Parkinson’s disease is a progressive nervous system disorder that effects the parts of the brain that control movement. The most common symptoms are tremor, rigid muscles and slowed movement. One way of reducing the symptoms is deep brain stimulation (DBS). In DBS surgery, electrodes are placed in specific brain regions to deliver electrical stimulation that can block the abnormal nerve signals causing the symptoms. The surgery requires careful planning and imaging in order for the electrodes to be placed exactly right. Even minor displacements might cause unwanted side effects.
In addition to Parkinson’s disease DBS is used in essential tremor and dystonia. The procedure has been performed on around 160 000 patients worldwide and in Linköping around 30 patients are treated every year.
MRI and Laser Doppler Technique Guide the Probe
Before the stereotactic surgery a preoperative magnetic resonance tomography (MRI) is performed to guide the surgeon. Using the stereotactic frame and the coordinates from the MRI the surgeon can use a probe to produce a canal for the electrode to reach the exact location.
Karin Wårdell is Professor in biomedical engineering and has been working with MRI protocols for stereotactic neurosurgery for many years. In addition, she has developed a tool based on laser doppler technique which can be used with the DBS probe to give more data on the surrounding tissue.
-The optical fibers can measure the blood flow in a small volume in front of the probe and warn the surgeon when the probe is about to pass a volume with high blood flow. We can also determine if the probe is in white or grey matter, Karin explains.
The data from the DBS probe, the preoperative MRI and a postoperative computed tomography (CT) or MRI are combined and used as input for patient-specific computer modelling and simulations of the electric field around the DBS electrode.Example of simulation of DBS electric field (in green) in the zona incerta (Zi) for a patient with essential tremor. Right image: Zoom in of the target region where the Zi is an area between the subthalamic nucleus and the red nucleus.
-The simulated electric field is visualized together with the preoperative MRI. This makes relative comparisons between simulations possible and the electric field can be related to the patient’s own anatomy, Karin says.
Investigating the Surroundings
The research group is investigating if it is important to know what type of tissue that surrounds the electrode. The results show that there is little difference between white and gray matter but if the electrode is placed close to interstitial fluid the electrical field may be distorted. This is highly relevant since Parkinson’s patients often have Virchow-Robin Spaces, interstitial fluid-filled spaces, in the brain.
-We believe that placing the electrode too close to Virchow-Robin Spaces might be the reason for some of the side effects seen in DBS treated patient, Karin continues.
Three of the most common targets for DBS are located close to white matter tracts in the brain. These tracts can be visualized with diffusion MRI (dMRI). Karin’s research group recently started up a project investigating if it would be possible to visualize these tracts and find out if the treatment is more effective when the electrode is placed on one of these tracts.
-You need to be able to show very thin white fibers and also when they intersect. We have developed a dMRI methodology for this using tractography and are working on refining the protocol to improve the visualization as much as possible, Karin says.
Karin has collaborations with research groups and neurosurgery clinics both in Sweden and in Europe.
-The close collaboration with the clinic and the neurosurgeons means everything to my research. Without them it would be impossible to continue, Karin explains.
Some of Karin’s research results have been commercialized, first a laser Doppler scanner developed during her PhD, and more recently the technique for optical measurements in the brain.
-It is not often that something in your research turns out to be a potential product but when it does, I think it is my obligation to try to commercialize so that my idea can benefit others, Karin concludes.