**3.1.3 Obstruction index**

174 Pulmonary Embolism

have indicated that the benefits from a separate ECG-gated CT scan for the evaluation of RV ventricular diameter are minimal and do not justify its routine clinical use instead of the

Although most studies have indicated that CT assessments of RV dilatation contribute to the risk stratification of patients with PE, two recent meta-analyses have found that CT findings of RV dilatation have limited prognostic importance for mortality among patients with nonhigh-risk PE (Coutance et al., 2011; Sanchez et al., 2008), and that the greatest value of this method appears to be the identification of low-risk patients based on the lack of RV dilatation. These analyses suggested that measurements made on the four-chamber view are more reliable than traditional measurements made on the minor axis. However, most of these studies were of retrospective design and in small numbers of patients with generally undefined clinical presentations. Hence, any conclusions about the usefulness of this marker must be treated with some caution, although future large clinical studies and standardized

If RV afterload suddenly increases, the interventricular septum, which normally bows toward the RV, may shift toward the LV because of its confinement within the pericardium. This phenomenon is readily visible on helical CT pulmonary angiography as straightening

Leftward bowing of the interventricular septum on CT has been related to severe PA obstruction (Fig. 3). This bowing was found to strongly predict admission to the intensive care unit for PE, but was not associated with in-hospital mortality (Araoz et al., 2003). Thus, this sign is likely not an indicator of outcome and is not specific for PE (van der Meer et al., 2005). This bowing has also been observed in patients with chronic pulmonary artery hypertension, although, in the latter condition, the RV wall is usually thickened (>6 mm),

standard measurements of the minor axis (Lu et al., 2009).

definitions of RV dilatation will be required in this patient subset.

whereas, in acute PE, the RV wall thickness is usually normal.

Fig. 3. Ventricular septal bowing (arrow) into the left ventricular lumen

**3.1.2 Interventricular septal straightening/bowing**

or bowing of the interventricular septum.

The PA obstruction index, or the percentage of vascular obstruction of the pulmonary arterial tree caused by PE, may be calculated as Σ (n × d) expressed as percentage vascular obstruction ([Σ (n × d)/40] × 100), where n is the value of the proximal clot site that equals the number of segmental branches arising distally, and d is the degree of obstruction, with partial obstruction scored as 1 and complete obstruction as 2. Values for n range from a minimum of 1 (obstruction of one segment) to a maximum of 20 (obstruction of both right and left pulmonary arteries) (Qanadli et al., 2001). With this scoring system, the maximum obstruction score is 40 (thrombus completely obstructing the pulmonary trunk), which corresponds to a 100% obstruction index. Using a cutoff of 60%, 83% of the patients with an index >60% died, whereas 98% of patients with a lower index remained alive (Wu et al., 2004). Patients with an obstruction index ≥ 40% were found to be at an 11.2-fold (95% CI: 1.3, 93.6) increased risk of dying from PE (van der Meer et al., 2005). However this index may not be practical for routine application without the aid of radiologists.

Another obstruction index, the pulmonary embolism severity index (PESI), was developed to estimate 30-day mortality in patients with acute PE. This index has also been used to identify patients with a low mortality risk who may be suitable for outpatient management of acute PE (Aujesky et al., 2007). The PESI contains 11 differently weighted baseline clinical parameters and is relatively complicated to administer and score. A simplified version of the PESI (sPESI) was therefore developed for ease of application. The sPESI showed similar prognostic accuracy and clinical utility as the PESI, although its use made it easier to identify patients at low-risk of adverse outcomes (Jimenez et al., 2010) (Table 2).

Fig. 4. Transverse contrast-enhanced chest computed tomographic scan showing pulmonary emboli (arrows) in both main pulmonary arteries (PAs). This patient had a PA obstruction index of 55%

#### **3.1.4 Pulmonary artery diameter measurement**

A pulmonary artery (PA) diameter greater than 30 mm indicates a PA pressure greater than 20 mmHg (Kuriyama et al., 1984). Moreover, the diameter of the central PA has been significantly correlated with the severity of PE (Collomb et al., 2003). In other studies, however, the diameter of the main PA and the ratio of the diameters of the main PA and the

Risk Stratification of Submassive Pulmonary Embolism:

Fig. 5. Saddle pulmonary embolism.

