Improved Activity Determination of a Radioactive Point Source in the Lungs

A modern Lung Counter system is based on four High Purity Germanium detectors. The basic assumption made when calibrating the system is that the deposition of the radioactive aerosols in lungs is homogenous and therefore the calibration is made with realistic phantoms containing lung sets with homogeneous contamination, but the deposition of the radioactive aerosols is mostly a function of the aerosol size and breathing rate. In some cases, the contamination can even be present in form of single 'hot' particles or in form of several particles with heterogeneous distribution. As the shape and the location of the aerosols distribution is unknown, there is an uncertainty which can reach more then one order in magnitude, depending on the photon energy which is emitted. The uncertainty can be reduced by taking into consideration the count rates ratios of the four detectors. The method employed is based on building a normalized reference library of the response of the 4 detectors to point source locations in the lungs of a phantom, and finding the optimal correlation between the system response to the unknown location of the point source and the response characterized by one of the library vectors. In the present work, the following aspects were studied: - Localization and activity determination of a point source in the lungs, based on the information of the count ratios from the 4 detectors, by employing the multi-parameter identification method. The use of the information from the several photon energies emitted by uranium (separately and simultaneously) was investigated. -The influence of the isotopic ratio on the accuracy of the evaluated activity