# Lung Ultrasound Imaging, a Technical Review
## 1 Introduction
B-lines are clinically defined as hyper-echoic vertical artifacts that originate from a point along the pleura-line and lie perpendicular to the latter, and are thought to originate from a reduction of the lung volume originally occupied by air in favor of liquid or tissue. This reduction locally diminishes the acoustic impedance mismatch between the intercostal tissues and the lung. Resulting open channels accessible to ultrasound waves ultimately generate these patterns. Differently, A-lines are visualized in those conditions where the lung surface behaves essentially as a perfect reflector to ultrasound, impeding propagation beyond the pleura line. A-lines are due to the multiple-reflections of ultrasound waves that occur between the probe and the lung surface and can be described as multiple equidistant hyper-echoic horizontal lines which are generally visualized across the entire image and that are parallel to the pleura line. The pleural-line (indicated by arrows in Figure 1) can be defined as the line that separates the intercostal tissue and the lung in the image. The possibility to discriminate between a diseased and a healthy lung boosted the clinical interest in lung ultrasonography (LUS) [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. However, these diagnoses are based on imaging-artifacts, rely  
Figure 1: B-line artifacts as observable with a linear (**left**) and convex (**center**) probe and A-line artifact as observable with a linear probe (**right**). The pleura line is also indicated for the three images. on the qualitative and subjective interpretation by the clinicians [34; 35; 36; 37], and are performed using equipment and modalities inadequate to investigate the lung [36; 37]. Indeed, standard ultrasound imaging is designed assuming a high-similarity between the acoustic properties (e.g., speed of sound) of soft-tissues, which is primarily due to their high water content [38]. This allows sufficient contrast for imaging and at the same time does not impede transmission through the different tissues, and hence propagation up to the required depths. Moreover, thanks to moderate variations in speed of sound throughout soft-tissues [39; 40], the position and shape of different objects in the field-of-view can be estimated with sufficient accuracy. The aforementioned conditions are definitely not applicable to the lungs, which are air-filled organs, and being air, a medium presenting significant differences in acoustic properties compared to liquids and soft-tissues [38; 41]. This implies that the straightforward application of standard ultrasound imaging to the inspection of the lung will obviously result in images that do not represent the actual anatomic structure but can only spot deviations from a healthy condition by displaying artifacts such as the aforementioned B-lines. The genesis of B-lines and their relationship with the alterations of the lung still remain an open question. In addition to medical/clinical-studies, several technical-studies have been performed since the 60's attempting lung characterization (on restricted frequency ranges) based on standard soft-tissue acoustic properties, i.e., speed of sound and attenuation [38; 39; 40; 41; 42; 43]. This type of characterization is however insufficient, and certainly not specific, for the accurate diagnosis of lung diseases. Once again, treating the lung as a conventional soft tissue does not provide optimal results. Instead, dedicated transmission schemes and signal processing algorithms should be designed and implemented in order to take into account the specific properties of the lung. In the next two sections, two approaches attempting to go exactly in this direction are presented and discussed.