Obtaining access uses the Seldinger technique by puncturing the vessel with a needle, placing a wire through the needle into the lumen of the vessel, and then exchanging the needle for a larger plastic sheath. Finding the vessel with a needle can be challenging and both ultrasound and fluoroscopy can be used to aid in finding and confirming access. Sheaths typically have a side port that can be used to withdraw blood or injection fluids/medications, and they also have an end hole that permits introducing the catheters, wires, etc. coaxially into the blood vessel.