Somatosensory-Evoked Potentials (SSEP) In SSEP, low doses of electrical current are given to a limb or the face. Electrodes placed on the surface of the brain record the weak electrical impulses as they travel along nerves to the brain. These impulses are called “evoked potentials” and are another tool for identifying critical areas. Functional MRI (fMRI) This scanning technique takes faster images of the brain than traditional MRIs and creates pictures of the brain’s use of oxygen. This helps distinguish between normal brain, which uses oxygen, and a tumor that contains dead cells and does not use oxygen. A functional MRI may also be referred to as real-time MRI, fast MRI, echo-planar MRI or dynamic MRI. Intraoperative Ultrasound Imaging During surgery, ultrasonic waves might be used to determine the depth of the tumor and its size. It works SURGERY www.abta.org 15 by sending ultrasonic pulses into the brain. These pulses are instantly reflected back. The time it takes for these “echoes” to bounce back is calculated by a computer and formed into an image on a screen. These images can help the neurosurgeon: • Determine the shortest surgical route to the tumor • Define the borders of the tumor • Distinguish the tumor from a cyst, swelling (edema), and normal brain Intraoperative MRI (iMRI) Some operating rooms have specialized MRI machines that allow scans to be performed during surgery. These images provide the surgeon with similar information as intraoperative ultrasound, but usually produce more detailed pictures. They are particularly helpful in determining how much of the tumor has been removed, and they allow the surgeon to navigate using updated brain images. Although once available at only a few centers, iMRI is now more widely available. EMBOLIZATION If a tumor has a large number of blood vessels, surgery can be difficult due to the bleeding that could result. Embolization is a technique neurosurgeons use to stop the blood flow to the tumor prior to removing it. A diagnostic test, called an angiogram, is performed to determine if a significant amount of blood is going to the tumor. If so, the neurosurgeon or neuroradiologist can insert a small “plug” made of wire or glue-like material into the vessel. This stops the blood flowing to the tumor, but not to normal parts of the brain. Tumor removal usually follows within a few days. This technique might also be used with tumors that contain a high number of blood vessels – referred to as “vascular” or “well-vascularized” tumors. Meningiomas, meningeal hemangiopericytomas and glomus jugulare tumors are typically well-vascularized tumors. 16 AMERICAN BRAIN TUMOR ASSOCIATION CONVECTION-ENHANCED DELIVERY (CED) One of the newest methods of delivering chemotherapy drugs or biologic therapies to a tumor is CED, or “convection enhanced delivery.” CED uses the principles of constant pressure to “flow” or “infuse” substances through brain tumor tissue. The procedure begins with a surgery, during which a catheter (or multiple catheters, depending on the tumor size) is placed into the tumor area. The neurosurgeon then connects a pump-like device to the catheter, filling it with the therapeutic substance. The fluid then flows, by use of pressure and gravity, through the tumor area. This “bulk flow” or “convective-delivery” method bypasses the blood brain barrier, placing the therapeutic substance in direct contact with tumor tissue. Clinical trials are exploring the use of CED as a way of placing immunotoxins, radioactive monoclonal antibodies and various chemotherapy drugs at the tumor site. As this technique is developing, researchers are simultaneously exploring ways to include “tracers” in the substances flowing into the brain. Those tracers can be viewed on an MRI scan performed during CED, and may allow researchers to make real-time observations of the movement of therapeutic substances in and around the tumor. Research is also underway to predict the flow pattern that will occur after catheter placement. An MRI scanner in the operative suite, called an iMRI, is seen on the right in the photo. Linda Liau, MD, UCLA Medical Center, Los Angeles © Eric Behnke SURGERY www.abta.org 17 LASERS A laser is a surgical tool. It is a device that emits a narrow beam of intense heat that can cut and vaporize tissue during brain surgery. The beams emitted by a laser do not pass through the dense bones of the skull. For a laser to be used effectively, it must first bypass the skull bones, using one of several surgical techniques. Lasers may be especially important with tumors located at the base of the skull, deep within the brain or those tumors that cannot be removed easily for any number of reasons. Lasers are frequently used in microsurgery, photodynamic therapy and for a variety of diagnostic purposes. Laser probes may also be directed into deep tumors to heat them in the LITT process. Whether or not a neurosurgeon uses a laser during surgery depends on his or her personal judgment and the best “tools” for removal of any given tumor. An operative microscope aids the neurosurgeon’s view of the tumor. John Sampson, MD, Duke University, Durham, North Carolina Some microscopes allow surgeons to share the same view. Carl Zeiss Surgical, Inc. NC33 Microscope 18 AMERICAN BRAIN TUMOR ASSOCIATION MICROSURGERY Microsurgery involves