Feasibility of Polestar N20, an ultra-low-field intraoperative magnetic resonance imaging system in resection control of pituitary macroadenomas: lessons learned from the first 40 cases.

OBJECTIVE: To evaluate the feasibility of PoleStar N20 (Medtronic Surgical Navigation Technologies, Louisville, KY), an ultra-low-field intraoperative magnetic resonance imaging (iMRI) system during resection control of pituitary macroadenomas and to compare intraoperative images with postoperative 1.5-T MRI images obtained 3 months after the procedure.
METHODS: Forty patients with a pituitary macroadenoma (mean size, 26.9 +/- 9.1 mm) underwent a surgical procedure to remove the tumor. The iMRI system was implemented in a standardized microsurgical procedure (endonasal, transseptal, transsphenoidal approach) using standard microsurgical instruments. Intraoperative imaging was performed for tumor visualization/navigation and resection control. If an accessible tumor remnant was suspected, surgery was continued for reexploration and, if necessary, continued resection. Total anesthesia time and operation time were compared with a historical cohort of 100 patients who underwent a surgical procedure on pituitary adenomas without iMRI. Sensitivity and specificity of the iMRI to detect residual tumor tissue was assessed in 33 patients (82.5%) after comparison with standard postoperative 1.5-T MRI 3 months after the procedure.
RESULTS: Preoperative tumor visualization with the ultra-low-field iMRI showed a very good congruency with the preoperative 1.5-T MRI scans. A three-dimensional reconstruction of the coronal scan enabled the surgeon to safely approach the tumor using the integrated navigation system. In seven patients (17.5%), iMRI resection control showed accessible residual tumors leading to further resection. After tumor resection, the final iMRI scan documented adequate decompression of the optic pathway in all patients. Implementation of iMRI led to a significant increase of anesthesia time (246.0 +/- 50.7 versus 163.4 +/- 41.2 min) and operation time (116.9 +/- 43.9 versus 78.2 +/- 33.0 min; P < 0.05, t test). Sensitivity of the iMRI was 88.9, 85.7, 93.3, and 100% for the suprasellar, intrasellar, and right and left parasellar regions, respectively, and the specificity was 90.5% in the suprasellar and 100% in the intra- and parasellar regions on both sides. In four patients, the intraoperative interpretation of iMRI was equivocal; thus, it was difficult to distinguish between very small tumor remnants and perioperative changes.
CONCLUSION: Ultra-low-field 0.15-T iMRI is a safe, helpful, and feasible tool for navigation and tumor resection control in patients with pituitary macroadenomas. Total anesthesia and operation times are prolonged, but iMRI adequately documents the extent of tumor resection. In this series, the PoleStar system increased the rate of resection without disrupting the neurosurgical workflow.