Research Focus: Intraoperative Technologies
Precision and safety: intraoperative technologies shape modern neurosurgery
No other field requires the precision needed for microsurgery – as low as 0.1 millimeters for neurosurgery in regions adjacent to vital areas of the brain. With the 20-fold magnification provided by an operating microscope, neurosurgeons can recognize structures as small as 0.005 millimeters (5 micrometers) in size. This degree of resolution is necessary in order to distinguish brain tissue, nerves or microarteries – for example during the removal of an adjacent tumor or repair of a vascular abnormality. Until recently, the impossibility of visualizing functionally important nervous tissue was the main limitation. In the brain, nerve fibers can be visually differentiated from nerve cells, but functionally important nervous tissue cannot be visually distinguished from less vital tissues.
Making the invisible visible
In traditional neurosurgery the surgeon depended upon his or her knowledge of anatomy and surgical experience, and received only delayed feedback – if any – regarding the functional (i.e. clinical) outcome. This made it difficult to know which procedures were most effective and thereby reduce the rate of neurological deficits. Risky operations remained risky, even for surgeons with extensive experience. Tumor tissue was difficult to see, and often difficult to distinguish from healthy nervous tissue. Functionally important brain regions could not be visually distinguished from less important regions, and the vascularization of these tissues could not be determined. New technological developments have changed all of this. Operations on the brain and spinal column can be carried out using intraoperative imaging methods and functional monitoring. With these intraoperative methods, the surgeon receives an immediate warning that he or she is operating close to functionally important tissues, and thereby gains more control over the surgical outcome.
Key technologies in the neurosurgical operating room
Being able to localize specific brain structures, determine function of specific areas, and visualize the vascularization of the tissue influences the entire neurosurgical procedure. Planning begins with a pre-operative summary of navigational data. This includes patient-specific corrections based on the MRI imaging, spectroscopic images, or functional images of cortical areas and subcortical nerve tracts (for example motor cortex, motor or sensory language areas, fornix, cingulate cortex, optic radiation, pyramidal tract, or the language-related fiber tracts in the arcuate fasciculus).
With the availability of this information, the pre-operative planning of the surgery can be done virtually, which helps to ensure an optimal approach for microsurgery. All of these sources of information are integrated into the navigation system and are available during the actual surgery.
The three pillars of modern intra-operative technology are:
1. Visual imaging and functional neuronavigation
Computer-assisted navigation has become a routine part of neurosurgery. The integration of functional information with anatomical images enables rapid generation of images and operational plans that are customized for each patient. Modern functional neuronavigation should not be carried out with a pointer, but rather with an integrated system that combines neuronavigation with traditional microscopic imaging, to distinguish the folds of the tumor from the functional areas within the microscope-enabled visual field of the surgeon. An advantage of neuronavigation is that the operating microscope does not need to be removed; the surgeon can continue to work, using a mouth control to adjust the position of the microscope and a foot control to adjust the focus.
2. Intraoperative monitoring and mapping
Continuous monitoring of functionally important areas requires more than imaging methods and neuronavigation. During the operation, some brain areas are displaced due to the opening of the dura mater and the tumor resection, which causes some loss of cerebrospinal fluid. This tissue displacement means that the pre-operative images already start to lose accuracy as brain tissue shifts during surgery. An intraoperative image taken with MRI or CT also reflects a specific moment during the surgery, which can make a difference in the success of tumor resection. In summary, such images do not enable continuous monitoring and cannot ensure safety. Continuous monitoring is only possible through electrophysiological monitoring or mapping. With cortical stimulation and monitoring of peripheral activity (motor evoked potentials [MEPs]), or through peripheral stimulation and monitoring of cortical activity (somatosensory evoked potentials [SEPs]), different nerve tracts can be carefully avoided, no matter what region is being operated upon (e.g., cortex, myelinated nerve fibers [white matter], brainstem, spinal cord, spinal column). The most valuable approach with regard to surgical treatment of tumors or aneurysms is the use of MEPs. The ‘gold standard’ is the use of continuous stimulation via pre-implanted electrodes.
In contrast, mapping involves the search for specific functional areas within the cortex. We use this method to localize the primary motor cortex and secondary motor cortex, which aids in the identification of the pyramidal tract. In addition, with surgery on awake patients, this is the method of choice for locating brain regions related to speech and language interpretation. Use of monitoring and mapping has improved patient safety. New techniques are constantly being investigated at the Inselspital.
3. Intraoperative imaging and monitoring of results
In the future, monitoring of results will rarely be performed after surgery. Instead, the monitoring will be performed during the operation. This change is important for the patient: with intraoperative monitoring, a problem can often be corrected during the operation. Intraoperative monitoring enables more frequent and closer monitoring of the procedure. Many different types of information are integrated into the neurosurgery navigation system: Hochfeld-MRI, CT, angiography, ultrasound, and fluorescence imaging. International advances that have been particularly important for intra-operative imaging and intraoperative monitoring are: the microscopic integration of 5-ALA for visualization of fluorescent tissue, and the Infrared-800 technology (ICG-angiography).