Synopsis

In this study, we monitored biological response in neutronexposed normal mouse brains using DW-MRI and 18F-FDG-PET. DW-MRI and 18F-FDG-PET were sensitive enough to detect physiological changes that were not identified in the H&E results. These results suggest that DW-MRI and 18F-FDG-PET can be used as imaging biomarkers to provide a quantitative indicator of complex in vivo pathological changes. Assessment of radiation exposure using imaging biomarkers is non-invasive and can be used repeatedly in clinical practice. In addition, it should be applied not only for evaluating the therapeutic effects of tumors, but also for evaluating the degree of tissue damage by radiation exposure.

Introduction

Materials and Methods

Animals : Ten female BALB/c nu/nu mice (age 6 weeks) were assigned into radiation groups (1 or 2 Gy, n = 5). Before and after neutron irradiation, DW-MRI and 18F-FDG-PET imaging were performed at different times (Pre, Day 0; day of irradiation, Day 1; after 24 h irradiation, Day 4 and Day 8).

Radiation sources : Neutrons were generated by the medical cyclotron (MC-50; Scanditronix, 1985) using the proton-beryllium reaction at KIRAMS. The irradiation time of neutron beams in our studies was typically performed as the standard point of about 1 Gy/12 min.

MRI : Images were acquired on 9.4T (Agilent Technologies, USA) using specific mouse brain coil. T2WI sequence with TR 2s, TE 10 ms, 256×256 matrix, 1 mm slice thickness, scan time 6 min 36 sec. DWI sequence with b-values 0 and 800 s/mm2, TR 2s, TE 10 ms, 128×128 matrix, 1 mm slice thickness, scan time 8 min 32 sec. ROIs were manually circumscribed for hippocampus and cortex. (Figure 1)

PET: Whole-body PET images were obtained on an animal PET/CT scanner (nanoScan, Mediso Medical Imaging Systems, Budapest, Hungary). Mice were anesthetized with 2.5% isoflurane in oxygen, and ~ 200 μCi of 18F-FDG in 100 μL was injected via tail vein. PET scanning was performed for 20 min (30 min post injection).

Histological Evaluation and Immunohistochemical Analysis: After neutron irradiation, animals were sacrificed and brain tissue was obtained then stained with hematoxylin and eosin (H&E). The immunohistochemistry was performed according to the avidin-biotinylated-HRP complex (ABC) method using the Vectastain Elite ABC kit. The sections were stained by standard method, using anti-glucose transporter GLUT1 antibody.

Results

MRI : On the T2WI there was no significant change in the signal intensity, while on the ADC maps the signal was changed, and the signal change was observed according to the intensity of the irradiation. Figure 2A and 2B shows the quantitative ADC comparison at the different time points for the two groups (1 Gy and 2 Gy) in the hippocampus and cortex. The ADC value of the radiation day (Day 0) was changed in both regions, and the radiation dose was compared with that of the day before radiation (Pre). After 24 hours of irradiation (Day 1), the mean ADC value increased most significantly, and then gradually decreased until Day 8. The mean ADC value of 2 Gy was higher than 1 Gy in both the hippocampus and cortex, and the signal change pattern of the two regions was similar.

PET : In group 1 Gy, the glucose uptake decreased until Day 1, then began to increase on Day 4. On the Day 8, the signals were restored to their original levels. However, group 2 Gy showed that the signal decreased on Day 0, then it seemed to recover again on Day 1, but decreased again by Day 8, and the signals did not recover to the original level. Overall, the glucose uptake value of group 2 Gy was lower than that of group 1 Gy in in the hippocampus and cortex, and the signal aspects of both regions were similar (Figure 3).

Histological evaluation and immunohistochemistry : In H&E, there were no apparent differences between the brain tissues from each group. No structural degeneration or cellular damage, such as demyelination and neuronal loss, was observed (Figure 4A). The GLUT1 expression was significantly decreased in both cortex and hippocampus regions and was more decreased with increasing intensity of radiation exposure (Figure 4B). The expression levels were then increased back again at Day 8, almost as high as the initial level. The recovery was also dependent on the intensity of radiation exposure, as the expression levels at Day 8 were lower in 2 Gy than 1 Gy in each region (Figure 4 C and D).

Conclusion

Acknowledgements

This work was supported by a grant from the Korea Institute of Radiological and Medical Sciences(KIRAMS), funded by the Ministry of Science and ICT(MSIT), Republic of Korea (No. 50461-2018).

References

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