A 32-year-old pregnant woman at 28 weeks gestation presents to a rural ED with sudden onset pleuritic chest pain and dyspnoea. She is tachycardic at 112 bpm, RR 24, SpO2 95% on room air and normotensive. There are no signs of DVT on leg examination. You suspect pulmonary embolism. Which investigation is most appropriate as the first-line imaging test in this scenario?