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PurposeTo quantify B0- and B1-induced imaging artifacts of braided venous stents and to compare the artifacts to a set of laser-cut stents used in venous interventions.MethodsThree prototypes of braided venous stents with different geometries were tested in vitro. B0 field distortion maps were measured via the frequency shift Δf using multi-echo imaging. B1 distortions were quantified using the double angle method. The relative amplitudes B1rel were calculated to compare the intraluminal alteration of B1. Measurements were repeated with the stents in three different orientations: parallel, diagonal and orthogonal to B0.ResultsAt 1.5 T, the braided stents induced a maximum frequency shift of Δfx<100Hz. Signal voids were limited to a distance of 2 mm to the stent walls at an echo time of 3 ms. No substantial difference in the B0 field distortions was seen between laser-cut and braided venous stents. B1rel maps showed strongly varying distortion patterns in the braided stents with the mean intraluminal B1rel ranging from 63±18% in prototype 1 to 98±38% in prototype 2. Compared to laser-cut stents the braided stents showed a 5 to 9 times higher coefficient of variation of the intraluminal B1rel.ConclusionBraided venous stent prototypes allow for MR imaging of the intraluminal area without substantial signal voids due to B0-induced artifacts. Whereas B1 is attenuated homogeneously in laser-cut stents, the B1 distortion in braided stents is more inhomogeneous and shows areas with enhanced amplitude. This could potentially be used in braided stent designs for intraluminal signal amplification.  相似文献   

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The intrinsically disordered, positively charged H4 histone tail is important for chromatin structure and function. We have explored conformational ensembles of human H4 tail in solution, with varying levels of charge neutralization via acetylation or amino-acid substitutions such as KQ. We have employed an explicit water model shown recently to be well suited for simulations of intrinsically disordered proteins.Upon progressive neutralization of the H4, its radius of gyration decreases linearly with the tail charge q, the trend is explained using a simple polymer model. While the wild type state (q=+8) is essentially a random coil, hyper-acetylated H4 (q=+3) is virtually as compact and stable as a globular protein of the same number of amino-acids.Conformational ensembles of acetylated H4 match the corresponding KX substitutions only approximately: based on the ensemble similarity, we propose KM as a possible alternative to the commonly used KQ.Possible effects of the H4 tail compaction on chromatin structure are discussed within a qualitative model in which the chromatin is highly heterogeneous, easily inter-converting between various structural forms. We predict that upon progressive charge neutralization of the H4 tail, the least compact sub-states of chromatin de-condense first, followed by de-condensation of more compact structures, e.g. those that harbor a high fraction of stacked di-nucleosomes. The predicted hierarchy of DNA accessibility increase upon progressive acetylation of H4 might be utilized by the cell for selective DNA accessibility control.  相似文献   

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PurposeTo assess the task-based performance of images obtained under different focal spot size and acquisition mode on a dual-energy CT scanner.MethodsAxial CT image series of the Catphan phantom were obtained using a tube focus at different sizes. Acquisitions were performed in standard single-energy, high resolution (HR) and dual-energy modes. Images were reconstructed using conventional and high definition (HD) kernels. Task-based transfer function at the 50% level (TTF50%) for teflon, delrin, low density polyethylene (LDPE) and acrylic, as well as image noise and noise texture, were assessed across all focal spots and acquisition modes using Noise Power Spectrum (NPS) analysis. A non-prewhitening mathematical observer model was used to calculate detectability index (dNPW).ResultsTTF50% degraded with increasing focal spot size. TTF50% ranged from 0.67 mm−1 for teflon to 0.25 mm−1 for acrylic. For standard kernel, image noise and NPS-determined average spatial frequency were 8.3 HU and 0.29 mm−1, respectively in single-energy, 12.0 HU and 0.37 mm−1 in HR, and 7.9 HU and 0.26 mm−1 in dual-energy mode. For standard kernel, dNPW was 61 in single-energy and HR mode and reduced to 56 in dual-energy mode.ConclusionsThe task-based image quality assessment metrics have shown that spatial resolution is higher for higher image contrast materials and detectability is higher in the standard single-energy mode compared to HR and dual-energy mode. The results of the current study provide CT operators the required knowledge to characterize their CT system towards the optimization of its clinical performance.  相似文献   

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