# Lung Ultrasound Imaging, a Technical Review
## 4 Discussion  
It appears clearly that the future of lung ultrasound lies in the development of robust and quantitative techniques specifically developed for lung tissue. This is what directed the recent research effort of the growing lung ultrasound research community. Lung ultrasound has remained limited to the critically ill and to emergency care. This is a missed opportunity that could be seized if more reliable, reproducible, and quantitative techniques were developed. We see multiple advantages to developing quantitative techniques. First, the potential availability of an absolute measure would enable standardized ultrasound lung evaluation across centers, potentially with thresholds that could be used to guide and inform treatment. Second, it would then be possible to follow the progress of a disease or to monitor the response to a treatment over time, for a given patient longitudinally. This would be of considerable help for the monitoring of pulmonary edema, which could be an indirect marker of cardiac health in the case of cardiogenic edema. It could also be an extraordinary tool for the evaluation of the efficacy of novel treatments for pulmonary fibrosis [51; 52]. It should be noted that significant and promising research efforts are also being made in the area of surface wave elastography and its applications to the lung. However, they are out of the scope of the present contribution, which focuses on the application of longitudinal waves in the parenchyma [53].  
Although much progress has been made in the recent years in the quantitative ultrasonic evaluation of the lung, potential bioeffects have not been fully investigated yet. Research efforts need to be conducted in order to ensure a fully responsible use of ultrasound for these new applications. Adverse effects of ultrasound in the lung are different than in other organs due to the presence of air. Cavitation effects leading to hemorrhage have been demonstrated in rodent lungs, but no such effects have been observed in the human lung [54; 55; 56]. On the contrary, lung ultrasound has been performed extensively in the clinic without any obvious adverse effect. However, [PERSON] et al. have demonstrated that the cavitation thresholds are much lower in the lung than in other organs [55]. Absorption by the air in alveoli may lead to thermal effects. The cavitation effects are more likely to occur at low frequencies while the thermal effects are more likely to occur at higher frequencies. [PERSON] et al. have established a new formulation of the mechanical index (MI) specific to the lung, which accounts for the frequency, the pulse repetition frequency, as well as the pulse duration. According to the literature, this index will enable us to predict risk. The MI, thermal index, and the modified MI defined by [56] should be fully investigated, although knowing the exact local pressure in the lung will not be trivial. Generally speaking, the relatively shallow depths to be investigated (the pleura lays approximately at a depth of 2 cm) allows for the use of a low MI without compromising the capability to gather information by ultrasound.  
Conceptualization, L.D. and M.M.; introduction, L.D. and M.M.; ultrasound spectroscopy LD.; ultrasound multiple-scattering characterization M.M. and T.E.; discussion L.D. and M.M.; writing--original draft preparation, L.D. and M.M.; review and editing, L.D., M.M. and T.E.; All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors want to acknowledge M.D. [PERSON] for providing the clinical ultrasound images presented in this paper.  
The authors declare no conflict of interest.