While potentially fatal, pulmonary embolism (PE) often has a confusing clinical picture and an uncertain outcome. This article reviews the imaging techniques for diagnosing PE with CT scanning. By understanding the possible pitfalls and artifacts, a radiologists will be able to make a more confident interpretation of positive and negative studies, and reduce the number of false-positive and false-negative examinations.
Dr. Ravenel is an Assistant Professor of Thoracic Imaging
in the Department of Radiology at the Medical University of
South Carolina, Charleston, SC. Dr. McAdams is an Associate
Professor of Radiology and Dr. Goodman is a Professor of
Radiology and Section Chief of Thoracic Imaging at Duke
University Medical Center, Durham, NC
Pulmonary embolism (PE) is a frequently suspected disease that
has a confusing clinical picture and an uncertain outcome. It has
been stated that PE is a major contributor to death in large
numbers of patients.
PE is said to be the sole cause or major contributor to death in up
to 37% of patients in autopsy studies where PE was present.
Furthermore, it has been stated that the untreated mortality rate
from PE may be as high as 30%
and that anticoagulation therapy may reduce this mortality rate to
These data are, however, controversial. Some investigators believe
that PE is overdiagnosed and that its significance is overstated.
Previously, ventilation-perfusion scintigraphy and pulmonary
angiography were the imaging tests of choice in cases of suspected
PE. Due to limitations in the accuracy of ventilation-perfusion
(V/Q) scanning and the invasive nature and perceived complications
of angiography, coupled with improvements in spiral computed
tomography (CT), a great deal of interest in using spiral CT for
screening and diagnosis of PE has been generated.
Since 1996, a number of studies evaluating spiral CT for the
diagnosis of PE have been performed, with a wide range of
sensitivities (53% to 92%) and specificities (78% to 100%)
However, in outcome-based studies, the negative predictive value
for CT approaches 100%. That is, a normal CT correlates with
extremely low rates of morbidity and mortality due to subsequent
While pulmonary angiography is still considered by many to be the
gold standard for diagnosis of PE, CT ultimately may achieve this
The implementation of CT into the diagnostic algorithm for PE at
hospitals has been variable and is related to the individual
clinician's acceptance of the technique as well as the
radiologist's skill in interpretation. The implication of
false-negative interpretations would be great if this occurred with
significant frequency. However, as mentioned previously, the
negative predictive value of a normal CT is virtually as high as a
normal V/Q scan and normal pulmonary angiogram. Perhaps more
important are false-positive diagnoses, which can also have
significant consequences. Oral anti-coagulation may result in fatal
and major bleeding in 0.25% to 1% and 1% to 5% of patients
and thrombolysis has a major bleeding incidence of up to 20%.
Fortunately, CT scanning is more accurate than V/Q scanning
and should alleviate this problem in many patients.
Nevertheless, in order to gain acceptance, both positive and
negative diagnoses must be made confidently. Our goal is to review
and demonstrate the various diagnostic pitfalls and artifacts that
can complicate the interpretation of CT pulmonary angiography.
Techniques for spiral CT pulmonary angiography have been
and vary from institution to institution. The most important
parameters are: 1) rapid bolus infusion of intravenous (IV)
contrast material through a large IV catheter; 2) appropriate scan
delay to optimize opacification of the pulmonary arteries; and 3)
thin collimation (1 to 3 mm). Representative protocols are listed
in Table 1.
In our practice, we typically infuse 150 cc of nonionic contrast
at a rate of 4 cc/second via power injector. This rapid infusion
requires placement of a large bore IV catheter, preferably
18-gauge, in a large vein. We usually require placement in an
antecubital vein. We will not power inject through a small-gauge
peripheral catheter (>20-gauge), a central venous catheter, a
peripherally inserted central catheter (PICC), or a catheter placed
in a more superficial (hand) vein.
In the majority of patients, peak enhancement in the pulmonary
arteries occurs between 15 and 25 seconds after beginning the
injection. A number of medical conditions, particularly congestive
heart failure, can delay pulmonary artery opacification. There are
three methods for optimizing pulmonary arterial opacification. Some
institutions, including ours, use a
standard timing delay
. We use a 25-second delay in most patients; if the patient has
known cardiac disease, we increase the delay by 5 seconds. We have
found that this approach works very well in our institution.
is a second method for optimizing pulmonary arterial opacification.
This technique is available on most CT scanners and is known by a
variety of proprietary terms (for example, "Smart Prep" on General
Electric scanners [GE Medical Systems, Milwaukee, WI]). Using this
technique, the patient is repeatedly scanned at low
milliampere (mA) at a single preselected level, usually the main
pulmonary artery. A region of interest is placed on the main
pulmonary artery and the scan is initiated at either a preset level
of opacification or at peak contrast opacification. This technique
is particularly useful when injecting from the lower extremities.
is a third method for optimizing pulmonary arterial opacification.
