Pulmonary embolism is one of the serious cardiovascular emergencies that are frequently seen in the clinical setting. However, its diagnosis is still probabilistic and is considered as a challenging diagnosis in many cases, thus an imaging confirmation is usually needed for this purpose. Many diagnostic modalities had been implemented in the diagnosis of PE, and each of them had its own benefits and limitations including the gold standard CTPA [5].
Early detection and treatment of PE is potentially lifesaving, and this necessitates the presence of a simple, non-invasive, and an accurate imaging modality that could be rapidly performed for immobile and critically ill patients as a bedside test. TUS may serve in this regard and had been recently used for this purpose; moreover, it avoids the hazardous exposure to radiation and contrast media that could be met with the use of the CTPA [3].
Many studies had tested the validity of the use of the TUS in the diagnosis of PE, but few of them had compared its results with those of the multi-slice CTPA [3].
In the present study, we investigated the validity of TUS—compared to and correlated with the multi-slice CTPA—in the diagnosis of the clinically suspected cases with PE, and its sensitivity and specificity were assessed.
The criteria to diagnose PE with TUS applied in this study were the presence of at least one typical pleural-based/subpleural hypoechoic lesion with or without pleural effusion (Figs. 3 and 4); these nodules are typically hypovascular on color Doppler imaging (Fig. 5). However, the presence of nonspecific subpleural lesions less than 5 mm in size and isolated free pleural effusion or the presence of normal sonographic findings should make the diagnosis of PE unlikely.
A differential diagnosis of the above-described lesions may encompass some other diseases including pneumonia, pulmonary neoplasm, and atelectasis; however, some sonographic criteria may be suggested for the aim of differentiation between them in the light of the clinical examination, where the carcinoma may have a rather rounded appearance with speculated borders, possible central breaking down, and can show internal vascularity. In pneumonia, the hypoechoic lesions are typically polygonal, inhomogenous with serrated margins and characteristically show an air-bronchogram or fluid bronchogram with increased internal vascularity. Atelectatic lesions are considered as the real challenge or the great mimicker lesions as they may have variable shapes and echogenicity patterns, but they often show internal vascularity inside (vascular sign), a feature that may help in the differentiation; pulmonary embolism lesions may present a hypoechoic rather homogenous lesions that may occasionally have central echoic area but often lack the internal vascularity and the air-bronchograms are seldom seen [6, 7].
The studies done by Pfeil et al., Comert et al., Abootalebi et al., and Ghanem et al. had adopted the same diagnostic criteria [8,9,10,11].
The current study included 25 patients, 15 males (60%), and 10 females (40%); thus, we had the same gender distribution as the Comert et al. and Ghanem et al. studies [9, 11].
In the current study, TUS demonstrated 17 subpleural hypoechoic lesions suggestive of PE in 12 out of 16 patients finally diagnosed as PE by CTPA with a mean of 1.3 lesion per patient which is lower than that was detected in Ghanem et al. [11], and this could be attributed to the difference in the sample size between the two studies where the latter was larger.
In agreement with Comert et al. and Ghanem et al. studies, the detected lesions were located mainly in the right basal lung region [9, 11], and unfortunately, we did not have an explanation for this.
Regarding the size of the lesions, our study showed that the mean size of the lesions was 15.3 × 10.5 mm while, in Comert et al. study, it was 22.9 × 31.2 mm [9] and, in Abootalebi et al. study, it was 16.4 × 11.1 mm [10]; thus, there was no big difference in the mean size of the lesions in the three studies, and all of them had a mean size that was more than 10 mm.
As regards the shape of the lesions, we found that the majority of lesions were wedge-shaped. This finding was also depicted by Abootalebi et al. and Ghanem et al. studies [10, 11].
The central hyperechogenicity inside the lesions was found in 2 lesions (15%) in two different patients. The central hyperechoic structure indicates the presence of an air inside the patent bronchiole in the affected segment and was considered as a sign of segmental involvement. Our result was higher than the Ghanem et al. study which reported central hyperechogenicity in only 4.5% of their patients [11].
The sensitivity, specificity, PPV, NPV, and accuracy of TUS for the diagnosis of PE in comparison with MDCT, the gold standard, in our study were 75%, 89%, 92%, 67%, and 80%, respectively (Figs. 1 and 2).
Pfeil et al. in Germany, for the first time, had compared the results of TUS with MDCT in the detection of PE among 33 patients who had symptoms of suspected PE. The design of our study was similar to theirs. They reported 70% sensitivity and 69.6% specificity, 84.25% NPV, and 50% PPV for TUS in this regard [8].
Comert et al. included 50 patients in their study and had found that the sensitivity, specificity, PPV, NPV, and the accuracy of TUS were 90%, 60%, 77.1%, 80%, and 78%, respectively [9].
Another study done by Abootalebi et al. included 77 patients and documented that the sensitivity, specificity, NPV, PPV, and the accuracy of TUS were 84%, 94.2%, 87.5%, 92.5%, and 91%, respectively [10].
A more recent study by Ghanem et al. included 60 patients and reported that the sensitivity, specificity, NPV, PPV, and the accuracy of TUS were 82%, 90%, 72%, 94%, and 85%, respectively [11].
Comparing our results with the forementioned studies done by Pfeil et al., Comert et al., Abootalebi et al., and Ghanem et al. [8,9,10,11], rather similar statistical parameters were met apart from the low sensitivity and NPV in our study which could be explained by the small sample size.
In our study, four cases were considered false negative, where they had a negative TUS scan while their CTPA was positive for PE (Fig. 6). This could be explained in that two of them had isolated central PE not reaching the periphery of the lung and the PE associated lesions can be detected by TUS only when they extend to the lung periphery, while the other two patients were bedridden and their general condition hindered proper patient positioning and exposure for the classic examination.
Beyond the scope of the current study, some studies had investigated the use of the trans-thoracic echocardiography (TTE) for the detection of acute pulmonary thrombo-embolism and described some echocardiographic findings including right ventricular reduced size and dysfunction and dilated pulmonary arteries, and sometimes, the thrombus may be visualized; moreover, the interventricular septum changes (like flattening and paradoxical movement) as well as inferior vena cava dilatation had been described; however, they reported some limitations regarding the acoustic window for depiction of such findings in addition to the low sensitivity for these findings [12, 13].
Although our study had reported encouraging results regarding the diagnosis of PE-related lesions by TUS, we do not believe that TUS can be considered as an efficient alternative to CTPA. As a considerable proportion (16%) of patients who were considered false negative by TUS and then PE could have been missed without CTPA, another pitfall is the false-positive results, where the TUS findings (subpleural hypoechoic nodules) were depicted in the absence of any emboli detected by CTPA (Fig. 7).
Some limitations were met for the use of TUS in the diagnosis of PE in our study including the small sample size, the central location of the thrombus as the lesions can be detected by TUS only when they extend peripherally to the lung surface, and the fact that ultrasonography is an operator-dependent technique.