首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2649752篇
  免费   192237篇
  国内免费   4983篇
耳鼻咽喉   34926篇
儿科学   87214篇
妇产科学   72848篇
基础医学   382125篇
口腔科学   76543篇
临床医学   242247篇
内科学   514161篇
皮肤病学   59323篇
神经病学   208309篇
特种医学   98733篇
外国民族医学   496篇
外科学   389909篇
综合类   59834篇
现状与发展   6篇
一般理论   1041篇
预防医学   208028篇
眼科学   62285篇
药学   194296篇
  13篇
中国医学   5871篇
肿瘤学   148764篇
  2019年   20886篇
  2018年   29539篇
  2017年   22403篇
  2016年   24640篇
  2015年   28227篇
  2014年   39214篇
  2013年   58440篇
  2012年   80525篇
  2011年   85211篇
  2010年   50075篇
  2009年   47278篇
  2008年   79750篇
  2007年   84879篇
  2006年   85163篇
  2005年   81988篇
  2004年   79089篇
  2003年   75465篇
  2002年   73016篇
  2001年   128280篇
  2000年   131213篇
  1999年   109488篇
  1998年   29800篇
  1997年   26408篇
  1996年   26700篇
  1995年   25430篇
  1994年   23498篇
  1993年   21838篇
  1992年   84304篇
  1991年   82066篇
  1990年   79157篇
  1989年   76532篇
  1988年   70158篇
  1987年   68703篇
  1986年   65048篇
  1985年   61508篇
  1984年   45966篇
  1983年   39596篇
  1982年   22830篇
  1979年   42062篇
  1978年   29829篇
  1977年   25239篇
  1976年   23512篇
  1975年   25005篇
  1974年   30406篇
  1973年   29655篇
  1972年   27681篇
  1971年   25758篇
  1970年   24181篇
  1969年   22721篇
  1968年   21167篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. Shielded and unshielded longitudinal gradient coils are used to exemplify this novel approach. To assist in the evaluation of transient eddy currents induced within a realistic cryostat vessel, a low-frequency finite-difference time-domain (FDTD) method using the total-field scattered-field (TFSF) scheme was performed. The simulations demonstrate the effectiveness of the proposed method for optimizing longitudinal gradient fields while taking into account the spatial and temporal behavior of the eddy currents.  相似文献   
113.
114.
115.
Continual loading and articulation cycles undergone by metallic (e.g., titanium) alloy arthroplasty prostheses lead to liberation of a large number of metallic debris particulates, which have long been implicated as a primary cause of periprosthetic osteolysis and postarthroplasty aseptic implant loosening. Long-term stability of total joint replacement prostheses relies on proper integration between implant biomaterial and osseous tissue, and factors that interfere with this integration are likely to cause osteolysis. Because multipotent mesenchymal stem cells (MSCs) located adjacent to the implant have an osteoprogenitor function and are critical contributors to osseous tissue integrity, when their functions or activities are compromised, osteolysis will most likely occur. To date, it is not certain or sufficiently confirmed whether MSCs endocytose titanium particles, and if so, whether particulate endocytosis has any effect on cellular responses to wear debris. This study seeks to clarify the phenomenon of titanium endocytosis by human MSCs (hMSCs), and investigates the influence of endocytosis on their activities. hMSCs incubated with commercially pure titanium particles exhibited internalized particles, as observed by scanning electron microscopy and confocal laser scanning microscopy, with time-dependent reduction in the number of extracellular particles. Particulate endocytosis was associated with reduced rates of cellular proliferation and cell-substrate adhesion, suppressed osteogenic differentiation, and increased rate of apoptosis. These cellular effects of exposure to titanium particles were reduced when endocytosis was inhibited by treatment with cytochalasin D, and no significant effect was seen when hMSCs were treated only with conditioned medium obtained from particulate-treated cells. These findings strongly suggest that the biological responses of hMSCs to wear debris are triggered primarily by the direct endocytosis of titanium particulates, and not mediated by secreted soluble factors. In this manner, therapeutical approaches that suppress particle endocytosis could reduce the bioreactivity of hMSCs to particulates, and enhance long-term orthopedic implant prognosis by minimizing wear-debris periprosthethic osteolysis.  相似文献   
116.
117.
OBJECTIVE: To evaluate the effect of statins on aneurysm growth in a group of consecutive patients under surveillance for infrarenal aortic aneurysms (AAA). MATERIALS AND METHODS: All patients (59 statin users, 91 non-users) under surveillance between January 2002 and August 2005 with a follow-up for aneurysm growth of at least 12 months and a minimum of three diameter evaluations were retrospectively included in the analysis. Multiple regression analysis, weighted with the number of observations, was performed to test the influence of statins on AAA growth rate. RESULTS: During a median period of 3.1 (1.1-13.1) years the overall mean aneurysm growth rate was 2.95+/-2.8 mm/year. Statin users had a 1.16 mm/year lower AAA growth rate compared to non-users (95% CI 0.33-1.99 mm/year). Increased age was associated with a slower growth (-0.09 mm/year per year, p = 0.003). Female gender (+1.82 mm/year, p = 0.008) and aneurysm diameter (+0.06 mm/year per mm, p = 0.049) were associated with increased AAA growth. The use of non-steroidal anti-inflammatory drugs, chronic lung disease, or other cardiovascular risk factors were not independently associated with AAA growth. CONCLUSIONS: Statins appear to be associated with attenuation of AAA growth, irrespective of other known factors influencing aneurysm growth.  相似文献   
118.
119.
120.
目的:通过16例经内窥镜及病理证实为Barrett食管的钡餐检查表现,初步探讨Barrett食管及其并发症放射学检查的意义。方法:回顾性分析16例Barrett食管的内窥镜和气钡检查的记录及表现,着重探讨气钡检查的X线表现。结果:食管内窥镜和组织学检查全部病例均有炎症改变,其他可见胃食管反流、食管溃疡和食管腺癌。放射学钡餐检查结果:Barrett食管炎4例,Barrett食管溃疡7例,Barrett食管癌5例。结论:Barrett食管及其并发症至今无明确放射学征象,改进对“Z”线的检查技术和方法,有可能提高其发现率。钡餐发现胃食管反流、食管裂孔疝同时并发食管炎、食管溃疡及食管癌者均应考虑到本症的可能。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号