[en] Monte Carlo simulations have been used to derive the relationship between the counting efficiency and both the chest wall thickness and adipose mass fraction as a function of photon energy. Adipose mass fraction can be eliminated by the use of the derived quantity muscle-equivalent chest wall thickness and thus simplify the relationship. As a result, the counting efficiency at any adipose mass fraction can be predicted from a single exponential plot of counting efficiency against muscle-equivalent chest wall thickness; therefore, it is unnecessary to have a family of curves for different adipose mass fraction values. Uncertainties due to the measurement of the subject's adipose mass fraction and the difference between the subject's chest (ribs, cartilage) compared to the phantom cannot be eliminated by the use of muscle-equivalent chest wall thickness. Adipose mass fraction measurements are only important for 17 keV photons and can be neglected at photon energies greater than 40 keV. (author)

[en] Designing and testing new equipment can be an expensive and time consuming process or the desired performance characteristics may preclude its construction due to technological shortcomings. Cost may also prevent other types of scenario being tested. An alternative is to use Monte Carlo simulations to make the investigations. This paper describes and exemplifies how Monte Carlo code calculations can be used to fill the gap. An example is given for the investigation of two sizes of germanium detector (70 mm and 80 mm diameter) at four different crystal thicknesses (15, 20, 25 and 30 mm) and makes predictions on how the size affects the counting efficiency and the Minimum Detectable Activity (MDA). (author)

[en] Designing and testing new equipment can be an expensive and time consuming process or the desired performance characteristics may preclude its construction due to technological shortcomings. Cost may also prevent other types of scenario being tested. An alternative is to use Monte Carlo simulations to make the investigations. This paper exemplifies how Monte Carlo code calculations can be used to fill the gap by describing two investigations: (1) the possible self- attenuation of homogeneously distributed natural uranium in a lung phantom; and (2) the effect of activity deposited in the ribs on the activity estimate from a lung count. (author)

No abstract available

[en] The Human Monitoring Laboratory which acts as Canada's National Calibration Reference Centre for in vivo monitoring, recently acquired a new lung counting system comprised of four 70 mm diameter and 30 mm thick germanium detectors. Prior to purchasing the system, calibrations were performed with an identical evaluation detector. The work was carried out using activity homogeneously distributed in lung sets which were placed in the torso phantom that was originally developed at Lawrence Livermore National Laboratory, USA. Results of the cross-talk and heterogeneously loaded lungs indicated that detectors used for low energy photon emitters should be calibrated individually instead of as an array, as is often the case. It was clear that no activity, particularly at 17 keV (²³⁹Pu), contributed to the counts of a detector which was not directly above this activity and that to use a summed array of detectors with a distributed source (i.e. two lungs) could lead to some very large errors. (Author)

[en] A fundamental assumption made in calibrating a lung counter is that the deposition of radioactivity in the lung is homogeneous; however, depositions rarely follow this pattern. There will be an uncertainty on the activity estimate if the lung counter is calibrated using a lung set that has the activity homogeneously distributed throughout the tissue substitute material and the deposition in the subject is heterogeneous. Monte Carlo code simulations have shown that the uncertainty on an activity estimate of an internal deposition that emits 17 keV photons can be underestimated by as much as a factor of four, overestimated by as much as a factor of 26, or missed completely. As the photon energy rises to 60 keV the uncertainty on the activity decreases by as much as a factor of 26, or missed completely. As the photon energy rises to 60 keV the uncertainty on the activity decreases so that the maximum overestimate (or underestimate) will be a factor of three. Detector arrays minimise the uncertainties arising from the geometry of the lung deposition and should be the preferred counting configuration. (Author)