[Development of a counting method for black spots caused by radioactive contamination on the imaging plate].

Due to accidents of the nuclear power plants in Fukushima prefecture, a lot of radioisotopes were diffused into the environment. They adhered onto the surface of the X-ray detector (imaging plate; IP) and many black spots were seen on the medical images. The process to count them is important to evaluate the degree of contamination and/or removal. In this study, we aimed to develop a counting method for black spots. Based on the analysis of the medical images having black spots, we summarized that areas affected by the certain black spots were limited to the eight pixels surrounding the most intensive pixel. The newly developed counting method was applied to these nine pixels (3×3 pixels) and selection rules were based on the following two information: 1. differences between the digital value of the most intensive pixel and those of the surrounding eight pixels, and 2. total summation of the digital values in the nine pixels. The estimated image based on our method showed a good concordance with the original image. Therefore, we summarized that our counting method is a powerful tool for estimating numbers of black spots.

[Radioisotope decontamination of X-ray detector (photostimulable phosphor plate)].

We tried to remove contamination of radioisotope (RI) for an X-ray detector (photostimulable phosphor plate; IP) and verified that our procedure suggested by Nishihara et al. was effective for decontamination. The procedure was as follows. First, the IP was kept for approximately twelve hours, and then it was processed [image (A)] as well as a clinical processing mode. Second, using a wet-type chemical wiper, we scavenged the IP to remove the adhered RI on its surface. Then, once again, the IP was kept for approximately fifteen hours and processed [image (B)] in order to check an effect of decontamination. Finally, the two images of (A) and (B) were analyzed using ImageJ, which can be downloaded as a free software, and a percentage of removal was calculated. The procedure was applied to two IPs using the FCR 5501 plus. In the present case, the percentage of removal was approximately 96%. The removed radioisotopes in the chemical wipers were analyzed by Ge detector. Then, (134)Cs and (137)Cs were found with activities of 2.9 4.3 Bq and 3.5 5.2 Bq, respectively. For three months after that, we cannot see black spots on the IPs owing to the contamination of the RI and there are no defects caused by decontamination using a wet-type chemical wiper.

[Estimation of activities caused by black spots in digital radiography systems: analysis of black spots by the low concentrate radioisotopes adhered directly to the surface of the phosphor plate].

Because of an accident of the nuclear power plants in the Fukushima, many radioisotopes (RI) have been diffused to the environment. As a result, black spots were appearing on the medical images which were taken by the phosphor plate. The aim of this study is to evaluate the activity (Bq) of radioactive contaminated IP based on the experiment using RI. The radioactive material ((134)Cs and/or (137)Cs) in the form of liquid was dropped on filter paper (25 mm(2)), and radioactive sources having 40-240 Bq activities were made. These sources were closely attached to the IP with irradiation times of 2-22 h. Then, we obtained the relationship between pixel values and products of activities and irradiation times. Using these relationships, we evaluated the activity in the contaminated IP. The evaluated value of approximately 7 Bq was in good agreement with a value which was inhered in a chemical wiper used for the decontamination of the IP. Based on the results, we summarized that almost all black spots were created by the RI adhered directly to the IP.