76937 is billed when US is used for visualization for vascular needle entry. It's also an add-on code that may not be billed alone.
If you're billing it with 37191, 37192, 37193, 37760, 37761 or 76942, it will definitely deny. As stated in the CPT manual, you may not report 76937 with any of those codes.
76942 is billed when US is used for needle placement for injections for pain management (some codes include visualization, so you will need to reference the CPT manual to see if it's bundled). Also, you may refer to page 460 of the 2017 CPT manual for the long list of codes you may not report with 76942.
Generally, each would require a modifier of 26 if performed by the surgeon.
Additionally, if there are two procedures done (ex, 2 pain injections), you may only bill US once.
I hope this is helpful.