Using this method, a small bolus (20 to 30 cc) of IV contrast is
injected, serial images are obtained over the main pulmonary
artery, and the time to peak opacification is determined. The scan
delay is then set at this time plus a few seconds (typically 3
seconds). This technique works quite well, but it is more
time-consuming to setup than the standard delay technique. The
choice of technique used in an individual practice is dictated by a
variety of concerns, including type of CT scanner, experience of CT
technologists, and patient population.
Choice of slice collimation and pitch depends on a number of
factors, including type of machine (manufacturer, single versus
multidetector, rotation speed), area to be covered and length of
breath-hold. Thin-collimation is absolutely essential for confident
exclusion of PE. On a single detector scanner, collimation should
be no greater than 3 mm; on a multidetector scanner, thinner
collimation is both possible and desirable. We perform all of our
PE studies on a multidetector scanner and typically use 2.5-mm
collimation with 15-mm/rotation table speed, 0.8-second rotation.
Some have found that 1.25-mm collimation with 7.5-mm/rotation table
speed improves depiction of subsegmental vessels. Whether or not
this improves detection of small emboli is not known. We currently
use 2.5-mm collimation because it allows us to complete the scan in
<10 seconds--a very reasonable breath-hold for all but the most
acutely ill patients. As CT scanners are upgraded to faster
rotation speeds (from 0.8 seconds to 0.5 seconds or less), it may
be feasible to acquire the studies in <10 seconds using 1.25-mm
Axial images should be reconstructed at intervals no greater
than, and preferably less than, slice collimation. Overlapping
reconstructions helps eliminate stair-step artifacts (see below)
and facilitates multiplanar reconstructions. We typically
reconstruct the axial images at 1-mm intervals. This results in a
large number of images per case (150 to 250), necessitating review
of the study on a workstation. We find it difficult and more
time-consuming to read the studies in hard-copy format. Scrolling
through individual images in stepwise fashion on a workstation
makes it easier to follow vessels travelling perpendicular to the
axial plane. Furthermore, two-dimensional (2D) images reconstructed
in coronal, sagittal, or oblique projections can be quite useful
for assessing vessels that run either parallel or slightly oblique
to the axial plane.
A variety of normal structures and related conditions may
complicate the interpretation of CT scans performed to evaluate for
PE. The usual appearance of PE on CT studies is a central filing
defect in a pulmonary artery surrounded by a rim of contrast,
complete cut-off of a vessel (figure 1), a serpiginous low
attenuation clot in larger vessels (figure 2), or, exceptionally,
as high attenuation thrombus on a non-contrast CT (figure 3).
Venous structures, lymphoid and perivascular tissue, vessel
orientation, and mucus or fluid in adjacent bronchi all can lead to
false-positive interpretations. Occasionally, chronic pulmonary
embolism may be confused with acute disease and rarely
nonthrombotic emboli may be seen macroscopically. In this section,
we will discuss the various normal and pathologic conditions that
can mimic PE on spiral CT scan.
Since scanning parameters are designed for optimal arterial
enhancement, enhancement of the pulmonary veins is quite variable.
Flow artifacts in poorly filled veins may simulate PE (figure 4A).
It is, therefore, important not to mistake a poorly filled vein for
an artery. Owing to their proximity to the central hilar vessels,
the superior pulmonary veins can be mistaken for arteries easily.
The right and left superior pulmonary veins descend anterior to the
left and right upper lobe pulmonary arteries at the bifurcation of
the anterior segmental artery (A2 segment). With careful viewing,
the veins can be traced into the left atrium. As a general rule,
arteries and bronchi run in parallel, while the veins return to the
left atrium individually. In cases where there is confusion, 2D
coronal and sagittal reformatted images (figure 4B) and viewing
images at lung window settings can be helpful (figure 5).
Normal lymphoid tissue can present difficulty in interpretation,
particularly at arterial branch points. Normal lymph nodes are
frequently present lateral to the A2 segment of the upper lobes
(figure 6), medial to the interlobar and lower lobe arteries, and
at take-off of the lower lobe segmental arteries.
