Monitoring of current density and electric field distribution during electroporation of heterogeneous tissues using MR techniques

Electrical properties of biological tissues have been of interest for over a century as they determine the pathways of current flow through the body, and are therefore important in the analysis of a wide range of biomedical applications. Numerous factors determine electrical properof intra- and extra-cellular fluids, concentration and mobility of ions in these fluids, temperature of the tissue, and their pathological conditions, to name only a few. Tissue is a heterogeneous material with important interfacial processes and cells of uneven size and different functions, and so from the electrical point of view, it is impossible to consider tissue as a homogeneous material. Measuring and evaluating electrical changes in real time is important for electroporation-based treatments and technologies. Therefore, an efficient approach is needed to monitor the electric field in studied tissue during the delivery of electroporation pulses. Electric field can be reconstructed from current density distribution data by magnetic resonance electric impedance tomography (MREIT). MREIT is enabled by Current Density Imaging (CDI), an MRI modality designed to detect electric currents via the temporal change of magnetic field that is induced by the currents. Since monitoring is performed during pulse delivery, the determined electric field distribution considers all heterogeneities and changes that occur in the treated tissue. This work will give an overview of monitoring of electric field distribution in tissues of different levels of structural complexity. Experiments were performed in apple fruit, potato tuber, and carrot tissue since these vegetable matrices are – in this order – of ever increasing complexity. Additionally, muscle tissue samples were taken from the neck and thigh region of a domestic pig (Sus domesticus) in order to evaluate anisotropy. Electroporation protocol consisted of two sequences of 4 pulses with a duration of 100 μs per pulse and with a repetition rate of 5 kHz. The voltage amplitude was adjusted to obtain a sufficient signal-to-noise ratio of the electric field in the sample. Application of electric pulses was triggered by an MRI control unit and synchronised with the CDI pulse sequence. MREIT demonstrated to be suitable for monitoring of the electric field distribution inside structures of plant tissues, whereas in muscle tissues we were able to detect different current density behaviour based on application of electric pulses relative to the muscle fibre orientation. These findings provide useful insights into the evaluation of electroporation and suggest that magnetic resonance techniques could be used as an efficient tool to improve the effectiveness of electroporation-based treatments.