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1 Department of Radiology, University Hospital, UMDNJ-NJ Medical School, 150
Bergen St., UH C-320, Newark, NJ 07103-2406.
2 Department of Radiology, University of Washington, Seattle, WA.
Received December 27, 2005;
accepted after revision December 27, 2005.
Address correspondence to P. D. Maldjian.
Abstract
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Keywords: cardiopulmonary imaging • CT angiography • embolism • lung • MDCT
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| QUESTION 1 In the setting of patients with suspected pulmonary embolism studied by MDCT pulmonary angiography, which statement is correctly implied by a negative predictive value of 99%?
QUESTION 2 Which of the following is the LEAST common positive finding on negative MDCT angiograms in the setting of suspected pulmonary embolism?
QUESTION 3 Which one of the following abnormalities is not consistent with a presumptive diagnosis of paradoxical embolism?
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| QUESTION 4 Concerning patent foramen ovale (PFO), which one of the following statements is true?
QUESTION 5 A CT pulmonary angiogram shows early enhancement of the thoracic aorta with decreased attenuation of the pulmonary arteries. Assuming that the IV contrast material was properly administered, which one of the following abnormalities is most consistent with these findings?
QUESTION 6 Concerning pulmonary embolism in individuals with patent foramen ovale (PFO), which one of the following statements is false?
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Solution to Question 1
The percentage of patients who have an MDCT angiogram that is positive for
pulmonary embolism among those patients who actually have pulmonary embolism
is called the sensitivity, not the negative predictive value. Most research
studies of helical CT angiography have found a sensitivity for the detection
of pulmonary emboli of 85% or greater
[1]; with MDCT, the sensitivity
appears to be greater. Option A is not the best response. The
percentage of patients with pulmonary embolism who have a negative study is
called the false-negative rate, not the negative predictive value. The
false-negative rate depends on both the sensitivity of the test and the
prevalence of the disease. Option B is not the best response. The
percentage of patients without the disease who have a negative test is called
the specificity. Most research studies have found a specificity of 90% or
greater for CT in the diagnosis of pulmonary embolism
[1]. Option C is not the
best response. The negative predictive value of a test is the probability that
patients with a negative test will not have the disease. Thus, in this
context, a negative predictive value of 99% implies that among patients with a
negative test, 99% will not have the disease and 1% will have the disease. The
negative predictive value of MDCT for the development of subsequent clinically
significant pulmonary embolism is about 99%, implying that about 1% of
patients with an MDCT study negative for pulmonary embolism will subsequently
develop clinically significant pulmonary embolism
[2,
3]. Option D is the best
response.
Solution to Question 2
In a study of patients with suspected pulmonary embolism who had MDCT
angiograms that were negative for pulmonary embolism, emphysema was found in
21% of cases, pulmonary consolidation in 18%, pleural effusion in 12%, and
congestive cardiac failure in only 1%
[2]. Option A is the
best response.
Solution to Question 3
Paradoxical embolism is defined as venous thrombosis producing systemic
embolism from clots passing through a right-to-left shunt. Definitive
diagnosis can be made if thrombus is visualized within the right-to-left shunt
(such as thrombus within a PFO or atrial septal defect). Because such direct
observation is rare, in most cases the diagnosis of paradoxical embolism is
presumptive and relies on the presence of the following: systemic embolism
without an apparent source in the left heart or proximal arterial tree (no
evidence of atrial fibrillation or severe atherosclerosis of the thoracic
aorta); venous thrombus or pulmonary emboli as an embolic source;
right-to-left shunting through an abnormal communication between the right and
left circulations (such as a PFO, atrial septal defect, or pulmonary
arteriovenous malformation) [4,
5]. A pulmonary arteriovenous
malformation is an abnormal communication between a pulmonary artery and a
pulmonary vein. This produces a right-to-left shunt and can be responsible for
paradoxical embolism [4].
Option A is not the best response. Venous thrombus is usually the
source for paradoxical emboli. Option B is not the best response. An
atrial septal defect is an abnormal communication that would allow emboli from
the venous circulation (right atrium) to pass into the systemic circulation
(left atrium). Option C is not the best response. Atrial fibrillation
results in formation of thrombi within the left atrium. Systemic embolism
associated with atrial fibrillation usually originates from a source in the
left heart and does not represent paradoxical embolism
[4]. Option D is the
best response.
