Figure 12.

When the US beam travels through an area with strong impedance, the delayed return of the US signals to the transducer would lead to overestimation of the depth of the object. In contrast, if the target is located in the area of low acoustic impedance, the object shown on the monitor would appear shallower that its actual depth (A). During deep peroneal (fibular) nerve block; as the propagation speed of sound waves in the muscle is faster than that in the fat, the needle shaft in the fat pad will be seen bended toward the tibia bone (B). White arrow: refraction artifact; asterisks: injectate. AT: tibialis anterior muscle; EHL: extensor hallucis longus muscle; EDL: extensor digitorum longus muscle; N: deep peroneal (fibular) nerve. Blue arrows: the projected and reflected US beams.