AJR 2003; 180:1661-1664
© American Roentgen Ray Society
Six Cases of Acute Central Pulmonary Embolism Revealed on Unenhanced Multidetector CT of the Chest
Jeffrey P. Kanne1,
Michael B. Gotway2,
Nisa Thoongsuwan3 and
Eric J. Stern3
1 Department of Radiology, University of Washington Medical Center, University
of Washington School of Medicine, 1959 N.E. Pacific St., Box 357115, Seattle,
WA 98195-7115.
2 Department of Radiology, Thoracic Imaging Section, San Francisco General
Hospital, University of California, San Francisco, 1001 Potrero Ave., Rm. 1X
55A, Box 1325, San Francisco, CA 94110.
3 Department of Radiology, Harborview Medical Center, University of Washington
School of Medicine, 325 Ninth Ave, Box 359728, Seattle, WA 98104-2499.
Received September 23, 2002;
accepted after revision November 13, 2002.
Address correspondence to E. J. Stern
(estern{at}u.washington.edu).
Abstract
OBJECTIVE. Our purpose was to describe the imaging findings of
central pulmonary embolism on unenhanced multidetector CT (MDCT) of the
chest.
CONCLUSION. Unenhanced MDCT of the chest is often performed for the
evaluation of nonspecific chest symptoms. Awareness of the rare finding of a
high-attenuation centrally located pulmonary embolism on unenhanced MDCT is
important because acute pulmonary embolism may be identified when it is not
suspected clinically, and such detection can determine further imaging needs
and allow the timely initiation of appropriate therapy.
Introduction
Despite recent advances in CT, the diagnosis of acute pulmonary embolism
can still be challenging for both clinicians and radiologists.
Contrast-enhanced multidetector CT (MDCT) of the chest has become an accepted
technique for the definitive evaluation of suspected acute pulmonary embolism
at many institutions. The most reliable criterion for the diagnosis of acute
pulmonary embolism on MDCT is the visualization of an intraluminal filling
defect surrounded by IV contrast material
[1]. To our knowledge, only one
case of acute pulmonary embolism visualized on unenhanced CT of the chest has
been reported in the peer-reviewed medical literature
[2]. Given that patients
undergo unenhanced CT of the chest for various cardiopulmonary symptoms, we
believe that awareness of this finding may help in detecting acute pulmonary
embolism when it is not suspected clinically. Such detection may be critical
for determining further imaging needs or instituting appropriate treatment in
a timely manner. We present five cases of acute central pulmonary embolism
detected on MDCT pulmonary angiography in which the high-attenuation central
thrombus was visualized retrospectively on unenhanced images and a sixth case
in which it was identified prospectively during data analysis.
Materials and Methods
After obtaining human subjects committee approval, we performed a
retrospective radiology information system review of reports from all CT
pulmonary angiograms obtained to evaluate acute pulmonary embolism at our two
major institutions, one a tertiary referral medical center and the other a
major urban trauma center. Only studies performed since the installation of
MDCT scanners were included, a period of approximately 2 years concurrent with
increased use of CT for evaluation of suspected acute pulmonary embolism.
All studies were performed on LightSpeed QX/i MDCT scanners (General
Electric Medical Systems, Milwaukee, WI), and the scanning protocol at the
time included both unenhanced and enhanced scans. Parameters varied among the
unenhanced examinations, with slice thickness ranging from 5-10 mm, but
contrast-enhanced studies were all performed with 2.5-mm detector collimation.
Central pulmonary emboli were defined as intraluminal filling defects
visualized in the main, right, or left pulmonary arteries. One radiologist
reviewed official radiology reports for the diagnosis of acute pulmonary
emboli, and images showing central acute pulmonary emboli were then viewed on
a PathSpeed PACS (picture archiving and communication system) workstation
(General Electric Medical Systems). In cases in which a high-attenuating
central clot was present on unenhanced images, average CT attenuation values
were measured for the clot and for the blood pool in the main pulmonary
artery, and a medical chart review was performed.
Results
Four hundred twenty-four pulmonary CT angiograms were obtained for acute
pulmonary embolism, and one CT angiogram was obtained for both aortic
dissection and acute pulmonary embolism. Eighty cases with findings positive
for acute pulmonary embolism were identified, 11 of which had findings of
central clots. Review of the contrast-enhanced and unenhanced images showed
that five (45% of scans with central clots, 6% of all scans positive for
pulmonary embolism) had a high-attenuating clot or clots detected on the
unenhanced scans that conformed in shape and location to the clots seen on the
contrast-enhanced scans. While these data were being analyzed, a sixth case of
acute, high-attenuation central pulmonary embolism was identified
prospectively. These six patients were the basis of this report.
The study group included one woman and five men with a mean age of 57 years
(range, 43-67 years), all of whom had risk factors for developing acute
pulmonary embolism, including malignancy, deep venous thrombosis, recent
surgery, or prolonged immobilization.
