The short-axis or out of plane approach is the technique used to visualize the vein in a cross-section while inserting the needle perpendicular to the beam axis.
In this view, the needle is only visible at the point where it crosses perpendicular to the beam path. Therefore the entire needle shaft and tip are not seen with this technique.
The vessel is positioned in the center of the screen. The needle is positioned at the center of the transducer at a 45 degree angle. Based on the Pythagorean Theorem, the needle punctures the skin at the same distance from the transducer as the depth of the vessel.
For example: if the vein is 1 cm below the skin surface, the needle should puncture the skin 1 cm away from the transducer (toward the operator) at a 45 degree angle (noble, nelson 281).
The needle is seen as a bright dot with a black shadow or reverberation artifact. The operator must angle the sound beam or slide the transducer to ensure the needle tip (not the needle shaft) is being visualized.
Ma, OJ, MD,James R. Mateer, MD, RDMS, Michael Blaivas, MD, RDMS, et al. Emergency Ultrasound, 2nd Edition. McGraw-Hill: 2007
Noble, V., Nelson, B. Manual of Emergency and Critical Care Ultrasound, 2nd ed. New York. Cambridge University Press: 2011