全文获取类型
收费全文 | 9904篇 |
免费 | 820篇 |
国内免费 | 381篇 |
专业分类
耳鼻咽喉 | 215篇 |
儿科学 | 138篇 |
妇产科学 | 25篇 |
基础医学 | 2234篇 |
口腔科学 | 142篇 |
临床医学 | 746篇 |
内科学 | 562篇 |
皮肤病学 | 84篇 |
神经病学 | 2001篇 |
特种医学 | 985篇 |
外科学 | 292篇 |
综合类 | 834篇 |
预防医学 | 608篇 |
眼科学 | 222篇 |
药学 | 547篇 |
2篇 | |
中国医学 | 1313篇 |
肿瘤学 | 155篇 |
出版年
2024年 | 18篇 |
2023年 | 164篇 |
2022年 | 196篇 |
2021年 | 311篇 |
2020年 | 322篇 |
2019年 | 286篇 |
2018年 | 282篇 |
2017年 | 331篇 |
2016年 | 338篇 |
2015年 | 359篇 |
2014年 | 546篇 |
2013年 | 859篇 |
2012年 | 533篇 |
2011年 | 671篇 |
2010年 | 492篇 |
2009年 | 495篇 |
2008年 | 538篇 |
2007年 | 468篇 |
2006年 | 447篇 |
2005年 | 402篇 |
2004年 | 352篇 |
2003年 | 300篇 |
2002年 | 278篇 |
2001年 | 225篇 |
2000年 | 194篇 |
1999年 | 163篇 |
1998年 | 139篇 |
1997年 | 139篇 |
1996年 | 119篇 |
1995年 | 156篇 |
1994年 | 140篇 |
1993年 | 137篇 |
1992年 | 93篇 |
1991年 | 106篇 |
1990年 | 61篇 |
1989年 | 75篇 |
1988年 | 56篇 |
1987年 | 46篇 |
1986年 | 33篇 |
1985年 | 46篇 |
1984年 | 34篇 |
1983年 | 29篇 |
1982年 | 23篇 |
1981年 | 17篇 |
1980年 | 20篇 |
1979年 | 13篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1974年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Arterial spin labeling (ASL) imaging is a powerful magnetic resonance imaging technique that allows to quantitatively measure blood perfusion non-invasively, which has great potential for assessing tissue viability in various clinical settings. However, the clinical applications of ASL are currently limited by its low signal-to-noise ratio (SNR), limited spatial resolution, and long imaging time. In this work, we propose an unsupervised deep learning-based image denoising and reconstruction framework to improve the SNR and accelerate the imaging speed of high resolution ASL imaging. The unique feature of the proposed framework is that it does not require any prior training pairs but only the subject's own anatomical prior, such as T1-weighted images, as network input. The neural network was trained from scratch in the denoising or reconstruction process, with noisy images or sparely sampled k-space data as training labels. Performance of the proposed method was evaluated using in vivo experiment data obtained from 3 healthy subjects on a 3T MR scanner, using ASL images acquired with 44-min acquisition time as the ground truth. Both qualitative and quantitative analyses demonstrate the superior performance of the proposed txtc framework over the reference methods. In summary, our proposed unsupervised deep learning-based denoising and reconstruction framework can improve the image quality and accelerate the imaging speed of ASL imaging. 相似文献
3.
