Stroke type detection by Multi-Frequency Electrical Impedance Tomography (MFEIT) - a feasibility study

Multi Frequency Electrical Impedance Tomography (MFEIT) is a recently developed non-invasive portable imaging technique. Acquisition is performed by injection of current at multiple frequencies through a set of scalp electrodes, and boundary voltages are measured over another set. 3D impedance distribution maps can be reconstructed by solving the inverse admittivity problem. Biological tissue impedance changes with frequency due to the frequency-dependent behaviour of cell membranes; each tissue is characterised by a unique spectroscopic signature. So far, all clinical EIT has been of differences over time in order to reduce modelling and instrumentation errors. MFEIT has the potential to distinguish between haemorrhagic and ischemic brain stroke in emergency situations where CT or MRI are impractical. Tissue plasminogen activator (t-PA) is a medication that can break up blood clots and restore blood flow when administered within 3 hours of the ischemic event. Whereas about 80% of the patients suffer from ischemia, only 2.5-5% of them are classified in time and treated . There are three main approaches by which this problem could be addressed: statistical analysis over the raw boundary voltages, absolute imaging or multi-frequency imaging. Due to lack of time referenced measurements, time difference imaging which is the most robust approach is not applicable for acute stroke. The first approach ignores, in a sense, spatial information related to the problem, absolute imaging does not account for inter-frequency trends and is also highly sensitive to geometrical discrepancies , multi-frequency imaging does bury a promise of enjoying the benefits of both approaches, and yet not much is known regarding the robustness of such approach. However, before deciding upon a strategy for recovery of the internal impedance changes, it is essential to know what are the expected boundary changes due to acute stoke pathologies, and by what extent these changes are subjected to systematic deviations. The purpose of this study was to model the size of the expected changes measured with scalp electrodes during acute stroke. This was achieved using an anatomically realistic FEM mesh and three different sizes of ischemic infarction or haemorrhage. In order to assess the likelihood in reality of being able to distinguish the resulting small changes over frequency, errors due to electrode position, normal variation in tissue electrical properties, electrode contact impedance and extracerebral shell thicknesses deviations were modeled too. At the current stage, this study is concentrate in quantifying the raw changes over frequency in light of performing statistical analysis over the boundary measurements.