The distinction can be more difficult in patients with neoplasm
where either tumor or lymph node metastases may distort the normal
arterial anatomy. Acute emboli often are surrounded by a rim of
contrast that can distinguish them from lymphoid tissue. At
bifurcation points, emboli may impact and not be completely
surrounded by contrast. In these cases, attention to vessel caliber
and a careful search for distal emboli often lead to the correct
The majority of upper and lower lobe vessels run in a
superior/inferior plane. Difficulty in tracing middle lobe and
lingular arteries exists as they generally run oblique to the axial
scan plane. Partial volume averaging can lead to the appearance of
low attenuation filling defect within these vessels (figure 7), and
as every hard-copy image may not be printed, attention to these
regions on the monitor is essential. Obliquely oriented vessels can
often be better evaluated with coronal and sagittal reformatted
Mucus or fluid in bronchi
Patients with chronic obstructive pulmonary disease, those that
are ventilator-dependent, and postoperative patients all may have
difficulty clearing secretions. This may lead to the accumulation
of mucus or fluid in segmental bronchi. On axial images,
fluid-filled bronchi have a low attenuation center with a rim of
higher attenuation (bronchial wall), mimicking the appearance of PE
(figure 8). Because the arteries and bronchi run parallel to each
other, a contrast-filled vessel should be apparent immediately
adjacent to the fluid-filled bronchus. Reformatted images can also
better depict the anatomy and differentiate fluid-filled airways
It is rare for nonthrombotic emboli to present with macroscopic
filling defects. Neoplasms that grow into the inferior vena cava
(renal, adrenal, and hepatic) and atrial myxomas can fragment and
present as visible filling defects. Because of similar Hounsfield
unit (HU) measurements, these are indistinguishable from thrombotic
emboli. The possibility of nonthrombotic emboli should be
considered in the appropriate clinical setting.
Exceptionally, macroscopic fat emboli can be encountered and
diagnosed by HUs consistent with fat.
Chronic versus acute embolus
Chronic PE may be present particularly in those with
long-standing deep venous thrombosis. The distinction is difficult
in the segmental and subsegmental arteries. In the main and lobar
arteries, chronic PE is applied closely to the vessel wall and has
obtuse angles at its periphery (figure 9). Abrupt vessel cut-off,
intravascular webs, intimal irregularity, and pouch-like defects
may also be seen.
Artifacts in CT imaging come from inherent problems with the
imaging system (beam hardening from high-density contrast), image
reconstruction (stair-step artifact) and patient cooperation
(breathing and motion artifact). All of these artifacts can make
studies difficult to interpret.
Artifacts of enhancement
Despite rapid infusion of contrast and a 20 to 30-second scan
delay, there is often marked enhancement of the superior vena cava
during the scan. Dense opacification of the superior vena cava can
cause beam hardening and streak artifact obscuring adjacent vessels
(figure 10). In order to minimize the artifact, some authors
suggest diluting the contrast. In one report, a 1:1 dilution of
300 mg/mL iodine results in less artifact compared to undiluted
contrast; however, further dilution results in unacceptable
In our experience, this artifact does not significantly impact the
quality of the images and we do not typically dilute the
A poor contrast bolus can similarly make interpretation of CT
scans for PE difficult (figure 10). The detection of PE relies on
HU differences between PE and the contrast bolus. Poor
opacification of the pulmonary artery decreases the attenuation
differences between the contrast bolus and a PE. At wider window
settings, the embolism may not be distinguished from contrast
within the vessel. Viewing at narrower window settings can help
accentuate subtle differences in film contrast, but in cases where
a firm diagnosis must be established, a repeat contrast-enhanced CT
scan or other imaging test (V/Q scan or conventional pulmonary
angiography) should be performed.
Stair-step artifacts result from data reconstruction and
misregistration. With loss of data along the z-axis, apparent areas
of both decreased and increased attenuation occur. This apparent
decrease in attenuation of the vessel lumen on multiple images can
mimic the appearance of PE. On axial images, the vessel lumen is
generally affected on alternating images with normal intervening
vessel enhancement. On reconstructed images, the "stair-step"
appearance can be appreciated (figure 11). Decreasing the slice
thickness and overlapping images will reduce the amount of artifact
along the z-axis; however, when slice thickness is too thin,
quantum mottle, scan time, and radiation dose to the patient can
provide significant problems.
Patients with suspected PE are usually short of breath. With a
24-second scan time on single-slice scanners, the patient often
cannot perform a full breath-hold. This is less of a problem with
multislice scanning due to the faster scan time. Breathing
artifacts cause blurring of the image (figure 12), making
assessment of the vessel lumen impossible. While the ultimate
solution is decreasing scan time, scanning from bottom to top
allows images to be obtained at the lung bases first, where motion
and breathing artifact will be most pronounced. More superiorly,
breathing has less of an effect and blurring of vessels is
generally not a problem.
The success of CT in the imaging of PE not only depends on the
interpreter, but also the quality of the technique. A knowledge of
the various pitfalls and artifacts allows for a more confident
interpretation of positive and negative studies as well as fewer
false-positive and false-negative examinations.