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1
Department of Radiology, Unité d'Imagerie
Thoracique et Cardiovasculaire, CHU Bordeaux,
Hôpital Cardiologique
Haut-Lévêque,
Ave. de Magellan, 33604 Pessac, France.
2
Department of Nuclear Medicine, CHU Bordeaux,
Hôpital
Haut-Lévêque,
33604 Pessac, France.
3
Department of Cardiology, CHU Bordeaux, Hôpital
Haut-Lévêque,
33604 Pessac, France.
4
Department of Pneumology, CHU Bordeaux, Hôpital
Haut-Lévêque,
33604 Pessac, France.
5
Laboratoire de physiologie cellulaire respiratoire, INSERM E 9937,
Université Victor
Ségalen, Bordeaux 2, France.
Received June 25, 1999;
accepted after revision September 14, 1999.
Address correspondence to F. Laurent.
Abstract
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SUBJECTS AND METHODS. Two hundred sixteen consecutive patients who were clinically suspected of having acute pulmonary embolism underwent helical CT angiography, ventilation—perfusion radionuclide lung scanning, and Doppler sonography of the veins of the legs. On the basis of concordance of the results for ventilation—perfusion radionuclide lung scanning and helical CT angiography and on the degree of clinical suspicion, certain patients underwent pulmonary angiography. Patients without pulmonary embolism at initial evaluation in whom no treatment was instituted were followed up for at least 3 months to determine the potential recurrence of thromboembolic disease.
RESULTS. Of the 216 patients, 37 (17%) were excluded because of
insufficient data to assess the initial event. Final diagnosis for the 179
remaining patients was pulmonary embolism in 68 (37.9%) and no pulmonary
embolism in 111 (62.0%), based on pulmonary angiography in 23 patients (12.8%)
and concordant imaging findings and outcome in the remaining patients.
Statistically significant differences (p < 0.05) were found
between sensitivity, specificity, positive predictive value, and negative
predictive value for helical CT angiography and ventilation—perfusion
radionuclide lung scanning (94.1% versus 80.8%; 93.6% versus 73.8%; 95.5%
versus 82%; and 96.2% versus 75.9%, respectively). Interobserver agreement was
excellent for helical CT angiography (
= 0.72) and moderate for
ventilation—perfusion radionuclide lung scanning ( = 0.22).
CONCLUSION. Helical CT angiography could replace ventilation—perfusion radionuclide lung scanning as the initial test for screening patients who are clinically suspected of having pulmonary embolism.
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Ventilation—perfusion radionuclide lung scanning is the most frequently performed noninvasive imaging study for the diagnosis of pulmonary embolism. A scan showing a normal or low probability has a high negative predictive value when the clinical suspicion of pulmonary embolism is low, and a high-probability scan has a high positive predictive value when the clinical suspicion is high. Unfortunately, only 34% of cases correspond to these two categories [7]. In addition, large differences (25-30%) in interpretation among expert observers have been reported, especially in the classification of low- or intermediate-probability scans [8]. The latter results necessitate further investigation to exclude or confirm pulmonary embolism [9]. Because deep venous thrombosis and pulmonary embolism are manifestations of the same process, noninvasive exploration of the veins has been included in algorithms for the diagnosis of pulmonary embolism [10]. Results in a large outcome-based study showed that the combination of ventilation—perfusion and noninvasive studies of the lower extremities enabled identification of 71% of patients who needed anticoagulation [11]. Noninvasive exploration of the legs, however, helps detect deep venous thrombosis in only approximately 50% of patients with angiographically proven pulmonary embolism [12].
At our institution, the practice before the advent of helical CT angiography was to perform both ventilation—perfusion radionuclide lung scanning and Doppler sonography as a first line for diagnosing pulmonary embolism. Recent reports have shown the value of helical CT angiography for diagnosing acute pulmonary embolism [13,14,15,16]. Nevertheless, the use of helical CT angiography as a screening test has not been extensively investigated, although some studies have suggested its potential role [17, 18]. To test the practice of replacing ventilation—perfusion radionuclide lung scanning with helical CT angiography as the first examination in patients suspected of having a pulmonary embolism, we undertook a prospective study in patients hospitalized in our institution. Ventilation—perfusion radionuclide lung scanning, CT angiography, and Doppler sonography of the lower extremity veins were performed within the first 48 hr of clinical presentation. Clinicians were free to decide who needed pulmonary angiography according to the results of these tests and the degree of clinical suspicion for pulmonary embolism.
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All patients underwent ventilation—perfusion radionuclide lung scanning, contrast-enhanced helical CT angiography, and Doppler sonography of the legs within 48 hr of clinical presentation. Initial results of CT angiography, ventilation—perfusion radionuclide lung scanning, and Doppler sonography were initially interpreted independently and tabulated. After initial interpretation, the need for pulmonary angiography was determined by the referring physician depending on the degree of suspicion of pulmonary embolism, Doppler sonography results, and concordance of the results of helical CT angiography and ventilation—perfusion radionuclide lung scanning.
