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排序方式: 共有1224条查询结果,搜索用时 15 毫秒
1.
Florian Wiesinger Peter Boesiger Klaas P Pruessmann 《Magnetic resonance in medicine》2004,52(2):376-390
The purpose of this article is to elucidate inherent limitations to the performance of parallel MRI. The study focuses on the ultimate signal-to-noise ratio (SNR), which refers to the maximum SNR permitted by the electrodynamics of the signal detection process. Using a spherical model object, it is shown that the behavior of the ultimate SNR imposes distinct limits on the acceleration rate in parallel imaging. For low and moderate acceleration, the ultimate SNR performance is nearly optimal, with geometry factors close to 1. However, for high reduction factors beyond a critical value, the ultimate performance deteriorates rapidly, corresponding to exponential growth of the geometry factor. The transition from optimal to deteriorating performance depends on the electrodynamic characteristics of the detected RF fields. In the near-field regime, i.e., for low B0 and small object size, the critical reduction factor is constant and approximately equal to four for 1D acceleration in the sphere. In the far-field wave regime the critical reduction factor is larger and increases both with B0 and object size. Therefore, it is concluded that parallel techniques hold particular promise for human MR imaging at very high field. 相似文献
2.
3.
Martin Buehrer Klaas P Pruessmann Peter Boesiger Sebastian Kozerke 《Magnetic resonance in medicine》2007,57(6):1131-1139
Arrays with large numbers of independent coil elements are becoming increasingly available as they provide increased signal-to-noise ratios (SNRs) and improved parallel imaging performance. Processing of data from a large set of independent receive channels is, however, associated with an increased memory and computational load in reconstruction. This work addresses this problem by introducing coil array compression. The method allows one to reduce the number of datasets from independent channels by combining all or partial sets in the time domain prior to image reconstruction. It is demonstrated that array compression can be very effective depending on the size of the region of interest (ROI). Based on 2D in vivo data obtained with a 32-element phased-array coil in the heart, it is shown that the number of channels can be compressed to as few as four with only 0.3% SNR loss in an ROI encompassing the heart. With twofold parallel imaging, only a 2% loss in SNR occurred using the same compression factor. 相似文献
4.
Werner I Hagens Peter Olinga Dirk K F Meijer Geny M M Groothuis Leonie Beljaars Klaas Poelstra 《Liver international》2006,26(2):232-239
BACKGROUND: Liver fibrosis is the common response to chronic liver injury, ultimately leading to cirrhosis. Several lines of evidence indicate that inducing apoptosis of hepatic stellate cells (HSC) may lead to regression of liver fibrosis. Recently, it was shown that gliotoxin (GTX) induces apoptosis of HSC. However, the clinical use of GTX may be limited because of the lack of cell and tissue specificity, causing a high risk of potentially severe adverse effects. The aim of this study, therefore, was to study the effect of GTX on different cells of the liver. METHODS: We used normal and fibrotic precision-cut rat liver slices to study the effect of GTX on the various resident liver cell types. In these slices, the complex cell-cell interactions are preserved, which closely mimics the in vivo situation. RESULTS: GTX exhibited a potent apoptosis-inducing activity in these slices. Both immunohistochemical stainings and real-time mRNA techniques showed that this apoptosis-inducing effect was seen in HSC. However, Kupffer cells and liver endothelial cells were also affected by GTX, whereas hepatocytes were only mildly affected. CONCLUSIONS: This study indicates that the apoptosis-inducing strategy to treat liver fibrosis has high potential, but it will be necessary to develop an HSC-specific therapy to prevent adverse effects. 相似文献
5.
