Differentiating acute and chronic pulmonary embolism

Acute PE: diameter of pulmonary artery may be increased
Chronic PE: diameter of pulmonary artery may be decreased

Acute PE: may be central or eccentric; if eccentric, the filling defect shows acute angle with vessel wall; if central, defect may be surrounded by contrast
Chronic PE: peripheral crescent defect making obtuse angles with vessel wall

Acute PE atteniuation value (about 30 HU) is less than chronic PE (about 85 HU); this may be due to thrombus enhancement, iron and calcium.

Additional signs in acute PE: dilated right ventricle, deviation of interventricular septum towards left ventricle (right ventricular strain)

Additional signs in chronic PE: dilated bronchial arteries, dilated intercostal arteries, right ventricular hypertrophy.

Reference:
http://radiographics.rsnajnls.org/cgi/content/abstract/29/1/31
Castañer et al. CT Diagnosis of Chronic Pulmonary Thromboembolism. RadioGraphics 2009;29:31-50