The ACR had a Q&A article on this in March/ April of 2010! This is a copy of it....
Q: How is an MRI of only the brachial plexus reported?
As noted in a coding Q&A in the February 2001 ACR Bulletin, the consensus of the Economics Committee on Coding & Nomenclature is that the choice of the appropriate CPT code for a magnetic resonance imaging (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, or in the evaluation of a patient with a brachial plexopathy (a non-specific symptom related to the nerve itself that might require imaging), an MRI of the upper extremity would be appropriate.
Hope this helps!