Effects of Irradiation on Brain Tumors Using MR-Based Electrical Conductivity Imaging

SIMPLE SUMMARY: The contrast mechanism of electrical conductivity is preliminarily determined by the concentration and mobility of ions that make up tissues. Recent magnetic resonance-based conductivity imaging has been reported as a highly sensitive tool for measuring and evaluating the responses of normal tissues to irradiation. To evaluate and assess its therapeutic effects in clinical practice, it is required to verify the response of malignant tissues to irradiation. In this study, the responses of tumor tissues following irradiation were quantified and compared with the responses of normal tissues. Conductivity at high frequencies provides information on the changes in cellularity and the amounts of electrolytes inside tumor tissues, showing potential as an imaging tool for quantifying the therapeutic effects of radiation on tumors by measuring absolute values and calculating percentage changes. For clinical applications, the imaging results of large samples and statistical analysis of the relationship between conductivity changes and tissue responses are required. ABSTRACT: Ionizing radiation delivers sufficient energy inside the human body to create ions, which kills cancerous tissues either by damaging the DNA directly or by creating charged particles that can damage the DNA. Recent magnetic resonance (MR)-based conductivity imaging shows higher sensitivity than other MR techniques for evaluating the responses of normal tissues immediately after irradiation. However, it is still necessary to verify the responses of cancer tissues to irradiation by conductivity imaging for it to become a reliable tool in evaluating therapeutic effects in clinical practice. In this study, we applied MR-based conductivity imaging to mouse brain tumors to evaluate the responses in irradiated and non-irradiated tissues during the peri-irradiation period. Absolute conductivities of brain tissues were measured to quantify the irradiation effects, and the percentage changes were determined to estimate the degree of response. The conductivity of brain tissues with irradiation was higher than that without irradiation for all tissue types. The percentage changes of tumor tissues with irradiation were clearly different than those without irradiation. The measured conductivity and percentage changes between tumor rims and cores to irradiation were clearly distinguished. The contrast of the conductivity images following irradiation may reflect the response to the changes in cellularity and the amounts of electrolytes in tumor tissues.