Helical CT Angiography
CT scans were acquired with a Somatom 4+S (Siemens, Erlangen, Germany)
scanner. A contrast-enhanced CT evaluation of the pulmonary arteries was
performed from the level of the aortic arch to at least 2 cm below the level
of the pulmonary veins. Scans were acquired during suspended inspiration or
shallow breathing, depending on the patient's ability to hold his or her
breath during the acquisition time. Technical parameters included 3-mm
(n = 82) or 2-mm (n = 134) collimation, 1.8-2.0 pitch, 120
kV, 170 mA, and 0.75-sec scan time. The choice of collimation was made
according to the patient's capacity to breath-hold. Images were reconstructed
at 2-mm intervals using a standard algorithm and a field of view adapted to
the patient's size. Contrast material was injected at 4-5 ml/sec with a power
injector (Medrad, Pittsburgh, PA) through an 18- to 20-gauge catheter in the
antecubital fossa. The injected arm was placed at the patient's side to
eliminate kinking of the subclavian vein at the thoracic inlet during
injection, and the other arm was placed above the patient's head. A total
volume of 120-150 ml of the nonionic contrast material iohexol 240 (Omnipaque
240; Nycomed Ingenor, Paris, France) was injected. A timing bolus was not
used, and scanning began 12-15 sec after the initiation of injection. The scan
delay and arm position allowed direct visualization of the IV site of the
first phase of injection and minimized the risk of interstitial injection.
Images were viewed at settings for pulmonary vasculature (window width,
350-400 H; window level, 50 H) and lung parenchyma (window width, 1200 H;
window level, -700 H) on hard copies. The entire examination could be reviewed
on a workstation. Images were assessed initially by one of the two experienced
observers. The presence or absence of an occlusive or nonocclusive clot in the
main, lobar, segmental, and subsegmental arteries was recorded on a study data
sheet. Helical CT studies were categorized as positive for pulmonary embolism
if a clot was observed; negative for pulmonary embolism if no clot was
observed; and indeterminate if poor examination, inadequate enhancement, or
motion artifacts precluded confident interpretation of the study. Acute
pulmonary embolism was diagnosed if a normal-sized or enlarged pulmonary
artery was obstructed completely by a nonenhancing thrombus, or if
nonocclusive filling defects were apparent centrally in the vessel. The
initial interpretation was used to assess the need for further investigations.
Subsequently, the images were independently interpreted by the second
radiologist for calculation of interobserver variation.
Ventilation—Perfusion Radionuclide Lung Scanning
Ventilation studies were performed after inhalation of Technegas (Tetley
Manufacturing, Sydney, Australia) (xenon-133 gas). Six images were acquired
with a minimum of 200,000 counts per incidence. Perfusion studies were
performed after IV administration of 3-5 mCi (111-185 MBq) of
99mTc-labeled macroaggregated serum albumin. Perfusion images were
acquired with a minimum of 400,000 counts per view in six projections.
Ventilation—perfusion studies were performed with a large-field-of-view
gamma camera equipped with a high-resolution collimator. All patients
underwent chest radiography on the same day as ventilation—perfusion
radionuclide lung scanning. The ventilation—perfusion scans were
interpreted by the nuclear medicine physician on service, and results were
tabulated using the original and revised criteria of the Prospective
Investigation of Pulmonary Embolism Diagnosis (PIOPED)
[8]. As with the initial
helical CT angiography, the initial interpretation was used to determine the
need for pulmonary angiography. The scans were later interpreted independently
by a second observer (who was unaware of the first interpretation result and
the final diagnosis) at the same institution to assess interobserver
variation.
Doppler Sonography
All sonographic examinations were performed by experienced radiologists or
cardiologists using a Doppler sonography scanner (Elegra; Siemens) and 7.5-
and 3.5-MHz linear display probes. The veins of both legs were examined with
color or duplex sonography from the calf to the inferior vena cava. The
criterion for deep venous thrombosis was the presence of an intraluminal
thrombus, incomplete compressibility of the veins, or both.
Pulmonary Angiography
When indicated, selective pulmonary angiography was performed with digital
recording. In all patients, at least four projections were acquired in the
left posterior oblique and the right posterior oblique projections. A pigtail
catheter was placed selectively in the right or left pulmonary artery, and
contrast material was injected at 20-25 ml/sec to a total of 30-50 ml. A total
of 150-200 ml of iohexol (Omnipaque 350; Nycomed Ingenor) or ioversol (Optiray
320; Guerbet, Aulnay-sous-Bois, France) was used. Angiograms were interpreted
by an experienced angiographer. Acute embolism was diagnosed if a persistent
intraluminal filling defect or a vascular cutoff of a pulmonary artery was
seen. Subsequently, the images were independently examined by the second
angiographer for calculation of interobserver variation.
