## 4 Summary and Outlook  
Lung ultrasound has been used for decades, becoming a valid clinical tool for characterizing peripheral-pleural lung consolidations. In contrast to CXR, which determines density variations, LUS can differentiate atelectasis, pneumonia, lung infarct, and is useful for the diagnosis of heart failure and pneumothorax. In order to achieve unimpaired ultrasound imaging of the entire lung, a replacement of gas by a liquid fraction is needed. So far, two methods have been investigated for the purpose of endobronchial liquid instillation. Herein, OLF using saline generates unimpaired sonographic imaging. The use of inert PFC has physiological advances although it is acoustically inappropriate. The liquid must have identical properties, with regard to speed of sound and impedance, as the alveolar tissue. Such properties so far can only be provided by physiological saline (see Table 1). Using PFC liquids with deviating acoustic properties, sound diffraction and scattering appear in the flooded alveolar parenchyma, which impairs any sonographic imaging. Therefore, no progress has been seen in the improvement of PFC based ultrasound applications in lung over processes performed with saline based OLF. There are no further needs in optimizing the acoustic conditions under saline OLF. Sonographic tumor detection rate, as well as visualization of intra-parenchymal vascular and bronchial structures are superior, and imaging in depth is slightly better than in abdominal organs. Given the valuable B Mode visualization during saline OLF, most of the accepted diagnostic and sonographic guided interventions are applicable in the lung as practiced for abdominal organs such as the liver. Particular for minimal invasive interventions, OLF is justifiable and beneficial. Performing OLF was initially motivated out of pulmonary oncology for the management of central lung nodules using sonographic guidance during VATS. Later work showed the feasibility of complete new minimal-invasive treatment strategies, such as non-invasive HIFU ablation of central lung nodules. However, despite the promising results, OLF related research was based on pre-clinical models. Therefore, studies showing the advances of OLF based sonographic guided interventions in a clinical setting are of high demand. The investigation of puncture hit- and complication rate of sonographic guided biopsy during OLF in comparison with CT would be the logical next step. Herein, all the techniques are clinically established, and the outcome is immediately measurable. Conceptualization, F.W. and T.G.L. ; Literature Research, F.W. and T.G.L. ; Writing, F.W. ; Reviewing-Editing, F.W. and T.G.L. All authors have read and agreed to the published version of the manuscript. There were no funding sources for this work. The authors  
* _[PERSON] et al. (2012)_ [PERSON]; [PERSON]; [PERSON] Intrathoracic tumors in contact with the chest wall--Ultrasonographic and computed tomography comparative evaluation. _Med. Ultrason._**2012**, _14_, 115-119. [PubMed]
* _[PERSON] et al. (2008)_ [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Ultrasound vs. CT in detecting chest wall invasion by tumor: A prospective study. _Chest_**2008**, _133_, 881-886. [CrossRef] [PubMed]
* _[PERSON] et al. (2018)_ [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON] Usability of Transthoracic Shear Wave Elastography in Differentiation of Subpleural Solid Masses. _Ultrasound Q._**2018**, _34_, 233-237. [CrossRef]
* _[PERSON] (2005)_ [PERSON]; [PERSON] Pulmonary applications of perfluorchemical liquids: Ventilation and beyond. _Pediatr. Respir. Rev._**2005**, \(6\), 117-127. [CrossRef]
* _[PERSON] et al. (2017)_ [PERSON]; [PERSON]; [PERSON] Whole lung lavage--Technical details, challenges and management of complications. _[PERSON]. Thorac. Dis._**2017**, \(9\), 1697-1706. [CrossRef]
* _[PERSON] et al. (2000)_ [PERSON]; [PERSON]; [PERSON]; [PERSON] From isoflurane to perfluorohexane?