Here the notion of causality is one of contributory causality as discussed above: If the true value 
  
    
      
        
          a
          
            j
          
        
        ≠
        0
      
    
    {\displaystyle a_{j}\neq 0}
  , then a change in 
  
    
      
        
          x
          
            j
          
        
      
    
    {\displaystyle x_{j}}
   will result in a change in y unless some other causative variable(s), either included in the regression or implicit in the error term, change in such a way as to exactly offset its effect; thus a change in 
  
    
      
        
          x
          
            j
          
        
      
    
    {\displaystyle x_{j}}
   is not sufficient to change y.
CTPA is non-inferior to VQ scanning, and identifies more emboli (without necessarily improving the outcome) compared to VQ scanning. Ventilation/perfusion scan
A ventilation/perfusion scan (or V/Q scan or lung scintigraphy) shows that some areas of the lung are being ventilated but not perfused with blood (due to obstruction by a clot). This type of examination is as accurate as multislice CT, but is less used, due to the greater availability of CT technology. It is particularly useful in people who have an allergy to iodinated contrast, impaired kidney function, or are pregnant (due to its lower radiation exposure as compared to CT). The test can be performed with planar two-dimensional imaging, or single photon emission computed tomography (SPECT) which enables three-dimensional imaging. Hybrid devices combining SPECT and CT (SPECT/CT) further enable anatomic characterization of any abnormality. Low probability diagnostic tests/non-diagnostic tests
Tests that are frequently done that are not sensitive for PE, but can be diagnostic. Chest X-rays are often done on people with shortness of breath to help rule-out other causes, such as congestive heart failure and rib fracture. Chest X-rays in PE are rarely normal, but usually lack signs that suggest the diagnosis of PE (for example, Westermark sign, Hamptons hump). Ultrasonography of the legs, also known as leg doppler, in search of deep venous thrombosis (DVT). The presence of DVT, as shown on ultrasonography of the legs, is in itself enough to warrant anticoagulation, without requiring the V/Q or spiral CT scans (because of the strong association between DVT and PE).