Multiple reflections (reverberations) can occur between two strong parallel reflectors or between the transducer and a strong parallel reflector (Feldman et al. 2009). The echoes are reflected back and forth repeatedly and may be strong enough to be detected by the instrument and displayed (Fig. 1.7). The spuriously reflected echoes are displayed beneath the real reflector at intervals equal to the distance between the transducer and the real reflector (Gibbs et al. 2009). Each subsequent reflection is weaker than the previous one due to attenuation. Comet-tail artifacts are usually seen in echo-free regions and are short, being less than 2 cm in length. Examples of reverberation artifacts include crystalline deposits of cholesterol in the near wall of the gallbladder, calcification within tissues, or metallic structures. Reverberation artifact may also occur at the anterior peritoneal interface of the abdomen (Fig. 1.8a, b).

Ring-down (or resonance) artifact has a similar ultrasonographic appearance to comet-tail artifact but is generated by a different mechanism (Feldman et al. 2009). When the transmitted ultrasound beam passes through small gas or fluid bubbles, the energy within the beam causes vibration of the gas or fluid bubbles which, as a result, emit and return sound waves back to the transducer (Fig. 1.9). Discrete echogenic lines or parallel bands of ring-down artifact extend from the point of origin. An example of ring-down artifact can be found with air or fluid in the duodenum (Fig. 1.10).

This artifact occurs when an object is located directly in front of a highly reflective smooth surface (Hedrick et al. 2005; Abu-Zidan et al. 2011). The ultrasound beam is totally reflected by the strong reflector towards the object. When the beam hits the object, part of the energy is reflected back to the strong reflector, which then redirects this echo towards the transducer. The true and false images are equidistant from but on opposite sides of the strong reflector (Fig. 1.11). Mirror-image artifacts are most commonly seen where there is a large acoustic mismatch, such as an air-fluid interface. The diaphragm, for example, is a strong reflective interface, and the liver parenchyma is displaced above and below the diaphragm. Similarly, during scanning of the urinary bladder, the rectum can act as a specular reflector leading to a mirror image of the urinary bladder being displayed posterior to the rectum (Fig. 1.12).

The speed of sound varies within different tissues (Feldman et al. 2009). However, ultrasound technology assumes that sound has a constant speed (1,540 m/s) within human tissue. If the average speed through the tissues is greater than 1,540 m/s, the returning echo will return faster to the transducer than calculated. The structure will thus be displayed more superficially on the image than expected. Conversely, if the average speed through the tissues is lower than 1,540 m/s, the structure will appear deeper than it is in reality (Fig. 1.13). Examples of velocity artifact include ultrasound imaging of a hyperechoic hepatic mass that leads to an ambiguous image of the diaphragm or ultrasound imaging of a silicon breast implant that results in a distorted image of the fibrous capsule surrounding the implant (Fig. 1.14). The image posterior to the hyperechoic lesion/silicone implant will appear deeper than it is in reality. As a result, the diaphragm or capsule appears to be distorted.

Refraction of the ultrasound beam at the boundary between two media with different acoustic velocities (due to different density and elasticity) will result in misregistration and defocusing artifact (Feldman et al. 2009). In refraction artifact, a non-perpendicular incident ultrasound beam will refract and change direction at a boundary between two materials of different acoustic impedance. The ultrasound display process assumes that the beam travels in a straight line and thus misplaces the returning refracted echoes to the side of the true object location (Fig. 1.15).