Imaging
The imaging results were taken from reports generated by specialist radiologists and nuclear medicine physicians. It was recorded whether patients had ventilation-perfusion (V/Q) lung scans, computed tomographic pulmonary angiograms (CTPA), both or neither.
V/Q scans were acquired by single photon emission tomography (SPECT) with a Symbia T6 scanner (Siemens AG, Munich, Germany). It is a large field-of-view, dual-head gamma-camera with a low-energy, all-purpose collimator. The ventilation-phase images were acquired following the inhalation of technetium-labelled ultrafine carbon particles (Technegas, Cyclomedica, Lucas Heights, Australia). The scanner was pre-programmed to 64 positions for 8-15 seconds per position (depending on body habitus). The perfusion phase images were acquired following the peripheral intravenous administration of technetium-labelled macroaggregated albumin (DraxImage MAA, Jubilant DraxImage Inc, Montreal, Canada). In this stage, the scanner was pre-programmed to 64 positions for 8 seconds per position. Iterative reconstruction with ordered subset expectation maximisation (OSEM) was applied and images could be viewed in all tomographic (coronal, transverse and sagittal) planes. Images were interpreted according to the European Association of Nuclear Medicine’s 2009 Guidelines14.
For CTPA studies, a routine protocol was utilised after injection of a 50–70-mL bolus of iopromide (300 mg iodine per millilitre, Ultravist; Bayer Schering Pharma, Berlin, Germany), which was followed by injection of 20–30 mL of a saline solution into an antecubital vein through an 18-gauge intravenous cannula (injection rate, 3.0–4.0 mL/sec). By using a bolus-tracking technique, a region of interest was placed in the main pulmonary artery, and image acquisition began 4 seconds after the signal attenuation reached the predefined threshold of 100 HU. The other acquisition parameters were as follows: tube voltages of 80 and 140 kVp and effective tube currents of 177 and 42 mAs for the two x-ray tubes; rotation time, 0.33 second; detector collimation, 32 × 2 × 0.6 mm; pitch, 0.75; and field of view, 500 mm for the large detector array.