How can acute pulmonary embolism be distinguished from the chronic one on CTA?
It may sometimes be difficult to differentiate acute from chronic PE, and chronic PE may be present with superimposed acute PE. However, an acute pulmonary embolus typically appears as a centrally located hypoattenuating filling defect within the artery and is often occlusive, whereas a chronic pulmonary embolus will typically be eccentrically located within the artery. Sometimes intraluminal linear or curvilinear hypoattenuating bands or webs may be seen in chronic PE. This can be conceptually understood because the appearance of chronic PE is due to progressive but incomplete recanalization of a prior acute PE. Chronic PE can also calcify, whereas acute PE does not, and may lead to diminution in pulmonary arterial caliber or luminal obliteration. Long-standing PEs are more likely than acute emboli to cause pulmonary hypertension, leading to right heart enlargement and enlargement of the central pulmonary arteries. Mosaic attenuation of the lungs, seen as sharply demarcated regions of decreased and increased attenuation in the lungs related to areas of hypoperfusion, blood redistribution, and associated pulmonary air trapping, may also be seen with chronic PE. Lastly, increased bronchial arterial collateral vessels can be seen in some cases of chronic PE