《Mayo Clinic proceedings. Mayo Clinic》2022,97(6):1114-1122
ObjectiveTo assess the frequency of silent brain infarcts (SBIs) and white matter disease (WMD) and associations with stroke risk factors (RFs) in a real-world population.Patients and MethodsThis was an observational study of patients 50 years or older in the Kaiser Permanente Southern California health system from January 1, 2009, through June 30, 2019, with head computed tomography or magnetic resonance imaging for nonstroke indications and no history of stroke, transient ischemic attack, or dementia. A natural language processing (NLP) algorithm was applied to the electronic health record to identify individuals with reported SBIs or WMD. Multivariable Poisson regression estimated risk ratios of demographic characteristics, RFs, and scan modality on the presence of SBIs or WMD.ResultsAmong 262,875 individuals, the NLP identified 13,154 (5.0%) with SBIs and 78,330 (29.8%) with WMD. Stroke RFs were highly prevalent. Advanced age was strongly associated with increased risk of SBIs (adjusted relative risks [aRRs], 1.90, 3.23, and 4.72 for those aged in their 60s, 70s, and ≥80s compared with those in their 50s) and increased risk of WMD (aRRs, 1.79, 3.02, and 4.53 for those aged in their 60s, 70s, and ≥80s compared with those in their 50s). Magnetic resonance imaging was associated with a reduced risk of SBIs (aRR, 0.87; 95% CI, 0.83 to 0.91) and an increased risk of WMD (aRR, 2.86; 95% CI, 2.83 to 2.90). Stroke RFs had modest associations with increased risk of SBIs or WMD.ConclusionAn NLP algorithm can identify a large cohort of patients with incidentally discovered SBIs and WMD. Advanced age is strongly associated with incidentally discovered SBIs and WMD. 相似文献
4.
《Gait & posture》2022
BackgroundOlder adults benefit most from engaging in higher-intensity physical activity, which is often determined using step rate thresholds. Fixed step rate thresholds that correspond to moderate (MPA) and vigorous-intensity physical activity (VPA) have been developed for heuristic activity promotion. The activPAL monitor uses step rate thresholds to determine activity intensity. Stepping thresholds may also vary based on body mass index (BMI) or aerobic fitness level in older adults. Despite the various thresholds used in the literature, it is unclear whether they produce similar outcomes.Research QuestionHow does time spent in physical activity intensities compare between different step rate thresholds in older adults?MethodsThirty-eight participants (24♀; 67 ± 4 years; BMI: 26.6 ± 4.4 kg/m2) wore an activPAL monitor 24-hr/day for up to 7-d (total: 205-d). Aerobic fitness (V̇O2max: 23 ± 8 ml/kg/min) was determined via indirect calorimetry during a maximal, graded cycling test. Time spent in each intensity category (light-physical-activity [LPA], MPA, VPA) was determined using the fixed (MPA/VPA) 100/130, 110/130, and activPAL step rate thresholds (74/212), as well as BMI-adjusted absolute (108.5 ± 2.5/134.0 ± 4.8) and BMI-adjusted relative (40%/60% V̇O2max; 111.4 ± 14.7/132.0 ± 19.0) cut-offs. Times spent in each intensity category were compared between methods.ResultsThe activPAL and 100/130 thresholds yielded less LPA and more MPA than all other methods. The activPAL had no time spent in VPA at all. The BMI-adjusted absolute and relative thresholds produced statistically equivalent time in LPA and MPA (via equivalence testing), but not VPA. No two methods yielded similar time spent in LPA, MPA, or VPA.SignificanceThe choice of step rate threshold has a major impact on physical activity intensity outcomes in older adults. Inherently, strategies that adjust for older adults’ body size and/or aerobic fitness level provide a more individualized data processing strategy than fixed thresholds that assume the same threshold for all older adults 相似文献
5.
6.
