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ung sonography is widely accepted and used in emergency medicine and critical care. [1][2][3][4][5] Moreover, many pulmonologists are interested in chest sonography for the study of pleural diseases and are increasingly discovering a role for sonography in parenchymal lung diseases. [6][7][8][9] For those physicians who are devoted to chest sonography, a clear dichotomy between usual sonography and aerated tissue sonography is obvious. Pleural sonography is effective under most circumstances, whereas lung sonography is effective only when certain physical properties of the lung (eg, the bubble system) are lost. In other words, the lung is sonographically explorable only when it is physically comparable with soft tissue. In particular, when using lung sonography, a lung that contains dispersed air and has a density that is not comparable with the density of water does not show anatomic images but rather artifactual images. 10 Therefore, lung artifacts are quite consistent with the physical properties of a lung that is not fully consolidated rather than with an anatomic image. 11 The physical properties of the subpleural nonconsolidated lung are the hallmarks of many pulmonary diseases, which can be roughly grouped into "interstitial diseases." If an ultrasound imaging system is used, all of these pulmonary diseases are classified by the generic term "sonographic interstitial syndrome" (B-lines with variable arrangements along the pleural line). 5 According to this view, it is not surprising that since 1997, 12 vertical lung artifacts, commonly named B-lines, have been associated with pathologic conditions ranging from pulmonary edema to fibrosis, which are characterized by a change in the subpleural physical features in terms of full and empty spaces. 11

B-lines are ultrasound-imaging artifacts, which correlate with several lung-pathologies. However, their understanding and characterization is still largely incomplete. To further study B-lines, lung-phantoms were developed by trapping a layer of microbubbles in tissue-mimicking gel. To simulate the alveolar size reduction typical of various pathologies, 170 and 80 µm bubbles were used for phantom-type 1 and 2, respectively. A normal alveolar diameter is approximately 280 µm. A LA332 linear-array connected to the ULA-OP platform was used for imaging. Standard ultrasound (US) imaging at 4.5 MHz was performed. Subsequently, a multi-frequency approach was used where images were sequentially generated using orthogonal sub-bands centered at different frequencies (3, 4, 5, and 6 MHz). Results show that B-lines appear predominantly with phantom-type 2. Moreover, the multi-frequency approach revealed that the B-lines originate from a specific portion of the US spectrum. These results can give rise to significant clinical applications since, if further confirmed by extensive in-vivo studies, the native frequency of B-lines could provide a quantitative-measure of the state of the lung.

Lung ultrasound imaging is a fast-evolving field of application for ultrasound technologies. However, most diagnoses are currently performed with imaging protocols that assume a quasi-homogeneous speed of sound in the volume of interest. When applied to the lung, due to the presence of air, this assumption is unrealistic. Consequently, diagnoses are often based on imaging artifacts and thus qualitative and subjective. In this paper, we present an image formation protocol that is capable of capturing the frequency dependence of well-known artifacts (B-lines) and visualizing it in real time, ultimately providing a quantitative assessment of the signals received from the lung. Previous in vitro studies have shown the potential of B-lines native-frequency for the characterization of bubbly medium, but this paper presents the first results on clinical data. The image formation process has been designed to work on lung tissue, and ultrasound images generated with four orthogonal bands centered at 3, 4, 5 and 6 MHz can be acquired and displayed in real time. Results show that B-lines can be characterized on the basis of their native frequency in vivo and open the way toward real-time quantitative lung ultrasound imaging.