Young female with cough and chest pain presents to the ED. CXR is normal, D-dimer is elevated and a CT PE study is performed.
There is no PE. But, there is a perfusion defect in the left lower lobe. Now why would that be?
Well, the answer is in the lung windows, where bronchi to the left lower lobe are filled with debris. This is an example of Euler-Liljestrand mechanism, with reflex vasoconstriction in response to hypoxia.
The left lower lobe posterior pulmonary veins are hypoenhancing, consistent with vasoconstriction in the vascular bed.
In 1946 Dr.'s Euler and Liljestrand from Stockholm postulated based on their experiments in cats that "regulation of the pulmonary blood flow is mainly mediated by a local action of the blood and alveolar gases leading to an adequate distribution of the blood through the various parts of the lungs according to the effeciency of aeration."
I think here we see a spectral demonstration of the Euler-Liljestrand mechanism. Bronchial obstruction leads to localized hypoxia, then vasoconstriction with reduced perfusion to the affected bronchovascular territory. Decreased contrast in the draining pulmonary vein proves the vasoconstrictive effect.