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Uniformly redundant array coded apertures have proven to be useful in the design of collimators for x-ray astronomy. They were initially expected to be equally successful in single-photon emission computed tomography (SPECT). Unfortunately, the SPECT images produced by this collimator contain artifacts, which mask the true picture and can lead to false diagnosis. Monte Carlo simulation has shown that the formation of a composite image will significantly reduce these artifacts. A simulation of a tumor in a compressed breast phantom has produced a composite image, which clearly indicates the presence of a 5 mm x 5 mm x 5 mm tumor with a 6:1 intensity ratio relative to the background tissue.

We measured population abundance and density of

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The temperature dependencies of four ternary positive ion–molecule association reactions yielding the products N+3, N+4, and (CO)+2 have been studied using a high pressure pulsed source. The pressure range 0.5 to 4.0 Torr and the temperature range 220 to 500 K were examined. When the experiments were conducted in a pure gas, the third-order rate coefficients were found to follow an overall T−n dependence with 2.1≤n≤2.4. For the case of N+4 formation in a mixture of helium and nitrogen, a power law dependence was not found. The measured pressure and temperature dependencies indicate that the low-pressure limit for all of the reactions extends to neutral number densities as high as 1017 cm−3 throughout the experimental temperature range. The measurements support the existence of a temperature dependence in the stabilization of the intermediate complex for both molecular and atomic third bodies. The results are discussed in light of current models of the ternary association process.