Molecular mechanisms that set the stage for DC-T cell engagement 总被引:2,自引:0,他引:2
The unsurpassed capacity of dendritic cells (DC) to prime naive T cells is thought to depend on the formation of an immunological synapse. DC-SIGN, a C-type lectin exclusively expressed at the cell surface of DC, functions as an adhesion receptor facilitating T cell binding and priming through recognition of glycosylated ICAM-3 on naive T cells. Yet, DC-SIGN also mediates binding to pathogens such as HIV by recognizing glycosylated gp120. The scope of the present study was to investigate whether DC-SIGN upon recognition of its cellular ligand and pathogenic ligand affects DC synapse formation and activation/mobilization of other adhesion receptors such as LFA-1 to the cell contact site. Using a DC-SIGN deletion mutant, we show that DC-SIGN is a constitutively active receptor that mediates ligand binding independent of signaling through the cytoplasmic domain. Surprisingly, initial binding of gp120 to DC-SIGN did not result in increased adhesion levels of LFA-1 to its ligand ICAM-1 in both immature DC and Raji-DC-SIGN cells. However, ligand binding to DC-SIGN induced recruitment of LFA-1 to the adhesion site. Moreover, we could demonstrate that activation of LFA-1 results in DC-SIGN-LFA-1 co-clustering in the cell membrane. This triggers binding of ligands to LFA-1 that are shared with DC-SIGN, such as ICAM-3, but not of ligands that are not shared with DC-SIGN, such as ICAM-1. Thus, we propose that upon ligand binding DC-SIGN recruits LFA-1 to the contact site, resulting in the formation of DC-SIGN-LFA-1 co-clusters, in which the initial DC-SIGN-mediated interactions with ligand are transient and eventually shift to more stable LFA-1-dependent interactions. 相似文献
6.
Parmentier HK Lammers A Hoekman JJ De Vries Reilingh G Zaanen IT Savelkoul HF 《Developmental and comparative immunology》2004,28(1):39-49
We studied the presence of Natural antibodies in plasma samples from individual birds from selected chicken lines at young and old age. Binding, specificity, and relative affinity to various antigens were determined in plasma from non-immunized female chickens at 5 weeks of age, and in plasma obtained from the same chickens one year later using indirect two-step ELISA. Birds were from three different lines. The lines were divergently selected for either high (H line) or low (L line) antibody titers to Sheep Red Blood Cells at 5 weeks of age, next to a random bred control (C line). Binding of plasma immunoglobulins (Ig) from all three lines was found with chicken-egg-white protein (CEP), ovalbumin (OVA), myoglobin (MYO), thyroglobulin (THYRO), keyhole limpet hemocyanin (KLH), bovine serum albumin (BSA), and transferrin (TRANS). Significantly higher binding to most antigens was found with plasma Ig from adult birds from the H line as compared to plasma Ig from the L line, whereas binding of plasma Ig from C-line birds was in between or similar to the H or L line, respectively. Binding of Ig to all antigens in all three lines was significantly higher in plasma obtained at one year of age as compared to plasma obtained at 5 weeks of age. A competitive ELISA with homologous and heterologous antigens was used for determining specificity of the antigen-binding antibodies. Nai;ve plasma samples were characterized by a broad binding to all antigens tested. Inhibition of binding to specific antigens was possible with a broad range of heterologous antigens, but highest competition of binding was obtained with homologous antigen. Both linear regression analysis of serial dilutions of the plasma Ig binding the antigens, as well as competitive ELISA with homologous antigen indicated that plasma Ig from the H line and plasma Ig from the L line had similar affinity characteristics to the antigens tested with the exception of OVA and KLH. Pooled non-immune plasma from H line birds bound to CEP, OVA, THYRO, TRANS, MYO, KLH, and salt-precipitated extracts and supernatants of extracts from chicken heart, spleen, liver, brain, bursa, thymus, and kidney, respectively, as determined by Western blotting. The increasing presence of antibodies in nai;ve chicken plasma binding heterologous and homologous (tissue) antigens indicates the presence of Natural antibodies in poultry. Apart from age, increasing levels of Natural antibodies may be related with the genetically based magnitude of specific antibody levels in the chicken lines studied. 相似文献
7.