目的 分析米泔水制桔梗(MZJ)浸泡液中的微生物群落组成及丰度变化规律,为基于微生物发酵及成分转化角度阐明MZJ炮制原理提供依据。方法 采用米泔水浸泡桔梗,于28 ℃浸泡72 h,借助16S rRNA测序技术和生物信息学分析技术分析不同浸泡时间液体中微生物群落结构及丰度变化。结果 MZJ浸泡液中微生物群落组成具有多样性,随着浸泡时间的延长,溶液中特有操作分类单元(OTU)数量、丰度和群落多样性均呈降低趋势。从门水平分析,MZJ炮制前期变形菌门(Proteobacteria)占优势,浸泡12 h时相对丰度最高(89.75%),随着炮制时间的延长,厚壁菌门(Firmicutes)相对丰度增加,由10.20%增至68.61%,48 h时趋于稳定。从属水平分析,炮制初期以欧文氏菌属(Erwinia)、肠杆菌属(Enterobacter)、泛菌属(Pantoea)为优势菌属,随着炮制时间延长,乳球菌属(Lactococcus)、魏斯氏菌属(Weissella)、明串珠菌属(Leuconostoc)、葡糖杆菌属(Gluconobacter)等为优势菌属,其中乳球菌属丰度由2.24%增至40.15%,魏斯氏菌属丰度由1.89%增至24.71%,明串珠菌属丰度由1.47%增至7.58%,而欧文氏菌属、肠杆菌属、泛菌属丰度均明显下降。结论 米泔水炮制桔梗过程中,浸泡液中的微生物种群结构及丰度发生明显改变,可能使桔梗中的成分发生生物转化从而改变桔梗的药性和生物活性,为阐明MZJ的炮制原理及桔梗新型饮片的制备提供依据。 相似文献
7.
目的:考察黑豆汁炖制时间对制何首乌外观性状及各类有效成分含量变化规律的影响。方法:采用HPLC同时测定不同炮制时间制何首乌样品中二苯乙烯苷、大黄素、大黄素甲醚、大黄素-8-O-β-D-葡萄糖苷、大黄素甲醚-8-O-葡萄糖苷的含量,选择Agilent ZORBAX Extend-C_(18)色谱柱(4. 6 mm×250 mm,5μm),甲醇(A)-水(B)为流动相梯度洗脱(0~30 min,5%~100%A; 30~40 min,100%A),流速1. 0 mL·min~(-1),柱温35℃,检测波长280 nm。结果:随炖制时间的增加,二苯乙烯苷含量逐渐降低,与8 h时相比,至64 h时含量降低76%;大黄素-8-O-β-D-葡萄糖苷和大黄素甲醚-8-O-葡萄糖苷2种蒽醌苷含量先上升后下降,至24 h时达最高值,炖制40 h时降至与8 h近似的水平,之后略有波动;大黄素、大黄素甲醚2种蒽醌苷元含量先上升后下降,炖制32 h时达最大值,之后缓慢降低并趋于稳定。结论:炖制时间对制何首乌中各类成分含量的影响显著,且变化趋势不尽相同,应规范制何首乌饮片的炮制时间;同时,仅以二苯乙烯苷及蒽醌类成分作为制何首乌的指标性成分依据不够充分,应考虑增加多糖类等质量控制指标。 相似文献
8.
基于主客观嗅觉评价结合挥发性成分分析优选美洲大蠊去腥矫臭炮制方法 总被引:4,自引:3,他引:1
目的通过主客观评价结合分析挥发性成分变化,筛选美洲大蠊去腥矫臭的炮制方法,研究去腥矫臭作用机制。方法分别制备美洲大蠊生品、醋炙品、酒炙品与麸炒品,采用志愿者感官评价法结合电子鼻系统评价美洲大蠊生品与炮制品的腥臭气味差异,筛选较优炮制品种类。采用顶空-固相微萃取-气相-质谱联用(HS-SPME-GC-MS)结合相对气味活度值(ROAV)分析关键性腥臭气味成分,并采用面积归一化法结合多元统计分析方法分析生品与炮制品挥发性成分差异,探究炮制去腥矫臭的作用机制。结果志愿者评分结果显示生品、醋炙品、酒炙品与麸炒品的平均评分分别为3.38、1.25、2.88与3.04。电子鼻结果显示,醋炙品、酒炙品、麸炒品与生品间的欧式距离分别为7.34、3.77、1.60,但醋炙品的散点分布方向与酒炙品、麸炒品方向相反,提示可能作用机制有异。整合分析主客观评价数据,确定醋炙为最优去腥矫臭炮制方法。ROAV法确认美洲大蠊关键性腥臭气味成分为3-甲基丁醛、己醛、壬醛、庚醛、癸醛、苯乙醛、(E,E)-2,4-壬二烯醛、2-戊基呋喃、(+)-柠檬烯、肉豆蔻醛。挥发性成分的PCA结果显示,4种炮制品可被明显区分开,挥发性成分及含量存在差异。正交偏最小二乘判别分析(OPLS-DA)结果表明,生品与醋炙品中存在7个差异性成分,分别为己醛、棕榈酸、油酸、油酸乙酯、醋酸、棕榈酸乙酯、亚油酸乙酯,醋炙品含量较生品均有减少,其中关键性腥臭味成分己醛减少最为明显。结论醋炙为改善美洲大蠊腥臭气味的较优方法,其炮制机制是通过降低己醛等挥发性腥臭气味成分实现的。 相似文献
9.