Clinical Follow-Up and Outcome
Patients with a diagnosis of pulmonary embolism were treated with
anticoagulant therapy and followed up clinically at 3 and 6 months. Patients
with a negative diagnosis of pulmonary embolism were followed up to determine
whether a recurrence of pulmonary embolism or of a venous thromboembolic event
had occurred. All these patients had a follow-u8p visit at 3 months and their
medical files were checked.
Gold Standard for Diagnosis of Pulmonary Embolism
The initial event was considered to be a pulmonary embolism when pulmonary
angiography was positive; when helical CT angiography,
ventilation—perfusion radionuclide lung scanning, and Doppler sonography
were concordant and led to anticoagulation therapy; or when recurrence of a
thromboembolic event occurred during follow-up in a patient whose blood had
not undergone anticoagulation therapy. The initial event was not considered to
be pulmonary embolism when pulmonary angiography findings were negative or
when the clinical outcome did not reveal any recurrence of a thromboembolic
event in a patient whose blood had not undergone anticoagulation therapy.
Statistical Analysis
Differences in the diagnostic accuracy between ventilation—perfusion
scintigraphic and CT angiographic findings were assessed using Pearson's
chi-square coefficient. Statistical significance was set at the 0.05 level.
Interobserver agreement was calculated using the kappa statistic.
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The final diagnosis of pulmonary embolism was made in 68 patients (37.9%) on the basis of pulmonary angiography in 12 and outcome in 56. It was excluded in 111 patients (62.0%) on the basis of pulmonary angiography in 11 and outcome in 100. The mean duration of the follow-up period was 192 days (range, 124-479 days)
Results of the initial interpretation of helical CT angiography are detailed in Table 2. Helical CT angiography findings were considered positive in 64 of 68 patients with pulmonary embolism. Three patients were proved during follow-up to have a recurrence of pulmonary embolism. There were three false-positive helical CT angiography results according to pulmonary angiography results—two caused by a partial volume effect on horizontally oriented vessels and one caused by small hilar nodes mimicking a mural thrombus—and one indeterminate result. In five patients (2.8%), CT angiography results were interpreted as indeterminate, four without pulmonary embolism and one with pulmonary embolism. Sensitivity, specificity, positive predictive value, and negative predictive value of the first interpretation were 94.1% (95% confidence interval [CI], 88.5-99.7%), 93.6% (CI, 89-98.1%), 95.5% (CI, 90.6-100%), and 96.2% (CI, 92.6-99.8%), respectively. Sensitivity, specificity, positive predictive value, and negative predictive value of the second interpretation were 94.1% (CI, 88.5-99.7%), 95.0% (CI, 91.7-99.3%), 95.5% (CI, 90.6-100%), and 97.2% (CI, 94.1-100%), respectively. No difference was found between sensitivity and specificity for helical CT angiography in patients in whom 2- and 3-mm collimation was used. The single patient with a pulmonary embolism restricted to subsegmental arteries was observed with the 2-mm collimation protocol.
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Among patients with pulmonary embolism and positive findings on CT angiography, eight had thrombi in the main pulmonary arteries (right, left, or both), 39 had thrombi restricted to the lobar arteries (Fig. 5A,5B), and 32 had thrombi restricted to the segmental and subsegmental arteries. A single patient had thrombi restricted to the subsegmental arteries. A mean of 6.3 thrombi were detected (range, 1-22). Thrombi were located in the upper lobe in 21 patients, in the middle lobe or lingula in 26, and in lower lobes in 35. Seven patients had a pulmonary embolism limited to a single visible thrombus.
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Results of the initial interpretation of ventilation—perfusion scans are detailed in Table 2. Sensitivity, specificity, positive predictive value, and negative predictive value of the first interpretation were 80.8% (CI, 71.6-90.1%), 73.8% (CI, 65.7-81.9%), 82% (CI, 72.8-91.2%), and 75.9% (CI, 67.9-83.9%), respectively. Sensitivity, specificity, positive predictive value, and negative predictive value of the second interpretation were 76.5% (CI, 66-86.4%), 80.2% (CI, 72.7-87.5%), 77.6% (CI, 67.6-87.5%), and 82.4% (CI, 75.2-89.6%), respectively.
The sensitivity, specificity, positive predictive value, negative predictive value, and number of indeterminate findings of helical CT angiography were significantly higher (p < 0.05) than those of ventilation—perfusion radionuclide lung scanning.
