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Original Report |
1 All authors: Department of Thoracic Imaging, Hospital Calmette, Blvd. Jules Leclerc, 59037, Lille Cedex, France.
Received July 29, 1999;
accepted after revision January 26, 2000.
Address correspondence to M. Remy-Jardin.
Abstract
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CONCLUSION. Thin-collimation helical CT provided technically acceptable examinations for pulmonary embolism in 360 patients (97%). In this population, CT revealed pulmonary embolism in 104 patients (29%), negative findings in 217 patients (59%), indeterminate findings in 39 patients (10%), and alternative diagnoses in 65% of patients with negative or inconclusive findings. Ventilation-perfusion scanning and Doppler sonography of the lower extremities were performed in 158 (44%) and 133 patients (37%), respectively, whereas pulmonary angiography was performed in 27 patients (7.5%). The estimated false-negative rate of helical CT was 5%.
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The presence of endoluminal clots on CT scans was considered diagnostic of embolism; central emboli included thrombi in main arteries, lobar arteries, or both; peripheral thrombi consisted of endoluminal clots in segmental and/or subsegmental branches. Isolated subsegmental embolism was assessed on concordant imaging studies (CT and pulmonary angiography) with positive findings or on CT with positive findings and rapid clinical improvement after the initiation of anticoagulation therapy. When helical CT did not depict thrombus down to the subsegmental level, the examination had negative findings; according to the degree of their clinical suspicion, the clinicians were responsible for referring this group of patients to undergo pulmonary angiography. When helical CT revealed negative findings without further pulmonary angiographic evaluation, the patients were followed up for at least 3 months to detect any clinical events that could be related to pulmonary embolism. If pulmonary embolism was diagnosed on pulmonary angiography or during follow-up, then the helical CT study was labelled false-negative. When the confident exclusion of pulmonary embolism on helical CT scans was limited to central pulmonary arteries, helical CT was interpreted as inconclusive; these cases consisted of technically limited CT scans that could not allow the assessment or exclusion of peripheral emboli.
All followed patients were clinically reexamined after a mean period of 3.2 months (range, 2.7-4 months). Patients were asked to return to the hospital if they experienced symptoms or signs that suggested pulmonary embolism. If the follow-up examination could not be performed at our institution, we contacted the patients' family physicians by telephone and asked about the occurrence of any clinical events that could be related to pulmonary embolism. These patients did not undergo further investigation. If the patient had died, the cause of death was determined by means of discussion with the physician in charge at the time of death. Ventilation-perfusion scanning and lower extremity Doppler sonography were not systematically included in the initial imaging techniques because of the specificities of the population studied (i.e., high proportion of patients with a history of cardiopulmonary disease or cardiac dysfunction and local equipment constraints).
Helical CT angiograms of the pulmonary arteries were obtained with a Somatom Plus A4 scanner (Siemens Medical Systems, Erlangen, Germany) with a 0.75 sec per revolution scanning time. According to the patient's breath-hold capabilities, two acquisition protocols were considered. When patients were able to hold their breath for a 20-sec period, they were scanned with a 2-mm collimation and a table feed of 4 mm per revolution (pitch of 2). When patients were only able to hold their breath for a shorter period of time (e.g., 12 sec), a 3-mm collimation and a table feed of 5 mm per revolution (pitch of 1.7) were selected. When patients were unable to maintain strict apnea, they were scanned while gently breathing with a 2-mm collimation and a table feed of 4 mm per revolution (pitch of 2). According to the acquisition parameters, the study group was divided into two groups of patients: those scanned with a 2-mm collimation and a pitch of 2 (n = 284) and those scanned with a 3-mm collimation and a pitch of 1.7 (n = 86). The patients received an injection of 120-140 mL of 24% (n = 143) or 30% (n = 227) iodinated contrast material at a rate of 4 mL/sec (n = 284) or 5 mL/sec (n = 86), respectively.
Interobserver agreement was expressed as a percentage of agreement and as a kappa coefficient, the latter accounting for the chance agreement between two observers. Kappa values were interpreted as follows: less than 0.20, poor; 0.21-0.40, fair; 0.41-0.60, moderate; 0.61-0.80, good; 0.81-1.0, very good.
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Agreement between the two interpreters for the diagnosis of pulmonary
embolism on a per patient basis occurred in 364 patients (98%; 
=0.97).
