The evolving MR appearance of structures in the internal auditory canal after removal of an acoustic neuroma

PURPOSE: To identify patterns of enhancement in the internal auditory canal (IAC) on MR studies after removal of an acoustic neuroma, including changes in those patterns with time; to evaluate signal and enhancement of the labyrinth; to differentiate normal postoperative findings from those suggesting residual tumor; and to describe MR hallmarks of surgical approaches. METHODS: We reviewed the postoperative MR studies obtained in 36 patients who had had surgery for acoustic neuroma (101 images total). Four patterns of IAC enhancement were evaluated, as was labyrinthine signal intensity before and after contrast administration, changes in findings over time, and anatomic alterations caused by surgery. RESULTS: All patients had enhancement of the IAC on the first postoperative study. In 30 patients, IAC enhancement remained the same or decreased over time. Seventeen patients had hyperintense cochlear signal and 15 had cochlear enhancement that decreased with time. Effects of retrosigmoid craniotomy, a translabyrinthine surgical approach, and middle fossa craniotomy were recognizable. CONCLUSION: Linear enhancement in the IAC is probably normal after surgery. Nodular and masslike enhancement and any progressive enhancement may require close follow up to monitor growth of residual tumor. Labyrinthine hyperintensity may reflect blood metabolites. An MR protocol is suggested for following up patients in the years after surgery.