Best investigation when there is clinical suspicion of pulmonary embolism in a patient is -

CT pulminary angiography is the first-line diagnostic test.It has the advantages of visualising the distribution and extent of the emboli,or highlighting an alternative diagnosis,such as consolidation,pneumothorax, or aoic dissection.A definitive diagnosis of PE depends on visualisation of an intraluminal filling defect in more than one projection.Secondary signs of PE include abrupt occlusion("cut-off") of vessels,segmental oligemia or avascularity,a prolonged aerial phase with slow filling,and touous,tapering peripheral vessels. Reference:Harrison's medicine-18th edition,page no:2174;Davidson's medicine-22nd edition,page no:722.