"Synthetic" comets: a new look at lung sonography.

Ultrasound interstitial syndrome is an echographic pattern of the lung characterized by the presence of multiple acoustic artifacts called "comets" or B-lines. It correlates to increase in extravascular lung water and to interstitial lung disease. From the physical and genetic point of view, the characteristics and the entity of this correlation have not yet been studied. The purpose of this study was to extrapolate past observations and demonstrate how comets or B-lines are artifactual images whose formation is linked to ultrasound interactions on discretely aerated tissues of variable density. Echographic comets were studied by scanning a wet synthetic, partially aerated polyurethane sponge (phantom). Density of the phantom in different drying phases was measured and correlated to the presence of echographic artifacts. Artifacts (comets) showed a different concentration from a completely white artifactual field to presence of rare comets. Their density correlates with porosity and geometry of the phantom. In our opinion, comets represent superficial, artifactual, density and geometry correlated phenomenon due to the acoustic permeability of a broken (collapsed) specular reflector, normally present when the phantom is dry.