A 55 year old man who has been on bed rest for the past 10 days, complains of breathlessness and chest pain. The chest X-ray is normal. The next investigation should be

Lung scanning has become a second-line diagnostic test for PE, used mostly for patients who cannot tolerate intravenous contrast. Small paiculate aggregates of albumin labeled with a gamma-emitting radionuclide are injected intravenously and are trapped in the pulmonary capillary bed. The perfusion scan defect indicates absent or decreased blood flow, possibly due to Pulmonary EmbolismRef: Harrison's 18/e p2173, 17/e p1652, 1663