Usually, it all starts with an abnormal finding in an echo suggestive of intracavitary mass. How can we tell one from the other? It can be confusing.
For artifacts, I did a Tweetorial already that describes the most common ones. Basic understanding of ultrasound physics is needed to be able to explain them: twitter.com/i/moments/1030…
Ultrasound waves need a media to travel. Our tissues are that media. All tissues have different acoustic impedances. Air, fat, water, blood, muscle, bone listed in increasing impedance. Always there will be reflected & refracted waves as it traverses different tissue layers.
Remember that the way the US distributes the waves in a phased array probe is similar to a folding fan, so reflection (rebound) & refraction (change in direction) occurs across the sector width (Triangle area where you see the images) look at panel B of above figure.