Dr. Reittner
and
Dr. Müller
are with the Department of Radiology, Vancouver General Hospital,
University of British Columbia, Vancouver, BC, Canada.
P
ulmonary embolism (PE) is a frequent cause of morbidity and
mortality.
1
It has been estimated that 650,000 cases of PE occur each year in
the United States, with approximately 50,000 to 100,000 deaths
resulting.
2
Clinically, the diagnosis of PE is often difficult to establish,
because the symptoms are nonspecific and may be confused with a
variety of other cardiopulmonary disorders.
1,2
Goldhaber et al
3
reported that only 16 (30%) of 54 patients with PE at autopsy had a
correct antemortem diagnosis.
For many years, ventilation-perfusion (V/Q) scintigraphy has
been the main imaging modality used in the evaluation of patients
with suspected PE.
1,4,5
A high probability V/Q scan provides sufficient certainty to
confirm the diagnosis of PE, while a normal or near normal scan
reliably excludes the diagnosis.
5
However, in the PIOPED (Prospective Investigation of Pulmonary
Embolism Diagnosis) study, indeterminate scans, present in 39% (364
of 931) of patients, showed a 30% incidence of PE; and
low-probability scans, seen in 34% (312 of 931) of patients, had a
14% incidence. Based on these data, the authors concluded that
indeterminate and low probability lung scans (i.e., two-thirds of
V/Q scans in the PIOPED study) were not useful in establishing or
excluding PE.
5
Furthermore, although there was good interobserver agreement for
high-probability and normal V/Q scans, there was a 25% to 30%
disagreement between observers in the interpretation of
intermediate and low-probability scans.
5
Currently, pulmonary angiography is considered the gold standard
for diagnosing PE.
1
Pulmonary angiography allows direct visualization of the pulmonary
arterial tree and detection of filling defects, typical for PE.
However, it is an invasive test with associated morbidity (6%) and
mortality (0.5%) and is underutilized.
6
It has been estimated that even in academic centers only 12% to 14%
of patients with nondiagnostic V/Q scan undergo pulmonary
angiography.
6,7
Although incidental detection of PE has been described with
conventional CT, slow data acquisition time precluded the inclusion
of conventional CT in diagnostic algorithms for the diagnosis of
PE.
8,9
With the introduction of spiral CT technology, it is now possible
to image the entire chest in a short period of time and analyze the
pulmonary arteries during the peak of contrast enhancement. Several
studies have demonstrated that spiral CT has a high sensitivity and
specificity for the diagnosis of PE.
9-15
The aim of this article is to review a practical approach for
the use of spiral CT in diagnosing pulmonary embolism and to
illustrate the characteristic imaging features.
Technique
Optimal assessment of the pulmonary vessels on spiral CT
requires careful attention to several parameters including scan
collimation, imaging volume, and contrast enhancement. We currently
use the following protocol: spiral CT during a 20 to 30 second
breath-hold using 3-mm collimation, a table speed of 5 to 6 mm/sec,
pitch 1.7 to 2.0, 120 kVp, and 180 to 320 mA. Images are
reconstructed at 1.5-mm intervals and a field-of-view appropriate
for the size of the patient.
The lung volume scanned should be large enough to include all
segmental and subsegmental pulmonary arteries. This can be achieved
by scanning from the top of the aortic arch to the dome of the
diaphragm. Although the scans can be performed in the craniocaudal
direction, it has been shown that scanning caudocranially helps to
minimize motion artifacts, particularly in patients unable to hold
their breath for the duration of the scan.
16,17
Non-ionic iodinated contrast material is administered through an
antecubital venous access or a central line using a power injector.
16,17
Injection rates from 2 to 7 mL/ sec have been reported.
4
We use 120 to 150 mL of 30% iodinated contrast material, injected
at a rate of 4 mL/sec. In hemodynamically stable patients, a 10- to
15-second scan delay provides optimal contrast enhancement of the
pulmonary arteries. This delay may need to be increased in patients
with severe pulmonary hypertension or right-sided heart failure. In
order to determine the optimal time delay, we perform a test
injection to assess the circulation time. A total of 20 mL of
contrast material is injected at a rate of 4mL/sec and serial scans
are performed at 3 to 5 second intervals for 20 seconds at the
level of the main pulmonary artery. To ensure opacification of the
peripheral arteries during the diagnostic study, 5 seconds are
added to the time to peak enhancement of the main pulmonary artery.
16,17
Images are viewed at settings for pulmonary vasculature (window
width, 250 HU; window level, 35 HU) and lung parenchyma (window
width, 1,500 HU; window level, -700 HU). In selected cases,
multiplanar reformatting may be helpful in demonstrating the extent
of the pulmonary emboli.
17
Image interpretation
Knowledge of the bronchovascular anatomy is essential for
correct assessment of the images. The segmental arteries are
located adjacent to the corresponding branches of the bronchial
tree and are situated either medially (upper lobes) or laterally
(lower lobes, lingula, middle lobe) to the bronchi.
18,19
Characteristic findings of acute PE are: 1) partial central or
marginal filling defect surrounded by a thin rim of contrast
material (figures 1 and 2); or 2) complete filling defect with
obstruction of an entire vessel section ("vessel cut-off sign,"
figure 3). Pulmonary arteries completely obstructed by an acute
embolus usually have an increased diameter. Diagnosis of acute PE
requires assessment of both the vascular and parenchymal findings.
Assessment of the lung windows is important, not only to identify
the pulmonary arteries by their proximity to the bronchi, but also
to assess for the presence of ancillary signs that may be helpful
in suggesting the presence of pulmonary embolism.
