eye, or a cyst, ultrasound waves do not become as attenuated as the neighboring waves passing through more solid tissues to either side of the structure. Therefore, the tissues on the far side of the fluid‐filled structure appear much brighter than the neighboring tissues at the same depth. Acoustic enhancement is obvious, looking past the fluid‐filled gallbladder and urinary bladder (Figure 3.3). On the other hand, by realizing how the artifact is formed, the acoustic enhancement artifact can be useful to the savvy sonographer in determining if a structure of interest is fluid filled (brighter through the far‐field having acoustic enhancement) or soft tissue (lacking acoustic enhancement) (Penninck 2002) (see Figure 3.3).
Figure 3.3. Acoustic enhancement artifact. Because there is less attenuation when sound moves through fluid, the area distal to a fluid‐filled structure will appear hyperechoic (brighter or whiter) to the surrounding tissue. Note the hyperechoic regions distal to the gallbladder (A) and distal to the urinary bladder (B) that are outlined with arrows (←) in (C) and by white bars in (D).
Artifacts of Velocity or Propagation
Mirror Artifacts: Strong Reflector (Air)
When we image a structure that is close to a curved, strong reflector such as the diaphragm (remember this is actually the lung–air interface following the curve of the diaphragm), a sound beam can reflect off the curved surface, strike adjacent tissues, reflect back to the curved surface, and then reflect back to the transducer. Because the processor only uses the time it takes for the beam to return home and cannot “see” the ongoing reflections, it will be fooled into placing (mirroring) the image on the far side of the curved surface. The classic place for a mirror artifact is at the diaphragm, and the classic mistake is interpreting the artifact as a diaphragmatic hernia (Penninck 2002) (Figure 3.4).
Figure 3.4. Mirror artifact. The gallbladder appearing to be on both sides of the diaphragm is the classic example of mirror artifact, created by a strong soft tissue–air interface. The mirror image artifact also may be generated under similar circumstances when the fluid‐filled urinary bladder lies against the air‐filled colon. (A) The white arrows illustrate the actual path of the sound beam reflecting off the curved lung–air interface against the diaphragm, while the black arrows illustrate the path perceived by the ultrasound processor. Note that the gallbladder falsely appears as if it is within the thorax, and should not be mistaken for a diaphragmatic hernia. (B) Mirror image artifact in which it appears that the liver and gallbladder are on both sides of the diaphragm (*). (C) Mirror image artifact in which it appears that liver (gallbladder not present in this view) is on both sides of the diaphragm (*).
Source: Courtesy of Robert M. Fulton, DVM, Richmond, VA.
Reverberation or A‐Lines: Strong Reflector (Air)
Reverberation occurs when sound encounters two highly reflective layers. The sound is bounced back and forth between the two layers before traveling back to the receiver. The probe will detect a prolonged traveling time and assume a longer traveling distance and display additional reverberated images in a deeper tissue layer. The reverberations can get caught in an endless loop and extend all the way to the bottom of the screen as parallel equidistant lines, referred to as A‐lines (also see Chapters 22 and 23). This artifact most commonly extends beyond air‐filled structures within the thorax, (e.g., lung) and within the abdomen (e.g., gastrointestinal tract), with varying width (Penninck 2002) (Figure 3.5; see also Figures 3.1 and 3.2).
Figure 3.5. Reverberation artifact. (A) Reverberation artifact, also known as A‐lines (think of it as "A" for air). A‐lines are seen as regularly spaced parallel lines illustrated by the arrows (←) outlined in (C). The most proximal arrow in the near‐field denotes the lung's surface, evident within the intercostal space between two ribs on either side (ribs [bone] creating the "clean shadowing" through the far‐field). (B) The very tight and distinct band of reverberation artifact, referred to as a comet‐tail or ring‐down artifact, caused by sound waves reflecting off a metal needle used during abdominocentesis in the near‐field. (D) The same image but with an arrow overlay to show the tight band of A‐lines as the comet‐tail or ring‐down artifact. Any strong reflector of ultrasound waves produces this artifact that typically involves bone, stone, or metal, such as implants, needles, and foreign bodies.
Source: Courtesy of Robert M. Fulton, DVM, Richmond, VA.
Comet‐Tail or Ring‐Down Artifact: Strong Reflector (Usually Metal or Bone But Can Be Air)
A comet‐tail artifact, also called a ring‐down artifact, is similar to reverberation. It is produced by the front and back of very strong reflectors with high acoustic impedance, such as metallic foreign bodies or implants, needles, and stylets during ultrasound‐guided procedures (see also Chapters 43 and 44), or strong reflectors with very low acoustic impedance relative to their adjacent soft tissues, such as gas in the lung, gas bubbles, or gas in the bowel. The reverberations are spaced very narrowly and blend into a small band. The greater the difference between the acoustic impedance of the reflecting structure and the surrounding tissues, the greater the number of reverberation echoes (Reef 1998) (see Figure 3.5).
B‐Lines: Strong Reflector (Air Immediately Next to Fluid)
Ultrasound lung rockets (ULRs), more recently termed B‐lines (Volpicelli et al. 2012), are vertical, narrow‐based lines arising from the near‐field's pulmonary–pleural line, extending to the far edge of the ultrasound screen, always obliterating A‐lines, and moving “to and fro” in concert with inspiration and expiration. Although B‐lines are similar to comet‐tail artifacts, they are specifically created by the strong difference in impedance of air adjacent to a small amount of water and are the ultrasound near‐equivalent of radiographic Kerley B lines (representing interlobar edema). Their clinical relevance is very important and explained later (see Chapters 22 and 23) (Lichtenstein and Meziere 2008; Lichtenstein et al. 2009; Lisciandro 2011; Volpicelli et al. 2012) (Figure 3.6).
