Detecting small pulmonary emboli with Dual Energy

CT angiography has emerged as the diagnostic standard for detecting pulmonary embolism. It has been shown that pulmonary emboli can be detected with high reliability even at segmental and subsegmental levels. Recent studies have shown the sensitivity of CTA to be 90 – 100 % and the specificity to be 89 – 94 % for the detection of pulmonary emboli to the level of the subsegmental arteries.

However, the relevance of smaller singular pulmonary emboli detected in CT remains somewhat unclear. Embolism cannot be found in many symptomatic patients, and the reason may be that the emboli are just too small. Then again, the condition of many patients with large emboli is surprisingly good, and this seems to be especially true for non-occlusive thrombi that are surrounded by contrast material. Altogether, it seems that there is some discrepancy between the clinical relevance and the size or location of pulmonary embolism.

Therefore, perfusion imaging may add relevant information, even though the initial fast diagnosis is usually made with CT angiography. So far, this was the domain of pulmonary perfusion scintigraphy, in which technetium-marked small albumin spheres are trapped in capillary vessels and thus map pulmonary perfusion. However, spiral dual energy CT angiography also provides the opportunity to assess pulmonary perfusion based on a three-material decomposition for iodine, air and soft tissue.

After contrast material administration, perfused and ventilated lung parenchyma will contain all three components. The iodine distribution can be mapped to visualize perfusion. This gives a new, functional aspect to CT, because even perfusion defects caused by small peripheral emboli can be detected, and the hemodynamic relevance of larger emboli can be assessed.

Authors: Thorsten R.C. Johnson, Christian Fink, Christoph R. Becker

See corresponding protocol: CTA lung perfusion
See corresponding cases: Acute Pulmonary Embolism, Pulmonary hypertension due to chronic recurrent pulmonary embolism
See corresponding news: CTA lung perfusion