Many of the TOS issues don't show up on MRI unfortunately. If a bony or some anomaly then it should show .
Makes harder to get a knowledgeable dx..
plus if laying down for MRI the compression is usually less as it could be something due to being upright and postural.

Sometimes it is not a large/main nerve that is involved but smaller nerves.

Actually an expert /advanced PT should be able to evaluate and test hands on to track it down , often more cost effective than imaging .