收费全文 | 18119篇 |
免费 | 704篇 |
国内免费 | 22篇 |
电工技术 | 215篇 |
综合类 | 18篇 |
化学工业 | 3772篇 |
金属工艺 | 322篇 |
机械仪表 | 404篇 |
建筑科学 | 1005篇 |
矿业工程 | 49篇 |
能源动力 | 501篇 |
轻工业 | 1676篇 |
水利工程 | 120篇 |
石油天然气 | 56篇 |
武器工业 | 1篇 |
无线电 | 1584篇 |
一般工业技术 | 3058篇 |
冶金工业 | 3033篇 |
原子能技术 | 188篇 |
自动化技术 | 2843篇 |
2023年 | 157篇 |
2022年 | 296篇 |
2021年 | 461篇 |
2020年 | 342篇 |
2019年 | 365篇 |
2018年 | 425篇 |
2017年 | 389篇 |
2016年 | 491篇 |
2015年 | 423篇 |
2014年 | 521篇 |
2013年 | 1073篇 |
2012年 | 857篇 |
2011年 | 1115篇 |
2010年 | 765篇 |
2009年 | 821篇 |
2008年 | 861篇 |
2007年 | 757篇 |
2006年 | 701篇 |
2005年 | 590篇 |
2004年 | 484篇 |
2003年 | 436篇 |
2002年 | 420篇 |
2001年 | 295篇 |
2000年 | 317篇 |
1999年 | 348篇 |
1998年 | 735篇 |
1997年 | 478篇 |
1996年 | 392篇 |
1995年 | 286篇 |
1994年 | 269篇 |
1993年 | 266篇 |
1992年 | 182篇 |
1991年 | 154篇 |
1990年 | 135篇 |
1989年 | 135篇 |
1988年 | 124篇 |
1987年 | 116篇 |
1986年 | 113篇 |
1985年 | 138篇 |
1984年 | 133篇 |
1983年 | 115篇 |
1982年 | 89篇 |
1981年 | 103篇 |
1980年 | 85篇 |
1979年 | 96篇 |
1978年 | 87篇 |
1977年 | 116篇 |
1976年 | 138篇 |
1975年 | 80篇 |
1974年 | 73篇 |
Object
Delta relaxation enhanced magnetic resonance (dreMR) is a new imaging technique based on the idea of cycling the magnetic field B 0 during an imaging sequence. The method determines the field dependency of the relaxation rate (relaxation dispersion dR 1/dB). This quantity is of particular interest in contrast agent imaging because the parameter can be used to determine contrast agent concentrations and increases the ability to localize the contrast agent.Materials and methods
In this paper dreMR imaging was implemented on a clinical 1.5?T MR scanner combining conventional MR imaging with fast field-cycling. Two improvements to dreMR theory are presented describing the quantification of contrast agent concentrations from dreMR data and a correction for field-cycling with finite ramp times.Results
Experiments demonstrate the use of the extended theory and show the measurement of contrast agent concentrations with the dreMR method. A second experiment performs localization of a contrast agent with a significant improvement in comparison to conventional imaging.Conclusion
dreMR imaging has been extended by a method to quantify contrast agent concentrations and improved for field-cycling with finite ramp times. Robust localization of contrast agents using dreMR imaging has been performed in a sample where conventional imaging delivers inconclusive results. 相似文献Object
Recent advances have allowed oscillating gradient (OG) diffusion MRI to infer the sizes of micron-scale axon diameters. Here the effects on the precision of the inferred diameters are studied when reducing the number of images collected to reduce imaging time for clinical feasibility.Materials and methods
Monte Carlo simulations of cosine OG sequences (50–1000 Hz) using a two-compartment model on a parallel cylinder (diameters 1–5 μm) geometry were conducted. Temporal diffusion spectroscopy was used to infer axon diameters. Three different gradient sets were simulated with different combinations of gradient strengths.Results
Five frequencies were adequate for d = 3–5 μm with single-sized cylinders and for effective mean axon diameters greater than 2 μm for cylinders with a distributions of diameters. There was some improvement in precision for d = 1–2 μm with 10 frequencies. It is better to repeat measurements at higher gradient strengths than to use a range of gradient strengths. The improvement tended to be greatest when using fewer frequencies and was especially noticeable at very high gradient strengths.Conclusion
Images can be collected with fewer gradient strengths and frequencies without sacrificing the precision of the measurements. This could be useful in reducing imaging time so that OG techniques can be used in clinical settings.Image post-processing corrects for cardiac and respiratory motion (MoCo) during cardiovascular magnetic resonance (CMR) stress perfusion. The study analyzed its influence on visual image evaluation.
Materials and methodsSixty-two patients with (suspected) coronary artery disease underwent a standard CMR stress perfusion exam during free-breathing. Image post-processing was performed without (non-MoCo) and with MoCo (image intensity normalization; motion extraction with iterative non-rigid registration; motion warping with the combined displacement field). Images were evaluated regarding the perfusion pattern (perfusion deficit, dark rim artifact, uncertain signal loss, and normal perfusion), the general image quality (non-diagnostic, imperfect, good, and excellent), and the reader’s subjective confidence to assess the images (not confident, confident, very confident).
ResultsFifty-three (non-MoCo) and 52 (MoCo) myocardial segments were rated as ‘perfusion deficit’, 113 vs. 109 as ‘dark rim artifacts’, 9 vs. 7 as ‘uncertain signal loss’, and 817 vs. 824 as ‘normal’. Agreement between non-MoCo and MoCo was high with no diagnostic difference per-patient. The image quality of MoCo was rated more often as ‘good’ or ‘excellent’ (92 vs. 63%), and the diagnostic confidence more often as “very confident” (71 vs. 45%) compared to non-MoCo.
ConclusionsThe comparison of perfusion images acquired during free-breathing and post-processed with and without motion correction demonstrated that both methods led to a consistent evaluation of the perfusion pattern, while the image quality and the reader’s subjective confidence to assess the images were rated more favorably for MoCo.
相似文献