Multidetector computed tomography (MDCT) has improved the diagnosis of pulmonary embolism (PE), especially the detection of small emboli in segmental and subsegmental pulmonary arteries. This article presents optimal scanning parameters and interpretation techniques for MDCT evaluation of PE, reviews the findings of acute and chronic PE in small vessels, and addresses potential interpretation errors.
is a Radiology Resident,
is a Radiology Fellow, and
is a Professor and the Director of Chest Radiology, Department of
Radiology, Drexel University College of Medicine, Philadelphia,
is an Assistant Professor of Radiology, Hospital of the
University of Pennsylvania, Philadelphia, PA.
Multidetector CT (MDCT) has become the gold standard imaging
modality for the diagnosis of pulmonary embolism (PE). Detection of
small emboli in segmental and subsegmental pulmonary arteries is
still a diagnostic challenge, however. The reported incidence of
isolated segmental PE ranges from 4% to 30% in various series.
Small segmental or subsegmental PE are of importance in patients
with limited cardiopulmonary reserve and for diagnosis of chronic
pulmonary hypertension. They may be an indicator of silent deep
venous thrombosis, which may predispose patients to more severe
This article will present optimal technical parameters and scan
interpretation techniques for MDCT evaulation of PE, will review
key imaging appearances of acute and chronic PE in small vessels,
and discuss potential interpretation errors.
Multidetector CT has extended the assessment of pulmonary
vessels to distal segmental and subsegmental levels primarily due
to a number of advantages of CT for this indication. CT provides
uniform arterial enhancement of vessels as small as 2 to 3 mm in
diameter. It also provides thinner slices with reduced partial
volume artifact and decreased noise as well as isotropic imaging
with almost equal resolution in all planes. The speed of MDCT
scanning offers faster scanning and results in fewer motion
artifacts. Finally, MDCT has a lower rate of inter-observer
variability and a lower error rate than does pulmonary angiography
for the assessment of PE in segmental and subsegmental vessels.
Currently available 16-slice MDCT scanners are used to acquire
images of the thorax after administration of nonionic intravenous
contrast media. The scanning is best performed in a caudocranial
direction since PE are more common in the lower lobes,
and images are less likely to be affected by breathing artifacts if
the scan is started from the caudal direction. The field of view is
the widest rib-to-rib distance acquired during a breath-hold after
inspiration. With the 1.25-mm detector thickness and submillimeter
reconstruction intervals available on a 16-slice CT scanner (Table
1), there is an excellent chance of detecting small emboli and
greater interobserver agreement in reading the PE studies.
An 18- to 20-gauge antecubital line or a central catheter is
ideal for contrast injection. With the shorter scan durations
offered by MDCT, an effective approach for optimal contrast
opacification is to reduce the contrast volume, increase the
injection rate, and increase the contrast medium concentration.
Contrast is ideally administered with the patient's arm
alongside the thorax, which offers physician control of the venous
access during the injection and avoids venous compression at the
The patients receive an injection of 100 to 140 mL of 270 to 360
mgI/mL iodinated nonionic contrast media at a rate of 3 to 4
mL/sec. Bolus tracking yields more homogeneous enhancement than
does the test bolus technique.
In one study, the use of an isosmolar contrast agent for MDCT
angiography to exclude PE did not significantly improve enhancement
quality compared with the use of a low-osmolar contrast agent.
To interpret the enormous number of thin slices acquired as part
of an MDCT PE study and evaluate small vessels in multiple planes,
a satellite console with a computer workstation is essential. In
our institution, we magnify and evaluate each lung separately
(Figure 1). Pulmonary vessels are best evaluated by scrolling every
segmental branch in and out from its origin to distal visible
branch. Images are displayed with 3 different gray scales for
interpretation of lung window (window width/level [HU] =
1500/-600), mediastinal window (350/40), and PE-specific (700/90)
The postprocessing of MDCT data sets is best termed "volume
In the era of MDCT, axial image review alone is rapidly becoming
not only impractical, but also suboptimal. Using radial multiplanar
reformatting of the MDCT data sets offers significant improvement
in the recognition of subsegmental PE.
Reformatted images through the longitudinal axis of a vessel are
often used to overcome various difficulties encountered with axial
sections of oblique- or axial-oriented arteries. These images help
to differentiate true PE and a variety of patient-related,
technical, anatomic, and pathologic factors (Figure 2) that can
Multiplanar images are also beneficial as communication tools for
our referring colleagues and can enhance the role of the
radiologist as part of the clinical team.
For documentation and interpretation purposes, it can be useful
to subjectively grade the level of contrast opacification as good
to excellent, diagnostic but of limited value, or not of diagnostic
quality, since it reflects clinical situations. In our departments,
we grade the opacification objectively as excellent, good, or poor,
depending upon the visualization of small peripheral arteries,
segmental arteries, and only central main and lobar arteries,
respectively. It is important to also make note of respiratory
motion and image noise.
