- MRI provides information that makes it uniquely suited as an imaging modality for guiding neurosurgical interventions.

- Several MRI systems have been designed or adapted to help guide neuro interventions; and specialized accessory hardware has been developed as well.

- Examples of specific interventional neurosurgical applications will be discussed.

Basic scientists and clinicians who are involved or who desire to become involved in MRI-guided neurosurgical applications.

- Understand equipment requirements for MRI-guided neuro interventions.

- Appreciate the imaging characteristics that enable MRI to be well-suited for guiding interventions.

- Understand several of the major applications of MRI-guided neuro interventions.

Due to many desirable features, MRI can be used to guide or assist in guiding neurosurgical procedures. Broadly speaking, applications often take advantage of the strengths of MRI to guide the trajectory of a device, to monitor a therapy in realtime, or to provide immediate feedback or confirm the results of an intervention performed outside the magnet; many applications use a combination of these.

MRI systems used for neuro interventions historically were low field configurations (0.35T-1.0T). Often, these systems had an open configuration; examples include vertical field magnets; the “double-doughnut” system; and small footprint magnets specifically designed for the neuro OR. In recent years, the advantages of higher field systems (1.5T and 3T) for imaging have lead to the development of neuro-interventional suites that can accommodate these magnets. Examples include the sliding magnet design with fixed OR position (IMRIS/Deerfield Imaging, Minneapolis, MN, USA) and fixed magnet position with transfer from the OR to the patient (GE Healthcare, Waukesha, WI, USA) (Figure 1). Conversely, many neurosurgical procedures can be performed in the magnet or just outside the magnet with appropriate suite design and/or specialized instruments.

Several specific MRI guided procedures have been developed for neuro interventions. These include device guidance with MRI visible guidance systems (Navigus, Medtronic, Minneapolis, MN, USA; Clearpoint, MRI Interventions, Irvine, CA, USA) (Figure 2); and these types of guidance systems are often used in procedures such as MRI-guided brain biopsy; deep brain stiumulator (DBS) lead implantation; laser ablation (Neuroblate, Monteris Medical, Minneapolis, MN, USA; Visualase, Medtronic, Minneapolis, MN, USA) ; focused ultrasound (FUS) ablation (Insightec, Tirat Carmel, Israel); convection-enhanced delivery (CED); and FUS-mediated drug delivery.

MRI is very often used in conventional neurosurgery, acquired pre-operatively; with the pre-operative images used in conventional OR neuronavigation (e.g. Stealth, Medtronic, Minneapolis, MN, USA). Intraoperative MRI has been used to help guide tumor resections, beginning in some centers in the 1990’s. Because the brain often shifts during craniotomy, intraoperative MRI can be used to provide updated imaging sets, and can often provide more complete resections, particularly for complicated surgical procedures (1).

Some early MRI-guided neuronavigation used frameless guidance systems; and have been used successfully for biopsy (2) and DBS lead placement, with millimeter to sub-millimeter accuracy. The more recently development Clearpoint system (MRI Interventions, Irvine, CA, USA) has been able to achieve sub-millimeter targeting accuracy (3,4) with outcomes for DBS similar to those for conventional OR techniques (5).

Currently in the USA, two companies have received FDA approval for laser-ablation devices for use in neurosurgery. Therapies have been most often performed for tumor ablation and epilepsy treatment. Due to the proton resonance frequency effect, MRI can used to monitor relative temperature change in real time and predict the thermal dose to tissue. Immediately post-procedure, MRI can confirm the extent of therapy on DWI, T2-weighted, and contrast-enhanced T1-weighted imaging, and subsequently monitor treatment volume progression and potential recurrence for tumors (6). Industry-sponsored trials are currently underway to determine the efficacy of laser treatments.

Focused Ultrasound (FUS) is being investigated as treatment for essential tremor, Parkinson’s Disease, and neuropathic pain; and likely applications include epilepsy (7). In addition, FUS has been used to interrupt the blood-brain barrier for the delivery of therapeutic agents in animal models (7). Another technique, convection enhanced delivery (CED), has used the slow delivery of agents directly through a catheter directly to targets in the brain, bypassing the blood-brain barrier. By co-injecting a Gd-based contrast agent or GBCA-infused liposomes, the injection can be visualized on MRI (8).

MRI has been used for many neuro-surgical interventions, for planning, navigation/trajectory guidance, real-time monitoring of therapy, and therapy confirmation. Several features of MRI can make its use in interventions appealing, include favorable soft tissue contrast, lack of ionizing radiation, the ability to account for intra-procedural tissue shifts, and the ability to map relative temperature changes during thermal therapies.

Despite its potential advantages, MRI-guided neuro interventions face competition from traditional and other new neurosurgical techniques. One challenge is the cost, either with a dedicated MRI system for use in the OR, or by using time on MRI systems that could otherwise be used for diagnostic procedures. Conventional neurosurgical techniques often have advantages compared to their MRI-based counterparts, and other recently developed non-MRI guided techniques may also offer benefits not easily obtained with MRI.

MRI provides information that makes it well-suited for neuro interventions. Several MRI systems have been designed or adapted to help guide neuro interventions; and specialized accessory hardware and associated software has been developed. Many advanced neurosurgical procedures are possible under MRI guidance.