A spiral Computed Tomograpy (CT) scan is a quick, less expensive CT scan which can scan the entire thorax in one breath-hold. It has a sensitivity ranging 64-93% with a specificity of 89-100%. Its accuracy increases with the size of the embolism. It directly visualizes the clot and has the added benefit of diagnosing other disease states in the differential diagnosis (e.g. lung cancer, vascular remodelling, and pleural effusion). Spiral CT is a useful adjunct to the majority of ventilation perfusion scans that have non-diagnostic results that require further testing (PIOPED Investigators 1990).