<prompt> Best investigation when there is clinical suspicion of pulmonary embolism in a patient is -D - Dimer assayMulti detector CT angiographyDoppler ultrasoundCatheter angiography  <response>: 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 occlusioncut-off of vessels,segmental oligemia or avascularity,a prolonged aerial phase with slow filling,and touous,tapering peripheral vessels. Reference:Harrisons medicine-18th edition,page no:2174;Davidsons medicine-22nd edition,page no:722. 