Performance of positron imaging systems as a function of energy threshold and shielding depth

The effect of energy discrimination and shielding on positron imaging data quality was investigated using a detector pair to simulate a ring positron emission tomograph. Formulas are presented relating the sensitivity, random fraction, and scatter fraction for a detector pair to the same parameters for a ring system. Data were fitted to detector pair expressions for the variation of the above parameters with shielding depth in order to obtain information on the effect of energy threshold level. These fitted curves were used to determine the sensitivity, random fraction, and scatter fraction, as well as an overall data quality factor as a function of energy threshold level and shielding depth. Data were obtained for both NaI(T1) and BGO detector types using activity levels in the range of 1.5 muCi/cm3. Results show that for NaI(T1) detectors, the lowest possible energy threshold level is optimal, with the corresponding optimal shielding depth determined by the level of activity to be imaged. For BGO detectors, a tradeoff exists between energy thresholds of 100-400 keV and shielding depths of 15-30 cm with smaller shielding depths requiring higher energy thresholds.     

Digital autoradiography: film and electronic multitracer techniques for heart imaging

A multitracer digital film autoradiography (DFA) and a single-tracer multiwire proportional chamber autoradiography (MWPCA) have been developed for the simultaneous study of regional myocardial blood flow and metabolism in the canine heart. Radioactive indicators of flow (plastic microspheres), metabolism (deoxy-2-D-glucose labeled either with (14)C or (3)H), and flow and metabolism ((201)T1), were used in the same experiment. Multiple tracers were discriminated by film autoradiography on the basis of their properties (particulate or nonparticulate, short or long half-life), and by multiple film exposure. A multiwire chamber was used for (3)H detection. Perfusional and metabolic maps were obtained in transverse microslices of the heart (40 mum thick) by digital image processing. The operation, the advantages and limits of the single techniques as well as their combined use are described. The two techniques are complementary: DFA, although time consuming and proportionality limited, allows the use of multiple tracers and it is mandatory for particulate tracer detection due to its high spatial resolution; MWPCA, in spite of a lower resolution provides a very fast and proportional detection as compared to DFA, but limited to only one tracer at the time.     

Carbon dioxide evolution of refrigerated meat

The carbon dioxide (CO(2)) evolution from pork loins stored in air or nitrogen at 5°C was measured. The loins were cut immediately after slaughter ('pre-rigor meat.'), or approximately 20 h after slaughter ('post-rigor meat'). CO(2) evolution rate was highest during the first day (2·3 × 10(-3)mlcm(-2)h(-1)) and then declined to approximately 0·5 × 10(-3)mlcm(-2)h(-1). The rate was fairly constant during this later phase, and there were no large differences between aerobically and anaerobically stored meat, or between pre-rigor meat and post-rigor meat. Four hypotheses for the origin of the CO(2) evolution are discussed: (1) diffusion from a pre-formed pool in the meat; (2) aerobic energy metabolism of the meat cells; (3) other biochemical reactions of the meat cells; and (4) microbial activity. None of the hypotheses completely supports the experimental results of the investigation. The impact of these findings on the vacuum packaging of meat is discussed.     

The Department of Defense Human factors standardization program

The Department of Defense (DoD) Human Factors Standardization Program is the most far-reaching standardization programme in the USA. It is an integrated component of the overall DoD Standardization Program. While only ten major documents are contained in the human factors standardization area, their effects on human factors engineering programmes are profound and wide-ranging. Preparation and updating of the human engineering standardisation documents have grown out of the efforts of several military agencies, contractors, consultants, universities and individuals. New documents, engineering practice studies and revision efforts are continuously planned by the Tri-Service (Army, Navy, Air Force) Human Factors Standardization Steering Committee in collaboration with industry groups and technical societies. The present five-year plan and other standardisation documents are readily available for review and input by anyone with relevant interests. Human factors specialists and other readers of this journal may therefore influence the direction of the human factors standardisation programme and the content of its military specifications, standards and handbooks.