Intraoperative imaging is an important tool in modern minimally invasive neurosurgery. Intraoperative imaging, especially in combination with some kind of neuronavigational device, correlating the virtual image data to the real patient world in the operating room, serves as intraoperative quality control, supporting the goal of maximum tumor resection with least morbidity.

Magnetic resonance imaging (MRI), in comparison with computed tomography (CT) and ultrasound, provides multiplanar imaging with a high soft tissue resolution. It is generally accepted that MRI is the method of choice for the pre-operative diagnostic evaluation of intracranial tumors and patients with epilepsy. However, when the MR technology was introduced into clinical diagnostics, the closed-bore superconducting cylindrical design of MR scanners, with relatively long imaging times and difficult patient access, prevented their intraoperative application.

Therefore, intraoperative imaging evaluating the extent of a tumor resection was introduced into neurosurgery by ultrasound imaging (1-3) and CT (4-9). Their efficacy was investigated in the early 1980s (7,8). However, imaging quality and lesional resolution, especially in CT for soft tissue, were not quite satisfactory.

In the mid 1990s, widespread interest arose in intraoperative imaging in the neurosurgical community, since the development of open configured MRI systems made it possible to adapt these systems to the operating room. Intraoperative MRI in neurosurgery was first introduced by Black and Jolesz in 1995 at the Brigham and Women's Hospital in Boston with a dedicated MRI system for intraoperative use (Signa SP, 0.5 T), which was developed in a collaboration with the General Electric Corporation (10). Removing the central segment of a cylindrical MR system has solved the problem of patient access. This so-called "double doughnut" design allows patient access through the vertical gap of the scanner. As an alternative approach, we adapted a diagnostic low-field MR scanner (Magnetom Open, 0.2 T) for intraoperative use: it was originally designed for diagnostic purposes only. This implementation was a cooperative

From: Minimally Invasive Neurosurgery, edited by: M.R. Proctor and P.M. Black © Humana Press Inc., Totowa, NJ

effort of the Siemens Corporation with the Departments of Neurosurgery at the Universities of Erlangen and Heidelberg (11,12) at the end of 1995. The scanner has a biplanar magnet design using a resistive magnet. The C-shape design with a horizontal gap allows a wide patient access.

Our primary concept was based on the installation of the MR scanner in a twin operating room, in combination with pointer- and microscope-based neuronavigation systems (Stealth, Medtronic and MKM, Zeiss). This setup allows frameless stereotaxy based on anatomical data. Furthermore, it is also possible to integrate functional data from magnetoencephalography (MEG) and functional MRI (fMRI) for intraoperative identification of eloquent brain areas, which is now known as "functional neuronavigation" (13-15).

Intraoperative image quality and sequence spectra of the various low-field MR scanners cannot compete with the routine pre- or postoperative imaging quality standards set by high-Tesla machines. This led to the development of a new concept to adapt a standard high-field MR scanner to our operating environment, while preserving the benefits of standard microsurgical equipment and microscope-based neuronavigational guidance with integrated functional data. The active magnetic shielding of modern high-field MR systems results in a steep decrease in the magnetic field, so that the 5-G line is close to the scanner, facilitating the intraoperative application of these high-field devices.

This chapter summarizes our concept and different setups for intraoperative low- and high-field MRI and gives an overview of our clinical experience in low-field intraoperative MRI, which is compared with our preliminary data using the new high-field system.

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