A 35-year-old previously well woman presents with sudden onset pleuritic chest pain and shortness of breath after a long-haul flight from Europe. She is tachycardic at 112 beats/min, respiratory rate 26 breaths/min, and SpO₂ 94% on room air. Her Wells score is high probability for pulmonary embolism. Pregnancy test is negative. According to current practice, what is the most appropriate initial imaging investigation?