CT findings included two cases with high-attenuation clots in the main
pulmonary artery, one case with high-attenuation clot in the right pulmonary
artery, and three cases with high-attenuation clots in both the left and right
pulmonary arteries (Figs. 1A,
1B,
2A,
2B,
3A,
3B,
4A,
4B). High-attenuation clots
were best seen with narrow window settings (width, 158-349 H; level, 15-50 H),
although no specific window setting was consistently used. Attenuation values
of the clots ranged from 53 to 77 H (mean, 65.8 H), and blood pool-attenuation
values ranged from 25 to 41 H (mean, 36.3 H). The smallest difference in
attenuation between clot and blood pool was 17 H, and the largest was 45 H.
Table 1 shows measurements in
detail, patient age, and hematocrit level at the time of imaging.
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Fig. 1A. –67-year-old man with syncopal episode 1 month after
hemicolectomy for colon adenocarcinoma. Unenhanced multidetector CT (MDCT)
scan shows bilateral central high-attenuation emboli (arrows).
Average CT attenuation is 70 H.
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Fig. 2A. –63-year-old man with diabetes, congestive heart failure, and
recent immobilization. Unenhanced multidetector CT (MDCT) scan shows
high-attenuation emboli in right main pulmonary artery (asterisk).
Average CT attenuation is 77 H.
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Fig. 4A. –65-year-old woman with Stanford type A aortic dissection.
Unenhanced multidetector CT (MDCT) scan shows bilateral high-attenuation
central pulmonary emboli (white arrows) and aortic intimal flap
(black arrow).
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Discussion
Contrast-enhanced helical CT is well established as the initial definitive
imaging study after chest radiography at many institutions
[3]. Visualization of complete
or partial intraluminal filling defects surrounded by the contrast-enhanced
blood pool in the pulmonary arteries is a direct sign of pulmonary embolism
[4]. Unenhanced MDCT of the
chest is performed for various reasons at our institutions. These reasons
include patients with allergies to iodinated contrast material or with
elevated serum creatinine levels; patients undergoing aortic dissection,
pulmonary embolism, and high-resolution CT protocols; and patients with
nonspecific cardiopulmonary signs and symptoms. Although not performed in many
centers, unenhanced imaging as part of the CT pulmonary angiography protocol
is useful for two reasons. First, it allows evaluation of the lung parenchyma,
pleura, and chest wall. Second, identification of calcified lesions such as
hilar lymph nodes or calcified thrombi is possible. The former may interfere
with interpretation of contrast-enhanced images, and the latter may go
undetected on contrast-enhanced studies
[1]. Indirect signs associated
with acute pulmonary embolism include a wedge-shaped subpleural consolidation,
dilated central or segmental pulmonary arteries, oligemia, and pleural
effusion [5], all of which can
be detected on unenhanced CT but may be nonspecific. Our study indicates that
some central pulmonary emboli are detectable on unenhanced scans. Further
research is necessary to determine the sensitivity and specificity of
unenhanced MDCT for pulmonary thromboemboli.
The prevalence of acute pulmonary embolism diagnosed on MDCT of the chest
on which the thrombus is visualized before administration of IV contrast
material is unknown. To our knowledge, direct visualization of an acute
pulmonary embolism on unenhanced MDCT of the chest has been reported in the
peer-reviewed literature in only one case
[2]. We retrospectively
identified five patients with high-attenuation clots among 11 patients with
central acute pulmonary emboli, and one patient with this finding was
identified prospectively during data analysis. Recognition of a
high-attenuation intraluminal thrombus may allow the diagnosis of acute
pulmonary embolism to be made in patients undergoing unenhanced MDCT of the
chest for other suspected conditions or in cases in which injection or
scan-timing errors render a CT pulmonary angiogram suboptimal.
Visualization of the clot is likely related to the age of the clot, the
patient's hematocrit level at the time of imaging, or a combination of the
two. Five of our six patients had subnormal hematocrit levels at the time of
imaging (normal range at our institutions, 38-50). Attenuation of unenhanced
blood is hematocrit-dependent and ranges from 20-30 H
[6]. As a thrombus retracts,
its water content decreases, concentrating the hemoglobin, and subsequently
raising the CT attenuation values of the thrombus to 50-80 H
[7]. A high-attenuation
extraluminal collection is an acknowledged CT manifestation of a hematoma
somewhere in the body, including in the peritoneal cavity after trauma
[8], in the brain parenchyma
and extraaxial space [9], and
in the adrenal gland [10].
The small number of cases and variation in scanning parameters of
unenhanced images, and the retrospective nature of this evaluation, limit our
study. Many patients with large central clots experience sudden death; thus,
many patients who survive central thromboembolism may have smaller,
nonobstructing clots that may not be well visualized on thicker collimation
unenhanced CT because of the combined effects of poor contrast resolution and
partial volume averaging.
Because unenhanced MDCT of the chest is performed for evaluation of
patients with cardiopulmonary symptoms, awareness of the appearance of acute
pulmonary embolism on unenhanced multidetector CT of the chest becomes
important because it may be the unsuspected cause of the patient's signs and
symptoms.
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Abstract
Introduction