Kok K Dijkhuizen T Swart YE Zorgdrager H van der Vlies P Fehrmann R te Meerman GJ Gerssen-Schoorl KB van Essen T Sikkema-Raddatz B Buys CH 《European journal of medical genetics》2005,48(3):250-CGH
In 2-8% of patients with mental retardation, small copy number changes in the subtelomeric region are thought to be the underlying cause. As detection of these genomic rearrangements is labour intensive using FISH, we constructed and validated a high-density BAC/PAC array covering the first 5 Mb of all subtelomeric regions and applied it in our routine screening of patients with idiopathic mental retardation for submicroscopic telomeric rearrangements. The present study shows the efficiency of this comprehensive subtelomere array in detecting terminal deletions and duplications but also small interstitial subtelomeric rearrangements, starting from small amounts of DNA. With our array, the size of the affected segments, at least those smaller than 5 Mb, can be determined simultaneously in the same experiment. In the first 100 patient samples analysed in our diagnostic practice by the use of this comprehensive telomere array, we found three patients with deletions in 3p, 10q and 15q, respectively, four patients with duplications in 9p, 12p, 21q and Xp, respectively, and one patient with a del 6q/dup 16q. The patients with del 3p and 10q and dup 12p had interstitial rearrangements that would have been missed with techniques using one probe per subtelomeric region chosen close to the telomere. 相似文献
8.
The Effect of Parenterally Administered Cyclodextrins on Cholesterol Levels in the Rat 总被引:1,自引:0,他引:1
Frijlink Henderik W. Eissens Anko C. Hefting Nanco R. Poelstra Klaas Lerk Coenraad F. Meijer Dirk K. F. 《Pharmaceutical research》1991,8(1):9-16
The inclusion complex formation of intravenously administered hydroxypropyl--cyclodextrin and -cyclodextrin with endogenous lipids was studied. We tested the hypothesis that complex formation of endogenous cholesterol with cyclodextrins in the bloodstream leads to extraction of cholesterol from the large lipoprotein particles. The relatively small cholesterol–cyclodextrin complexes then leave the bloodstream via capillary pores, and dissociation of the complex in the extravascular compartment finally causes redistribution of cholesterol from blood to tissue. This hypothesis is supported by the following experimental findings. Intravenous administration of cyclodextrins led to a transient decrease in plasma cholesterol levels in a dose-dependent manner, and in vitro cholesterol-cyclodextrin complexes passed dialysis membranes with a molecular weight cutoff of 6000–8000. Further, cyclodextrins increased protein binding of the steroidal drug spironolactone, probably through removal of cholesterol from plasma protein binding sites. Finally, extravascular redistribution was directly demonstrated in histological studies of the kidneys. Glomerular filtration of the cholesterol–cyclodextrin complex is followed by dissociation of the complex in the ultrafiltrate, resulting in cholesterol accumulation in the proximal tubule cells. The cholesterol--cyclodextrin complex has a limited aqueous solubility. Crystallization of this complex in renal tissue might explain the nephrotoxicity of parenterally administered -cyclodextrin. The absence of such crystallization might explain the lower nephrotoxicity of hydroxypropyl--cyclodextrin after intravenous administration. 相似文献
9.
Joachim Marr Klaas Heinemann Andrea Rapkin 《International journal of gynaecology and obstetrics》2011,113(2):103-107
Objective
To determine the effects of ethinyl estradiol (EE)/drospirenone in a 24/4 regimen (24 days of active and 4 days of inactive pills) on functional impairment (affecting work, partnership, and social activities) in women with premenstrual dysphoric disorder (PMDD).Methods
The present study was a secondary analysis of a double-blind, randomized, parallel-design multicenter trial. Women received EE 20 μg/drospirenone 3 mg (n = 232) or placebo (n = 218) and completed the Daily Record of Severity of Problems (DRSP) scale daily.Results
The decrease in mean scores for all 3 DRSP functional impairment items (work, partnership, and social activities) from baseline to cycle 3 mirrored changes in the total DRSP symptom score; the greatest decreases were observed in cycle 1 with further small reductions through to cycle 3. The proportional mean decreases from baseline to cycle 1 for the 3 functional items ranged from 47% to 48%. For all 3 functional items, the mean reductions from baseline to cycle 1 (but not from cycle 1 to cycles 2 and 3) were significantly greater with EE/drospirenone than with placebo (P < 0.05).Conclusion
Ethinyl estradiol 20 μg/drospirenone 3 mg in a 24/4 regimen significantly improved functional impairment in women with PMDD. Symptoms improved in parallel. 相似文献10.