When only the main and lobar arteries were considered, the interobserver
agreement was 99%, with a kappa value of 0.99. When only the segmental
arteries were considered, the interobserver agreement was 99%, with a kappa
value of 0.98. When only the subsegmental arteries were considered, the
interobserver agreement was 98%, with a kappa value of 0.97. No significant
difference was found between the kappa values on 2-mm and 3-mm collimation
scans for central and segmental arteries. At the subsegmental level, the kappa
value was significantly better on 2-mm ( = 0.98) than on 3-mm (
= 0.94) collimation scans (p < 0.05). Overall, discord was most
frequent for the subsegmental arteries of the right and left upper lobes.
Pulmonary angiography was performed in 27 (7.5%) of 360 patients with a technically adequate helical CT scan (group 2, n = 5; group 3, n = 2; group 4, n = 20). In group 2, pulmonary angiography revealed negative findings in four patients and depicted an isolated segmental thrombus in the right lower lobe of one patient. In group 3, pulmonary angiographic findings confirmed the CT findings of two patients in whom CT had revealed an isolated subsegmental thrombus. In group 4, pulmonary angiography revealed positive findings in one patient and showed an isolated subsegmental thrombus in the apical segment of the right upper lobe that was not visible in retrospect on helical CT because of the focal suboptimal enhancement of the first centimeter of the z-axis coverage (Fig. 3A,3B). Overall, acute pulmonary embolism was diagnosed in 106 (29%) of 370 patients at the time of the initial evaluation.
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In 165 patients (22/39 in group 2 and 143/217 in group 4), alternative diagnoses were considered that could explain the clinical presentation but were unrelated to pulmonary embolism (Fig. 4) (Table 1). These diagnoses were made by clinicians at the time of the initial evaluation and recorded in the medical charts. Pneumonia (n = 64) was proven by bacterial culture; acute respiratory failure caused by bronchial infection (n = 58), by resolution of clinical symptoms after treatment with specific medication; malignancy (n = 18), by histologic or cytologic findings; chronic obstructive pulmonary disease (n = 16) and lung fibrosis (n = 4), by functional and imaging techniques; postoperative bronchopleural fistula (n = 4), by fiberoptic endoscopy; and pneumomediastinum (n = 1), by imaging. Apart from the 58 patients diagnosed with acute respiratory failure caused by bronchial infection, helical CT provided diagnostic information in the remaining 107 patients (29% of the study group; 41.8% of patients with negative or inconclusive findings on CT).
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Ventilation—perfusion scans were obtained in 158 (44%) of 360 patients with technically adequate helical CT scans (group 2, n = 12; group 3, n = 39; group 4, n = 107). In 12 patients in group 2, ventilation—perfusion scanning revealed high probability in two patients and an intermediate probability in 10. In 39 patients in group 3, ventilation—perfusion scanning revealed high probability in 22 patients and low probability in 17. In 107 patients in group 4, ventilation—perfusion scanning revealed high probability in 25 patients, intermediate probability in 76, and normal findings in six. A combined interpretation of ventilation—perfusion scans and helical CT scans in the 25 patients with ventilation—perfusion scans with a high probability and helical CT scans with normal results led to the identification of focal lung and/or bronchial abnormalities on CT scans in 21 of 25 patients. The following changes were depicted: lung consolidation (n = 9), focal bronchiectasis (n = 6), diffuse emphysema with more severe lesions in the areas of scintigraphic high probability (n = 4), and bronchopulmonary carcinoma (n = 2). Among the 25 patients with discordant CT and ventilation—perfusion scan results, eight underwent pulmonary angiography, which revealed negative findings in every patient. The remaining patients underwent clinical follow-up, which did not reveal recurrent pulmonary embolism. Doppler sonography of the lower extremities was performed in 133 (37%) of 360 patients with a technically adequate helical CT scan (group 2, n = 13; group 3, n = 65; group 4, n = 55). Positive findings were revealed in four patients in group 2, 42 patients in group 3, and 14 patients in group 4. Therefore, a total of 18 patients from groups 2 and 4 received anticoagulants because of deep vein thrombosis in the absence of CT findings of pulmonary embolism.
