# Lung Ultrasound Imaging, a Technical Review
## 1 Introduction  
Lung diseases are among the most common pathological conditions worldwide. As an example, interstitial lung diseases (ILDs) group over 200 diseases including pneumonia, tuberculosis, and pulmonary fibrosis. Pneumonia alone is the first cause of death in children under five years of age worldwide, with 808,694 deaths in 2017 (15% of all deaths of children under five) [1]. Chronic obstructive pulmonary disease (COPD), classified by WHO as an under-diagnosed life-threatening lung disease, and lower respiratory infections are the third and fourth leading causes of death, respectively, and are together responsible for six million deaths each year [2]. Idiopathic pulmonary fibrosis (IPF) is a major public health problem, with 50,000 new cases diagnosed each year [3]. Although it is not a lung pathology per se, because it often originates from congestive heart failure, pulmonary edema is also a major health issue, affecting over six million in the U.S. [4], resulting in affected lung compliance and dyspnea. Accurate diagnosis and monitoring of lung diseases are concerns of global scale. For some pathologies, the standard imaging techniques used to diagnose and monitor lung pathologies are chest x-ray and computed tomography (CT). Unfortunately, these modalities are based on ionizing radiation, representing a hazard to patient's health in case of high dose or frequent exposure, especially in the context of frequent monitoring [5; 6; 7; 8; 9]. Amongst the risks, the United States Food and Drug Administration reports an increase in the possibility to develop cancer later in life. This represents a serious concern for children and adolescents, who are significantly more sensitive to radiation exposure than adults [10]. Moreover, CT is expensive, often inaccessible, and bedside-unavailable. Some of the diseases, such as pulmonary edema can't even be monitored using CT and are currently monitored by invasive intracranial hemodynamics and devices, biomarker measurements or chest X-Rays [11]. Ultrasound technologies can potentially overcome the limitations of current monitoring techniques offering a safer, portable, and cost-effective alternative. Firstly, being a radiation-free modality, ultrasound is a diagnostic option especially relevant to children, pregnant women and patientssubjected to repeated investigations. Secondly, ultrasound devices are easily transportable to patient's site, including in remote and rural areas, and developing countries. Thirdly, devices and examinations are significantly cheaper as compared to CT or MRI, making ultrasound techniques accessible to a much broader range of facilities, thus reaching more patients. This makes ultrasound especially relevant for sustainable healthcare issues related to aging-society and increased chronic diseases. The capability of ultrasound imaging to provide access to relevant diagnostic information on pulmonary-tissue has been suggested since the 1990s [12; 13; 14; 15]. For example, sonographic interstitial syndrome is a recognizable pattern observable from lung ultrasound-investigation and characterized by the appearance of several acoustic artifacts known as \"comet-tails\" or \"B-lines\" [14; 15; 16; 17]. A correlation exists between these artifacts and the increase in extravascular lung water [17], ILDs [18; 19; 20], cardiogenic and non-cardiogenic lung edema [21], interstitial pneumonia [22; 23], and lung contusion [24]. Moreover, a clearly detectable difference exists between the aforementioned artifacts and the artefactual pattern generally observable with ultrasound imaging when inspecting a healthy lung. Figure 1 shows an example of lung ultrasound images with B-lines (vertical artifacts), as obtained with a linear and a convex probe. An example of A-line artifact (horizontal artifacts) is also shown.