NEUROSURGERY ARTICLES

INTRAOPERATIVE MRI AT UCI MEDICAL CENTER

by Mark Linskey, M.D.

The latest generation of portable intraoperative magnetic resonance imaging (IOMR) should be ready for patient care at UCIMC by the end of September 2006. This latest addition to UCI will have significant impact in improving the care, safety, and clinical outcomes of patients undergoing surgery for brain tumors, pituitary tumors, epilepsy, and other neurosurgical disorders. UCI is the first medical center south of Los Angeles to be able to offer this cutting edge intraoperative technology to our patients. It is now the closest available IOMR service for patients in Orange, San Diego, Imperial San Bernadino, and Riverside Counties.

The brain is a critical, unforgiving structure surrounded by delicate and important nerves and blood vessels. Tumors can be difficult to locate, can reside very close to dangerous eloquent brain regions, and can sometimes be difficult to distinguish from surrounding normal brain tissue. At many institutions the best that can be done to assist the surgeon is something called frameless stereotactic image guidance. With this procedure, the patient's "virtual head" is reconstructed on a computer workstation based on pre-operative MR images. This virtual space is correlated ("registered") to the patient's actual head once they are asleep and fixed in position within the operating room. A probe or instrument is then tracked using triangulation cameras in the operating room to tell the surgeon where the probe is at any time in three dimensions within the computer virtual space. Unfortunately, the brain is composed of tissue with a consistency similar to gelatin and supported and surrounded by cerebral spinal fluid. Once the skull is opened, the fluid drains away and the effects of gravity and tumor pressure take hold. Therefore, the internal brain structures may shift significantly relative to their original position on the preoperative MR images. This shifting becomes even more pronounced as time elapses and as the mass effect of the tumor is relieved by tumor removal. Eventual navigational errors with frameless stereotaxis as large as 5-10 millimeters are common and can reach up to 2 centimeter magnitude in extreme cases. For brain surgery tissue removal and patient safety, every single millimeter counts.

IOMR allows for direct, real-time, intraoperative brain imaging and accurate three-dimensional neuronavigation despite brain shifting. It can also be used in a complementary fashion to improve frameless stereotactic image guidance by updating the computer workstation "virtual head" to account for the brain shift. In this capactiy, IOMR and frameless stereotaxis can be used in tandem, with frameless stereotaxis utilized during less critical or dangerous stages of the operation and IOMR utilized to keep the frameless stereotaxis as accurate as possible. Direct imaging and navigation with real-time intraoperative neuroimaging can therefore be done during the most critical portions of the case. The use of IOMR can maximize completeness of brain tumor or lesion removal, reduce the need to return to the operating room to remove unexpected residual tumor found on diagnostic postoperative MR imaging, minimize the chance of operative complications, and potentially shorten the time a patient needs to stay in the hospital after surgery.

Many studies at multiple centers utilizing IOMR have clearly demonstrated that IOMR increases the completeness of brain tumor resection without increasing neurological complication risk compared with surgeries performed using standard techniques. For patients with benign tumors, this can mean the difference between permanent tumor cure and later tumor progression or recurrence, or the need for additional supplemental therapies or procedures. For patients with low grade primary brain tumors, this might mean that cure is possible, or that longer survival time can be realized before progression or recurrence. Indeed, for patients with benign or low grade primary brain tumors, the finding of residual tumor on a postoperative MR scan obtained one or two days after surgery is often an indication for immediate return to the operating room to complete the removal, before the wound has time to heal or the remaining lesion to scar into place. Direct demonstration of complete tumor removal at the end of the initial operation by IOMR before closure significantly reduces this possibility. For patients with high grade primary brain tumors, mounting evidence suggests that the fewer tumor cells left behind after surgical resection, the greater the patient's chances of an optimal outcome after postoperative radiation and/or chemotherapy.

Specific tumor types where IOMR has made the most difference include primary brain gliomas and pituitary tumors. Without the benefit of IOMR, surgeons often rely on their appreciation of color andtexture differences between tumor and normal brain to know where the edge of a tumor exists under the operating microscope. Unfortunately, since tumors such as gliomas arise from the brain, they can have very similar color and texture compared with normal brain at surgery.

Multiple studies have now confirmed that for removal of large pituitary tumors, IOMR will detect additional unsuspected residual tumor while still in the operating room in 40% of cases. The use of IOMR in these instances will increase the likelihood of complete tumor resection confirmed by diagnostic postoperative MR imaging by approximately one and a half times. Large pituitary tumors that extend to optic nerves are often treated in a staged fashion with transsphenoidal removal followed by Gamma Knife stereotactic radiosurgery. This can be done if the initial removal is able to create the safe distance between the tumor and the optic nerves for subsequent radiosurgery. IOMR maximizes the chance of successfully pursuing this staged approach by confirming the extent of removal near the optic nerve before leaving the operating room.

IOMR can also be very useful for epilepsy surgery. This is particularly true for temporal lobe epilepsy where multiple studies have now shown that the success of seizure control after epilepsy surgery is directly related to the completeness of removal of several deep temporal lobe structures that are damaged in cases of epilepsy. Given their deep location and their color and texture similar to normal brain, IOMR can be very useful to confirm the completeness of anatomical removal prior to leaving the operating room.

Unlike standard diagnostic MR scanners and some high field IOMR units, low-field portable IOMR scanners do not require the patient to move through the bore of an enclosed "doughnut" or slide between the slabs of an open diagnostic MR scanner. Instead, the patient stays stationary, allowing the surgical team full access without compromising surgical technique, surgical access, or sterility, and the magnet simply moves into position around the anesthetized patient whenever it is needed during surgery. The IOMR being installed at UCIMC is a compact portable unit that can be brought into a standard operating room and used as needed during surgery. Appointments for consultation to see whether your case might benefit from this advanced technology can be made by calling (714) 456-6392.

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