
graphic images of the body. Early on, CT played only a minor role in clinical
vascular imaging, as it was mostly used for stationary anatomic imaging due
to slow acquisition speed. However, the development of helical scanning
technique and multidetector row technology in the early 1990s revolutionized
the role of CT in angiographic applications.
3, 5
Slip-ring technology allows
continuous rotation of the gantry without the need to unwind power/ electricity
cables, allowing modern gantry rotation speeds of approximately 0.33 s per
rotation. When combined with a moving table, the data acquisition changes
from axial 2D imaging to volumetric 3D imaging. The overall scan time is
reduced and the acquired data can be reconstructed into traditional axial slices
or other reformatted planes. In the early implementation of this technology,
the anatomy recorded by detectors at the beginning of the gantry rotation
was different than at the end of the gantry rotation (due to continuous table
movement), leading to exaggerated volume-averaging effects and decreased
through-plane resolution. The development of multiple detector rows on the
circulating gantry addressed this problem, thereby allowing multiple rows
of data to be acquired in a single rotation.
6
With advances in detector tech-
nology, thinner detector elements were developed with subsequent gains in
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38 P.S. Liu and D.M. Williams
through-plane resolution (individual detector size) as well as overall z -axis
coverage (total number of rows on the gantry). Currently, detector element
width is approximately 0.5–0.625 mm. The combination of helical scan-
ning and multiple detector rows is the foundation for modern multidetector
CT (MDCT) scanners, which can provide data sets with equal spatial and
contrast resolution in all three planes and ultrafast imaging times (complete
vascular interrogation from arch to toes in less than 30 s).
7
CT studies focused on vascular disease often involve MDCT scanners with pro-
tocols consisting of multiple imaging phases, including a precontrast image series
and at least one contrast-enhanced image series. Although the contrast-enhanced
images provide exquisite luminal imaging, it is important to not overlook the pre-
contrast images. Precontrast images are critical for the detection of blood, as acute
hemorrhage/hematoma is usually hyperdense on noncontrast images (Fig. 4a and b).
Although CT attenuation values are absolute numbers related to the constituents that
make up a given voxel, subtle areas of acute hemorrhage can be less conspicuous
when dense intraluminal contrast is present (Fig. 5a and b ). From the standpoint
of clinical management, the presence of extraluminal high attenuation in a
patient suspected of ruptured aneurysm should obviate the need for further
imaging and decrease the delay time to definitive treatment.
7
Additionally,
atherosclerotic calcifications, calcified old hematomas, surgical grafts, and
felt pledgets/rings are often more reliably seen on precontrast images than
postcontrast images
8
(Fig. 6a and b ). Identification of these components will
aid in rendering a final opinion about the patient’s disease status.
The contrast-enhanced portion of a CTA study is typically performed with
mechanical dual-chamber power injectors. Given the potential coverage speed of
MDCT, contrast bolus geometry and injection protocols have become areas
of considerable research with the rapid evolution of MDCT technology; a
detailed discussion of this research is beyond the scope of this chapter and
Fig. 4 ( a ) Axial noncontrast computed tomography ( CT ) image through the abdomen in standard abdominal
soft tissue windowing (window = 400, level = 40) demonstrates retroperitoneal high attenuation ( arrows ) with
concomitant fascial thickening around the anterolaterally displaced left kidney, indicating retroperitoneal hemor-
rhage. ( b ) Axial noncontrast CT image with narrow window settings (window = 100, level = 30) demonstrates
increased conspicuity of high attenuation related to suspected retroperitoneal hemorrhage ( arrows ). The patient
expired during attempted open repair of abdominal aortic aneurysm rupture
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Chapter 3 Contemporary Imaging of Aortic Disease 39
Fig. 5 ( a ) Axial noncontrast computed tomography ( CT ) image through the chest demonstrates crescentic high
attenuation ( arrow ) associated with the left lateral aspect of the descending thoracic aorta, compatible with an
intramural hematoma. There is also a suggestion of a dissection flap on the medial aspect of the aorta ( arrow-
head ). ( b ) Axial contrast-enhanced CT image at the same level easily demonstrates the suspected dissection flap
( arrowhead ). However, the visual conspicuity of the intramural hematoma on the left-lateral aspect of the aorta is
decreased ( arrow ) versus the dense contrast bolus in the aortic lumen
includes consideration of physiological and anatomical factors, injection rate,
total volume, and iodine concentration.
