**9. Ventilation/perfusion scanning (V/Q scan)**

An alternative diagnostic study to CTPA is V/Q scanning. V/Q scanning involves imaging of pulmonary perfusion and ventilation to evaluate for areas of mismatch that suggest the presence of PE. The average radiation exposure for a V/Q scan is 1.2 mSv. PIOPED evaluated the accuracy of V/Q scanning in the assessment of pulmonary embolism compared with the gold standard pulmonary angiogram (PIOPED 1990). Patients with a high clinical probability of PE and a high probability V/Q had a 95% likelihood of truly having a PE. Patients with a low clinical probability of PE and a low probability VQ scan had a 4% likelihood of having a PE (Gottschalk 2007). A normal V/Q scan virtually excluded PE (PIOPED 1990). Unfortunately, in patients with other combinations of clinical risk and V/Q results, the diagnostic accuracy of V/Q ranged from 15-86%. Therefore, additional diagnostic testing is required to determine the presence of PE in this group (Calvo-Romero 2005). (Table 3)


**Table 3.** Likelihood of pulmonary embolism according to scan category and clinical probability (based on PIOPED study 1990)
