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61.
Potential of coded excitation in medical ultrasound imaging 总被引:6,自引:0,他引:6
Misaridis TX Gammelmark K Jørgensen CH Lindberg N Thomsen AH Pedersen MH Jensen JA 《Ultrasonics》2000,38(1-8):183-189
Improvement in signal-to-noise ratio (SNR) and/or penetration depth can be achieved in medical ultrasound by using long coded waveforms, in a similar manner as in radars or sonars. However, the time-bandwidth product (TB) improvement, and thereby SNR improvement is considerably lower in medical ultrasound, due to the lower available bandwidth. There is still space for about 20 dB improvement in the SNR, which will yield a penetration depth up to 20 cm at 5 MHz [M. O'Donnell, IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 39(3) (1992) 341]. The limited TB additionally yields unacceptably high range sidelobes. However, the frequency weighting from the ultrasonic transducer's bandwidth, although suboptimal, can be beneficial in sidelobe reduction. The purpose of this study is an experimental evaluation of the above considerations in a coded excitation ultrasound system. A coded excitation system based on a modified commercial scanner is presented. A predistorted FM signal is proposed in order to keep the resulting range sidelobes at acceptably low levels. The effect of the transducer is taken into account in the design of the compression filter. Intensity levels have been considered and simulations on the expected improvement in SNR are also presented. Images of a wire phantom and clinical images have been taken with the coded system. The images show a significant improvement in penetration depth and they preserve both axial resolution and contrast. 相似文献
62.
Gottfried J. Palm Maren Thomsen Leona Berndt Winfried Hinrichs 《Molecules (Basel, Switzerland)》2022,27(22)
The bacterial chalcone isomerase (CHI) from Eubacterium ramulus catalyses the first step in a flavanone-degradation pathway by a reverse Michael addition. The overall fold and the constitution of the active site of the enzyme completely differ from the well-characterised chalcone isomerase of plants. For (+)-taxifolin, CHI catalyses the intramolecular ring contraction to alphitonin. In this study, Fwe perform crystal structure analyses of CHI and its active site mutant His33Ala in the presence of the substrate taxifolin at 2.15 and 2.8 Å resolution, respectively. The inactive enzyme binds the substrate (+)-taxifolin as well defined, whereas the electron density maps of the native CHI show a superposition of substrate, product alphitonin, and most probably also the reaction intermediate taxifolin chalcone. Evidently, His33 mediates the stereospecific acid-base reaction by abstracting a proton from the flavonoid scaffold. The stereospecificity of the product is discussed. 相似文献
63.
Holger Lange María Machón Mikhail Artemyev Ulrike Woggon Christian Thomsen 《固体物理学:研究快报》2007,1(6):274-276
We investigated the influence of an epitaxially grown ZnS shell on the phonon spectra of CdSe nanorods of different sizes. The CdSe related Raman peaks shift with addition of a ZnS shell. The longitudinal optical phonon shifts slightly due to strain and the low‐energy shoulder shifts stronger, which can be explained within a model for surface optical phonons. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
64.
Bjrn Thomsen Heidemarie Oelze-Karow Christa Schuster Hans Mohr 《Photochemistry and photobiology》1993,58(5):711-717
In the mustard seedling (Sinapis alba L.) the appearance of photodetectable phytochrome and synthesis of relatively abundant cytosolic hemoproteins (nitrite reductase, ascorbate peroxidase) are stimulated rather than impaired by a photooxidative treatment of the plastids. While the ability to synthesize protoporphyrin IX from exogenous 5-aminolevulinic acid was preserved in the photooxidatively damaged plants, protochlorophyll and chlorophyll accumulation was no longer possible. It appears from our data that in higher plants the pathway of tetrapymole synthesis up to protoporphyrin IX is not adversely affected by a photooxidative treatment of the plastids that destroys the capacity of the organelle to synthesize chlorophyll. 相似文献
65.
66.
Kristoffer Rypdal Jens Juul Rasmussen Knud Thomsen 《Physica D: Nonlinear Phenomena》1985,16(3):339-357
Similarity transformations of the cubic Schrödinger equation (CSE) are investigated. The transformations are used to remove the explicit time variation in the CSE and reduce it to differential equations in the spatial variables only. Two different methods for similarity reduction are employed and the significance of similarity in the evolution of a collapsing wave packet is investigated. Numerical solutions in radial symmetry demonstrate that the similarity behaviour is local in space and time, and that some similarity solutions must be classified as improper solutions. The nature of the collapsing singularity is reexamined. 相似文献
67.
The T1 relaxation times were estimated for 31P-metabolites in human skeletal muscle. Five healthy volunteers were examined in a 1.5 Tesla wholebody imaging system using an inversion recovery pulse sequence. The calculated T1 relaxation times ranged from 5.517 sec for phosphocreatine to 3.603 sec for -ATP. 相似文献
68.
M.V. Abrashev C. Thomsen M. Surtchev 《Physica C: Superconductivity and its Applications》1997,280(4):5078-303
Polarized micro-Raman spectra of different compounds belonging to the Sr14Cu24O41-type structure were studied. In the spectra with parallel polarization along the plane crystal axes of the insulating samples Sr9R5Cu24O41 (R = La, Y) a broad peak near 3000 cm−1, similar to the well-known two-magnon peak in the layered cuprates, was observed. In addition to the Raman lines characteristic for this structure, we observed only in the spectra with parallel to the chains numerous lines between 100 and 1200 cm−1, probably originating from Raman-forbidden infrared-active only LO phonons and their combinations. In the spectra of the conducting compounds Sr14−xCaxCu24O41 (x = 0,7) these features were very weak or disappeared. We interpret these results as Raman evidence for hole doping of the Cu2O3 “spin-ladder” planes and for a redistribution of holes between chains and planes through Ca substitution in the case of the rare-earth-free samples. 相似文献
69.
70.
A. P. Litvinchuk C. Thomsen P. Murugaraj M. Cardona 《Zeitschrift für Physik B Condensed Matter》1992,86(3):329-335
The frequencies of allB
1u
(z) phonon modes predicted by a group-theoretical analysis were measured and found to agree well with recent lattice dynamical calculations for this compound. We report also the determination of two superconducting gap values in YBa2Cu4O8 through phonon self-energy effects in the normal and superconducting conducting state. The gap-to-T
c
ratios obtained from an analysis of these effects are 2
1/kT
c
2.5 and 5.82
2/kT
c
9.2. This coincides with previous results of both phononic and electronic Raman scattering where values of 2.1 and 6.3 were found. We further find anomalous softenings of two phonon modes 40 Kabove T
c
, which correlate with an observed deviation from the linear temperature dependence of the average plasma frequency
p
(T). 相似文献