The author s experience at the New Jersey Medical School has revolved around the PoleStar N20, a compact, mobile iMRI developed in Israel by Odin Medical Technologies. This system is built around a permanent 0.15 Tesla magnet (the first generation of this iMRI contained a magnet with 0.12T strength). It is designed to sit under a regular operating table, and is raised as needed for imaging sessions before and during surgery. A portable shield that opens over the table is used, or the entire room is shielded against radiofrequency interference. The PoleStar N20 has an integrated infrared navigational tool, with stereotactic accuracy comparable or better than conventional image guidance systems.

Case Illustration
A 35-year-old man complained of headaches and gait difficulty. Diagnostic MRI had revealed an enhancing mass and cyst in the right cerebellar hemisphere. He was positioned prone for surgery and the PoleStar N20 was parked under the head of the operating table (see Figure 1). Reformatted images obtained before surgery showed the lesion in multiple planes (see Figure 2).A hemangioblastoma was removed with an operating microscope. Repeat MRI confirmed that the resection was complete and that the fourth ventricle was opened (see Figure 3).

Discussion
The ability to image the brain in the OR was a logical innovation in the development of neurosurgery, which has been marked by a steady decrease in the amount of guesswork . Currently, and for the foreseeable future, MRI is the best means of brain imaging, although seven years after the groundbreaking introduction of iMRI by Peter Black and colleagues, this technology has yet to gain widespread acceptance. Currently, there are fewer than 50 active iMRI units in the world.A review of the reported experience with various iMRI systems and on iMRI applications may shed light on this issue.

Experience with Other iMRI Units

New Operating Environments
The largest published iMRI experience to date has been with the 0.5T General Electric Sigma iMRI, also known as the double donut . The greatest number of patients (over 800) reported is from the initial site at the Brigham and Women s Hospital. A large series of stereotactic tumor biopsies as well as intracerebral hematoma evacuations in this iMRI at the University of Zurich have also been described.

Besides the concept of iMRI itself, the major innovation in the Sigma design was the ability to operate within the imaging space itself, therefore 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.Two centers that had installed the Sigma iMRI in essence abandoned its use, favoring the PoleStar N-10.The Brigham group and GE, recognizing these concerns, are planning a new generation of iMRI. This will employ a 3T magnet, have wider surgical access, and potentially allow for true realtime imaging without the interruption of surgery.

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.

Evidence from another study of 346 operations, including 101 tumor resections and 140 brain biopsies, has been gathered using this system. Based on a 1.5T magnet, this unit is housed in the radiology suite, and is converted as needed into an OR.As a dual-resource 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, and diffusion tensor imaging (DTI) are available.