9
Generally, CTA studies utilize con-
trast media with iodine concentrations of 300–400 mmol iodine/ml, injection
rates of 3–5 ml/s, and a total volume of 70–150 ml. However, the fundamental
challenge in CTA is optimal delivery of the appropriate amount of contrast to
the volume of interest. This often requires some degree of protocol specificity
particular to the scanner being used. The scan time to cover a certain distance
Fig. 6 ( a ) Axial contrast-enhanced computed tomography ( CT ) image of the chest demonstrates irregularity of the
anterior aspect of the ascending aorta ( arrow ). The patient had prior surgical graft repair of the ascending aorta for
aneurysm. Tiny clips are present (arrowheads) around the ascending aorta as well. ( b ) Axial noncontrast CT image at
the same level demonstrates very high attenuation in the area of questioned abnormality on the contrast-enhanced CT
image ( arrow ). This area was stable for over 5 years and had been present on every postoperative scan, probably repre-
senting felt pledgets buttressing closure of an aortotomy wound. Surgical clips are again demonstrated ( arrowheads )
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40 P.S. Liu and D.M. Williams
can vary considerably depending on the number of detectors available on
a given MDCT system, and therefore the contrast injection protocol must
vary to meet the ability of the scanner. As an example, a 4-detector scanner
may take 21 s to cover a 25 cm length with standard parameters, whereas a
64-detector scanner could achieve similar or better imaging parameters in
4 s.
10
Additionally, delayed postcontrast images can be valuable portions of a
complete aortic protocol, especially in patients who have had prior endovas-
cular therapy for aneurysms.
11
Therefore, it is critical to tailor scan protocols
for the specific task toward the actual device being used for acquisition.
There are specific advantages and disadvantages of CTA when compared to
other cross-sectional techniques such as MRA. In general, CTA requires less
technical sophistication to achieve a diagnostic quality examination than does
MRA, which leads to more generalized reproducibility among studies obtained
at different institutions.
12
CTA is also superior to MRA in terms of spatial resolu-
tion and temporal acquisition, making it the test of choice for trauma or clinically
unstable patients (Fig. 7 ). As opposed to the high degree of background suppres-
sion obtained in MRA, the extraluminal detail is retained during the angiographic/
Fig. 7 Four contiguous axial contrast-enhanced computed tomography ( CT ) images through the upper abdomen
in a patient after motor vehicle collision demonstrate a hypodense linear tract through the spleen ( arrow ), compat-
ible with a splenic laceration. There are scattered foci of very high attenuation within the laceration bed ( arrowheads )
that represent active extravasation of iodinated contrast material into left upper quadrant, or hemoperitoneum (*).
The patient was taken for emergent embolization of active hemorrhage in order to maintain splenic function.
Because of its short scan times, CT is ideally suited for the initial imaging of trauma patients
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Chapter 3 Contemporary Imaging of Aortic Disease 41
Fig. 8 Axial contrast-enhanced computed tom-
ography ( CT ) image demonstrates a 6.8-cm
abdominal aortic aneurysm. Note how CT
demonstrates the luminal and extraluminal
components of the vessel wall, including the
patent lumen (*), incorporated mural throm-
bus ( arrowhead ), and intimal calcifications
( arrow ). The patient underwent subsequent
successful endovascular repair
contrast-enhanced portion of the CTA, which offers a single comprehensive evalu-
ation of the vascular system
7
(Fig. 8 ). The major clinical disadvantages associated
with CTA include the use of ionizing radiation (which limits dynamic or real-time
imaging of contrast bolus propagation) and a nephrotoxic contrast agent, while
other technical disadvantages include artifact related to the injected contrast mate-
rial, indwelling metallic prostheses, or adjacent osseous structures.
