
Artifacts can be defined as any structure in an ultrasound image that does not have a corresponding anatomic tissue structure.
Artifacts may be classified into four main categories: missing structures, degraded images, falsely perceived objects, and structures with a misregistered location.
Read more here: http://www.scahq.org/sca3/events/2012/echo/fundamentals/submissions/mon/Artifacts%20od%20Ultrasound%20-%20Heller.pdf
Learn more about the physics of ultrasound at Gulfcoast Ultrasound Institute