2022 Volume 53 Pages 127-138
Specific internally exposed radionuclides such as uranium are known to migrate into bone and accumulate over a long period of time. However, there is a lack of fundamental data on how uranium migrates into bone and how it affects bone development, particularly during juvenile period when bones are actively developing. Aiming at establishing a method to evaluate uranium migration into bone, as a first step, the possibility of visualizing and quantifying uranium migrated into rat femurs was investigated using a synchrotron radiation (SR)-μCT-based absorption-edge subtraction method. Two sets of radiographic images were taken at X-ray energies just below and above the L3-absorption edge (17.17 keV) of uranium (16.9 and 17.5 keV, energy gap of 600 eV) at the BL20B2 beamline of SPring-8, Japan, and reconstructed into microtomographic images by a convolution back-projection algorithm, followed by their subtraction, to leave a set of the differential images (voxel size of 2.73 μm). In addition to 1- and 10-week-old juvenile rat femurs treated with or without uranyl acetate, uranium calibration standards of solution and hydroxyapatite (HAP) were prepared and subjected to the μCT imaging.
For the standards, non-uniform distribution of uranium was observed both in solution and HAP, especially at higher concentrations of uranium, which was possibly caused by the cryo-condition for the solution and by the grain diameter larger than the pixel size for the HAP, respectively. This suggests that the μCT imaging for the standards should be performed at room temperature by using HAP with a grain diameter much smaller than the pixel size.
For the rat femurs, the histograms of the pixel values in the bone area of the differential images showed no significant difference between the uranium-treated group and the control group, confirming that uranium was not detectable in the present experimental setup. This was mainly due to the large standard deviation of the histograms for the control group. The detection limit of uranium under the present experimental condition was estimated to be several tens of thousands of ppm. On the other hand, the histograms of the pixel values in the bone area of the reconstructed images confirmed that the bone density of 10-week-old rat femurs increased by 20% compared with that of 1-week-old rat femurs. They also demonstrated that the three-dimensional microstructure of the bone was clearly visualized with a high spatial resolution of a few micrometers.
