Once a probe is applied on an intercostal space, only artifacts (from bones and lungs) are visible. These artifacts were always considered undesirable. Let us see them with more attention. For the sake of rapid communication, they were given short names using alphabetic classification (we describe 12 of them at the pleural line: A-, B-, C-, F-, I-, J-, N-, O-, P-, T-, X-, and Z-lines). This is simpler than seemingly at first view. Other artifacts are described above the pleural line (E-, S-, W-line), in other parts of the body (G-, M-, R-, V-, U-lines), or outside the body (H-, K-lines). Most are either horizontally or vertically oriented.

The normal artifact arising from the pleural line, i.e., displayed in Merlin’s space, is the repetition of the pleural line, a roughly horizontal hyperechoic fine line parallel to the pleural line (Fig. 9.1). We coined this artifact the A-line, following the alphabetic logic in a nascent discipline. Air blocks the ultrasound beam, which comes back to the head, yielding this regular artifact. The distance between the pleural line and the A-line is equal to the skin-pleural line distance. Several equidistant A-lines can be visible. They can be called A1-lines, A2-lines, etc., according to the number of observed lines, with little clinical relevance. Of same relevance, horizontal artifacts are sometimes seen between two A-lines and called “sub-A-lines” and even “sub-sub-A-lines” (Pi-lines, see Fig. 40.​4).