COMET TAIL ARTIFACT: a COMMON MISUNDERSTANDING in LUS

 

The ring down artefact (RDA) 

Arvuch et al demonstrated in experiments that emitted waves from the resonant vibrations within a bubble tetrahedral complex (four air bubbles arranged in a tetrahedron formation and trapping a bugle-shaped column of fluid within), in response to insonating US, creates the RDA [11].

Morphologically, the RDA is a linear artefact in the axis of the insonated ultrasound wave that starts from the point of origin and “ringsdown” to the end of the screen without fading. 

In contrast, a CTA, which is created via a reverberation mechanism dependent on acoustic impedance differences between mediums, fades and tapers with increasing depth. Depending on the gain setting on the US, the RDA may appear as continuous or as successive stacks of short horizontal bands. The thickness of the horizontal bands and the distance between them (periodicity) is always constant, unlike in CTA (fig 1).

B-lines: a Ring-down artefact [RDA]

 B-lines is a specific term for RDA found in LUS and they must have the following characteristics, in addition to the standard properties of RDA: 

• it originates at the pleural line (PL), 

• it has a strong light ray-like appearance, obliterating other background LUS artefacts, 

• it moves with lung sliding. 

Physiologic B-lines 

The normal lung is characterized by the absence or presence of very few B-lines; less than three per field of view. These “physiologic” B-lines, seen in only 10 % of young healthy subjects [21], are often a transient phenomenon and change with posture. The dependence of B-lines creation on availability of a medium that could create a resonating vibration (bubble-tetrahderal complexes or equivalent) probably explains why they are more likely found in the better perfused dependent areas of the lung or at the thicker interlobular septa [5,15]. Age related changes in the lung parenchyma such as fibrosis and sub-pleural lesions account for increased B-lines observed in 37% of the elderly [21]. 

Pathologic B-lines 

In pathological processes involving the lung parenchyma, fluid, inflammatory infiltrates or cellular content progressively increase, greatly enhancing the environment for the generation of B-lines (fig 2). As a guide, a finding of a cluster of three or more B-lines per intercostal space may be pathological. However, the number of B-lines and the number of chest areas positive for (multiple) B-lines increases with age [21-23] and therefore one must be cautious of a false positive interpretation in older persons. 

The often-used term “interstitial syndrome” [13,24] is a description of ultrasound findings of pathological B-lines. The term is not specific or synonymous to acute interstitial lung disease [6,25]. These are examples of conditions that produce a generalized and often bilateral interstitial syndrome: cardiogenic pulmonary edema [13], acute or chronic interstitial lung disease [26,27] or acute lung injury / acute respiratory distress syndrome [28]. Focal interstitial syndrome may be observed in relation to pneumonia [13,29], pulmonary contusion [30,31], lung tumors [29,32] or other pulmonary consolidating processes [29,33]. As B-lines are sensitive but non-specific for pathological lung parenchyma changes, the findings of interstitial syndrome must be correlated with the following information [34] to make a clinical diagnosis:

 • distribution of B-lines (interstitial syndrome) in the lung fields: patchy, uniform, symmetry,

• evaluation of PL morphology [35,36]: lung sliding, thickening, unevenness,

 • other LUS features: consolidation, sub-pleural lesions, effusion, 

• clinical information of the patient: oxygen saturation, blood gases, other laboratory findings,

 • medical history of the patient: e.g. history of pleuritis, thorax injury or surgery, interstitial lung disease or connective-tissue/ rheumatic disease. In the clinical context of acute dyspnea and desaturation, the utility of B-lines in LUS diagnosis is limited in patients with pre-existing interstitial syndrome unless prior LUS reports and images are available for comparison. 

Additional US studies such as echocardiography are often helpful in clarifying the cause of the interstitial syndrome. 

What kinds of CTA can be seen in LUS? 