20
The most helpful ancillary sign is the presence of a non-enhancing
pleural-based wedge-shaped pulmonary opacity.
20
Linear opacities, presumably representing plate-like atelectasis,
are also seen with increased frequency on CT in patients with acute
PE.
20
In a study of 98 patients assessed due to clinical suspicion of
acute PE, non-enhancing pleural-based wedge-shaped opacities were
seen on CT in 62% of patients with pulmonary embolism compared to
27% of patients without PE, and linear opacities were seen in 46%
and 21% of patients, respectively.
20
Other findings, such as areas of decreased attenuation and pleural
effusion, were found with equal frequency in patients with and
without acute PE.
Findings suggestive of chronic PE include: 1) filling defects
with obtuse angles to the vessel wall (figure 4); 2) irregularity
and narrowing of the arteries; 3) evidence of recanalization of the
thrombus; and 4) presence of linear filling defects ("arterial
webs," figure 5). In patients with chronic PE, occlusion of small
pulmonary arteries leads to blood flow redistribution to uninvolved
vessels and a mosaic pattern of attenuation and perfusion on CT
(figure 6).
Interpretive pitfalls and artifacts
A number of technical, anatomical, and patient-related pitfalls
may lead to misinterpretation of the CT images. Technical failures
occur in 1% to 5% of scans, and usually are due to motion artifacts
in dyspneic patients or insufficient vascular enhancement.
10
In patients with severe dyspnea, motion artifacts can produce
respiratory misregistration and inadequate sampling of the
pulmonary vessels, resulting in focal areas of decreased
attenuation which can mimic a clot.
Streak artifacts originating from dense intravenous contrast
within the superior vena cava may obscure the right main and upper
lobe pulmonary arteries. These streak artifacts are frequently
observed when using a craniocaudal image acquisition and highly
concentrated contrast material. These artifacts can be minimized or
eliminated by reducing the iodine concentration or scanning
caudocranially.
16,21
The lymphatic and connective tissue located adjacent to the
pulmonary arteries may mimic the appearance of pulmonary emboli.
This pitfall can be minimized by careful review of the images and
the use of additional imaging rendering tools such as cine-viewing
(which we use routinely) and multiplanar reconstructions.
Diagnostic accuracy of spiral CT
The reported diagnostic accuracy of spiral CT has varied
depending on the technique used, the patient population, and
whether the authors have limited the analysis to the central
pulmonary arteries down the level of the segmental vessels or have
included subsegmental arteries.
9-15
The results of the various studies are summarized in Table 1. In a
study of 139 patients in whom the CT scans were assessed
prospectively, the sensitivity of spiral CT for the detection of
pulmonary embolism was 87% and the specificity was 95%.
13
Isolated subsegmental pulmonary emboli accounted for two of the six
false-negative spiral CT scan interpretations. Recently, a
presentation of preliminary data from
the prospective European Multicentre Study (ESTIPEP), which
included 391 patients, demonstrated a sensitivity of spiral CT of
95% and a specificity of 97%.
15
Other investigators have reported lower sensitivities for spiral
CT, however, particularly in studies in which CT scans were
performed only in patients with indeterminate V/Q scans. In one
study, the authors compared spiral CT with pulmonary angiography in
20 patients with indeterminate V/Q scans.
10
When only the central arteries were analyzed, the sensitivity of CT
was 86% and the specificity 92%. When subsegmental vessels were
included, the sensitivity of CT decreased to 63%. Similar results
were reported in a study of 54 patients with indeterminate V/Q
scans.
14
These results indicate that although spiral CT has a high
sensitivity in the detection of central emboli, it is of limited
value in the diagnosis of subsegmental emboli. It should be noted,
however, that the clinical significance of isolated subsegmental
emboli, especially in patients with no underlying disease, is
controversial.
5
Furthermore, it has been shown that even though pulmonary
angiography is considered the gold standard for the diagnosis of
pulmonary embolism, the interobserver agreement for the diagnosis
of subsegmental emboli on angiography is only 66%.
5
Preliminary results suggest that the accuracy of spiral CT in the
diagnosis of subsegmental emboli will be improved with the use of
thinner sections such as 1 or 2 mm collimation.
17
Diagnostic algorithm
Given the data in the literature, the following algorithm is
recommended for the evaluation of patients suspected of having
acute pulmonary embolism
22
: 1) All patients should have a chest radiograph, the main role of
which is to exclude abnormalities, such as acute pneumonia, that
may mimic pulmonary embolism clinically; 2) Patients with symptoms
or signs of deep vein thrombosis should undergo evaluation of the
leg veins, the most recommended technique being Doppler ultrasound.
If Doppler is positive, the patient can be considered to have acute
pulmonary embolism and usually does not require further
investigation; 3) Patients who have no symptoms or signs of deep
vein thrombosis and symptomatic patients who have a negative
Doppler ultrasound examination, and who do not have extensive
underlying parenchymal lung disease or COPD, should undergo
ventilation-perfusion scintigraphy. A high-probability or normal
V/Q scan can
be considered diagnostic. All other patients should undergo further
evaluation with contrast enhanced spiral CT; 4) Patients who have
extensive pulmonary parenchymal disease or COPD and patients who
have non-diagnostic V/Q scan should undergo contrast-enhanced
spiral CT; and 5) Patients in whom the CT scans are suboptimal and
patients in whom the CT scan results are negative, but who have a
high clinical index of suspicion for acute pulmonary embolism,
should undergo pulmonary angiography.
Conclusion
Several studies have shown that spiral CT can play a major role
in the diagnosis of pulmonary embolism. Optimal assessment requires
knowledge of anatomy, careful attention to technique, and awareness
of potential diagnostic pitfalls.
AR
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