**4. Dual E CT** 

scanning DECT.

The Role of Chest Computed Tomography as an Alternative to Echocardiography 177

Physiologic changes in patients with acute PE, such as RV overload, may be more closely related to the extent of the pulmonary perfusion defect than to the burden of intravascular emboli. Several studies have reported that the extent of the pulmonary perfusion defect, as assessed by perfusion scintigraphy or SPECT, is an important risk factor for recurrence of pulmonary embolism and RV dysfunction (Wolfe et al., 1994; Palla et al., 2010). However, CT angiography has shown better performance than perfusion scintigraphy or SPECT in the initial diagnosis of pulmonary embolism, since the former has several advantages, including

Recent advances in CT technology have included dual-energy CT (DECT), using two tubes (dual-source CT) or a single tube with a rapid kVp switching technique. As iodine has unique spectral properties and X-ray absorption characteristics at higher and lower photon energies (e.g., 140-kVp and 80-kVp), iodine map images can be generated using DECT angiography (DECTA). These images may represent the regional perfusion status of lung parenchyma and showed good correlation with scintigraphic findings in patients with PE (Pontana et al., 2008; Thieme et al., 2008). Weighted-average 120-kVp equivalent CT angiography obtained from DECTA can be used for the direct visualization of a thromboembolism and for the evaluation of CT signs of RV dysfunction, similar to standard CT angiography. That is, information about regional lung perfusion status, as well as the burden of intravascular emboli and right-sided heart failure, can be evaluated by single

RV dysfunction is predictive of a poor prognosis in patients with acute PE. On CT angiography images, RV/LV diameter ratio is a reliable marker of RV dysfunction, with higher RV/LV diameter ratios related to poor clinical outcomes (Quiroz et al., 2004; Schoepf et al., 2004; van der Meer et al., 2005). Furthermore, the extent of perfusion defect on DECTA has been correlated with RV/LV diameter ratio (Zhang et al., 2009; Chae et al., 2010; Bauer

high spatial resolution, rapid scanning time and easy availability.


a The total point score for each patient is calculated as the sum the patient's age in years and the points for each predictor when present.

Scores corresponding to risk classes include: 65 or less, class I; 66 to 85, class II; 86 to 105, class III; 106 to 125, class IV; and more than 125, class V. Patients in risk classes I and II are defined as being at low risk. bThe total point score for each patient is calculated as the sum of the points. Scores correspond to the

following risk classes: 0, low risk; 1 or more, high risk. c The variables were combined into a single category of chronic cardiopulmonary disease.

Table 2. Original and simplified pulmonary embolism severity index (PESI)

aorta were not indicators of mortality or severity of acute PE (Araoz et al., 2003; van der Meer et al., 2005).

#### **3.1.5 Saddle pulmonary embolism**

Saddle PE is defined as a visible thromboembolus stradding the bifurcation of the main PA (Fig. 5). Saddle PE occurs at frequency of (5.2%) in all patients with PE (Pruszczyk et al., 2003). Such proximal thrombus may be regarded unstable, large clot burden in the PA, and "in-transit" embolus, which can fragment spontaneously or secondary to treatment and obstruct multiple, distal pulmonary arteries (Pruszczyk et al., 2003).

Debate has been going on regarding the size of the clot and prognosis. Some studies suggest that simple distinction of saddle versus non-saddle PE by CT findings was associated with death within 1 year (OR 7.4, 95% CI 1.7-31.5) and may provide a straight forward method for risk stratification (Yusuf et al., 2010). Whereas other studies found that saddle PE was not associated with mortality rate and may not necessitate aggressive medical management (Ryu et al., 2007; Musani, 2010). Therefore the prognosis value of saddle PE is not well established.

Fig. 5. Saddle pulmonary embolism.
