In standard B mode imaging a probe transmits brief pulses and receives the echoes reflected by every structure of the explored medium. A set of consecutive pulses are used to reconstruct a two-dimensional image even though some of the assumptions needed to do this are not completely satisfied. Consequently, numerous visual artefacts are present in ultrasound images and physicians are aware that significant discrepancies between the ultrasound images and the anatomy of the examined medium may exist. Nonetheless, such artefacts are usually analyzed accurately since they also provide clinical information and understanding the physical mechanisms which are at the basis of their formation is important. In the case of lung images, everything we see beyond the chest wall represents artefactual information since the aerated spaces of the lung reflect most of the pulse energy. Herein, we will describe the genesis of the most important lung artefacts by means of examples and mathematical models. We will discuss how the so called A lines may be generated by the reverberation effects between the probe and the pleura plane and how the so called B lines may be generated by acoustic energy which is firstly trapped and subsequently re-radiated by pathological alveoli distributions.

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