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, includ ing changes in those patterns with time; to evaluate signal and enhanc ement 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 studie s obtained in 36 patients who had had surgery for acoustic neuroma (10 1 images total). Four patterns of IAC enhancement were evaluated, as w as labyrinthine signal intensity before and after contrast administrat ion, changes in findings over time, and anatomic alterations caused by surgery. RESULTS: All patients had enhancement of the IAC on the firs t postoperative study. In 30 patients, IAC enhancement remained the sa me or decreased over time. Seventeen patients had hyperintense cochlea r signal and 15 had cochlear enhancement that decreased with time. Eff ects of retrosigmoid craniotomy, a translabyrinthine surgical approach , and middle fossa craniotomy were recognizable. CONCLUSION: Linear en hancement in the IAC is probably normal after surgery. Nodular and mas slike enhancement and any progressive enhancement may require close fo llow-up to monitor growth of residual tumor. Labyrinthine hyperintensi ty may reflect blood metabolites. An MR protocol is suggested for foll owing up patients in the years after surgery.