G, Elbarbary M, Blaivas M, et al. 2012. International evidence‐based recommendations for point‐of‐care lung ultrasound. Intensive Care Med 38:577–591.
Chapter Three POCUS: Basic Ultrasound Artifacts
Robert M. Fulton
Introduction
In this chapter, we'll look at the fundamental laws governing wave dynamics and see how ultrasound artifacts are created. Ultrasound machines rely on several physical assumptions to assign the location and intensity of each received echo. These assumptions are that the received echoes have originated from within the main ultrasound beam, an echo returns to the transducer after a single reflection, the depth of an object is directly related to the amount of time for an ultrasound pulse to return to the transducer, the speed of sound in tissue is constant (1540 m/sec), the sound beam and its echo travel in a straight path, and the acoustic energy in an ultrasound field is uniformly attenuated (Feldman et al. 2009). Artifacts may be grouped by the most important principles leading to their formation, including attenuation, velocity or propagation, and artifacts associated with multiple echoes. Many artifacts can be grouped by their association with air or fluid, making learning them easier.
What POCUS Basic Ultrasound Artifacts Can Do
Provide a basic understanding of how artifacts are formed to allow better interpretation of the ultrasound image.
What POCUS Basic Ultrasound Artifacts Cannot Do
Cannot provide an in‐depth discussion of ultrasound artifacts.
Cannot account for what artifacts your ultrasound machine's software is most biased towards.
Indications
Provide a basic understanding of ultrasound principles and common artifacts to maximize accurate image interpretation.
Objectives
Provide a basic understanding of how common ultrasound artifacts are formed to avoid image misinterpretation of artifacts for abnormalities.
Artifacts of Attenuation: Strong Reflectors (Bone, Stone, Air)
Shadowing, “Clean” and “Dirty”
Clean shadows and dirty shadows result from strong reflectors (bone, stone, and air). We know from differences in acoustic impedance at soft tissue–air and soft tissue–bone (stone) interfaces that most of the sound waves will be reflected, albeit in different degrees (Figures 3.1 and 3.2A).
Bone or Stone Interface: Clean Shadowing
When the ultrasound wave strikes bone (and stone), most of the waves are reflected back so there will be an area of intense hyperechogenicity (whiteness) at the soft tissue–bone (stone) interface. Because the surface of bone is often smooth, there is little scattering or reverberation of the ultrasound wave and a nice, clear‐cut, anechoic (blackness) “clean shadow” is produced beyond the reflector (bone or stone) (see Figure 3.1).
Figure 3.1. Dirty and clean shadowing. (A) "Dirty shadowing" created by air, a gas bubble within a fluid‐filled distended loop of small bowel. "Dirty shadowing" is generated because some ultrasound waves pass through the structure. Contrast the "dirty shadowing" with the "clean shadowing" of the cystourolith (urinary bladder stone) in (B) in which all ultrasound waves are reflected back to the transducer. (B) "Clean shadowing." The smooth surface of the cystourolith (urinary bladder stone) generates the "clean shadowing" typical of bone or stone with a hyperechoic (bright white) reflective surface in the near field, completely blocking all echoes and thus resulting in an anechoic (dark or black) shadow extending from it through the far‐field. In (C) and (D), the images have arrows (←) pointing out the artifact.
Source: Courtesy of Dr Sarah Young, Echo Service for Pets, Ojai, California.
Air Interface: Dirty Shadowing
On the other hand, soft tissue–air interfaces are more variable in their degree of reflection, with some of the ultrasound waves incompletely moving through the air‐filled structure, unlike the complete reflection at bone (or stone). Thus, reverberations occur distal to the air interface, creating a “dirty shadow” (Penninck 2002) (see Figure 3.1).
Artifacts of Attenuation: Fluid‐Filled Structures
Edge Shadowing: Fluid‐Filled Structures
When the ultrasound waves strike the edge of a fluid‐filled structure with a curved surface (its wall), such as the stomach wall, urinary bladder, gallbladder, eye, or cyst, ultrasound waves are reflected to a small degree and these reflected sound waves therefore do not return to the transducer. As a result, a thin hypoechoic (darker) to anechoic (black) area lateral and distal to the edge of the curved structure is formed. For example, the novice may mistake this artifact for a “rent” in the urinary bladder wall when in fact it is an artifact created by the ultrasound machine (Nyland et al. 2002) (see Figure 3.2).
Figure 3.2. Edge shadowing artifact. (A) An edge shadowing artifact is seen arising from the curved edge on the left side of the stomach wall in this image, making its wall appear to extend distally as an anechoic (dark or black) line. A dirty gas shadow is also produced from gas within the stomach lumen that appears as "pseudo B‐lines." (B) An edge shadowing artifact at the apex of the urinary bladder makes it falsely appear to have a rent, which can fool the hasty sonographer into thinking the free fluid is from a ruptured bladder; however, note the smooth expected normal contour of the urinary bladder. In (C) the edge shadowing artifact is outlined with arrows (←). In (D) the edge shadowing creates a false rent in the wall of the urinary bladder (circled) and the superimposed curved line points out the edge shadowing from the curved surface of the urinary bladder wall extending through the far‐field.
Source: Courtesy of Dr Sarah Young, Echo Service for Pets, Ojai, California.
Acoustic Enhancement: Fluid‐Filled Structures
When the sound beam passes through a fluid‐filled