Seventy-one patients (group 2, n = 12; group 4, n = 59) with unresolved suspicion of pulmonary embolism did not receive anticoagulant drugs and were followed up. All five patients who fulfilled the following criteria were gathered under the label of unresolved suspicion of pulmonary embolism: absence of pulmonary embolism on helical CT or pulmonary angiography, no alternative diagnosis at the time of the initial evaluation, and no anticoagulation therapy for deep vein thrombosis. Three patients from group 2 died within 3 days of the initial helical CT examination. Autopsy was not performed in these patients. The cause of death was related to myocardial infarction in one patient and to end-stage cancer in the remaining two. Sixty-eight patients were alive at the time of follow-up. Among the nine patients from group 2 alive at the time of follow-up, pulmonary embolism was diagnosed in two on helical CT scans at the 1- and 2-month follow-up examinations. None of the 59 patients in group 4 experienced clinical symptoms suggestive of deep vein thrombosis and/or acute respiratory symptoms.
In summary, thin-section helical CT depicted acute pulmonary embolism in 104 patients (29%), including six with isolated subsegmental pulmonary embolism (2%). Initial helical CT results were false-negative in four (two pulmonary embolisms were angiographically revealed at the time of the initial evaluation and two recurrences of pulmonary embolism were diagnosed during the 3-month follow-up examination) of 256 patients without pulmonary embolism on the initial helical CT.
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Helical CT revealed negative findings down to the subsegmental level in 217 patients and inconclusive findings in 39. In the latter group, CT scans were of technically limited quality, enabling the interpreters to exclude central pulmonary embolism but not peripheral thrombi. Ten examinations (3%) were technical failures, precluding any confident depiction of acute pulmonary embolism even at the level of the central pulmonary arteries. This proportion of technical failures is similar to that (2-4%) previously reported in the literature. The failures were variably related to suboptimal enhancement, poor signal-to-noise ratios, and/or marked motion artifacts [1,2,3]. Among patients with inconclusive or negative findings on helical CT, alternative diagnoses were made by the clinicians in 165 patients on the basis of specific diagnostic clues and/or the resolution of clinical symptoms after treatment with specific medication. Among these patients, helical CT provided diagnostic information that helped explain the clinical presentation in 107 cases: namely, 30% of the study group and 65% of the patients with negative of inconclusive findings on CT. Our results are in agreement with those of three studies that have investigated the role of CT as a provider of alternative diagnoses [3,4,5]. In our study, the results of these patients' clinical follow-up examinations would have helped us classify their helical CT scans as those with true-negative findings.
This study reflects our experience with helical CT for the diagnosis of pulmonary embolism. The study has one major limitation: the absence of a systematic comparison with another imaging technique. The high percentage of patients with poor respiratory reserve explains why this investigation was not included as a first-line diagnostic test.
The second limitation of our study was the absence of a systematic search for deep vein thrombosis because of local equipment limitations at the time of the initiation of this study, a situation that underlines the influence of local equipment availability on the treatment of pulmonary embolism. Moreover, pulmonary angiography was indicated by the clinicians in only 27 (7.5%) of 360 patients examined with helical CT. This low rate confirms the well-known reluctance of clinicians to use the gold standard of acute pulmonary embolism. In the absence of systematic pulmonary angiographic confirmation, analysis of the outcome was of crucial importance in patients with negative or inconclusive findings on helical CT. Among these patients, the absence of anticoagulant treatment was associated with a low incidence of later pulmonary embolism (2/71; 2.8%) diagnosed during the 3-month follow-up studies that involved patients clinically suspected of having acute pulmonary embolism but without thrombi on pulmonary angiography [6,7,8,9,10,11]. In the final analysis of the false-negative rate of helical CT, deep vein thrombosis was found in 14 of 217 patients in whom helical CT scans revealed negative findings and in four of 39 patients with inconclusive findings on CT. Considering that asymptomatic pulmonary embolism may be present in up to 50% of patients with deep vein thrombosis [12], one may theoretically hypothesize that nine of these 18 patients had pulmonary embolisms overlooked on helical CT scans that were fortuitously detected via lower extremity Doppler sonography. Therefore, this interpretation would increase the possible false-negative rate of helical CT in our study from four (1.6%) of 256 patients to 13 (5%) of 256 patients. Despite different levels of expertise between the two interpreters, an excellent interobserver agreement was observed in the retrospective interpretation of the cases (98%), suggesting a direct influence of a thin collimation in the analysis of the pulmonary artery bed.
Our study illustrates our experience with thin-collimation helical CT in the diagnostic workup of patients with pulmonary embolism. Thin-collimation helical CT images the population suspected of pulmonary embolism with better spatial resolution, subsequently rendering the peripheral pulmonary artery bed accessible to evaluation with helical CT angiography.
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