II. PHYSICS AND KNOBOLOGY
Q1: Name the three types of artifact indicated by the three types of arrows in this image of an ocular ultrasound.
Q1 Answer: Reverberation artifacts occur where there are two reflective surfaces that are perpendicular to the direction of the ultrasound beam and parallel to one another. The surface of the transducer itself can act as such a surface, creating reverberation artifacts that appear to arise from the skin, as in this case (small arrows). Behind the globe there is an area of posterior acoustic enhancement, which makes the retro-orbital soft tissues between the arrowheads appear brighter than the same tissue to the left. The left side of the eyeball (right side of screen) appears to be creating an acoustic shadow (larger arrows). If this darker area was continuous from the skin surface it could be attributed to absence of gel in this location, which can also cause shadowing on the image. However, in this case, the anterior soft tissues immediately under the probe appear symmetrical on both sides except where there is interference from reverberation artifact. The diminished echo signal in the "shadow" is due to a combination of factors and is frequently seen with circular structures such as this. The factors include the disadvantageous angle of insonation for structures that are parallel with the beam (leading to a paucity of echoes), and increased absorption of sound in the dense connective tissue walls of the globe.
Q2: Name the three types of artifact indicated by the three types of white arrows in this image of a thoracic ultrasound (bonus points for the type of artifact indicated by the black arrows).
Q2 Answer: Artifacts are used extensively in diagnostic ultrasound to identify both normal and abnormal conditions. This is particularly true of lung ultrasound. The accompanying still image serves as a guide to the artifacts that can be seen at various moments in this clip. Behind the ribs, shadowing can be seen (between the three white arrows). At the lung surface, multiple small reverberation artifacts (small white arrowheads) can be seen throughout the clip. Because they barely reach the bottom of the image, adjusted to the very superficial depth of 4.9 cm, they do not qualify as B-lines (see Chapter 6), but rather are clinically insignificant Z-lines, caused by irregularities on the surface of the visceral pleura. An A-line (also a reverberation artifact: see text) is indicated by the large white arrowheads. There is mirror artifact caused by the plural surface giving rise to the appearance of the superficial fascia of the intercostal muscles (black arrowheads) within the lung parenchyma (black arrows).
Q3: Define the following terms: