Development of sealed sheet sources for calibration of whole-body counters

Whole body counters are usually calibrated with the aid of a whole body phantom assembled with simply-shaped plastic vessels that are filled with an aqueous solution of the relevant radioisotopes. Most vessel-type phantoms represent only a human body in which radioisotopes are homogeneously distributed, whereas the radioisotopes in vivo are sometimes localized to specific organs. Each set of the vessels is usually applicable only to a specific combination of radioisotopes, because the replacement of radioisotopes requires troublesome procedures. Possible leakage of the solution is another disadvantage of the vessel-type phantom. The authors are developing a new-type calibration phantom that is free from these disadvantages, in which sealed sheet sources are sandwiched between sections of a sliced anthropomorphic phantom. This paper describes a method to prepare sealed sheet sources for this calibration phantom. Instead of γ-ray emitters a pure β-ray emitter 32P was used. This isotope is suitable for autoradiography and is easy to handle as its half-life is relatively short. An ink-jet printer was used to spread the solution of 32P mixed with ink on a sheet of paper. The surface concentration of radioactivity was regulated by the function of color density adjustment of an image processing software. The radioisotope-printed paper was laminated for sealing. Through the measurement of surface concentration of radioactivity with a liquid scintillation counter, the autoradiographical investigation of the pattern of the radioactivity distributed on the sheet sources, the immersion test of the sealed sheet sources and the monitoring of the concentration of 32P in air during the printing, it was demonstrated that sealed sheet sources for the calibration phantom can be prepared safely by the method described in this paper. Furthermore, by using sheet sources of 99mTc prepared as a trial it was confirmed that discrete arrangement of sheet sources in a phantom at a uniform interval did not cause systematic fluctuation in spatial distribution of γ-ray fluences outside the phantom. (author)