Utility of Intraoperative MRI in Skull Base Surgery

Introduction: Intraoperative MRI (iMRI) has gained importance in recent years for use in selected cases, particularly for resection of gliomas. However, the use of iMRI in skull base surgery has not been well documented. There are several questions regarding the use of iMRI in skull base surgery that remain unanswered: Is complex patient positioning (lateral, park bench, concorde) possible? What is the impact of the additional time involved with obtaining an intra-operative MRI, on an already lengthy surgical procedure? Can electrophysiologic neural monitoring remain in place during an iMRI? Should monitoring electrodes be removed during the MRI part of the procedure? The objective of this study is to evaluate the feasibility of and to provide our experience with iMRI in skull base surgery.

Methods: We retrospectively reviewed our single center experience with treating skull base pathology in the iMRI suite from April 2014 through July 2015. Patient demographics and radiographic data were collected for review. Lesions were subdivided by anatomic location as follows: anterior fossa, sphenoid wing, middle fossa, cavernous sinus, sella, clivus, internal auditory canal, jugular foramen, foramen magnum, and non-skull base lesions. Vascular lesions were not included. Descriptive statistical analysis was performed.

Results: Over a period of 16 months, the iMRI suite was utilized for 211 unique cases. There were 125 tumor resections in this time, 74 of which met the criteria for skull base tumor. The most common cases were meningiomas (31), pituitary adenomas (15), and acoustic neuromas (12). Intraoperative neural monitoring was utilized in 34 cases and included: somatosensory evoked potentials (SSEP), motor evoked potentials (MEP), Ojemann Stimulators, electromyography (EMG), brainstem auditory evoked responses (BAER) and lower cranial nerve monitoring. An iMRI was completed in 11 patients with monitoring in place: 6 with SSEP, 1 with MEP, 5 with cranial nerve VII monitored (CN VII), and 5 patients with BAER. Three patients were scanned with MRI-compatible platinum-iridium electrodes in place and 8 were scanned with stainless steel electrodes. iMRI was successfully performed in the prone position in 2 patients, park bench in one patient, and supine position in 49 cases. No adverse events were noted while using implanted electrodes (platinum iridium or stainless steel) during iMRI and no patients were unable to obtain an iMRI due to positioning. There were 18 patients who went on to have further surgery after the iMRI, 19 patients had a complete resection confirmed by intra-operative MRI and 12 patients had residual tumor but no attempt at further resection.

Conclusion: Despite the challenges posed by operating on skull base tumors, such as positioning and electrophysiological monitoring, iMRI can be safely obtained without adverse events. The major challenge in using the iMRI derives from the shift in the decision-making process: from the surgeon’s intra-operative impression of “maximum safe resection” to the immediate radiographic evidence of tumor residual. The immediate evidence of residual tumor with a patient still in position to have additional resection may influence the surgeon to alter the surgical plan and attempt further resection in a critical area.