Normal perfusion scintigraphy in pulmonary embolism. Causes and diagnostic alternatives

Diagnosing pulmonary embolism (PE) is a challenge for many physicians as it is a frequently occurring disease with nonspecific symptoms and signs. Ventilation-perfusion (V/Q) scintigraphy is widely used as the first step in diagnosing PE since it is non-invasive and highly sensitive. With a normal perfusion scan, clinically relevant pulmonary thrombo-emboli are considered to be absent. In an ongoing study assessing the value of spiral CT in the diagnosis of PE, we encountered a patient who had a normal perfusion scan while a large partially occluding thrombus in the right lower lobe artery and its branches was depicted by spiral CT and pulmonary angiography. In this article, we discuss the significance of normal findings in perfusion scintigraphy, the causes of false-negative perfusion scans and the role of alternative techniques such as spiral CT and pulmonary angiography.