CT Pulmonary Angiography > Pulmonary Embolus > Diagnostic Alternatives > V/Q Scan

V/Q Scan

Pulmonary perfusion scanning was the most widely utilized screening test to rule out clinically important pulmonary embolism prior to the advent of computed tomography pulmonary angiography (CTPA). It has been shown that a normal test result almost certainly excludes the presence of pulmonary embolism. A high probability ventilation-perfusion (VQ) scan usually indicates the presence of clinically significant PE. 88% of patients with a high probability VQ scan had angiographic evidence of PE in the PIOPED I study. However, in patients with a prior history of PE and high probability VQ scans the presence of acute PE was proven in only 74% angiographically. In addition, of all patients diagnosed with acute PE in the PIOPED I study, only 41% had a scan pattern thought to represent a high probability VQ scan.

Patients presenting in either the intermediate or indeterminate group of VQ scans are difficult to interpret and the technique is also not helpful in patients with a low probability scan in the setting of a high clinical suspicion. Unfortunately, the majority of patients undergoing perfusion scanning have nondiagnostic results (more than 60% of all patients in the PIOPED I study). Also, in the PIOPED II study, VQ scanning has been used for the diagnosis or exclusion of PE evaluated by CTA. Only 30% of all patients had a high probability and normal VQ findings. It is for this reason that other screening techniques for PE have been developed. The addition of a ventilation scan improves the overall test performance only marginally.

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