Solution to Question 4
A PFO is a communication between the atria that extends from an opening in
the septum secundum on the right atrial side (the foramen ovale) to an opening
in the septum primum on the left atrial side (the ostium secundum). The ostium
secundum is slightly superior to the foramen ovale so the communication takes
the shape of a short tunnel through the atrial septum. In the fetus, blood
flow passes from the right atrium to the left atrium via the PFO to bypass the
nonfunctioning lungs. In the neonate, ventilation of the lungs results in
pulmonary vasodilation and reduction of right-sided cardiac pressure. Hence,
when left atrial pressure exceeds right atrial pressure, the septum primum (on
the left side of the septum) acts as a flap valve and presses against the
septum secundum (on the right side of the septum), physiologically closing the
foramen ovale. In most individuals the septum primum fuses with the septum
secundum, permanently closing the defect. Failure of fusion results in a PFO,
which represents a potential communication across the atrial septum
[6]. A PFO is a common
abnormality. Echocardiographic and autopsy studies have shown a prevalence of
about 25% [6,
7]. Option A is not the
best response. Paradoxical embolism requires the presence of a communication
between the right and left circulations. This is usually a PFO, atrial septal
defect, or pulmonary arteriovenous malformation. Of these, PFO is the most
common congenital abnormality associated with paradoxical embolism
[4]. Option B is the
best response. The septum secundum acts as a flap valve and closes the PFO
when left atrial pressure exceeds right atrial pressure. Thus, left-to-right
shunting through a PFO does not usually occur. Option C is not the best
response. Although pulmonary arterial hypertension increases the amount of
right-to-left shunting through a PFO, common maneuvers such as inspiration,
cough, or Valsalva can produce transient elevation of right atrial pressure
sufficient to allow right-to-left shunting resulting in paradoxical emboli
[8]. Thus, pulmonary
hypertension is not a prerequisite for right-to-left shunting through a PFO.
Option D is not the best response.
Solution to Question 5
On CT pulmonary angiography, decreased enhancement of the pulmonary
arteries with early enhancement of the thoracic aorta can occur secondary to
intracardiac shunting via a PFO or atrial septal defect. Deep inspiration at
the beginning of the CT scan can cause increased right-to-left shunting
through the atrial septal conduit, allowing contrast material to pass directly
from the right atrium to the left atrium and bypass the pulmonary circulation
[9]. Option A is the
best response. Although a pulmonary arteriovenous malformation can
theoretically decrease the circulation time of contrast material through the
pulmonary vasculature, the pulmonary arteries should enhance normally.
Option B is not the best response. Massive pulmonary embolism can
result in decreased attenuation of the pulmonary vessels as a result of
intraluminal clots. However, this does not result in early opacification of
the thoracic aorta. Option C is not the best response. Right
ventricular dysfunction can occur from pulmonary embolism due to pressure
overload. This can result in decreased cardiac output from the right
ventricle. This would prolong the circulation time of contrast material
through the pulmonary vessels and would not cause early enhancement of the
thoracic aorta. Option D is not the best response.
Solution to Question 6
Pulmonary embolism can result in elevated right-sided cardiac pressures.
With a coexistent PFO, the amount of right-to-left shunting and the risk of
paradoxical embolism are increased
[10]. Because the question
asks for identification of the false statement, Option A is not the
best response. In a study by Konstantinides et al.
[5] in patients with acute
major pulmonary embolism, right-to-left shunting through a PFO was shown to be
an independent predictor of adverse outcome. Patients with pulmonary embolism
and PFO had a significantly higher mortality rate than those without a PFO
(33% vs 14%, respectively) [5].
Option B is the false statement and represents the best response. In
that same study, Konstantinides et al. also found that patients with acute
major pulmonary embolism and PFO also had a significantly higher incidence of
ischemic stroke compared with patients without PFO (13% vs 2.2%, respectively)
[5]. Because the question asks
for identification of the false statement, Option C is not the best
response. On CT pulmonary angiography, PFO can result in decreased enhancement
of the pulmonary arteries with early enhancement of the thoracic aorta. This
is caused by augmentation of right-to-left shunting through the defect from
deep inspiration. This can lead to insufficient attenuation of the pulmonary
arteries and impair detection of pulmonary emboli
[9]. Because the question asks
for identification of the false statement, Option D is not the best
response.
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