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   21970篇
  免费   1040篇
  国内免费   168篇
耳鼻咽喉   253篇
儿科学   379篇
妇产科学   434篇
基础医学   3275篇
口腔科学   354篇
临床医学   1978篇
内科学   4141篇
皮肤病学   715篇
神经病学   2711篇
特种医学   1196篇
外科学   2728篇
综合类   152篇
一般理论   6篇
预防医学   930篇
眼科学   504篇
药学   1739篇
中国医学   28篇
肿瘤学   1655篇
  2023年   80篇
  2022年   133篇
  2021年   274篇
  2020年   233篇
  2019年   278篇
  2018年   375篇
  2017年   296篇
  2016年   419篇
  2015年   465篇
  2014年   653篇
  2013年   815篇
  2012年   1285篇
  2011年   1364篇
  2010年   777篇
  2009年   787篇
  2008年   1432篇
  2007年   1480篇
  2006年   1555篇
  2005年   1577篇
  2004年   1442篇
  2003年   1480篇
  2002年   1397篇
  2001年   286篇
  2000年   232篇
  1999年   301篇
  1998年   328篇
  1997年   269篇
  1996年   211篇
  1995年   176篇
  1994年   196篇
  1993年   155篇
  1992年   119篇
  1991年   122篇
  1990年   119篇
  1989年   103篇
  1988年   107篇
  1987年   72篇
  1986年   74篇
  1985年   86篇
  1984年   93篇
  1983年   80篇
  1982年   83篇
  1981年   94篇
  1980年   73篇
  1979年   64篇
  1977年   60篇
  1976年   65篇
  1974年   58篇
  1955年   48篇
  1954年   47篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
52.
Cardiac resynchronization therapy (CRT) is a treatment option in patients with severe heart failure and left bundle-branch block (LBBB). This study evaluated the effects of 4 and 13 mo of CRT on myocardial oxygen consumption (MVO2) and cardiac efficiency as compared with mild heart failure patients without LBBB. METHODS: Sixteen patients with severe heart failure and LBBB due to idiopathic cardiomyopathy were studied at baseline and after 4 and after 13 mo of therapy. Thirteen patients with mild heart failure without LBBB served as a comparison group. The clearance rate (k2) of 11C-acetate was measured with PET to assess MVO2. Stroke volume was derived from the dynamic PET data according to the Stewart-Hamilton principle and, furthermore, cardiac efficiency using the work metabolic index. RESULTS: After 4 mo of CRT, stroke volume index (SVI) increased by 50% (P = 0.012) and cardiac efficiency increased by 41% (P < 0.001). Global k2 remained unchanged but regional k2 demonstrated a more homogeneous distribution pattern. The parameters showed no significant changes during therapy. Under CRT, cardiac efficiency, SVI, and the distribution pattern of regional k2 did not differ from mild heart failure patients without LBBB. CONCLUSION: CRT improves cardiac efficiency for at least 13 mo, as demonstrated by a higher SVI, whereas MVO2 remains unchanged. Cardiac efficiency, SVI, and the MVO2 distribution pattern reach the level of patients with mild heart failure without LBBB. The unfavorable hemodynamic performance in heart failure with LBBB is effectively restored by long-term CRT to the level of an earlier disease state.  相似文献   
53.
PET with (R)-[O-methyl-11C] metomidate ([11C] MTO) is an attractive method for the characterisation of adrenal masses discriminating lesions of adrenal cortical origin from noncortical lesions. [11C] MTO was prepared by the reaction of [11C] methyliodide with the corresponding free acid. Three purification methods have been compared. The method of choice uses preparative HPLC with a ready-to-use weak acidic solvent.  相似文献   
54.
To extend the signal longevity of magnetically excited spins in flowing fluids while in a state of global coherent free precession (GCFP), a refocusing radiofrequency (RF) pulse and bipolar gradient waveforms were combined with the GCFP sequence. The data demonstrate that RF refocusing in the presence of flowing blood is possible, but the improvement in signal amplitude depends on the static magnetic field homogeneity along the direction of motion and the displacement of the spins between the excitation and the RF refocusing pulse, as well as displacement during subsequent RF refocusing pulses. The least amount of phase dispersion and thus the longest lasting signal is obtained with the shortest echo spacing where only one line of data is recorded between two RF refocusing pulses. This approach was successfully used in a phantom and in vivo to image fast and slow blood flow. Depending on the experimental conditions, signal persistence is improved significantly compared to playing the same sequence without RF refocusing, but the improvement is limited by the product of blood flow velocity and the time between RF refocusing pulses.  相似文献   
55.
We report a case of a prenatally diagnosed infratentorial subdural hemorrhage. The hematoma located in the posterior cerebral fossa was detected by conventional ultrasound at 24 weeks of gestation. Intrauterine magnetic resonance imaging confirmed the diagnosis. Autopsy of the fetus revealed a well-circumscribed subdural hematoma. The prenatal findings of intracranial bleeding located in the posterior fossa and the prognosis of such cases are discussed.  相似文献   
56.
57.
58.
59.
Decrease of glucose in the human visual cortex during photic stimulation.   总被引:4,自引:0,他引:4  
Localized proton NMR spectroscopy was used to study cerebral metabolism in the visual cortex of healthy adults during rest and photic stimulation. Basal lactate levels showed considerable interindividual differences ranging from below detectability (less than 0.3 mM) to about 1 mM without consistent alteration during photic stimulation. Local brain glucose levels were significantly reduced (approximately 50%) during the entire period of photic stimulation and recovered to resting levels (approximately 0.8 mM) within 10 min after the end of stimulation. This decrease reflects the establishment of a new equilibrium due to enhanced delivery (blood flow) and enhanced consumption. The absence of lactate accumulation supports the hypothesis of a rapid efflux of lactate from brain tissue under activated conditions.  相似文献   
60.
The patterns of cell death and of macrophages were investigated in the forebrain and eyes of the tree shrew Tupaia belangeri during five phases of optic cup formation. Seventeen embryos were studied. Three- dimensional reconstructions were made from one embryo of each phase. In phase 1 (V-shaped optic evagination) a midline band of cell death passes through the closing anterior neuroporus. From phases 2 (optic vesicle) to 5 (far-advanced invagination) the midline band of cell death extends in the dorsal wall of the forebrain to its rostral pole and, further, into its ventral wall. At the approximate future position of the optic chiasm this ventral pycnotic area, predicted but so far unidentified by others, is connected to a previously described second band of cell death passing through the optic anlagen. Recently, evidence has been presented that chicken embryos develop holoprosencephaly and cyclopia when ventral forebrain structures are lost secondary to experimentally induced apoptosis. Our findings in Tupaia suggest that, in cases of spontaneous malformations of this kind, such an atypical pycnotic area in the ventral telencephalon might result from the defective regulation of cell death processes during optic cup formation. In the forebrain and eyes of Tupaia, the occurrence of bands of cell death precedes the appearance of the earliest intraepithelial macrophages. From phase 3 (onset of invagination) onwards almost all of them are concentrated along the band of cell death.  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