Intraoperative Magnetic Resonance Imaging

Neurosurgeons rely on image-guidance to perform safe, successful surgery efficiently and cost-effectively. Neuronavigation involves rigid frame-based or fiducial-based frameless stereotactic guidance that derives its accuracy from computed tomography (CT) or magnetic resonance imaging (MRI). This imaging can be obtained days or immediately before surgery. Fundamentally, however these systems do not address the brain shift that results from the loss of cerebrospinal fluid once the cranium is opened. Intraoperative MRI (ioMRI) systems were developed in part to compensate for such brain shift during surgery. Like its predecessors, ioMRI affords excellent soft tissue discrimination and the ability to view the surgical site in three dimensions. Moreover, as all of these approaches use CT or MR imaging, being able to view the tumor being biopsied or resected allows the surgeon to preoperatively determine a surgical corridor that avoids critical structures. However, unlike its predecessor, ioMRI also allows the surgeon to see in real-time what the surgery has accomplished, allowing the surgeon to revisit the surgical field to exclude the presence of intraoperative hemorrhage. Although all ioMRI systems offer T1- and T2-weighted imaging, only high-field (≥ 1.5 Tesla) systems are routinely capable of MR spectroscopy (MRS), MR angiography (MRA), MR venography (MRV), diffusion- and susceptibility- weighted imaging, as well as ultrastructural (diffusion and fiber tracking) and physiologic imaging by means of MR perfusion and brain activation (fMRI) studies. Identifying vascular structures with MRA and MRV may prevent their injury during surgery. Biopsying areas of elevated phosphocholine on MRS may improve the diagnostic yield for brain biopsy. Locating brain function may affect the surgical path chosen to biopsy or resect a tumor. Despite these advantages, however, there remains debate over the optimal field strength and configuration for an ioMRI-guided system.