10.
目的:探讨蜜丸酒煎下瘀血汤与饮片酒煎下瘀血汤抗肝纤维化作用是否存在异同。方法:40只Wistar大鼠随机分为正常组、模型组、蜜丸酒煎下瘀血汤组、饮片酒煎下瘀血汤组、扶正化瘀方组。以50%CCl_4皮下注射9周制备肝纤维化模型。第7周首日开始,正常组与模型组给予0.3%羧甲基纤维素钠灌胃,其余3组分别给予蜜丸酒煎下瘀血汤(0.58 g·kg~(-1)),饮片酒煎下瘀血汤(0.43 g·kg~(-1)),扶正化瘀方(2 g·kg~(-1))灌胃,第9周结束后收集肝组织和血清样本。分别采用苏木素-伊红(HE)染色观察各组大鼠肝组织病理学,天狼猩红染色观察纤维化程度,碱水解法测定肝组织羟脯氨酸含量情况,全自动分析法检测大鼠血清肝功能,实时荧光定量聚合酶链式反应(Real-time PCR)检测肝组织α-平滑肌肌动蛋白(α-SMA)和Ⅰ型胶原(COL-I)mRNA的表达情况,蛋白免疫印迹法(Western blot)和免疫组化测定肝组织α-SMA蛋白的表达,酶联免疫吸附测定(ELISA)测定炎症因子与氧化指标。结果:与正常组比较,模型组大鼠肝肝纤维化程度较严重,肝组织天狼猩红阳性染色面积比、羟脯氨酸含量,α-SMA和COL-I蛋白及mRNA表达显著增加(P0.01),大鼠血清丙氨酸氨基转移酶(AST),天门冬氨酸氨基转移酶(ALT),总胆红素(TBil),肿瘤坏死因子-α(TNF-α),白细胞介素(IL)-1β,IL-17A及丙二醛(MDA)的水平显著增加(P0.01),超氧化物歧化酶(SOD)及谷胱甘肽过氧化物酶(GSH-Px)的水平显著降低(P0.01);与模型组比较,蜜丸酒煎下瘀血汤与饮片酒煎下瘀血汤可明显减轻肝纤维化程度,肝组织天狼猩红阳性染色面积比、羟脯氨酸含量,α-SMA和COL-I蛋白及mRNA表达,大鼠血清AST,ALT,TBil,TNF-α,IL-1β,IL-17A,MDA的水平明显降低(P0.05,P0.01),SOD及GSH-Px水平明显增加(P0.05,P0.01)。其中蜜丸酒煎下瘀血汤组肝组织病理变化的改善显著优于饮片酒煎下瘀血汤,且肝组织在天狼猩红阳性染色面积比、羟脯氨酸含量、肝组织COL-I mRNA以及α-SMA mRNA和蛋白的表达降低也较饮片酒煎下瘀血汤明显(P0.05,P0.01)。血清肝功能、炎症因子及氧化相关指标2种制法下瘀血汤组间差异无统计学意义,但蜜丸酒煎下瘀血汤改善趋势更为明显。结论:蜜丸酒煎下瘀血汤与饮片酒煎下瘀血汤均具有较好的抗肝纤维化作用,其中以蜜丸酒煎下瘀血汤作用更为显著,提示制蜜丸一定程度上能够增加下瘀血汤抗肝纤维化作用。 相似文献