Bayonet artifact is a type of artifact encountered in ultrasound guided needle techniques, that results in the apparent bending of the needle as it passes into or adjacent to tissue with different sound transmission speeds.
In constructing an ultrasound image, the machine assumes a constant speed of sound, but some tissues conduct sound at slightly higher or lower speeds 1. When the needle passes into the vessel or a lesion surrounded by fat, there can appear to be bending of the needle towards the transducer (in the case of the vessel) or away from the transducer (in the case of fat).