Diagnostic ultrasound machines operate according to several assumptions about the way the beam interacts with the patient. these are necessary in order for the machine to operate. These assumptions may be incorrect, the image may not correspond to the true anatomy of the scan plane. Shadowing is useful for classifying calculus-type lesions. i. e. gallstone. However fluid-filled cysts do not attenuate sound and calculus shadow depends on size relative to the beam width. For shadowing enhancement and edge effect artefacts, the artefact always run parallel to the beam direction and can be identified by changing the beam angle and seeing if the artefact remains parallel to the beam. Echoes can only be received from a volume of tissue if the beam passes through it. If the beam is greatly attenuated at an interface, and negligible sound energy is transmitted, then no echoes ill be received deep to the interface.