# Lung Ultrasound Imaging, a Technical Review
## 2 Ultrasound Spectroscopy  
The main hypothesis behind the genesis of B-lines, is that they represent how ultrasound scanners visualize the signals originating from the multiple reflections experienced by ultrasound waves when trapped within channels that can form between the lung air spaces in the presence of a pathological condition [31; 36]. In fact, in presence of many alterations of the lung structure, such as edema, fibrosis, contusions, the volume originally occupied by air can be replaced by fluids, blood or tissue. These alterations locally reduce the mismatch in acoustical properties with respect to the intercostal-tissue, and open channels for ultrasound to propagate. Most importantly, the dimensions and shapes of these channels regulate the spatial distribution and dimensions of the remaining air-spaces and vary depending on the severity and type of disease. In this context, the periodicity of the reflections, and hence the frequency content associated to the B-lines, could thus be explored as a way to quantitatively characterize the channels. This characterization can then be utilized to indirectly assess and grade the condition of the lung. Preliminary results obtained on lung mimicking phantoms already proved the possibility to characterize bubbly structures by analyzing the B-line \"native-frequency\" [44]. In this study, different lung-phantoms designed to mimic different pathological conditions where scanned using standard ultrasound imaging and a dedicated multi-frequency imaging scheme. In particular, two types of phantoms where fabricated using two distinct mono-disperse microbubble populations. The first phantom type was made with microbubbles with a diameter equal to 170 \(\upmu\)m, the second with microbubbles with a diameter equal to 80 \(\upmu\)m. This allowed for the analysis of B-lines in a controlled environment where the alveolar size reduction, which is typical of various lung pathologies, was mimicked. In fact, the typical alveolar diameter in a healthy lung is around 280 \(\upmu\)m [45]. In total, ten phantoms were fabricated, five for each type. These phantoms were scanned using the ULA-Op research platform in combination with a LA332 (ESAOTE, Flore, Italy) Linear Array probe. Two different imaging modalities have been tested. First, standard ultrasound imaging at 4.5 MHz. Second, a multi-frequency imaging method was implemented and tested. In this case, images were sequentially generated using orthogonal sub-bands centered at different center frequencies, i.e., at 3, 4, 5, and 6 MHz. The driving signals were designed such that frequency-orthogonality, at \(-10\) dB, was guaranteed. At the same time the same bandwidth excited by the 4.5 MHz pulse was (globally) covered. The analysis of these data revealed that B-lines were only visible on phantom-type 2 (smaller bubbles) when standard ultrasound imaging was used. This suggests a relation between bubble size (the air-space size) reduction and increased artifact formation. This observation is in line with the correlation between B-lines and a pathological condition of the lung. Moreover, this result also indicates that, with standard ultrasound imaging, B-lines may be visualized only at an advanced stage of the pathology, since phantom-type 2 was made with microbubbles with a dimeter much smaller than the typical diameters observed in a healthy lung. On the contrary, when the multi-frequency imaging method was used, more B-lines were visible (15 vs. 7). In particular, B-lines were also visible for phantom-type 2. Additionally, the multi-frequency imaging method allowed to observe that the B-lines obtained from phantom-type 2 were generated at higher frequencies (4 to 6 MHz) as compared to those emerging from phantom-type 1 (3 to 5 MHz). This implies that the native frequency of the B-lines may be used to quantitatively evaluate the state of the lung surface. Figure 2 shows microscope images of the generated microbubbles used to fabricate the phantoms, together with a schema and a picture of a lung phantom.