Comparison of first interpretations of ventilation—perfusion radionuclide lung scanning and helical CT angiography showed concordant positive findings in 55 of 179 patients and concordant negative results in 19 of 179 patients (Table 3). In the 18 patients with intermediate probability ventilation—perfusion radionuclide lung scans, CT angiography enabled correct identification of six patients with pulmonary embolism and 12 without. Interpretation of ventilation—perfusion radionuclide lung scans and CT angiography was discordant in 105 cases. In the five patients with indeterminate findings on CT angiography, ventilation—perfusion radionuclide lung scans were of high probability in two and of low probability in three. This enabled correct identification in one patient with pulmonary embolism and in none of the four patients without pulmonary embolism.
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Among patients with pulmonary embolism and negative sonography results (n = 20 [29.4%]), there were 17 (85%) concordant findings on CT angiography and ventilation—perfusion scintigraphy and three (15%) discordant findings on first interpretations. Three patients had false-negative ventilation—perfusion radionuclide lung scan interpretation (two intermediate and one low probability), and none had a false-negative CT angiography interpretation.
Interobserver agreement was excellent for helical CT angiography ( =
0.72) and pulmonary angiography ( = 0.83) and moderate for
ventilation—perfusion radionuclide lung scanning ( = 0.22). No
disagreement occurred among CT interpretations concerning thrombus located in
lobar arteries (Fig.
5A,5B).
Reasons for disagreement between observers on helical CT angiography were
examinations showing a single thrombus of small size (n = 3),
examinations with several thrombi limited to segmental arteries but impaired
by motion artifacts (n = 5), partial volume averaging (Fig.
6A,6B)
on horizontally oriented pulmonary arteries (n = 4), and technically
suboptimal examinations that at least one of the observers considered
indeterminate (n = 12).
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An important problem in evaluating tests for diagnosing pulmonary embolism is the lack of a reference method when pulmonary angiography is not performed systematically. Concordance of initial tests when they show positive results for pulmonary embolism and event-free survival without the administration of anticoagulant medication were criteria used in patients who did not undergo pulmonary angiography. A few studies have used these references at least partly to assess the value of helical CT angiography [14,15,16,17,18,19,20,21,22,23,24]. A 3-month follow-up seems adequate because most deaths related to recurrent emboli occur within 2 weeks of diagnosis [2]. Nevertheless, if a patient had negative findings on imaging initially and was found to have pulmonary embolism a month later, that does not necessarily mean that the first test was false-negative, nor does it necessarily indicate a recurrence. It is equally possible that on day 1 and after 1 month the patient had the same high risk factor and, indeed, had the first embolus at a later date. So the rate of a thromboembolic event is an indirect indicator of the false-negative rate of a test. We found that three patients had a thromboembolic event in the months after the initial event. In the study of Mayo et al. [20], the conditions of two patients (3%) interpreted as negative on both helical CT angiography and ventilation—perfusion radionuclide lung scanning were proved subsequently to recur. In the study of Garg et al. [16], 28 patients were followed up clinically and did not experience a recurrence after negative findings on CT angiography. In the study of Ferretti et al. [21], three of the 112 patients without pulmonary embolism on the initial CT angiography experienced recurrent pulmonary embolism (with one death), and the false-negative rate was 5.4% (CI, 1.3-9.7%). Our rate of negative findings on helical CT angiography and recurrence is within this range of CI.
Our main result is the statistically significant improvement in diagnosing pulmonary embolism with helical CT compared with ventilation—perfusion radionuclide lung scanning. This has also been reported by Mayo et al. [20] and Cross et al. [17], who had different study designs. Cross et al. randomized 78 patients who underwent either CT angiography or ventilation—perfusion radionuclide lung scanning as an initial investigation. It was possible to make a confident diagnosis in a significantly larger proportion of patients when CT angiography was used as the initial investigation, 90% versus 54%, respectively (p < 0.001), in the study by Cross et al. Mayo et al. showed significant improvement in the sensitivity of CT angiography compared with that of ventilation—perfusion radionuclide lung scanning with a gold standard systematically using pulmonary angiography when CT angiography and ventilation—perfusion radionuclide lung scanning results were discordant.
Our results support the proposal that although prior clinical probability and noninvasive tests are currently used as the first-line technique to make the diagnosis of pulmonary embolism, CT angiography might be a better initial imaging technique than ventilation—perfusion radionuclide lung scanning, which carries a higher rate of indeterminate results and lower accuracy. However, when helical CT angiography has negative results and when clinical suspicion of pulmonary embolism remains high, pulmonary angiography is still indicated. Further investigations are necessary to assess the effectiveness of CT angiography compared with that of ventilation—perfusion radionuclide lung scanning in particular patient populations such as patients with chronic obstructive lung disease or other coexistent morbid conditions.
Acknowledgments
We thank Ray Cooke for writing assistance,
Séverine Triconnet for secretarial work, and
Joël Parisse for photography.
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