The wide availability, low radiation dose and short acquisition time of Cone-Beam CT (CBCT) scans make them an attractive source of data for compiling databases of anatomical structures. However CBCT has higher noise ...The wide availability, low radiation dose and short acquisition time of Cone-Beam CT (CBCT) scans make them an attractive source of data for compiling databases of anatomical structures. However CBCT has higher noise and lower contrast than helical slice CT, which makes segmentation more challenging and the optimal methods are not yet known. This paper evaluates several methods of segmenting airway geometries (nares, nasal cavities and pharynx) from typical dental quality head and neck CBCT data. The nasal cavity has narrow and intricate passages and is separated from the paranasal sinuses by thin walls, making it is susceptible to either over- or under-segmentation. The upper airway was split into two: the nasal cavity and the pharyngeal region (nasopharynx to larynx). Each part was segmented using global thresholding, multi-step level-set, and region competition methods (the latter using thresholding, clustering and classification initialisation and edge attraction techniques). The segmented 3D surfaces were evaluated against a reference manual segmentation using distance-, overlap- and volume-based metrics. Global thresholding, multi-step level-set, and region competition all gave satisfactory results for the lower part of the airway (nasopharynx to larynx). Edge attraction failed completely. A semi-automatic region-growing segmentation with multi-thresholding (or classification) initialization offered the best quality segmentation. With some minimal manual editing, it resulted in an accurate upper airway model, as judged by the similarity and volumetric indices, while being the least time consuming of the semi-automatic methods, and relying the least on the operator’s expertise.展开更多
Edge plasma characteristics were studied by a fast-scanning 4-probe array and a Much/Reynolds stress/Langmuir 10-probe movable array in the boundary region. These probes could measure the edge plasma temperature, dens...Edge plasma characteristics were studied by a fast-scanning 4-probe array and a Much/Reynolds stress/Langmuir 10-probe movable array in the boundary region. These probes could measure the edge plasma temperature, density, poloidal electric field, radial electric field, Reynolds stress, poloidal rotation velocities and their profiles, which could be obtained by changing the radial positions of the probe array shot by shot. The measured results were used to analyse plasma confinement, turbulent fluctuations and correlations. The fixed flush 3-probe arrays were mounted on the 4-divertor neutralization plates at the same toroidal cross-section in the divertor chamber. These probes were used to measure the profiles of the electron temperature, density and float potential in the divertor chamber. Edge plasma behaviours in both limiter configuration and divertor configuration are compared. The decay lengths of the edge temperature and density were measured and is emphasized for plasma behaviours of the supersonic molecular beam injection and lower hybrid current drive. The dependence of the radial gradient of Reynolds stress on the poloidal flow and the radial gradient of the electric field on turbulent loss are discussed.展开更多
Outflowing ion beams forming four successive inverted-V structures in the energy-time spectrograms of H+, He+, and O+ were observed at an altitude of 3.4 RE by Cluster satellites travelling above the auroral accelerat...Outflowing ion beams forming four successive inverted-V structures in the energy-time spectrograms of H+, He+, and O+ were observed at an altitude of 3.4 RE by Cluster satellites travelling above the auroral acceleration region (AAR) in the southern hemisphere on February 14, 2001. Energization by negative U-shaped potential structures in the AAR is believed to be responsible for the formation of these outflowing ion inverted-V structures. Thus, utilizing the different motion properties of the three ion species, the altitude of the upper boundary of the AAR is estimated to be ~11100 km. Moreover, based on multi-satellite observations, each of these U-shaped potential structures involved in this event crosses the latitudinal direction at ~0.4°–1° invariantlatitude (ILAT), moving poleward at an average speed of ~0.2° ILAT per minute, before disappearing at ~71.5° ILAT.展开更多
文摘The wide availability, low radiation dose and short acquisition time of Cone-Beam CT (CBCT) scans make them an attractive source of data for compiling databases of anatomical structures. However CBCT has higher noise and lower contrast than helical slice CT, which makes segmentation more challenging and the optimal methods are not yet known. This paper evaluates several methods of segmenting airway geometries (nares, nasal cavities and pharynx) from typical dental quality head and neck CBCT data. The nasal cavity has narrow and intricate passages and is separated from the paranasal sinuses by thin walls, making it is susceptible to either over- or under-segmentation. The upper airway was split into two: the nasal cavity and the pharyngeal region (nasopharynx to larynx). Each part was segmented using global thresholding, multi-step level-set, and region competition methods (the latter using thresholding, clustering and classification initialisation and edge attraction techniques). The segmented 3D surfaces were evaluated against a reference manual segmentation using distance-, overlap- and volume-based metrics. Global thresholding, multi-step level-set, and region competition all gave satisfactory results for the lower part of the airway (nasopharynx to larynx). Edge attraction failed completely. A semi-automatic region-growing segmentation with multi-thresholding (or classification) initialization offered the best quality segmentation. With some minimal manual editing, it resulted in an accurate upper airway model, as judged by the similarity and volumetric indices, while being the least time consuming of the semi-automatic methods, and relying the least on the operator’s expertise.
基金supported by the National Natural Science Foundation of China(Nos.10375020 and 10235010)
文摘Edge plasma characteristics were studied by a fast-scanning 4-probe array and a Much/Reynolds stress/Langmuir 10-probe movable array in the boundary region. These probes could measure the edge plasma temperature, density, poloidal electric field, radial electric field, Reynolds stress, poloidal rotation velocities and their profiles, which could be obtained by changing the radial positions of the probe array shot by shot. The measured results were used to analyse plasma confinement, turbulent fluctuations and correlations. The fixed flush 3-probe arrays were mounted on the 4-divertor neutralization plates at the same toroidal cross-section in the divertor chamber. These probes were used to measure the profiles of the electron temperature, density and float potential in the divertor chamber. Edge plasma behaviours in both limiter configuration and divertor configuration are compared. The decay lengths of the edge temperature and density were measured and is emphasized for plasma behaviours of the supersonic molecular beam injection and lower hybrid current drive. The dependence of the radial gradient of Reynolds stress on the poloidal flow and the radial gradient of the electric field on turbulent loss are discussed.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41031065, 41421003)by the Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences
文摘Outflowing ion beams forming four successive inverted-V structures in the energy-time spectrograms of H+, He+, and O+ were observed at an altitude of 3.4 RE by Cluster satellites travelling above the auroral acceleration region (AAR) in the southern hemisphere on February 14, 2001. Energization by negative U-shaped potential structures in the AAR is believed to be responsible for the formation of these outflowing ion inverted-V structures. Thus, utilizing the different motion properties of the three ion species, the altitude of the upper boundary of the AAR is estimated to be ~11100 km. Moreover, based on multi-satellite observations, each of these U-shaped potential structures involved in this event crosses the latitudinal direction at ~0.4°–1° invariantlatitude (ILAT), moving poleward at an average speed of ~0.2° ILAT per minute, before disappearing at ~71.5° ILAT.