CT signs of PE
Acute small PE
There are a number of very useful signs on MDCT that can help to
identify acute small PE.
Figure 3 illustrates complete arterial occlusion and Figure 4
depicts an artery that appears enlarged compared with adjacent
patent vessels. A partial arterial occlusion may appear as a
"donut" sign on images ac-quired perpendicular to the long axis of
a vessel (Figure 5A) or as the "tram track" sign on longitudinal
images of the vessel (Figure 5B). Figure 6 provides an example of a
peripheral intraluminal filling defect that formed an acute angle
with the arterial wall. A small PE may also appear as a peripheral
wedge-shaped infarct (Figure 7), discoid atelectasis, or pleural
reaction. Infarcts, right ventricle dilatation, and
interventricular septal deviation are uncommon with small emboli.
In contrast to acute small PE, there are distinct CT signs that
may be seen in cases of chronic PE.
It may appear as a complete occlusion of a vessel that is smaller
than adjacent patent vessels or as the abrupt cutoff of small
arteries (Figure 8). Figure 9 presents an illustration of
peripheral intraluminal defects that form obtuse angles with the
vessel wall. Thickened irregular small arteries may be seen because
of recanalization with or without calcification. Dilated collateral
vessels, such as the bronchial or internal mammary arteries (Figure
8), may also indicate chronic PE. Indirect signs may be seen, such
as an enlarged central pulmonary artery with narrowing of
peripheral arteries (Figure 10A), mosaic perfusion (Figure 10B),
right heart enlargement with wall thickening, and bronchial
dilatation. In our experience, compared with CT findings in larger
vessels, webs and flaps are difficult to visualize in subsegmental
In some patients, an MDCT study may be contraindicated because
of the lack of available venous access, renal failure, or contrast
allergy. Also, artifacts and mimics should be identified to prevent
misdiagnosis (Table 2). Respiratory motion is the most common cause
of artifacts in MDCT PE studies. Such artifacts are best recognized
on lung window settings as the blurring of vessels and rapid change
in the position of vessels on contiguous images (Figure 11). With
faster imaging in MDCT, motion artifacts are less problematic.
Imaging in large patients is frequently limited by excessive
quantum mottle, which leads to image noise (Figure 12). The
protocol can be modified by increasing detector width and kVp.
However, this increased detector width also decreases sensitivity
for the detection of PE caused by decreased spatial resolution.
Lower lobe flow-related artifacts caused by admixture of blood and
contrast material can result in poor vessel opacification at
segmental and subsegmental levels (Figure 13A). A flow-related
artifact can be confidently diagnosed by identifying its
This may become pronounced in cases of suboptimal timing of
contrast bolus or sluggish flow resulting from underlying vascular
congestion, such as in cases of congestive cardiac failure.
Peripheral increase in vascular resistance can be caused by
underlying lung consolidation, atelectasis, or peripheral
The poorly enhanced blood within the affected vessel may mimic PE
(Figure 13B). A repeat CT with an increased delay time may be
required to clarify the inconclusive findings. Mucus-filled bronchi
may show peripheral wall enhancement related to inflammation and
can mimic acute PE (Figure 14). The true nature of the artifact may
be determined by identifying the normally enhancing accompanying
pulmonary artery or viewing the bronchus on contiguous images.
Edema caused by congestive heart failure can produce
perivascular interstitial thickening, which mimics chronic
pulmonary embolism. Other CT signs of heart failure (including
ground-glass attenuation, diffuse interlobular septal thickening,
third spacing of fluid, and bilateral pleural effusions) help in
diagnosis. With a 1.25-mm detector width, there is a reduction in
partial volume averaging of lymphatic tissue and vessel that can
simulate PE. Lymph nodes can be easily distinguished from PE by
their extramural location and the normal smooth contour of the
Multiplanar reformatted images also help in difficult cases. As the
images are obtained in the early arterial phase, spurious filling
defects may be seen within the pulmonary veins. Pulmonary arteries
are accompanied by bronchi, whereas pulmonary veins run separately
within interlobular septa. In addition, pulmonary veins may be
followed se-quentially to the left atrium (Figure 15).
The appropriate window width and level settings are important
for identifying small emboli, webs, or flaps.
Pulmonary-embolism-specific or bone windows are helpful, especially
near the hila, as very bright vessel contrast can obscure small PE
(Figure 16). Partial volume artifacts may result in an apparent
filling defect that mimics acute PE. A stairstep artifact consists
of parallel lines on reformatted images and is accentuated by
cardiac and respiratory motion. These artifacts are less apparent
with overlapping reconstructions on MDCT.
The advent of multidetector row spiral CT has greatly expanded
our diagnostic capabilities in patients who are suspected of having
PE. Multidetector CT is an ex-citing modality for the diagnosis of
small segmental and subsegmental PE. When evaluating small
intravascular filling defects, radiologists must be familiar with
relevant CT artifacts and imaging pitfalls to prevent