Fundamental Physical Principles and History of a Vertical Artefact in Chest Ultrasound: The Current “B Lines”—A Comprehensive Narrative Review for Clinicians

Over the past 40 years, thoracic ultrasound (TUS) has relied predominantly on the analysis of artefacts generated by the marked acoustic impedance mismatch between the chest wall and pulmonary air. However, ambiguous terminology has often conflated distinct physical phenomena, compromising their diagnostic utility. This review explores the physical basis underlying the genesis of vertical artefacts (B-lines), starting from the theory of ultrasound propagation in aerated parenchyma. The first comprehensive description of vertical artefacts in pulmonary pathology, termed “radiating streaks”, dates back to 1986. Furthermore, a strict distinction is emphasised between pure, rapidly decaying reverberation phenomena (comet-tail) and resonance phenomena (ring-down or B-lines). By analysing different scenarios, we demonstrate that semantic confusion, combined with an alphabetical nomenclature, has led to an overestimation of the diagnostic potential of TUS artefacts. Although B-lines represent physical artefacts rather than direct anatomical images, they retain valuable clinical utility. For instance, they serve as a rapid screening and diagnostic triage tool in emergency settings, raising the suspicion of underlying pulmonary pathologies (e.g., acute pulmonary oedema, pulmonary fibrosis, or ARDS). “B-lines” on TUS are the manifestation of acoustic phenomena (artefacts) that are also present in the residual cavity of patients who have undergone pneumonectomy, as well as in the abdominal region. These artefacts depend primarily on the soft tissue/air–fluid interface—as demonstrated by the absence of vertical artefacts during intraoperative ultrasound—and not on any actual interstitial anatomical alteration. By analysing different scenarios, this review criticises the overestimation of the diagnostic potential of TUS artefacts. We must move beyond the semi-quantitative counting of these artefacts in favour of an integrated interpretation that acknowledges the physical limitations of the method, thereby avoiding the transformation of a simple TUS software error into a specific pathological measurement.