A radial sampling strategy for uniform k‐space coverage with retrospective respiratory gating in 3D ultrashort‐echo‐time lung imaging
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The purpose of this work was to develop a 3D radial‐sampling strategy which maintains uniform k ‐space sample density after retrospective respiratory gating, and demonstrate its feasibility in free‐breathing ultrashort‐echo‐time lung MRI. A multi‐shot, interleaved 3D radial sampling function was designed by segmenting a single‐shot trajectory of projection views such that each interleaf samples k ‐space in an incoherent fashion. An optimal segmentation factor for the interleaved acquisition was derived based on an approximate model of respiratory patterns such that radial interleaves are evenly accepted during the retrospective gating. The optimality of the proposed sampling scheme was tested by numerical simulations and phantom experiments using human respiratory waveforms. Retrospectively, respiratory‐gated, free‐breathing lung MRI with the proposed sampling strategy was performed in healthy subjects. The simulation yielded the most uniform k ‐space sample density with the optimal segmentation factor, as evidenced by the smallest standard deviation of the number of neighboring samples as well as minimal side‐lobe energy in the point spread function. The optimality of the proposed scheme was also confirmed by minimal image artifacts in phantom images. Human lung images showed that the proposed sampling scheme significantly reduced streak and ring artifacts compared with the conventional retrospective respiratory gating while suppressing motion‐related blurring compared with full sampling without respiratory gating. In conclusion, the proposed 3D radial‐sampling scheme can effectively suppress the image artifacts due to non‐uniform k ‐space sample density in retrospectively respiratory‐gated lung MRI by uniformly distributing gated radial views across the k ‐space. Copyright © 2016 John Wiley & Sons, Ltd.Keywords:
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The streak instability is examined experimentally by artificially generating spanwise-periodic low-speed streaks in a laminar boundary layer on a flat plate. Fundamental and subharmonic modes are excited for each of the sinuous and varicose instabilities and their development is compared with the corresponding result of a single low-speed streak. The development of subharmonic sinuous mode does not strongly depend on the streak spacing and it grows with almost the same growth rate as that for the single streak. By contrast, the development of fundamental sinuous mode is very sensitive to the streak spacing and is completely suppressed when the streak spacing is smaller than a critical value, about 2.5 times the streak width for the low-speed streaks examined. On the varicose instability, the fundamental mode is less amplified than the subharmonic mode, but the growth of both modes is weak compared with the case of the single streak.
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Recordings with the View Streak supply the full spatial as well as a highly time-resolved information about an event. Studies on simultaneous initiation devices (SID) are presented as an example. Such recordings can be made with a rotating-mirror streak camera, the streak slit of which can be removed, or also with an image converter camera, in this example with an IMACON type camera. The streak records give an exact information on the time and location at which a detonation in a channel is interrupted. Thus; Open View Streak Technique provides a means of detecting and eliminating sources of failure in the design and manufacture of such SID arrangements. This analog record makes it also possible to determine the detonation velocity during the entire event and in the individual portions of the path of propagation.
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The first step of streak image processing is noise suppressing,but noise suppressing algorithms for images of traditional scanning lidar are not applicable for streak image.Through the configuration and imaging principle of streak tube imaging lidar,noise sources,noise characteristics,influencing factors,contribution to streak image and suppression methods are discussed in details.Noise source analyzing result gets verified through streak tube imaging experiment for staircase tagets.For the special processing purpose of streak image,K Nearest Neighbor Mean Filter indicates lower time and space complexity and better noise filtering performance among classical edge preserving filters.As N=7,K=25,K Nearest Neighbor Mean Filter can improve the signal-to-noise of streak image in a certain extent in tolerance time.Then threshold algorithm is used to filter background noise and streak data is obtained finally among complex noise.This work lays a foundation for the following target image reconstruction and points out the direction and keystone of the next step.
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The streak instability is examined experimentally through artificially producing spanwiseperiodic low-speed streaks in a flat-plate laminar boundary layer. The observation focuses on sinuous instability modes which are responsible for the regeneration of quasi-streamwise vortices in near-wall turbulence. Effects of the interaction between disturbances developing along the neighboring streaks on the instability are examined through comparing the disturbance development for various streak spacing. The instability characteristics are also compared with those for a single low-speed streak. The development of subharmonic modes does not strongly depend on the streak spacing and they grow with almost the same growth rate as that for the single streak. By contrast, the development of fundamental modes is very sensitive to the streak spacing, and is completely suppressed when the streak spacing is less than 2.5 times the streak width.
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Recordings with the "Open View Streak Technique" supply the full spatial as well as a highly time-resolved information about an event. Studies on simultaneous initiation devices (SID) are presented as an example. Such recordings can be made with a rotating-mirror streak camera, the streak slit of which can be removed, or also with an image converter camera, in this example with an IMACON type camera. The streak records give an exact information on the time and location at which a detonation in a channel is interrupted. Thus; Open View Streak Technique provides a means of detecting and eliminating sources of failure in the design and manufacture of such SID arrangements. This analog record makes it also possible to determine the detonation velocity during the entire event and in the individual portions of the path of propagation.
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An unsteady numerical simulation of turbine stage on the S1 flow surface with hot streak was performed firstly.After analysis of the results,some assumptions for the hot streak migration were made.And then a separate section calculation method and an integral calculation method for the hot streak migration avenue were presented.Comparison with the unsteady calculation,the results show that,these two methods have high precision on the prediction of the hot streak migration avenue,and the differences of migration in circumferential direction are within one degree.
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An x-ray streak tube (XRST) for soft x-ray (0.1 - 10 keV) research and a streak camera with the XRST as a part of it are described. The streak tube is attached to a radiation source chamber, operating at 1 Pa pressure, via a pumping-out device with an ion pump, which provides operating pressure of 10-4 Pa in the streak tube. The pumping-out speed is 6 1/s. The pumping-out device has an inspection window through which changeable Au, CsI and CuI photocathodes and a cassette with a film (which separates the volume of the radiation source chamber from that of the streak tube) are installed into the streak tube. Production technology of these photocathodes and their characteristics are discussed in the paper. The streak camera operates in remote and manual control modes and has discrete sweep duration values in the 5 - 500 ns range. The image from the output screen of the streak camera is recorded by a contact type camera or a TV read-out system. The possibility of sweep duration values calibration directly before making measurements is provided in the streak camera.
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