**1. Introduction**

74 Pulmonary Embolism

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Pulmonary thrombo-embolism (PTE) remains a common cause of morbidity and mortality worldwide. Annually, as many as 300,000 people in the United States die from acute pulmonary embolism and the diagnosis is often not made until autopsy (Tapson, 2008). Obstruction of the pulmonary artery or one of its branches by material other than thrombi is commonly referred to as Non-thrombotic Pulmonary Embolism (NTPE). The lungs are a prominent target for the embolization of any material larger than approximately 10 microns that gains access to the venous circulation. This includes thrombi, air, amniotic fluid, fat, injected foreign material, and tumor (Fig 1). In comparison with pulmonary thrombo-embolism, NTPE is a less common condition (M. B. King & Harmon, 1994). Its complex and diverse etiologies renders it more difficult to accurate diagnosis and characterization. Therefore, there is a gross scarcity of epidemiologic data pertaining to this group. We speculate that it is often underestimated (both due to under-recognition and under-diagnosis). In contrast to pulmonary thrombo-embolism, the complex and diverse pathogenesis of different subtypes of emboli is subject to continuing speculation and extends beyond "simple'' mechanical obstruction of pulmonary vasculature. Non-thrombotic emboli may also be associated with a severe inflammatory response both in the systemic and pulmonary circulation, unlike pulmonary thromboemboli.

The diagnosis of NTPE is even more challenging given lack of specific clinical features, heterogeneity of radiographic findings as well as lack of specific laboratory blood tests. Nonetheless, NTPE can be associated with some specific radiographic findings and familiarity with these features should aid in prompt diagnosis (Han et al.,2003). High index of clinical suspicion in the appropriate clinical setting often paves the way to prompt diagnosis. It is important that the correct type of pulmonary emboli be identified, since treatment and prognosis vary considerably. In this chapter, we have summarized the current concepts of various types of NTPE.
