**4. Imaging studies**

Multi-detector computed tomography pulmonary angiography (CTPA) is the current best imaging study for the diagnosis of acute PE (Mos 2012, Klok 2011). Image acquisition is timed to occur during opacification of the pulmonary arterial bed after an intravenous injection of contrast. This technique has both high sensitivity, 96-100%, and specificity, 97-98% (Remy-Jardin 2007). The use of intravenous contrast may be contraindicated in patients with allergies to iodinated contrast and those with renal insufficiency or who are at risk for contrast-induced nephropathy. A prospective study showed that 14 of 174 patients (8%) developed contrastinduced nephropathy after CTPA which significantly contributed to the adverse outcomes of patients suspected of having an acute PE (Mitchell 2012). The radiation exposure during a CTPA has been estimated to cause 150 excess cancer deaths per million scans (Remy-Jardin 2007). RV function can also be assessed during multi-detector CTPA and abnormal position of the interventricular septum, inferior vena cava contrast reflux, RV diameter to left ventricle (LV) diameter ratio > 1.0 and RV volume to LV volume ratio > 1.0 are predictive of poor outcomes and RV diameter to LV diameter ratio > 1.0 and RV volume to LV volume ratio > 1.0 are also predictive of 30 day mortality after acute PE (Kang 2011).

In patients who are unable to undergo CTPA, ventilation perfusion scintigraphy (V/Q scan) is the next best imaging procedure for acute PE (Mos 2012). A normal V/Q scan effectively identifies patients who do not need anticoagulation treatment whereas a high probability V/Q scan has a positive predictive value >90% (PIOPED 1990). However, up to half of patients with suspected PE may have intermediate scans and require further testing to exclude or establish a diagnosis of PE (Anderson 2007). Intermediate scans occur less often in patients with a normal chest x-ray and no prior cardiopulmonary disease (Calvo-Romero 2005).

#### **4.1. Echocardiography**

Transthoracic and transesophageal echocardiography (ECHO) can be used to image cardiac structure and function in patients with suspected PE. Rarely, intravascular thrombus can be visualized if the clot is large and located within the proximal pulmonary artery (Goldhaber 2002). Studies correlating perfusion lung scans with ECHO findings suggest that approxi‐ mately 92% of patients with PE and occlusion of greater than one third of the pulmonary vasculature demonstrate right ventricular hypokinesis (Wolfe 1994, McConnell 1996). Other echocardiographic findings in acute PE include right ventricular dilation, septal flattening and paradoxical septal motion, pulmonary arterial hypertension, and patent foramen ovale (Goldhaber 2002).

ECHO may be used to risk stratify hemodynamically stable patients with acute PE. In a metaanalysis, Sanchez and colleagues found the unadjusted relative risk of RV dysfunction for predicting death in patients with acute PE and normal hemodynamics was 2.4 (95% CI 1.3-4.4) (Sanchez 2008). In a prospective study of patients with acute PE, RV dysfunction determined by ECHO had an odds ratio of 1.2 (95% CI 1.1-1.4) for adverse events including death, cardiogenic shock, and recurrent venous thromboembolism (Sanchez 2010).
