Sodium MRI clocks pre-necrotic alterations in rat glioma model without changes in tumor diffusion

Introduction In-vivo MRI biomarkers are crucial for drug development and tailoring individualized therapy. Sodium MRI and proton diffusion (ADC mapping) show strong potentials for evaluating changes inside tumors. In this study, for the first time, high resolution sodium MRI with voxel size less than 1 μL was applied for assessment of tumor therapy using a rodent glioma model. The goal was to monitor alteration of intact tumors and their response to chemotherapy. Results from two physiological windows (sodium and diffusion) represent cellular changes inside the tumor and demonstrate the ability of sodium MRI to grade the pre-necrotic development of non-treated tumors. This grading power of sodium MRI is especially important because, during the same time, there were no observable changes in tumor diffusion. Materials and Methods Male Fisher 344 rats with 9L brain tumors were subjects for MRI study when their tumor size reached ~50μl (n=6). Tumor treatments were performed by chemotherapeutic agent 1,3 bis(2-chloroethyl)-1-nitrosurea (BCNU). Single IP injections of BCNU (26.6 mg/kg) were applied ~ 14 days after tumor implantation. Animals in the control group (n=3) remained untreated. Tumor alterations were observed in-vivo by both high resolution sodium MRI and proton diffusion mapping. Imaging experiments were performed on Varian MRI scanner 9.4T. Three-D sodium images were acquired by a backprojection GE pulse sequence with an echo time of 1 ms, TR = 100 ms, matrix 64x128x128, FOV 64 mm and acquisition time of 2 h. For proton diffusion mapping the isotropic DW SE pulse sequence was used with “high-b” (b=1082 s/mm2) and “low-b” (117 s/mm2), 15 axial slices, FOV 30x30 mm, slice thickness 1.0 mm, TR/TE = 3000/40 ms. All measurements were repeated every 2-3 days. MR images were co-registered in 3 dimensions in order to monitor the same area of the tumor over time. Animal experiments were conducted according to the protocols approved by the University of Michigan LARC. Results During tumor growth, diffusion in untreated gliomas remained unchanged at (1.1 ± 0.05)*10 mm/s. These values were above diffusion in normal contra-lateral brain (0.78 ± 0.02) * 10 mm/s (Fig. 1). Sodium MRI, performed simultaneously with diffusion, showed elevated and consistently increasing Na content in tumor relative to a normal brain. The central area of tumors with high Na becomes more noticeable with time (Fig. 1). Furthermore, the rate of tumor sodium increase accelerated with tumor progression. The glioma response to BCNU treatment was dramatic, exhibiting a consistent correlated increase of Na and ADC. The time of the largest values for tumor sodium and diffusion correlated with intensive tumor cell destruction. Sodium T1 relaxation time dramatically increased during this period from 30 ± 3 ms in normal brain to 57 ± 6 ms in treated gliomas (Fig. 2).