Non-contrast enhanced molecular characterization of C6 rat glioma tumor at 7T

Purpose: The aim of this study was to investigate, how to assess the relevant magnetization changes in the rat brain tissue due to the present of glioma tumor and its growth at a 7 T animal magnetic resonance imaging (MRI) system. Material and methods: For this study, a custom-built two dimensional (2D) chemical exchange saturation transfer (CEST) pulse sequence was optimized for different tissue properties using fresh and cooked quail eggs. C6 tumor cells were investigated by in-vivo and post-mortem measurements in six Wistar rats using the optimized CEST sequence up to 5 weeks. Magnetization transfer ratio (MTR)- and asymmetric MTR (MTRasym)-maps of rat brains were created at different frequency offsets. In-vivo results were verified by H-1 spectroscopic, histological and also in-vitro C6 cell culture examinations. Results: The CEST module for the optimal visualization of magnetization effects consists of five EF-pulses, each with a duration of 20 ms and a flip angle of 180 degrees. In-vivo and post-mortem z-spectra of the cerebrospinal fluid (CSF), cortex, myelinated/demyelinated, healthy and tumorous tissue and tumor rim were obtained. The magnetization level and shape of the z-spectra on the upfield and downfield from the water peak were not the same. The magnetization dips on the upfield and downfield from the water peak of the z-spectra disappeared due to the thermal denaturation in cooked quail eggs and due to formaldehyde-induced fixation in post-mortem rat brains. The z-spectra of the rat brain in a range of +/- 2 to +/- 4 ppm displayed valuable information about the differentiation of various brain regions from the tumor tissue. Histological examinations confirmed our results. The C6 cell culture examinations showed that the observed magnetization changes in the rat brain occurred only due to the interaction between glioma cells and their environment in the rat brain and not from the C6 tumor cells. Conclusions: Based on our in-vivo and post-mortem results, it is to be recommended to create the MTR-maps at a special offset frequency depending on the aim of research project instead of MTRasym-maps. Otherwise, the desired effect attenuates or vanishes.