γ-Labelling with ⁸⁵Sr for in vivo Measurement of ⁹⁰Sr

—An Attempt for the in vivo Measurement of ⁹⁰Sr and ⁹⁰Y in Internally Exposed Animals—

Abstract

In the study on the biological effect of internal exposure, the evaluation of absorbed dose due to deposited radionuclide in the organ of interest is essential.
⁹⁰Sr is one of the most well studied radionuclides among the bone seeking isotopes, but its behavior in the living body is not clearly understood yet.
The in vivo measurement of ⁹⁰Sr is rather difficult since this nuclide emits only low energy (E_β: 0.545 MeV) β-rays. The daughter nuclide, ⁹⁰Y, can be measured in vivo by detecting bremsstrahlungs of its high energy (E_β: 2.26 MeV) β-rays, and if ⁹⁰Y exist in equilibrium with ⁹⁰Sr, the amount of ⁹⁰Sr can be determinded by the bremsstrahlung counting technique. However, it is expected that detailed behavior of ⁹⁰Sr and ⁹⁰Y in the living body is not so quite identical.
To determine the amount of ⁹⁰Sr in vivo, a new“γ-labelling method”has been developed: A mixture of ⁹⁰Sr and small amount (2-5% in activity) of ⁸⁵Sr is administered to the animal, and the γ-rays of 0.513 MeV from ⁸⁵Sr is measured with a scintillation counter. The counting efficiency should be determined beforehand with the original mixture solution.
⁹⁰Y activity existing with ⁹⁰Sr is measured by its bremsstrahlung effect if necessary.
Disscussions were given as to the several technical problems in the application of γ-labelling: characteristics of bremsstrahlung measurement, optimum mixing ratio, its practicability and dose contribution, from labelling isotopes.
This method was applied to the study of internal exposure of ⁹⁰Sr on the whole-body retention of ⁹⁰Sr itself. An example of the results obtained with mice was showed.
It was demonstrated that the whole-body retention of ⁹⁰Sr is influenced by administered dose level of ⁹⁰Sr itself.
The γ-labelling method is also applicable to the determination of the equilibrium ratio, ⁹⁰Y/⁹⁰Sr, by using the fact that the amount of ⁹⁰Y can be determined separately by the bremsstrahlung counting technique with a scintillation spectrometer. By this method the equilibrium ratio, ⁹⁰Y/⁹⁰Sr. in a rabbit bone was determined as a function of time after admininstration of equilibrated ⁹⁰Sr-⁹⁰Y soeution.A 13/4inφ×2 in NaI (Tl) crystal combined with a 2 in photo-tube was used as a detector. The detector was surrounded with 1mm copper in order to cut the low-energy part of the bremsstrahlungs and to reduce the effect of the characteristic X-rays emitted from the lead shield. The pulse-height distribution was obtained with a 100 channel pulse-height analyzer. The results obtained with oral administration and with i.v, injection was showed. Although the ratios reached unity at about two weeks after administration, their transition curves showed quite different pattern depending upon the type of administration.