Diagnostic ultrasound ( PDFDrive )

438 PART II Abdominal and Pelvic Sonography
FIG. 12.7 False-Positive Abdominal Aortic Aneurysm. The deep
cursor has been placed on the echo caused by the anterior edge of
lumbar vertebral body, posterior to the aorta. Mistaking the anterior
lumbar spine for the posterior wall of the aorta may cause a false-positive
exam, particularly if visualization is poor because of obesity. Transverse
imaging can usually detect the error. Longitudinal imaging in the coronal
plane also helps avoid this error.
False-Positive/False-Negative Results
Screening for aortic aneurysm should be highly accurate.
Nonetheless, we have encountered two sets of circumstances
where it is possible to have a false-positive study. he irst can
happen when the aorta at the diaphragmatic hiatus measures
3.0 cm or greater and is mistakenly called “aneurysmal.” he
normal size of the supraceliac aorta is 2.1 to 2.7 cm. 4 Based on a
deinition of aneurysm that requires the diameter to be 1.5 times
or more the normal diameter, a small aorta would not become
aneurysmal until it reached approximately 3.2 cm in diameter,
whereas the larger supraceliac aorta would not be aneurysmal even
at 4.0 cm.
he second circumstance where we have seen several falsepositive
studies is when the spine is mistaken for the posterior
wall of the aorta. his is particularly likely to occur when visualization
is poor, as in a very obese patient. he true posterior wall
of the aorta may then be diicult to determine. Usually, transverse
imaging or imaging from the let lank will help deine the true
posterior wall to help determine if an aneurysm is present
(Fig. 12.7).
False-negative studies may occur if visualization of the
infrarenal aorta is incomplete. If the entire infrarenal aorta is
well seen in longitudinal imaging, the diagnostician can exclude
the presence of a fusiform aneurysm, the most common type
of AAA. However, if the entire infrarenal aorta is not well seen
in the transverse plane, an eccentric aneurysm is not excluded
(see Figs. 12.5 and 12.6).
Ultrasound Versus Computed Tomography
for Evaluation of Rupture
As previously mentioned, rupture of aortic aneurysms is catastrophic,
usually resulting in the patient’s death. Although it
played a role in the rapid diagnosis of ruptured AAAs in the
emergency room 30 years ago, 38 ultrasound no longer has such
a place at most institutions. he near ubiquitousness of high-speed
multidetector CT scanners has made CT diagnosis extremely
rapid. CT has a number of other advantages in comparison to
ultrasound. CT, even without intravenous contrast, is diagnostic
of AAA in all cases, is highly diagnostic of retroperitoneal
bleeding associated with aneurysm rupture, and is not
operator dependent.
Because of these factors, obtaining a formal ultrasound in
patients with an emergent need for diagnosis in most settings
is unwise. If the patient has a ruptured AAA, time is of the
essence, and the patient needs the most complete information
that can be obtained in a reliable way. Performing ultrasound
risks wasting precious minutes that may be the diference between
patient survival and death (Fig. 12.8).
Treatment Planning
Treatment of large AAAs involves either open surgical repair or
endovascular aortic repair (EVAR). Endovascular repair involved
the placement of an endoluminal grat (also known as a stent
grat) to create a new channel for blood low through the
aneurysm isolating the aneurysmal sac.
Computed tomography angiography (CTA) is the most
appropriate imaging method for AAA treatment planning. In
the setting of open repair, CTA accurately assesses the degree
to which the iliac arteries may be involved with the aneurysm.
In addition, evaluation of the mesenteric arteries is of value to
determine whether reimplantation of the inferior mesenteric
artery (IMA) is needed. For EVAR, the distance of the renal
arteries from the top of the aneurysm and the presence and
location of accessory renal arteries are important. Knowledge
of the three-dimensional anatomy of the aneurysm and size and
coniguration of the iliac arteries is critical.
Postoperative Ultrasound Assessment
Ater open surgical repair of an aortic aneurysm, imaging surveillance
generally is not performed. Imaging is used to assess
postoperative complications, including thrombosis, infection,
stenosis (grat kink, neointimal hyperplasia, atherosclerosis), and
anastomotic pseudoaneurysm. Ultrasound may play a role in
the diagnosis of any of these complications. Another complication
is aortoenteric istula, generally to the third portion of the
duodenum, which is potentially catastrophic and presents most
oten with upper gastrointestinal bleeding. Ultrasound has no
role to play in the diagnosis of aortoenteric istula.
Ultrasound has a larger, potentially much larger, role to play
ater repair of AAAs with EVAR. Current recommendations
include lifelong imaging surveillance for endoleaks and grat
migration in patients who have undergone EVAR for AAA. 39
An endoleak is an area of the AAA that has been excluded by
the stent grat that nonetheless continues to have blood low.
here are four types of endoleaks, categorized by four diferent
sources of blood lowing into the aneurysmal sac (Fig. 12.9).
Egress of blood from the sac is usually through patent aortic
branches in all types of endoleak.
With a type 1 leak, one of the ends of the stent grat is not
tightly apposed to the arterial wall, allowing blood to enter the
aneurysmal sac (Fig. 12.10). A type 2 leak is caused by retrograde