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1.
Quo vadis elasticity imaging?   总被引:7,自引:0,他引:7  
Konofagou EE 《Ultrasonics》2004,42(1-9):331-336
In the past decade, an important field that has emerged as complementary to ultrasonic imaging is that of elasticity imaging. The term encompasses a variety of techniques that can depict a mechanical response or property of tissues. In ultrasound, its premise is built on two important facts: (a) that significant differences between mechanical properties of several tissue components exist and (b) that the information contained in the coherent scattering, or speckle, is sufficient to depict these differences following an external or internal mechanical stimulus. Parameters, such as velocity of vibration, displacement, strain, strain rate, velocity of wave propagation and elastic modulus, have all been demonstrated feasible in their estimation and have resulted in the accurate depiction of stiffer tissue masses, such as tumors, high-intensity focused ultrasound (HIFU) lesions and atherosclerotic plaques. More recently, through the development of ultrafast algorithms tailored to suitable hardware as well as the familiarity of the physician with the sensitivity of the methods used, one elasticity imaging technique in particular, elastography, has been shown applicable in a typical clinical ultrasound setting. In other words, elastograms can currently be obtained at quasi real-time (approximately at a frame rate of 8 frames/s) and with the use of a hand-held transducer (as opposed to the previously used frame-suspended setup) during and simultaneously with an ultrasound exam of, e.g., the breast or the prostate. The higher frame rate available with certain clinical ultrasound scanners has also resulted in the successful application of elasticity imaging techniques on the myocardium and monitoring its deformation over several cardiac cycles for the detection of ischemic regions. As a result, elasticity imaging with its ever increasing number of applications and demonstrated applicability in a typical, clinical ultrasound setting promises to make an important contribution to the ultrasound practice as we know it.  相似文献
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Multiple collisions and induced gluon bremsstrahlung in QCD   总被引:2,自引:0,他引:2  
Induced soft gluon bremsstrahlung associated with multiple collisions is calculated via perturbative QCD. We derive the non-abelian analog of the Landau-Pomeranchuk effect that suppresses induced soft radiation with formation times exceeding the mean free path. The dependence of the suppression effect on the SU(N) representation of the jet parton as well as the kinematic variables is expressed through a radiation formation factor. The soft radiation with k < μ, where μ is the infrared screening scale in the medium, is shown to lead to an approximately constant radiative energy loss per unit length.  相似文献
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In NMR spectra of complex proteins, sparse isotope enrichment can be important, in that the removal of many (13)C-(13)C homonuclear J-couplings can narrow the lines and thereby facilitate the process of spectral assignment and structure elucidation. We present a simple scheme for selective yet extensive isotopic enrichment applicable for production of proteins in organisms utilizing the Entner-Doudoroff (ED) metabolic pathway. An enrichment scheme so derived is demonstrated in the context of a magic-angle spinning solid-state NMR (MAS SSNMR) study of Pf1 bacteriophage, the host of which is Pseudomonas aeruginosa, strain K (PAK), an organism that uses the ED pathway for glucose catabolism. The intact and infectious Pf1 phage in this study was produced by infected PAK cells grown on a minimal medium containing 1-(13)C d-glucose ((13)C in position 1) as the sole carbon source, as well as (15)NH(4)Cl as the only nitrogen source. The 37MDa Pf1 phage consists of about 93% major coat protein, 1% minor coat proteins, and 6% single-stranded, circular DNA. As a consequence of this composition and the enrichment scheme, the resonances in the MAS SSNMR spectra of the Pf1 sample were almost exclusively due to carbonyl carbons in the major coat protein. Moreover, 3D heteronuclear NCOCX correlation experiments also show that the amino acids leucine, serine, glycine, and tyrosine were not isotopically enriched in their carbonyl positions (although most other amino acids were), which is as expected based upon considerations of the ED metabolic pathway. 3D NCOCX NMR data and 2D (15)N-(15)N data provided strong verification of many previous assignments of (15)N amide and (13)C carbonyl shifts in this highly congested spectrum; both the semi-selective enrichment patterns and the narrowed linewidths allowed for greater certainty in the assignments as compared with use of uniformly enriched samples alone.  相似文献
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The Planck theory of blackbody radiation imposes a limit on the maximum radiative transfer between two objects at given temperatures. When the two objects are close enough, near-field effects due to tunneling of evanescent waves lead to enhancement of radiative transfer above the Planck limit. When the objects can support electromagnetic surface polaritons, the enhancement can be a few orders-of-magnitude larger than the blackbody limit. In this paper, we summarize our recent measurements of radiative transfer between two parallel silica surfaces and between a silica microsphere and a flat silica surface that show unambiguous evidence of enhancement of radiative transfer due to near-field effects above the Planck limit.  相似文献
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Reactions produced by stopped negative pions on targets of 165Ho, 175Lu, 176Lu, 179Hf, and 181Ta are investigated by measuring photons coincident with the stopped pions. These photons were detected with an 80 cm3 Ge(Li) detector and identified as pionic X-rays or rotational γ-rays from the residual nuclei. Pion capture is found to result in the excitation of high-spin states. Yields for the residual nuclei are inferred from the observed γ-transitions and compared to calculations based on the exciton model for pre-equilibrium emission followed by particle evaporation. Preliminary results of a pion stop, γn coincidence experiment are presented and compared to the calculations.  相似文献
8.
Using the CUSB detector we have measured the production of electrons from the decay B→eνX. We obtain a branching ratio for B→eνX of (13.2±0.8±1.4)%. The observed energy spectrum of the electrons implies that the coupling (bc)W dominates over (bu)W. An upper limit Γ(B→eνXu)/Γ(B→eνXc)<5.5% at 90% confidence level is obtained, where Xc contains a c-quark and Xu contains no charmed particles.  相似文献
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A method of resumming perturbation theory is used to re-analyze previous Monte Carlo data of Bhanot and Dashen. We find no inconsistency with universality. There appears to be a relatively large region where Monte Carlo studies can reliably be done.  相似文献
10.
We report inelastic light scattering experiments in the fractional quantum Hall regime at filling factors . A spin mode is observed below the Zeeman energy. The filling factor dependence of the mode energy is consistent with its assignment to spin flip excitations of composite fermions (CF) with four attached flux quanta (φ=4). Our findings reveal a CF Landau level structure in the φ=4 sequence.  相似文献
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