According to the ACR, it depends on the indication.
If a MRI of only the brachial plexus is performed, would a MRI of the upper extremity be the appropriate code to use? Again, this is a specific structure that is viewed, not the entire upper extremity.
The consensus of the ACR's Committee on Coding and Nomenclature is that the choice of the CPT code for an MRI study of the brachial plexus depends significantly on the clinical indications. For example, an MRI of the chest, focusing on the brachial plexus, is most commonly used in cases of apical lung cancers (Pancoast tumors), while an MRI of the orbit, face and neck may be used to identify head and neck cancers to the level of the thyroid, including the brachial plexus. In the evaluation of a tumor of the shoulder girdle or axilla, including the brachial plexus region, an MRI of the upper extremity would be appropriate.