Optimization of a new scintillation gas detector used to localize electrons emitted by 99mTc

We have developed a scintillation gas detector to localize electrons emitted by 99mTc. This type of detector allows direct quantification of images and so provides a clear advantage over autoradiographic film. We have optimized the device to give an image spatial resolution that closely approximates that of typical autoradiographic film. To improve this resolution, it was necessary to select only low-energy electrons (2 and 15 keV) and to devise novel detection and localization techniques for the ionizing particles.

A parallel-plate proportional avalanche chamber is subject to a uniform electrical field and amplifies the number of released electrons through collisions of ionizing particles in the gas mixture. Light emitted by the gas scintillator during the avalanche process is collected by a highly intensified charge coupled device camera. The centroid of each resulting light distribution is calculated, resulting in a quantitative mapping of the sample's activity. Insertion of the sample within the gas volume improves the efficiency and so provides a method that is both very sensitive and linear.

We have shown that in a parallel-plate structure, the application of a high electrical field to the surface of the sample and the selection of appropriate light spots, according to their morphology, can overcome localization errors due to the particles' trajectories. We have obtained a resolution of the order of 30 microm, using electrons from 99mTc.

This detection technique allows considerable improvement in image resolution. This "electron camera" is a serious rival to existing autoradiographic techniques, because it provides certain other advantages, including direct quantification, linearity, high dynamic range and low noise levels. Thus, new perspectives are made available in quantitative double tracer autoradiography, because electrons can be selected for imaging as a function of their energy.