Lung comets 

In the normal lung, a specific type of CTA can be seen. These are short (almost always less than 1cm) vertical artefacts that taper and fade with increasing depth. Similar to B-lines, they arise from the PL and move with lung sliding; these signs point to their origin from a reverberation mechanism occurring at the peripheral lung parenchyma or the inter-pleural layer and the dependence on the apposition of the visceral and parietal  pleura for their creation. They are found in all areas of the lung [3,14,18,19], best visualized with higher frequency transducers. It is interesting to note that studies that use low frequency transducers for LUS do not specifically mention them. Numerous CTA seen together sometimes gives the PL a “beads on a string” appearance. As there is currently no specific term to describe this type of CTA, the authors propose the term ‘lung comets’ for the purpose of discussion henceforth. 

Z-Lines 

Z-lines are observed as vertical artefacts in LUS, especially in thin individuals. Z-lines do not originate at the PL and do not move with lung sliding [37,38]. They are typically weak in appearance, blend with surrounding artefacts (e.g. A-lines) and fade with increasing depth. These characteristics and the varying periodicity and thickness of the horizontal bands that comprises the Zline are testimonial to a reverberation mechanism occurring outside the lung. They have no clinical significance, but it is important not to confuse them with B-lines.

Lung comets versus B-lines in disease diagnosis 

The visualization of lung comets or B-lines in LUS gives proof that the visceral and parietal pleura are in contact. They are therefore important supporting signs, besides lung sliding, in excluding a pneumothorax. Verification of B-lines is often taught as a method for ruling out pneumothorax. However, this criterion is limited by low sensitivity because B-lines are rarely found in the normal lung [17,19], especially in the upper parts where pneumothorax tends to occur. This fact was clearly expressed in Lichtenstein’s validation of B-lines (then called “CTA”) in ruling out pneumothorax [38], where none of the subjects with normal lungs had B-lines. B-lines are more useful if their presence is already demonstrated prior to the pneumothorax event, e.g. in an intubated patient with pulmonary edema, and comparisons of scans show the subsequent disappearance. In contrast, the use of lung comets in this context is much more effective simply because of their ubiquitous nature. In the challenging situation of determining the cause of absent lung sliding (e.g. pneumothorax vs non-ventilation in critical care), one should actively look for lung comets and be less reliant on B-lines. This approach suggested by the authors has not been validated in research but it is already well known as evidenced by images of lung comets used in several discussions on pneumothorax diagnosis [39,40]. Due to their common occurrence in the normal lung, lung comets (c.f. B-lines) should not be used to assess for interstitial syndrome and to do so is to make a diagnosis when there is none.

RDA other than B-lines 

RDA at pathological interfaces 

RDA that do not originate from PL are not B-lines. A classic example is seen at the shred line; the interface between a consolidation and the aerated part of the lung [37]. This region has the right conditions for forming bubble tetrahedral complexes and RDA often arise from it. A lung compressed under the weight of a pleural effusion sometimes generates RDA (named sub B-lines by Lichtenstein) from the visceral pleural. In both these scenarios, these RDAs do not necessarily indicate the presence of interstitial syndrome. 

I-lines

 I-lines [37] share all the same characteristics as B-lines except they are short, about 3-4 cm in length. They are occasionally seen at the sites of the inter-lobar fissures. They could represent a partially visualized B-line, and currently have no clinical significance assigned to them. 

Other described vertical artefacts in lung ultrasound

 E-lines 

Air in the subcutaneous tissues of the chest wall can cause LUS artefacts that simulate the lung. The sonographer may mistake the layer of air as the PL. On closer examination, the pseudo-PL is uneven and varying in thickness; the ribs are not visible and there is no bat sign. Uncommonly, small pockets of air may create RDA or CTA. These vertical artefacts that arise from the subcutaneous layer are collectively called E-lines [37,41] 

W-lines 

W-lines [37] is just a variation of E-lines where the point of origins of the vertical artefacts are scattered at different planes.