Prior to the development of intra-operative imaging tools, surgery relied on images acquired pre-operatively. However, during the surgical procedure, dynamic changes of the intracranial contents regularly occur, which means that the neurosurgeon is faced with a continuously changing intra-operative field. This phenomenon is often referred to as brain shift . As such, only intra-operatively acquired images will provide the surgeon with the information required to perform real intra-operative image-guided surgery.

With the introduction of intra-operative imaging tools, the ability to image the brain during surgery is now a reality. Although intra-operative ultrasound and intraoperative computed tomography (CT) units have been successfully utilized in neurosurgery, iMRI confers a number of distinct advantages. Its excellent imaging qualities, combined with the avoidance of ionizing radiation, suggest that iMRI will remain the gold standard of neuroimaging for the foreseeable future.

iMRI
MRI was first brought into the operating room (OR) by Peter Black et al. at the Brigham and Women s Hospital, working in concert with General Electric Medical Systems (GEMS). Their device, the General Electric Signa magnetic resonance therapy (MRT) system, employed two superconducting magnets to create a 0.5T magnetic field. The Brigham and Women s Hospital has reported over neurosurgical 800 procedures with the Signa iMRI. These included a wide variety of intracranial operations, aimed mostly at the treatment of patients with brain tumors.

Besides the concept of iMRI itself, the major innovation in the Signa design was the ability to operate within the imaging space itself. Thus, no magnet or patient movement needs to occur in order to acquire an image. The major limitations of this iMRI have been the 56cm gap between the vertically oriented magnets, limiting surgeon access to the operative field, variable image quality, and the need for a fully non-ferromagnetic operating room. The Brigham group and GE, recognizing these concerns, are planning a new generation of iMRI. This system will employ a 3T magnet, provide wider surgical access, and potentially allow for true realtime imaging without the interruption of surgery.

In the wake of this pioneering effort other groups reported on the implementation of other interventional MRI concepts, with a variety of magnetic field strengths, versatility, and ergonomic pros and cons. Another iMRI design that, in essence, required moving an OR to a radiology suite, was developed with Philips Medical Systems at the University of Minnesota. Based on a 1.5T magnet, this unit is housed in the radiology suite, and is converted as needed into an OR.As a dualresource MRI it is used for diagnostic imaging when not used for surgery. Surgical images are of diagnostic quality, and such functional applications as MR spectroscopy, functional MRI (fMRI), and diffusion tensor imaging (DTI) are available.