Fig. 2

Determination of the skin entry site. The skin entry site is easily determined from the depth of the vessel (D) and angle of the needle (θ). Note the distance from the edge of the probe and the ultrasound-generating site. This distance may differ to some extent between probes. For example, if tanθ is 1.5 and D is 6 mm, L is calculated as 5 mm. It is necessary to know the operator’s own approximate insertion angle in advance