V/Q-scanning/SPECT for the diagnosis of pulmonary embolism

Pulmonary embolism (PE) is visualized indirectly by perfusion (Q) scanning as a parenchymal defect related to the embolized artery, and is confirmed by combined ventilation (V) scanning, showing a normal regional ventilation. Based on its principle of detection, V/Q scanning is able to show even small emboli and this ability is enhanced by SPECT. Compared to angiography, V/Q scanning seems to be burdened with low specificity, whereas its sensitivity is sufficiently high. Correlation of V/Q scanning with single-slice CT is comparable with that with angiography. The positive predictive value of V/Q scanning cannot be assessed by the reference standard angiography or by single-slice CT because both have turned out to be inadequate. Correlation of V/Q scanning with high-resolution CT is very promising, revealing high sensitivity as well as specificity of V/Q scanning, but this needs further evaluation in larger studies. Nevertheless, the use of V/Q scanning as a one-step diagnostic approach to PE is unrestricted in patients with normal chest radiograph, whereas V/Q scanning in combination with spiral CT is recommended in patients with abnormal chest radiograph. The negative predictive value of V/Q scanning for recurrent PE is unsurpassed high and by this, V/Q scanning tends to be used primarily to exclude rather than confirm PE.