13
With respect to aneurysmal disease, CTA provides a fast, reproducible study
that yields important clinical information. CTA is frequently the first-line tool
for the emergent evaluation of patients with suspected aortic aneurysm rupture;
noncontrast studies can be performed rapidly and can render a diagnosis that
expedites definitive surgical treatment.
14
CTA can be used to follow patients
with known aneurysms for rate of interval growth, to facilitate the preoperative
management of such patients when their aneurysm reaches critical dimen-
sions, and to follow up patients in the postoperative period for complication
development
13
(Fig. 9a and b ). In the era of endovascular repair, CTA has
become even more valuable as the acquired data set from MDCT scanners
Fig. 9 ( a ) Axial contrast-enhanced computed tomography ( CT ) image demonstrates an abdominal aortic aneurysm ( arrow )
with maximal axial dimensions of 3.88 × 4.72 cm. ( b ) Axial contrast-enhanced CT image of the same patient in a similar
location approximately 1 year later demonstrates interval growth of the aneurysm, now measuring 4.85 × 5.56 cm
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42 P.S. Liu and D.M. Williams
Fig. 10 ( a ) Coronal maximum intensity projection from contrast-enhanced computed tomography angiography
( CTA ) study demonstrates a 5.5-cm abdominal aortic aneurysm ( arrow ). ( b ) Curved reformatted image from the
same CTA data set using a center-line automated approach allows continuous visualization of the arterial anatomy
through the left external iliac artery ( arrowhead ) for endovascular treatment planning. Abdominal aortic aneu-
rysm is again seen ( arrow ). ( c ) Single oblique image from postprocessing software package demonstrates how
the vessel is sized at this specific level. The user may modify the included area based on visual inspection. ( d )
Final vessel analysis provides maximal and minimal luminal diameter at various levels to assist in planning the
endovascular operative approach
can be loaded into postprocessing software to generate the salient preopera-
tive parameters for stent-graft deployment (Fig. 10a–d ). Additionally, CTA is
a robust modality for following endograft patients in the postoperative period,
as CT can directly visualize the intraluminal graft and look for stent graft
complications such as endoleaks
15
(Fig. 11a and b ). With the increasing use of
endovascular systems for the aortic aneurysmal disease, CTA has become the
definitive imaging modality used for both planning and follow-up of surgery.
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Chapter 3 Contemporary Imaging of Aortic Disease 43
Fig. 11 ( a ) Axial noncontrast computed tomography ( CT ) image of the abdomen demonstrates an abdominal
aortic aneurysm with indwelling endovascular stent graft ( arrowhead ). There is no abnormal high attenuation in
the excluded aneurysm sac on these noncontrast images. ( b ) Axial contrast-enhanced CT image at the same level
demonstrates a blush of contrast within the aneurysm sac ( arrow ), which is outside the stent graft itself ( arrow ).
Since this was not present on the precontrast images, this area fills through an endoleak. Contrast angiography
confirmed a type II endoleak from retrograde flow through the inferior mesenteric artery ( IMA ) .
Magnetic Resonance Imaging
Magnetic resonance imaging allows imaging of the body utilizing a magnetic
field and radiofrequency pulses. The patient is placed within a strong mag-
netic field (modern clinical scanners function at 1.5–3.0 T) that aligns the net
magnetic field of the protons within a patient’s body along the bore of the
main magnetic field.
16
When a radiofrequency pulse is sent out from a trans-
mitter, the net magnetization is deflected from its resting axis along the main
magnetic field. As the protons relax back to their original state in the main
magnetic field, a detectable signal – called an echo – is generated and local-
ized by a receiver coil.
17
The frequency data (“k-space data”) received by the
coil is converted into an image via the Fourier transform.
18
Magnetic recovery
after an RF pulse has been sent out by the transmitter actually has two dis-
crete components: a longitudinal relaxation (parallel to the main magnetic
field) and a transverse relaxation (perpendicular to the main magnetic field).
