Objective assessment of low contrast detectability in computed tomography with Channelized Hotelling Observer.

PURPOSE: Iterative algorithms introduce new challenges in the field of image quality assessment. The purpose of this study is to use a mathematical model to evaluate objectively the low contrast detectability in CT. MATERIALS AND METHODS: A QRM 401 phantom containing 5 and 8 mm diameter spheres with a contrast level of 10 and 20 HU was used. The images were acquired at 120 kV with CTDIvol equal to 5, 10, 15, 20 mGy and reconstructed using the filtered back-projection (FBP), adaptive statistical iterative reconstruction 50% (ASIR 50%) and model-based iterative reconstruction (MBIR) algorithms. The model observer used is the Channelized Hotelling Observer (CHO). The channels are dense difference of Gaussian channels (D-DOG). The CHO performances were compared to the outcomes of six human observers having performed four alternative forced choice (4-AFC) tests. RESULTS: For the same CTDIvol level and according to CHO model, the MBIR algorithm gives the higher detectability index. The outcomes of human observers and results of CHO are highly correlated whatever the dose levels, the signals considered and the algorithms used when some noise is added to the CHO model. The Pearson coefficient between the human observers and the CHO is 0.93 for FBP and 0.98 for MBIR. CONCLUSION: The human observers' performances can be predicted by the CHO model. This opens the way for proposing, in parallel to the standard dose report, the level of low contrast detectability expected. The introduction of iterative reconstruction requires such an approach to ensure that dose reduction does not impair diagnostics.

Anthropomorphic model observer performance in three-dimensional detection task for low-contrast computed tomography.

X-ray medical imaging is increasingly becoming three-dimensional (3-D). The dose to the population and its management are of special concern in computed tomography (CT). Task-based methods with model observers to assess the dose-image quality trade-off are promising tools, but they still need to be validated for real volumetric images. The purpose of the present work is to evaluate anthropomorphic model observers in 3-D detection tasks for low-contrast CT images. We scanned a low-contrast phantom containing four types of signals at three dose levels and used two reconstruction algorithms. We implemented a multislice model observer based on the channelized Hotelling observer (msCHO) with anthropomorphic channels and investigated different internal noise methods. We found a good correlation for all tested model observers. These results suggest that the msCHO can be used as a relevant task-based method to evaluate low-contrast detection for CT and optimize scan protocols to lower dose in an efficient way.

Le retour des villes : les phénomènes de déprise et de reprise démographiques dans les villes suisses

Partant de la littérature sur le déclin des villes d'une part et sur le regain d'attractivité des centres urbains d'autre part, cet article analyse l'évolution démographique des villes suisses au cours des dernières décennies. Dès les années 1960/1970, la majorité d'entre elles enregistrent une diminution de leur population, une déprise interprétée comme le signe d'une attractivité déficiente et que beaucoup d'observateurs jugent inéluctable. Et pourtant, depuis les années 2000, une nouvelle phase de croissance (réurbanisation) est constatée. En prenant en compte les différentes composantes de l'évolution de la population, cet article identifie les mécanismes et facteurs économiques, démographiques, socio-culturels et politiques à l'origine de ce « retour des villes ». Drawing on the literature on urban decline on the one hand and on the renewed attractiveness of urban centres on the other, this paper addresses the demographic evolution of Swiss cities during the last decades. In the years 1960/1970 most of them experience a decrease of their population, a loss interpreted as the sign of a deficient attractiveness and that many observers assess as ineluctable. Nonetheless, since 2000 a new period of growth (reurbanisation) has been noted. Taking into account the various components of the population evolution, this paper identifies the mechanisms and the economic, demographic, socio-cultural and political factors at the origin of this « returns of cities ».