Special articleAnesthesia for brain tumor resection using intraoperative magnetic resonance imaging (iMRI) with the Polestar N-20 system: experience and challenges
Introduction
Intraoperative magnetic resonance imaging (iMRI) is thought to provide substantial benefits for neurosurgery compared with the conventional system of preoperative and postoperative high-field imaging. iMRI can maintain navigational accuracy as brain shifts occur during the standard craniotomy, and it can be used to continuously update the Stealth navigation coordinates (Medtronic Navigation, Louisville, CO) during surgery. Such brain shifts are common and can displace anatomy up to one cm [1], rendering the preoperative MRI-generated navigational coordinates inaccurate. Accurate real-time imaging allows the surgeon specifically to remove lesions while at the same time protecting function [2], [3]. In comparison to other modalities, T1, T2 and fluid-attenuated inversion recovery (FLAIR), images obtained with a low-field iMRI provide superior soft tissue images in real time compared with fluoroscopy, computed tomography (CT), and ultrasound (US) [4].
Another advantage of iMRI is that neurosurgeons have control over this imaging modality and can obtain a new scan at any point during the surgery. iMRI can be used to assess the integrity and dynamic changes of the lesion and surrounding tissue when microscopic visualization proves to be inadequate. Further resections can be performed if, at the end of the case, the iMRI scan reveals more lesions, which eliminates the need for repeat resections in the days following the initial surgery. The surgeon can also scan the patient during surgery, if the exact extent of the resection is unknown, and stop the resection if the lesion is completely removed. Such advantages of using iMRI may enhance clinical outcomes, improve patient care, and possibly provide economic savings if repeated surgeries can be avoided [5].
Using iMRI technology in the operating room (OR) presents new challenges for the anesthesiologist providing care during the procedure. The introduction of a magnetic field requires modification of equipment that provides ASA monitoring, ventilatory support, volatile anesthetic, drug infusion, and patient warming (Fig. 1). Any machine that incorporates ferrous material, or generates a substantial electromagnetic signature, cannot be used inside the OR without distorting the MRI scan. Another obstacle for the anesthesiologist is obtaining access to the patient while that patient is inside the bore of a MRI machine for extended periods of time (Fig. 2). Even with newer, portable MRI scanners, the patient is still confined in an enclosed space for an extended period of time during the scan.
Section snippets
Imaging system
The PoleStar N-20 iMR Imaging System (Medtronic Navigation, Louisville, CO) is an open, compact, mobile low-field MRI that uses two vertical disc-shaped permanent magnets with field strength of 0.15 Tesla producing a 5 Gauss line at 7.2 feet. Imaging accuracy of the system ranges from a mean error of 1.2 mm to two mm depending on the type of image (ie, T1, T2), which is clinically insignificant for surgical purposes [2]. The N-20 has a 27 cm aperture with a 20 × 16 cm field of view. The magnets
Case report
We studied 65 patients retrospectively who had anesthesia and surgery with the iMRI from April 2005 to December 2006. The patients were brought into the OR where Veris MR monitoring, including ECG, temperature, blood pressure, pulse oxygen monitor, CO2, O2, and anesthetic gases were attached. Anesthesia was induced and maintained using a combined intravenous (IV) and inhalation (isoflurane) anesthesia technique. Intravenous infusion was maintained in iMRI environments up to three Tesla.
Patient population
Our study population included 65 patients, 18 to 86 years of age, undergoing craniotomy for tumor resection using the iMRI from April 2005 to December 2006, with 5 different surgeons, 4 different anesthesiologists, and iMRI support staff. This retrospective chart review study was performed with institutional approval and with waiver of informed consent. Diagnoses included 13 glioblastoma multiforme cases, 14 glioma cases, 4 meningioma cases, 13 pituitary adenoma cases, and other miscellaneous
Discussion
The iMRI is clearly an innovative breakthrough. With improved efficiency of the iMRI setup, iMRI suites are becoming more practical and financially feasible. In fact, 45% of our iMRI cases required further intraoperative resection, which in turn may have minimized the need for repeat resection in the days following the initial surgery. In a retrospective study with an earlier version of our iMRI system, Hall et al. [7] reported that on average, iMRI patients had shorter hospital stays and lower
Conclusion
The Polestar N-20 iMRI allowed for the patient to be maintained in the same position and location during the entire procedure. With the Polestar, surgeons have substantial freedom of movement and position since the machine is recessed below the operating table during surgery. Use of the mobile shielding enabled the use of the iMRI in any neurosurgical OR without the need for expensive infrastructure or shielding. Monitoring with anesthesia equipment, including infusion pumps, must be iMRI
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Intraoperative MRI for Adult and Pediatric Neurosurgery
2021, Anesthesiology ClinicsCitation Excerpt :During intraoperative imaging, the discs are raised up to the level of the patient’s head, and an accordion-shaped radiofrequency (RF) shield (StarShield; Medtronic) slides over the entire patient, minimizing the effects of external electrical noise on image quality. As a result, the anesthesiologist is unable to directly visualize the patient during intraoperative imaging, which can take up to 20 minutes.4 Low-field iMRI systems are portable and much less expensive than high-field iMRI systems.
Anesthetic challenges and outcomes for procedures in the intraoperative magnetic resonance imaging suite: A systematic review
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“Low-field” intraoperative MRI: a new scenario, a new adaptation
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