16
Tissue-specific parameters related to longitudinal and transverse recovery are
called T1 and T2 relaxation times, respectively. Depending on the parameters
of the imaging sequence, the acquired data set (and subsequent generated
images) can demonstrate T1- or T2-weighting to highlight these characteris-
tics.
18
Additionally, the imaging contrast can be altered either by changing the
intrinsic contrast of an organ (the addition of Gadolinium-based intravenous
contrast) or by selectively nullifying certain tissues based on known T1/T2
values (fat saturation techniques), particularly in evaluation of the vascular
tree.
19
In general, the more specific lesion characterization that is offered by
MRI relies on the integrated spectrum of tissue appearances on various types
of T1- and T2-weighted sequences.
The use of magnetic resonance imaging in noninvasive vascular imaging
has been termed MRA. Similar to other applications in MRI, angiographic
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44 P.S. Liu and D.M. Williams
applications require more than one sequence to generate a complete diagno-
sis. Precontrast T1-weighted images can be used to obtain information about
anatomy, including delineation of the aortic wall and adjacent mediastinal
or retroperitoneal pathology
20
(Fig. 12a ). Fat-saturated T1-weighted images
often increases the conspicuity of extracellular methemoglobin in hematomas,
which is particularly important in angiographic applications (Fig. 13a and b ).
T2-weighted images are used less in angiographic applications than in other
anatomic regions, mainly due to their lengthy time of acquisition, though these
sequences remain useful in cases of vasculitis or suspected vascular malforma-
tion
20,
21
(Fig. 14a and b ). Black-blood vascular imaging technique is a tech-
nique based on fast spin-echo imaging that uses a set of two inversion pulses
Fig. 12 ( a ) Axial precontrast T1-weighted in-phase gradient echo image of the abdomen demonstrates a 4.9-
cm abdominal aortic aneurysm ( arrow ). Note that the complete/full diameter of the aneurysm ( arrowheads ) is
depicted on this sequence. ( b ) Axial fat-suppressed postcontrast T1-weighted spoiled gradient echo image at the
same level as panel a during the arterial phase of enhancement demonstrates that the aneurysm lumen ( arrow )
is most conspicuous with relative paucity of signal in the wall and background tissues . ( c ) Axial fat-suppressed
postcontrast T1-weighted spoiled gradient echo image at same level as panel a , obtained 5 min after injection,
demonstrates aneurysm ( arrow ) with increased conspicuity of the vascular wall ( arrowheads ). There is higher
signal in the background tissues as well
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Chapter 3 Contemporary Imaging of Aortic Disease 45
Fig. 13 ( a ) Oblique sagittal fat-suppressed precontrast T1-weighted spoiled gradient echo image of the thoracic
aorta demonstrates curvilinear foci of high attenuation (arrows) which parallel the expected aortic channel, suspect
for a thrombosed false lumen in a thoracic aortic dissection. The use of fat-suppression techniques allows greater
conspicuity of the blood products versus the background tissues, as the mediastinal fat is nullified. ( b ) Oblique
sagittal fat-suppressed postcontrast T1-weighted spoiled gradient echo image at the same location as panel a con-
firms suspected false lumen in aortic dissection. Note that the high signal intensity on the precontrast image is less
conspicuous on this postcontrast image, which validates the need to look carefully at the precontrast image
Fig. 14 ( a ) Axial T2-weighted fast spin echo image of the
lower leg in a patient with suspected vascular malformation.
There is infiltration of the subcutaneous fat ( arrowheads ) with
numerous small curvilinear foci of signal, likely representing
small vessels. Larger area of flow-void ( arrow ) likely represents
a high-flow channel. ( b ) Digital subtraction angiography on the
same patient reveals high-flow within numerous subcutaneous
vascular malformations in the lower leg, supplied by discrete
arterial feeding vessels. Note high arterial flow in matched
lesion ( arrow ) to magnetic resonance imaging ( MRI ) image
in panel a
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