Chronic Pulmonary Embolism

Fig.1: Perfusion lung scan (posterior view) with Tc-99m MAA shows multiple small peripheral perfusion defects in both lungs. There is a matched defect in the right lung that corresponds to abnormality on chest radiograph (not shown).
Fig. 2: Ventilation scan (posterior view) shows normal ventilation in areas of perfusion defects, except only in the right lung. Combined with perfusion scan findings, there are multiple small mismatches in both lungs and a matched defect in the right lung. This patient was considered having intermediate probability for pulmonary embolism. Subsequent pulmonary CTA did not show an acute PE but evidence of mosaic perfusion, typical of chronic pulmonary embolism.

Facts:

• Natural course of pulmonary emboli includes complete resolution, partial resolution, persistent defects or presence of new abnormalities
• Lung scans may show false positivity in patients with unresolved pulmonary emboli
• Those patients likely to delay clearing of pulmonary emboli: elderly, with underlying chronic lung disease, heart failure or having very extensive pulmonary infarction
• Serial lung scans may be helpful in management of patients with thromboembolic disease(decision whether to continue anticoagulant therapy, new abnormalities may influence more invasive therapy, provision of new baseline)
• If an embolus is going to clear, it will do so by three months. What is left after three months will persist indefinitely

Reference:

Fogelman I, Maisey MN, Clarke SEM. An atlas of clinical nuclear medicine, 1994, 2nd edition.

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