**2. Evolution and types of ioMRI**

The first intraoperative MRI for neurosurgical operations, was developed in 1991 by the combined efforts of the Departments of Neurosurgery and Radiology of the Brigham & Woman's Hospital of the Harvard Medical School in Boston and the General Electric Medical Systems [2]. It was 0.5 T open type. Low field

open type with Horizantal gap MRI was used for interventional procedures by Gronemeyer and colleagues [2, 3] which provided access to patients. Later Vertical gap MRI was developed which increased patient access and was used for various interventional, endoscopic and open surgeries. Many MRI compatible equipment had been developed since then along with ioMRI, with all ferromagnetic instruments replaced with titanium. The main drawback for open type configuration has been low field strength of magnet which does not yield good image resolution. Both vertical and horizontal systems had double doughnut magnets. The IMRIS system was developed later by a neurosurgeon, Dr. Garnette Sutherland of Calgary, Alberta, Canada. This system offered a uniquerailmounted MRI system in which the scanner could be mobilized to the patient. It was closed type 1.5 T ceiling mounted rail system, which was moved between two rooms [4].

ioMRI scanner can be open or closed type. Open type (with horizontal gap or vertical gap) has better access to patient while compromising on image quality. Closed type (small bore or long bore) has better quality of images with no access to the patient.

Based on field strength ioMRI are classified as low field (0.2 T), mid field (0.5 T) and high field (1.5 T & 3 T). Low-field systems are the GE Signa, the Hitachi 0.3 tesla system, and the Polestar0.15 tesla system. SIGNA SP 0.5 T is a midfield ioMRI system. High-field systems are Siemens Brain Suite, IMRIS system, and the Philips systems. High field ioMRI provides good quality of images with better spatial and contrast resolution with precision and some of the studies like perfusion, DTI and fMRI are possible compared to low or mid field ioMRI [5, 6].

The imaging in ioMRI can be truly intraoperative or interoperative in nature. Intraoperative imaging is done while surgery is ongoing in the scanner without any interruption of procedure. Horizontal and vertical donut models were used for it. These are mid field ioMRIs. These are the actual real time imaging which were performed during the surgery similar to fluoroscopy. The draw backs of this system were poor image quality, need of MRI compatible instruments including microscope, navigation which were very expensive and space constrains for movement. In interoperative imaging, surgery is stopped temporarily and either patient or gantry mobilized to acquire images. This type has been developed more commercially as there is no need for continuous intraoperative imaging and imaging is required only for certain periods like, to confirm extent of resection, location of residue or guiding further resection. This type allows installation of high field strength MRI in the area near the operating room or within the same room with operating table outside of the 5 gauss line. This allows surgeon to use non-MRI compatible instruments during surgery. Patient transportation is the main drawback which takes 20–40 minutes and requires proper trained staff [4].

High field ioMRI operative rooms (OR) are of different types. First type was IMRIS. In this type MRI was rail mounted and placed between two operating rooms and mobilized into the OR when required. During imaging instruments moved beyond the 5 gauss line. The operating rooms had to be RF shielded to acquire good images. Second type was RF shielded OR in which MRI and operating table are in the same room, when imaging is required, patient is moved into gantry to require images. This type has a rotating table, during surgery head end of the table is beyond 5 gauss line. When imaging is planned table is rotated so that head goes into the gantry. This was developed by Siemens and BrainLab companies combined. BRAIN SUITE is an integrated operative area away from the magnet in which interventional procedures can be done. MRI compatible instruments are not required.

**Figure 1.**

*Representational images. A. RF shielded ioMRI OT in which MRI moved towards operating table B. Nearby OT model, in this model patient shifted on mobile MRI compatible table top.*

It also has incorporated Neuronavigation with auto registration. All of these systems are possible in high volume centers or in institutes with research interest, as the cost is prohibitive [4]. Other more commercially viable concept is "nearby OT type" –in which MRI machine is fixed in separate room adjacent to OR and operating table is transferred into MRI suite. In nearby OT type, the MRI can be used for imaging other patients through separate entry to access the MRI which can be cost effective. Whenever ioMRI is planned the MRI is room is cleaned and sterilized, the outside entry is closed and OR entry is opened to receive the patient in operative position draped, under anesthesia. All these require special protocols, trained staff, and proper communication between MRI technician, surgeon, anesthesiologist and OT staff.

Authors are using "Nearby OT model" – Siemens 3 T ioMRI and have found to be cost effective. Initially it took around 85.6 minutes for shifting in and out ioMRI and restart surgery which latter reduced to 37.4 minutes by multiple mock drills and continuous training of involved staffs [7, 8] (**Figure 1**).
