Ventilation–perfusion (V/Q) scintigraphy is a non-invasive technique for the assessment of the distribution of pulmonary blood flow and alveolar ventilation and has primarily been used for the diagnosis of pulmonary embolism. Lung scintigraphy remains part of the diagnostic algorithm in the investigation of patients with pulmonary embolism, and guidelines suggest that it may be considered, subject to its availability, as the initial imaging investigation provided the chest radiograph is normal and there is no significant symptomatic concurrent cardiopulmonary disease.
Perfusion scintigraphy is performed following the intravenous injection of Tc-labelled protein microparticles which, because of their size, undergo microembolisation in the pulmonary vascular bed. Agents for ventilation scintigraphy include krypton-81m, Tcdiethylenetriaminepentaacetic acid, Tc-labelled carbon microparicles (Technegas) and xenon. Krypton-81m is in many ways the ideal agent of choice for ventilation imaging but it has a very short half-life, is expensive to produce and accumulates progressively in regions of lung with a low ventilatory turnover. The lung can be imaged in multiple projections and in each projection, perfusion and ventilation images can be acquired sequentially, or, with the newer digital cameras, simultaneously. Technegas is an ultrafine and scintigraphically more efficient aerosol that is considered to behave truly like a gas because the mean aerodynamic diameter of the particles are between 30 and 90 nm.
Ventilation–perfusion lung scintigraphy performed using single photon emission CT (SPECT) technique (as opposed to merely using planar acquisitions) has shown that diagnostic accuracy for pulmonary embolism is at least comparable with, and may exceed, that of CT pulmonary angiography with MDCT (4- to 16-detector MDCT). However, the lack of availability of this technique means it is unlikely to be frequently used for patients with suspected pulmonary embolism.
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