Saline can be used in place of US gel

A study comparing sterile saline as a conduction agent with ultrasound gel showed adequate visualization of anatomic structures for ultrasound-guided vascular access. The authors state that given sterile saline’s theoretical advantages over gel in terms of availability, cost, safety and ease of use in the procedural field, it should be considered as a viable alternative to gel as a conduction agent.

Saline - so many uses we hadn't thought of

Use of sterile saline as a conduction agent for ultrasound visualization of central venous structures
Emerg Med Australas. 2010 Jun;22(3):232-5