A 63-year-old woman is seen in the emergency room with acute shortness of breath. There is no history of heart or lung problems in the past. She was recently diagnosed with breast cancer and is undergoing active treatment. On examination, her blood pressure is 120/80 mm Hg, pulse 100/min, and heart and lungs are normal. There are no clinical signs of deep venous thrombosis (DVT). Which of the following investigations is most likely to rule out a pulmonary embolism (PE)?

The perfusion lung scan is most valuable in ruling out a PE. If properly performed early in the course of symptoms, a normal scan rules out the diagnosis. High-probability scans are usually considered enough evidence of PE to warrant definitive treatment. Intermediate- or low-probability scans may require further investigation (e.g., with pulmonary angiography), depending on the prior probability of disease. More recent data suggest that a normal high-resolution chest CT with contrast rules out clinically significant PE and is replacing perfusion scanning since the lung images may provide an alternate diagnosis for the patient's symptoms.