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Tumor Treating Fields (TTFields) are low intensity, intermediate frequency alternating electric fields and a TTFields frequency of 200 kHz is currently used to treat glioblastoma multiforme. Previous data have shown that the efficacy of TTFields not only depends on a specific frequency, but also strongly on intensity. Therefore, a maximal TTFields dose at the site of the tumor is important. The distribution of TTFields in the brain is dependent on the tissue electric properties (EP) which are heterogeneous throughout the brain. Water content based EP tomography (wEPT) is a technique that uses the ratio of two T1w images with different relaxation times (TRs). Here we report on wEPT adaptation in order to map EPs in the 100-1000 kHz range.

An empirical model was generated using 23 tissue samples from 3 juvenile bovine brains and 1 porcine CSF sample. This model linked water content (WC), T1 images and EPs in the range of 100-1000 kHz. For our analysis, we used T1w MRIs with TRs {700, 4000} ms. In order to estimate the water content, the dry and wet mass of the samples were measured and the differences between the two measurements were calculated. Curve fitting was then used to obtain empirical models that link Ir, WC, and EPs. Such empirical curves were then used in order to map EP and WC in tumor-bearing rat brains which were also imaged by T1w MRIs. In addition to that, 6 excised samples of each imaged brain were used to measure EPs and WC. Finally, the WC and EPs of the wEPT map were compared to measured values.

The wEPT maps of rat brains allowed to identify anatomical structures and the tumor. The data also showed that the estimated WC determined by wEPT was in line with measurements done on excised sample. In addition, the results indicate that EPs that are estimated with the wEPT method are connected with experimentally measured values. However, wEPT- derived EP values and measurements differed in some of the samples and this was particularly found for the conductivity of samples taken from the rat tumors.

Our results show that wEPT can be applied to map WC in brain tissues between 100-1000 kHz. On the contrary, wEPT did not yield reliable estimates of conductivity within tumors and therefore further examination is required to clarify the relationship between WC and EP between 100-1000 kHz. This method might then be used non-invasively to measure electrical properties within brain tissue and to further understand the distribution of TTFields.