You already know that a stroke, a severe head injury (such as occurs in a vehicle accident), or bleeding into the brain from a broken blood vessel has immediate negative effects, including loss of memory, speech and motor functions. But you probably are unaware of a secondary effect that often occurs several days later: Large scale waves of electrical disturbances rip through the brain, destroying more brain tissue. Just as a tsunami is a delayed wave response to the energy released in an earthquake, these Cortical Spreading Depolarizations (CSDs) are a delayed electrical wave response to a brain injury.
Our goal at CerebroScope is to develop a noninvasive technique for detecting CSDs so that medical staff can respond to them and save as much brain function as possible for these patients. ANSYS electromagnetic simulations, obtained through the ANSYS Startup Program, were instrumental in designing our device, the CerebroPatch, conceptualized in Figure 1 (left). This proof-of-concept prototype will be placed on the skin directly above brain surface electrodes (Figure 1, middle) at the border of the area of injured brain (Figure 1, right) after the skull portion has been replaced.
Figure 1. Conceptual placement of the CerebroPatch (left). X-ray image of a patient showing an invasive brain surface electrode strip (middle), and the region where the skull was removed (red dashed line) to remove an aneurysm and the placement of the CerebroPatch on the scalp over the site of the injury (right).
Designing from the Ground Up
CSDs were observed in animals as early as 1944 using the highly invasive method of placing electrodes on the brain. In human patients, CSDs have been observed since 2002 in Neuro-Intensive Care Units using research protocols that involve placing electrode strips on the brain during a medically necessitated neurosurgical procedure. Medical specialists have expressed a desire for a non-invasive system to detect CSDs without exposing the brain. To develop a system that is not implanted, scientists and engineers at CerebroScope had to determine how the electrical signal from a CSD traveling on the brain surface propagates to the scalp. CerebroScope leveraged the power of the ANSYS Electronics Desktop to better understand this signal propagation and to properly develop the CerebroPatch sensor system for detecting the CSD signal on the scalp surface.
Scalp Voltage from a CSD
The first challenge was to implement a complex, multilayered model to represent the layers between the brain surface and the scalp. Modeling the human head has the additional challenge that the thicknesses of each layer varies with location. Furthermore, there are large deviations in material parameters, not only from person-to-person but also over time. The parametric solver available in the Electronics Desktop allowed CerebroScope to investigate a variety of topologies and material parameters to fully understand their design.
The Electronics Desktop also allowed CerebroScope engineers to implement the complex, spatiotemporal mathematical representations of the CSD voltage wave as it travels across the surface of the brain (Figure 2). This surface solution avoided implementing the full geometry of the brain, which would have considerably increased the time and complexity of this work.
Figure 2. The simulated voltage field at the scalp generated by a CSD traveling along the surface of the brain was modeled using a “disk” representation of the layers between the brain surface and the scalp. The voltage field becomes much expanded on the surface.
CSD Detection System
The successful simulation of the signal propagation from CSD on the brain surface allowed CerebroScope engineers to determine the optimal design parameters for the CerebroPatch by fully understanding the temporal and spatial characteristics of its voltage distribution on the scalp. Additionally, the interaction of the CerebroPatch detection system with the invasive brain surface electrode strip to be used in our validation protocol was simulated (Figure 3).
After the successful simulation, the development of the detection software necessary for identifying this signal was started. Using the Python scripting features provided by ANSYS solutions, the simulated data was combined with actual EEG data to create training data. This combined data was used to start production of CerebroLab™, a data processing suite that will be used for identifying the CSD from the scalp (Figure 4).
Figure 3. A simulation of the scalp voltage as the CSD interacts with an implanted electrode strip placed in the path of the CSD in the upper right quadrant. The platinum electrode in the electrode strip initially enhances the signal, as the CSD approaches the 2 o’clock position at 1380 sec, but silicone material in the strip eventually diminishes the signal at the 1:30 position (1500 sec). Then again, the signal is enhanced by another platinum electrode at 12 o’clock (1800 sec).
Figure 4. In the left panel, the stationary voltage field representing a residual voltage caused by the injury to the brain is combined with the voltage field of a CSD, obtained using ANSYS simulation. The CSD signal is just barely visible as a very slight bump at the 2:30 position on a clock face. In the right panel, the CSD voltage field becomes fully visible after subtraction of the temporal mean of the complete voltage field.
With the completion of the initial design phase, CerebroScope is producing several CerebroPatch prototypes for use in a proof-of-concept trial in human patients that will begin this year.