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.