When the beam encounters structures much denser than

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deeper into the tissue. When the beam encounters structures much denser than liquid (such as bone or calcifications) or much less dense than liquid (air), a great deal of scatter and reflection occur. This leads to a loss of signal penetration distal to the air or bone. Thus, structures with a density much different than liquid are often referred to as strong reflectors. Acoustic shadowing extends from the strong reflector to the edge of the screen, much as the shadow from someone standing up during a movie extends all the way from that person to the movie screen. Reverberation occurs when the sound beam “bounces” between two highly reflective structures ( Figure 1.26 ). It can appear as recurrent bright arcs that are displayed at equidistant intervals from the transducer. These arcs – called A-lines when visualized on imaging the lung – represent reflections between the skin/transducer interface and the pleura or other bright reflectors in the Figure 1.25 Shadowing caused by rib (R, left ), gallstone (G, middle ), and air (bowel gas, B, right ). Note the dark shadows cast from the object toward the far end of the screen. Figure 1.26 Reverberation artifact (arrows, left ) and comet tail artifact or B-lines (arrows, right ). Fundamentals 17 Fundamentals
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body. Thus, artifactual “pleura” lines appear on the screen at depths which are multiples of the true distance from skin to pleura. A second clinically important variation on this is when sound gets trapped between two highly reflective structures that are closely opposed, such as visceral and parietal pleura. The fibrous tissue traps the sound beam, and it “bounces” infinitely back and forth such that the reflected echo is interpreted as a straight bright white echo also known as a comet tail . This concept is reviewed again in subsequent chapters because these artifacts are clinically important when evaluating the lung. Reverberation can also refer to the property of some materials to vibrate in response to the energy of the ultrasound beam. Metallic objects such as needles create bright white lines distal as they vibrate and create additional signal interpreted by the ultrasound machine as additional iterations of the same object, deeper than the true object. This phenomenon has also been described as a ring-down artifact. Refraction occurs when a sound beam obliquely crosses a boundary between tissues with different propagation speeds ( Figure 1.27 ). It appears as an acoustic shadow, originating from the point where the sound beam changes direction. This is often seen at the edge of a blood vessel in transverse orientation. Mirror images occur when an ultrasound beam undergoes multiple reflec- tions and an incorrect interpretation results ( Figure 1.28 ). When the beam encounters a bright reflector ( R ), some of the acoustic energy is reflected backward. When this beam path encounters an object ( A ), information about its relative brightness is relayed back to the transducer. However, its depth is miscalculated because the machine assumes the ultrasound beam took a straight path toward the target object. Because the reflected path (solid
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