Abstract 4111: Water content based Electrical Properties Tomography (wEPT) for modelling delivery of Tumor Treating Fields to the brain
Authors: Tempel-Brami Catherine , Wenger Cornelia , Hershkovich Hadas S. , Giladi Moshe , Bomzon Ze'ev 
Published: August 2018

Abstract
Objective: The purpose of this study was to investigate the application of Water content based Electrical Properties Tomography for mapping electrical properties (EPs) of brain tissues in the frequency range of 100-1000 kHz.
Background: TTFields are electric fields with frequencies of 100-500 kHz that disrupt mitosis. TTFields are approved for the treatment of glioblastoma multiforme. Determining the EPs of brain tissues is important for understanding how TTFields distribute within the head. The EPs of tissues are heterogeneous, especially in the region of the tumor. Therefore methods that map EPs within the brain with high spatial resolution are desired. Water content based EP tomography (wEPT) is a method that utilizes the ratio of two T1w MRI images with different relaxation times (TRs) to map EPs based on empirically derived relationships between T1, water content (WC) and EPs. wEPT has been applied to map EPs of healthy brain at 128 MHz using typical WC and EP values of healthy tissues reported in the literature to derive the empirical models. Here we adapted wEPT to map EPs in the 100-1000 kHz range utilizing in-house measurements of WC and EPs from healthy bovine and tumor-bearing rat brain tissue.
Methods: The empirical model connecting MRI images, WC and EPs in the 100-1000 kHz range were created using 32 tissue samples derived from three 3 calf brains and 1 CSF sample of a pig. For each sample, T1w MRIs with TRs {700, 4000} ms were acquired and the image ratio (Ir) between the images was calculated. EPs of samples were measured using parallel plates, and WC was measured by the wet-dry weight method. Curve fitting yielded empirical models connecting Ir, WC and EP. Next, T1w MRIs of in-vivo tumor-bearing rat brains and 6 ex-vivo pieces of calf brain were acquired, and the empirical curves described above were used to map WC and EP within the rat brains and the pieces of calf brain. EPs and WC were measured on 6 small samples excised from each imaged brain. For each sample, measured values were compared to the median WC and EPs extracted from the corresponding Regions of Interest (ROIs) in the wEPT map.
Results: Anatomical structures and the tumor were clearly visible in wEPT maps. WC estimated using wEPT agreed well with measurements on excised samples. There was a clear connection between EPs estimated with wEPT and the measured values. However, in some samples large differences between wEPT-derived EP values and measurements were found. In particular, the differences between tumor and healthy tissues conductivity estimated using wEPT was significantly higher than the measured difference in conductivities within the corresponding excised samples.
Conclusion: wEPT maps WC in healthy and tumor brain tissues and provides information on local electrical properties at frequencies of 100-1000 kHz. Further investigation is needed to clarify the relationship between WC and EP within this frequency range.
Citation Format: Catherine Tempel-Brami, Cornelia Wenger, Hadas S. Hershkovich, Moshe Giladi, Ze'ev Bomzon. Water content based Electrical Properties Tomography (wEPT) for modelling delivery of Tumor Treating Fields to the brain [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4111.