Brain tumor resections guided by magnetic resonance imaging

AORN Journal, March, 2003 by Angela Kanan, Beth Gasson

Image-guided surgery began at the turn of the nineteenth century when x-rays were discovered accidentally in 1895. (1) Since this time, physicians and scientists have been striving to improve the technique of being able to see inside the body without actually cutting into the patient. Neurosurgeons long have used image guidance systems, such as fluoroscopy, ultrasound, and, more recently, three-dimensional stereotaxy, to achieve maximum resection while preserving normal brain tissue and function.

According to the National Cancer Institute, more than 17,000 people are diagnosed with brain tumors annually in the United States. (2) The cause of brain tumors is not known, but certain risk factors increase a person's chance of developing one. Some of these risk factors include environmental exposure to pesticides and solvents, family history, and diet. One form of treatment for brain tumors is surgical resection. Studies of malignant brain tumor excision, however, show that as many as 80% of neurosurgical procedures to remove brain tumors leave part of the tumor behind. (3)

The conventional stereotactic systems in use today allow intracranial lesions to be found based on previously acquired imaging data sets. These stereotactic devices, however, do not allow for the correction of brain shift (ie, shifting of intracranial soft tissue during surgery). Brain shift occurs when the dura is opened, cerebrospinal fluid leaks out, and the brain either shrinks or swells. This causes deformity of brain anatomy from the preoperatively acquired images (Figure 1). This shift could result in inadequate treatment (eg, incomplete resection) or damage to the nervous system. In an attempt to eliminate errors that can occur when using a conventional stereotactic system, the intraoperative image guidance system has evolved (Table 1).

[FIGURE 1 OMITTED]

INTRAOPERATIVE MAGNETIC RESONANCE IMAGING

In 1990, Ferenc A. Jolesz, MD, a neurosurgeon at Brigham and Women's Hospital in Boston, realized that magnetic resonance imaging (MRI) could be used not only to diagnose disease but also to guide treatment during surgery. (4) The ability to image with real-time MRI in an OR setting can reassure physicians that they are targeting the correct area within the tumor while visualizing critical adjacent structures. The intraoperative MRI system can reacquire images during brain tumor resection so surgeons can monitor their surgical progress. In addition, using the real-time images allows surgeons to avoid critical areas that may have shifted during surgery (Table 2). Intraoperative MRI also is particularly helpful in determining tumor margins and monitoring potential intraoperative complications (eg, hemorrhage).

Magnetic resonance imaging is an imaging technique that provides superior tissue visualization of human anatomy. Magnetic resonance imaging is more sophisticated than the human eye and clearly can distinguish normal versus abnormal tissue. Magnetic resonance imaging is preferred in image-guided surgery because of its high tissue contrast and spatial resolution. The technique is different from computed tomography (CT) and x-rays because it uses a magnetic field instead of ionizing radiation. Before imaging, a transmit receive coil (ie, antenna) is placed around the area to be imaged. The magnetic field causes the patient's hydrogen atoms to align with the magnetic field. Radio frequency (RF) waves then are transmitted to the anatomy of interest via the coil. The RF waves cause the hydrogen atoms to flip out of and back into the aligned state within milliseconds. The energy that is created by this process produces a signal that then is processed by the computer and reconstructed into an image. Varied tissue properties cause normal and abnormal tissue to appear differently on images, which allows a physician to detect disease (ie, tumor tissue will differ from surrounding normal tissue).

The intraoperative MRI scanner was designed by making modifications to a conventional MRI unit. These modifications resulted in a double donut-shaped, open magnet with a 56-cm vertical gap that allows direct patient access for two physicians (Figure 2). Intraoperative magnetic resonance images are displayed on monitors within the gap. The anesthetized patient is placed in the center of the magnet and will remain there throughout the surgery. The sides of the magnet are draped sterilely, as is the patient, which allows surgeons to work within a sterile field. Magnetic resonance images are taken at various points throughout the resection. All this is accomplished without having to move the patient, disturb the sterile field, or remove instrumentation from the surgical area.

[FIGURE 2 OMITTED]

The scanner is housed in an OR fully equipped with MR-compatible instrumentation, surgical equipment, and anesthesia and patient monitors. Visual and two-way audio communication is installed in the room to enhance interaction among the surgeon and radiology staff members in the control area outside the OR.