Best investigation when there is clinical suspicion of pulmonary embolism in a patient is

CT of the chest with intravenous contrast is the principal imaging test for the diagnosis of PE . Multidetector-row spiral CT acquires all chest images with <=1 mm of resolution during a sho breath hold. Sixth-order branches can be visualized with resolution superior to that of conventional invasive contrast pulmonary angiography. CTPA is the first-line diagnostic test . It has the advantage of visualising the distribution and extent of the emboli or highlighting an alternative diagnosis, such as consolidation, pneumothorax or aoic dissection. The sensitivity of CT scanning may be increased by simultaneous visualisation of the femoral and popliteal veins, although this is not widely practised. As the contrast media may be nephrotoxic, care should be taken in patients with renal impairment, and CTPA avoided in those with a history of allergy to iodinated contrast media. In these cases, either V/Q scanning or ventilation/perfusion single photon emission computed tomography (V /Q SPECT) may be considered Colour Doppler ultrasound of the leg veins may be used in patients with suspected PE, paicularly if there are clinical signs in a limb, as many will have identifiable proximal thrombus in the leg veins. Ref Davidson edition23rd pg 620