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We like to think of the ultrasound beam as extending from the probe in a very thin fan or rectangle, and this is exactly what the processor thinks it sees. In reality, there are smaller beams that travel laterally to the main beam. When one of these smaller side beams is of sufficient strength and bounces off a highly reflective surface, such as the wall of the urinary bladder, it will be interpreted as coming from the main beam and the processor will place the resulting image within the main beam image, often mimicking sediment. The resulting image is usually weaker in intensity than the main image. Often, the artifact can be altered by changing probes or lowering the focal point, or lowering the gain setting – all ultrasonographic manipulations that will not remove true pathology (i.e., bladder sediment, bladder stones, etc.) (Penninck 2002) (Figure 3.7).

Figure 3.7. Sediment versus side‐lobe and slice‐thickness artifact. (A) Sediment will be affected by gravity and lead to a flat surface, as seen with the sludge within this gallbladder's lumen. True sediment can be stirred up by repositioning the patient or through ballottement with the ultrasound probe, whereas artifacts mimicking sediment cannot. (B) Slice‐thickness and side‐lobe artifacts mimic sediment in the gallbladder shown here; however, the artifact will not be altered by moving the patient's position or by ballottement. (C) True sediment in a urinary bladder with ballottement gives a snow globe appearance. (D) In contrast, the slice‐thickness and side‐lobe artifacts mimicking sediment shown here will fail to ballot (will not "snow globe") or change position to the gravity‐dependent side of the urinary bladder when the patient is moved. Other helpful tricks that discriminate true sediment from artifact include lowering the gain and/or moving the focus cursor. Generally speaking, artifacts can be eliminated by these maneuvers, but true sediment cannot.

Source: Courtesy of Robert M. Fulton, DVM, Richmond, VA.